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avai lab le at www.sc iencedi rec t .com

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eview

tate-of-the-art: Dental photocuring—A review

rederick A. Rueggeberg ∗

ental Materials, School of Dentistry, The Medical College of Georgia, Augusta, GA, USA

r t i c l e i n f o

rticle history:

eceived 7 October 2010

ccepted 22 October 2010

eywords:

hotocuring

a b s t r a c t

Light curing in dentistry has truly revolutionized the practice of this art and science. With

the exception bonding to tooth structure, there is perhaps no single advancement that has

promoted the ease, efficiency, productivity, and success of performing dentistry. Like most

every major advancements in this profession, the technology underlying the successful

application of light curing in dentistry did not arise from within the profession, but instead

was the result of innovative adaptations in applying new advances to clinical treatment. One

cannot appreciate the current status of dental photocuring without first appreciating the

history and innovations of the science and industry underlying the advances from which

uartz–tungsten–halogen

lasma arc

rgon laser

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eam homogeneity

it developed. This review will place the current status of the art within the context of its

historical progression, enabling a better appreciation for the benefits and remaining issues

that photocuring has brought us. Lastly, the manuscript will present thoughts for future con-

siderations in the field, offering suggestions as to how current advances in light-generating

science might yet be adapted for dental use.

emy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41ernative initiators” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42els. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

© 2010 Acad

ontents

1. Introduction to the review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2. Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.1. Abbreviated history of light curing in dentistry. . .2.2. UV-curing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.3. Visible light curing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.4. Quartz–tungsten–halogen lights . . . . . . . . . . . . . . . . . . .2.5. Vital tooth bleaching and introduction of the “alt2.6. The argon-ion laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.7. Plasma arc lights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.8. High-intensity QTH lights. . . . . . . . . . . . . . . . . . . . . . . . . .2.9. Issues resulting from application of high light lev

2.10. Light emitting diodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.11. First generation leD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.12. Second generation LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author.E-mail address: frueggeb@mail.mcg.edu

109-5641/$ – see front matter © 2010 Academy of Dental Materials. Puoi:10.1016/j.dental.2010.10.021

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

blished by Elsevier Ltd. All rights reserved.

40 d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 39–52

2.13. Third generation LEDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442.14. LED construction styles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442.15. Summary of history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

3. Contemporary issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453.1. Adequacy of exposure durations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453.2. Custom, in-office exposure guide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453.3. Photocuring training device and precision curing unit evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

4. Current methods and findings in curing unit characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464.1. Hand held curing radiometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464.2. Conventional irradiance measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464.3. Spectral irradiance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464.4. Determining beam irradiance inhomogeneity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474.5. Importance of beam imaging to curing light characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474.6. Nonuniformity of wavelength distribution within the beam . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 484.7. Curing tip movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

5. Future developments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48. . . . .. . . . .

6. In summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction to the review

There is hardly a clinical procedure performed in contem-porary dentistry where photocuring of some material isnot required. The process seems so simple, perhaps eventrivial: just point and shoot. What could be so compli-cated? However, there is much more to this story thanthat. In order to appreciate the convenience and benefitstoday’s state-of-the-art offers, it is first necessary to under-stand and value how this technology developed over time.This manuscript will provide a short review of the historyunderlying development of photocuring in dentistry, focus-ing more on issues related to construction and operationof the light sources themselves. The perspective for appre-ciating current technology will be established, the issuesthat we currently face will be addressed, and those we haveyet to overcome. A summary of recent key findings is pro-vided, and thoughts on important factors affecting the futureof development in dental photopolymerization are devel-oped.

2. Literature

2.1. Abbreviated history of light curing in dentistry

From the late 1940, with power/liquid, self-curing direct,acrylic-based restorative materials, to the late 1960s whenthe dual-paste, self-cure composite systems were popular[1,2], use of direct polymer-based restorative materials haschanged the way dentistry is performed and perceived. Theearly paste–paste systems, although rivaling their prede-cessors with respect to appearance and durability, still leftmany clinicians wanting for a faster process of fabricat-ing tooth-mimicking restorations, where the working time

was almost infinite [3], the setting time was only secondsand not minutes, and where colors were stable and long-term wear allowed successful use in the posterior segment[4,5].

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

2.2. UV-curing

Like many advances in dentistry, the technology for using lightto polymerize resin-based materials did not originate withinthe profession, but instead was an existing technology thatwas adapted for dental use [6–9]. The first photocuring unitswere designed to emit ultraviolet light (about 365 nm) througha quartz rod from a high pressure mercury source and wereintroduced in the early 1970s [10]. This development was seenas a revolutionary step in dentistry, for it allowed a “cure ondemand” feature, which was previously unattainable usingthe self-curing products. Typical exposure durations were 20 s,but 60 s provided enhanced results [11]. Filled, photocurablecomposites as well as sealant materials were available [11,12],and were used in very innovative ways to not only restore car-ious processes, but to also repair tooth fractures and provideeasily performed esthetic results [13,14]. The phoinitiatingsystem relied on benzoine ether-type compounds [1,15], whichbroke down into multiple radicals, without need of an inter-mediary component [16]. The spectral distribution of lightsources of the time with the absorption spectrum of the ini-tiator help to correlate these two parameters [17]. Althoughsome of the restorations placed using this early technologyhave proven to be remarkably successful [18], in general theprocedure was fraught with many issues. First, because of thelimited ability of light to penetrate deep within the material,incremental buildups were required instead of bulk place-ment, and were limited in depth [19,20]. In addition, concernswere voiced about the potential for harmful effects of the shortwavelength energy being exposed to human eyes (cornealburns and cataract formation) as well as possible changes inthe oral microflora [1,21].

2.3. Visible light curing

Amazingly, only a few years following the introduction of UV

radiation for curing dental restoratives, the ability of using vis-ible radiation was introduced: February 24, 1976. On that day,Dr. Mohammed Bassoiuny of the Turner School of Dentistry,Manchester, placed the first visible light-cured composite

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estoration on Dr. John Yearn, the then head of developmentf this effort by Imperial Chemical Industries (ICI) of England

22]. Again, development of the chemistry for doing so did notrise from the dental field, but instead adapted from use inhe field of polymerizing thin resin films for printing, metalnd plastic coatings, and paints [23,24]. The optimization of aisible light-curing photoinitiation system composition usingamphorquinone and a tertiary amine co-initiator was key tohe success of this effort, and remains the most popular basicormulation in use today [15,25–28]. The unit consisted of auartz–tungsten–halogen (QTH) source having heat absorb-

ng glass and a bandpass filter allowing only light between00 and 550 nm to pass: the wavelengths [29,30] required toctivate the photoinitiator, camphorquinone. ICI went on toartner with Johnson and Johnson to introduce the first light-ured resin composite and light: the FotoFil system [29,31].he advantages of using visible light were that 2-mm thick

ncrements could typically be placed using from 40 s to 60 sxposure from a QTH source, and that the potential formationf cataracts and oral microflora alteration were minimized

31]. However, direct retinal burning and advancement of mac-lar degeneration were now a potential for ocular damage,s the wavelength needed to initiate the visible-light initi-ted systems fell directly within the frequencies known toause immediate, and permanent damage resulting from reti-al burning [32,33]. Thus, practitioners were advised to placefiltering film between their eyes and the curing light to pre-

lude ocular damage, while also allowing high levels of longeravelengths to pass in order to adequately visualize the treat-ent field while curing: the so-called “blue-blockers” [34,35].

.4. Quartz–tungsten–halogen lights

he QTH light source was not developed for this purpose, butas, instead, advanced by engineers at General Electric forse in aircraft lights, where small but very bright and durableources were needed [36]. However, the QTH light becamehe mainstay of dental light curing for many years, into theate 1990s. During that time advances included a wide rangef adaptations for convenience and efficacy. Bulb wattage

ncreased from 35 W up to 100 W for hand-held units [3], andp to 340 W for table-top models [3]. Output values rangedrom an average of 400 to 500 mW/cm2 up to an extreme of000 mW/cm2 from one unit: The Swiss Master Light, Elec-ro Medical Systems, Nyon, Switzerland. The filtered spectralmission matched the absorption profile of camphorquinoneery well [17]. Units were either hand-held (gun style) usinglight source within the gun, from which light was emitted

o the target through a bundled glass fiber light guide. Theun was connected to a base power unit using a flexible cordontaining electrical wires. The adaptation of a curing guno accept a wide variety of sizes and styles of different lightuides fitting with similar focal points lead to the customiza-ion of specific types of light delivery to oral locations [37].he other basic model was a table top unit, which containedhigh-Wattage source directing filtered radiation to a flexible

lass bundle cord [38], or a liquid light guide. The QTH sourcesn the hand-held units lasted from 30 to 50 h [30], and utilizedhe halogen cycle to remain clear from tungsten contamina-ion on the inner wall of the quartz envelope [30]. Sources were

( 2 0 1 1 ) 39–52 41

readily available, easy to install, and relatively inexpensive.Typical exposure duration times to provide uniform proper-ties in a 2-mm thick composite increment ranged from 40 s to60 s [39], and controls allowed selected exposure intervals [40].Units had to be fan-cooled in order to extend the working timeof the source and allow the halogen cycle to operate correctly.

2.5. Vital tooth bleaching and introduction of the“alternative initiators”

Vital tooth bleaching was becoming an overnight success inthe early 1990s. However, following this type treatment, man-ufacturers were not able to provide direct esthetic restorativematerials that were of high enough value to match the newlybleached teeth. This situation arose because the photoinitiat-ing system of that time only used camphorquinone, whichis a bright canary yellow, and photobleaches only slightlyupon exposures within clinically relevant times [16,41]. Thus,after curing, the restoration tended to have a yellow, resid-ual tinge. In order to provide restorative materials of highvalue, manufacturers resorted to utilizing other photoinitia-tors that were used for the UV coating and printing industries,which had a small portion of their absorption range withinthe short, visible light spectrum [42]. These compounds werea class of initiator that directly broke into multiple radicalswithout need for any co-initiator, and, although they werepale yellow in color, photobleached to clear once utilized [43].Examples of such compounds are bis(2,3,6-trimethylbenzoyl)-phenylphosphineoxide [15,44] (Ciba Specialty Chemicals, Inc.,Basel, Switzerland), also known as Irgacure 819, and 2,4,6-trimethylbenzoyl-diphenylphosphine oxide [15,45] (BASF Cor-poration, Charlotte, NC), commonly referred to as Lucerin®

TPO. Later, an yellowish oily initiator (1-phenylpropane-1,2-dione, PPD), was introduced to help broaden the overallabsorption of initiators between 400 nm and 500 nm [15].When combined with CQ, a synergistic effect was found,allowing the concentration of CQ to be reduced, thus decreas-ing the yellow residual color following photocuring [46–48],however, others found no reduction in yellow [48]. Also, toreduce the time needed to obtain maximal bleaching effect,manufacturers of bleaching agents were advocating exposureof the solutions to bright light in order to heat the compo-nents and hasten their breakdown into peroxy radicals. Thewavelengths required for this process were short, near the vio-let and near ultraviolet, and as well as within the blue region.Thus, light curing units now serve a dual-fold function: to pho-toactivate resin-based restorative materials, and provide anenergy source to hasten vital tooth bleaching.

2.6. The argon-ion laser

As is typically the case, the use of the argon laser to photopoly-merize acrylics did not arise within the dental community,but was instead a product of industrial development in thecoating industry. The argon-ion laser was marketed first toenhance the effects of vital tooth bleaching in Europe, and

is still used for that purpose today [49]. However, when firstintroduced to the United States [50], only the clinician couldoperate the unit, and not auxiliary personnel [51], so the lightbecame more profitable to use as a dental polymerization

l s 2

42 d e n t a l m a t e r i a

source. The initial delivery of power from the laser to the toothwas directly through the end of a fiber optic cable. However,due to the divergent nature of that radiation, other methodswere developed in an attempt to make a fairly collimated beamof coherent energy, whose target power was not related tothe tip-to-tooth distance [52], as was the case with conven-tional QTH light guides. The devices proved very effective inproviding high physical properties in dental restorative mate-rials [53], and because of the unit’s tremendous radiant output,some noted that shorter exposure times were needed to pro-vide similar properties as a QTH source [54], while some didnot [55]. The size and footprint of the unit were very largeat first [56], but a single source could be adapted so that onelaser could feed multiple operatories using a network of fiberoptic cabling [56]. Over time, the unit became much smaller,and could easily fit into an operatory, but because of weight,needed to be on a cart, if moved from room-to-room. However,owing to the high expense (near $5000) of a typical unit, theinability to replace the source by office personnel, and the ele-vated room temperatures resulting from the device use, thiscuring system became outdated in a short time. The emis-sion profile of argon-ion lasers marketed in the Unites Stateseliminated the strongest output at 514 nm for some reason.This wavelength was typically used to provide hemostatis [52],and was present as a selectable feature on units marketedin Europe. Excluding the 514 nm peak, the major output fromdental argon lasers in the US occurred at 488 nm, just on thesharply declining, long wavelength leg of camphorquinoneabsorption. Other minor emission peaks at 457.9 nm, 468 nm,and 476.5 nm do occur within high CQ absorption levels, andare thus effective in curing, but others emissions are not:496 nm [52]. High speed pulsing of the light was found toprovide enhanced surface and depth conversion over con-tinuous exposure [57], and some researchers advocated thateach brand and shade of composite required an individual-ized energy delivery for optimal performance [58]. Designsfor a portable diode laser for dental light curing have beendeveloped, but to date, no commercial product has arisen [59].

2.7. Plasma arc lights

Because dental auxiliaries were prohibited from using theargon laser in the US, technology was imported from Europe(French patent #2,305,092, 1976), that had already met with agreat amount of success: the plasma-arc curing light. Devel-oped in the mid 1960s [60], this type source was not initiallydeveloped for dental purposes, but instead was used to providebroad-banded radiation for visualization of operating fields(endoscopy and colonoscopy, for examples) as well as for mini-mally invasive medical procedures. The source consists of twotungsten electrodes separated by a small distance, encased ina high pressure gas-filled chamber, having a synthetic sap-phire window through which the light emission was directedfrom a parabolic reflective surface [30]. A high electrical poten-tial is developed between the two electrodes, which thenforms a spark, ionizing the gas, and providing a conductive

path (a plasma) between the electrodes. Once the initial sparkis established, electronics then adjust the operating current tomaintain light generation through a variety of sophisticatedfeedback system [30]. The gas originally used in dental PAC

7 ( 2 0 1 1 ) 39–52

lights in Europe contained argon, and was extremely high inoutput, allowing claims of “sub-second” exposures in adver-tisements to be used to replace conventional 40–60 s exposuresusing the QTH light. These units had to be highly filtered, asthey generated tremendous amounts of infrared light (result-ing in heat generation at the target) as well as ultraviolet(having dangerous ozone formation potential). In dental appli-cations, these types of light sources were pulsed and initiallydeveloped for curing UV-polymerizable resins, although nocommercial unit could be found [19]. They were later adaptedfor use in visible-light cured products [61]. Thus, the table topunit contained the conventional heat-absorbing glass and vis-ible bandpass filters, but the light emitted after this entereda three-foot-long liquid light guide [62], which acted as a verylong optical filter, eliminating UV and IR radiation, and pass-ing only visible light [30]. The typical output from these typesof lights was near 2000 mW/cm2, and was broad banded: from380 to 500 nm, with a naturally occurring peak near 460 nm,where CQ has its optimal absorption. Originally designed inEurope as the Argo HP, claiming the ability to cure compos-ite using “sub-second exposures”, the unit was attempted tobe marketed in the US by DMDS, Inc, of Salt Lake City, UT,as the Apollo 9500. It is conjectured that the source in thisunit contained argon, which allowed it to emit higher thanconventional xenon, but did not last as long. However, issueswith licensing led to the US version of the light introducedin August, 1998 as the Apollo 95e [51]. This unit was quitesmall, and touted provision of adequate exposure times of3 s as being equivalent to that of a 40 or 60 s exposure froma QTH light. However, the reason why this unit provided nolonger continuous exposure than 3 s did not arise from itsadequacy of providing sufficient energy to optimally cure acomposite within that time, but instead was attributed to thepower supply driving the source not containing sophisticatedenough feedback to allow continuous output for any longertime. To perform adequately clinically, multiple 3-s exposureswere typically required [63,64]. The unit offered a variety ofremovable curing tips at the end of the light guide: curing tips:430 nm or 460 nm for resin curing and 400–500 nm for bleach-ing. It was from the confusion concerning tip selection forcuring specific restorative materials that the confusion startedrelated to the issue of what light will work with which com-posite or bonding resin. Subsequent PAC units overcame manyof the initial shortfalls of the Apollo 95e: broad banded output(not requiring a special bandpass tip end), longer, continuousexposure times, and a general tendency to provide adequatecomposite curing using a single 10-s exposure [30].

2.8. High-intensity QTH lights

The QTH light was then competing with PAC units for mar-ket share. In order to respond to the new, high output device,manufacturers of QTH lights used different mechanisms toincrease the overall output of their units, and claimed theywere “equivalent” to a PAC unit with respect to performance.First, source settings were available to “boost” light output

over normal values. This mechanism really just drove the fila-ment at a higher voltage, exceeding accepted tolerance valuesthan the bulb manufacturer suggested. Settings in such out-put modes were no longer than 10 s, as longer exposures would

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everely degrade the operating lifetime of the source. Anotherechanism of increasing output was the development of the

turbo-tip”. This device was a rigid glass fiber bundle, butad the individual fibers drawn while hot, so that the bun-le diameter was smaller at the distal, emitting end than athe proximal, receiving end of the guide. By doing so, the samemount of power was present at both ends, but was distributedver a much smaller area at the emitting end, resulting inbout a 1.6 times increase in irradiance [65]. This type tip istill widely used in contemporary LED units to help increaseverall output values. However, even with both of these fea-ures in operation, the output of the QTH did not match that oftypical PAC light of the day [30]. The future for the QTH light

s not “bright”, as many governmental agencies are banninghis tremendously inefficient incandescent light sources fromeneral usage soon. Thus US government has stipulated dead-ines by which incandescent light sources will stopped being

arketed: elimination of the 100-W bulb in 2012 and endingith removal of the 40-W source by 2014 [66].

.9. Issues resulting from application of high lightevels

uring this time of application of ever increasing irradi-nce levels to teeth and gingiva, issues arose related to thepplication of excessive heat to the target. Clinicians weredmonished to direct light output on their thumb nail to sensehe level of “heat” evolved from a light. In addition, with suchigh intensity levels, the PAC lights were now noted to causehigh degree of polymerization stress, as the resin was poly-erized so fast that relaxation was not allowed to occur in

he polymer network before it became vitrified. To overcomehis problem, researchers found that if the light intensity waselivered in such a manner as to control the rate of curing,tress relief would occur by allowing the resin to flow prior toitrification. It was hoped that in so doing the potential forebonding between the restoration and the tooth would beuch lower [67]. In addition, less heat would generate in the

arget during the restorative process [68]. However, little dif-erence was noted in microleakage studies [69–71]. Thus arosehe concept of “soft-start” polymerization.

When first introduced, this exposure method was addeds a selectable feature on QTH lights, and resulted in an ini-ial short (10-s) low-level output near 100 mW/cm2, which wasollowed by an immediate jump to full output power for theemainder of the exposure: step curing [72]. Later changesesulted in a time-based increase of the light initially applied,ith the remainder of exposure providing full output: ramp

uring [73]. Still another modification was provided as thepulse-delay” technique, where, for the last composite incre-

ent to be cured, a very low level, short duration exposureas given (3 s at 200 mW/cm2), The clinician was advised

o treat another patient (for 5–10 min) while the compositeas allowed to flow and relieve stress, after which a bolusf higher output power was provided to complete the cure

30 s at 500 mW/cm2) (BISCO VIP). However, the results did not

roduce any remarkable differences to what was being usedonventionally [74].

An issue with use of this type exposure delivery is the ade-uacy of energy delivery at the bottom of an increment. Thus,

( 2 0 1 1 ) 39–52 43

the possible correlation between enhanced marginal adapta-tion, reduced shrinkage, and lowered curing stress using thiscuring protocol might have been related to an overall lowerextent of cure at the bottom surface [75,76]. As long as an ade-quate exposure is provided during the high-output phase ofthis technique, concerns about under polymerization are notan issue [77].

Variations of a soft-start feature are still offered on manycontemporary light curing units, even LEDs. Comparisonof material conversion between conventional 40 s exposureand that when using a ramped soft-start procedure did notindicate any significant different difference [73], however,contraction strain and the polymerization exotherm weredecreased [73,78]. To remain competitive, manufacturers ofPAC lights also attempted to utilize a soft-start mode to reducethe tremendously high stresses and temperature rises devel-oped from their use. However, the operational characteristicsof the PAC light do not provide for low irradiant output, asthe spark arising from even the minimal voltage needed tomaintain its presence produces more irradiance than does aconventional QTH light using regular output. Thus, benefits ofsoft-start curing with these types of units could not be realized[79]. However, one manufacturer incorporated a modulatedlevel of output during exposure in order to maintain an overallhigh irradiant output, but also to decrease target temperature[80].

2.10. Light emitting diodes

The 50th anniversary of the invention of the light emittingdiode (LED) is fast approaching [81]. Use of this technology hastruly changed our lives, and will continue to do so, as it provesto be an efficient, cost-effective lighting source. These solidstate devices rely on the forward-biased energy difference(band gap) between two dissimilar semiconductor substrates(n-type conduction band, and p-type valence band), to deter-mine the wavelength of emitted light [30]. They are muchmore efficient than previous types of dental light sources, arelight weight, and can be easily battery powered for portabil-ity. During the middle-to-late 1990s, tremendous strides weremade in the video display, optical communications and solid-state lighting industries with the introduction of light emittingdiodes (LEDS). At that time, the red laser was used for pro-viding the source of reading optically encoded video discs.Because the wavelength of red is longer than that of blue, thephysical size of the hole through which the light had to passin order to create a digital signal was also large. With devel-opment of shorter wavelength, blue LEDs, smaller sized holeswere required than with red, and literally many more openingscould be placed into the same physical area occupied by fewerand larger holes that transmitted red light. Thus, more digi-tal information could be stored on smaller sized media, anddevelopment in LEDs within this wavelength range became afocus of interest [82].

Blue emissions were developed using indium–gallium–nitride (InGaN) substrates in the early 1990s [81]. This

was also a color needed to activate phosphors to emit yellow,enabling the first white-appearing LED in history [81,83]. Itwas thus coincidental that the output range of the newlydeveloped blue LEDs fell within the maximal absorption of

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CQ, enabling the possibility of using blue LEDs as a dentallight-curing source. As has been mentioned previously, it didnot take long before the blue LED made its way into a dentalcuring light [30,84].

2.11. First generation leD

These lights were first introduced to the commercial mar-ket at the end of 2000 [85], with the introduction of theLUXoMAX light (Akeda Dental A/S, Lystrup, Denmark), how-ever the concept was developed much earlier: 1995 [86,87]. Atypical design of first generation LED curing lights used anassemblage of multiple, individual LED, single-emitting “5 mmlamp” LED elements (each chip providing 30–60 mW) into afocused, axial [88,89] or planar array [87], arranged such thatthe combined output of the luminaire was sufficient enoughto provide sufficient energy to activate CQ. Arrays of fromseven (Freelight, 3 M/ESPE, St. Paul, MN) to 64 (GC-e-light, GCAmerica, Alsip, IL) were available, but even with those num-bers, the extent of radiation was not sufficient to warrantcompetition with respect to providing decreased exposuretime over that of the standard QTH light. In addition, bat-tery technology relied upon use of NiCad cells, which wereplagued with poor performance and memory effects. How-ever, because some photo-curable restorative products usedonly the short wavelength, alternative initiators, the new blueLED lights would not polymerize them, nor would the argon-ion laser. This situation started the great confusion over whatlight will adequately polymerize which restorative material, aphenomenon that lasts even to this day. Needless to say, theeffectiveness of LED lights over that of conventional QTH unitstook a while to understand, but it had to do with the amount ofradiation emitted within the maximal absorption needs of CQ:with blue LEDs providing much more output within this area(450–470 nm) than from a typical QTH light [90]. As a result,it was possible to provide equivalent resin curing in shortertime with an LED light that provided less measurable outputthan from a longer exposure from a QTH light emitting moretotal light. In general, however, the overall curing potential ofthis first-generation LED light was much less than those ofconventional, competitive source types of the time [91].

2.12. Second generation LED

Advances in the LED chip industry in the early 2000s led tothe ability of placing multiple emitting die pads on a sin-gle chip, greatly increasing overall light output [84]. Again,dental manufacturers incorporated the new 1-W chips intocuring lights, developing what is referred to as the “secondgeneration” LED lights. Chip manufacturers were now fabri-cating chips specifically within the wavelength requirementsfor dentistry, and labeling them as “dental LED blue”. Luxeonis Philips Lumileds Lighting Company’s trade name for theirhigh-power LEDs which dissipate 1 W or more. At first, therewere two types of Luxeon chips used in second generationlights, according to the development of the technology: the

1 W chip (Luxeon LXHL-BRD1 or –MRD1 generating 140 mWoutput), and the 5 W chip (Luxeon LXHL-PRD5 or –MRD5, gen-erating 600 mW output). Characteristic of these units is thegreat increase in output power over that of the first gener-

7 ( 2 0 1 1 ) 39–52

ation [81], with one 5-W chip providing similar luminanceas 10–20 of the individual 5-mm types of the first genera-tion. However, a similar wavelength range output persisted aswith the first generation, resulting in a continued inability tophotocure restorative materials using only short wavelengthinitiators. Battery technology also improved, and NiMH energypacks became the typical power source. However, because ofthe generation of tremendously high power in such a smallarea, the temperature within the chip was now a concern, astoo high a temperature would result in permanent chip dam-age, if not thermostatically cut off prior to that point [84]. Thus,units had heavy metal heat sinks or large metal surfaces todissipate chip heat [81,92]. In addition, the return of fans [92]to curing lights was seen – something everyone thought wasgone forever. Recently, a 10 W blue LED has become availableespecially for dental applications (LZ4-00DB10, LedEngin, Inc.,Santa Clara, CA, as well as a 15 W unit (LZ4-00CB15, LedEn-gin, Inc.), and can provide up to 4.2 W and 5.6 W of radiantflux, respectively. With the dramatically increased chip out-put, second generation LED lights were now reliably able tooutperform their commercial competitive curing light typesusing shorter exposure times [93,94].

2.13. Third generation LEDS

In order to enable polymerization of restorative materialsutilizing more than only CQ as initiator, curing light manufac-turers resorted to providing LED chip sets that emitted morethan one wavelength [95]. The first of these units utilized a sep-arate 5 W blue LED central chip surrounded by four low powerviolet (around 400 nm) LEDs: the Ultralume 5, Ultradent Prod-ucts, South Jordan, UT. Thus, by combining the output fromthese two wavelengths, light was provided at wavelengthsthat were effective for not only CQ, but for the alternativeset of initiators as well, creating the equivalent of a “broadbanded LED” curing light. Other manufacturers incorporatedthe violet chips located next to other blue chips within a sin-gle LED element: LZ4-00D110, High Efficiency Dental Blue + UVLED Emitter, Led Engin, Inc. with 1 UV die emitting 0.76 W and3 blue dies each emitting 3 W. This ability to generate multiplewavelengths from a single LED light led a “third generation”of LED dental curing lights. Typically, these units use eitherNiMH or Li-ion battery technology. One manufacturer cur-rently incorporates three different wavelength chips: VALO,Ultradent Products. Units of this generation are quite effectivein providing sufficient irradiance at appropriate wavelengthsto be able to polymerize any type of dental restorative material.

2.14. LED construction styles

Contemporary LED lights are either those styled after the oldQTH guns, with a turbo-tip fiber optic bundle, or are a pencil-type unit, having the emitting chip set at the distal end ofthe unit, or a similar fiber optic guide. The lights are eithertotally driven by mains connection, are battery operated, orboth. Those with the chips at the distal end of the unit either

have no coverage over the elements, a sealed or removableclear plate covering it, or utilize a fixed lens. Advantages ofthe pencil style body include the ability to gain access to manymore locations in the mouth than possible with the hard, long

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lass bundled guide, as well as less possibility of breakage,hould the unit drop.

.15. Summary of history

ven with all the advantages that solid state light curing hasrought to dentistry, many of the issues correlated with otherypes of light sources (polymerization shrinkage and stress,ntrapulpal temperature rise, and confusion over method ofight intensity application) persist at present. Misunderstand-ng and frustration still exist over compatibilities betweenights and restorative resins, as well as the advantages and dis-dvantages of using curing units with such tremendously highutput levels [96–98]. Even with all the improvements made

n photocuring over the last 40 years, after 10 years of service,nly 43% of almost 100,000 resin restorations were still in ser-ice, with performance described as “comparable only to theorst performing amalgam restoration [99]”. Only with thisackground, can the most contemporary findings and issuesan be better placed in context of the entire field.

. Contemporary issues

.1. Adequacy of exposure durations

here is still much confusion over the ability of a given light orspecific composite combination to be used together to pro-

ide optimal polymerization within a short clinical timeframe.linicians desire to perform quality dentistry using minimalhair time. Curing light manufacturers advertize use of theirights for a single duration of output, often regardless of com-osite type, shade, or clinical distance the tip is held fromhe restoration. Likewise, manufacturers of composite recom-

end specific exposure durations for their products that ofteno not match those of lights made by competitive companies

93,100–104]. Thus, the clinician is left wondering which sug-ested time is “correct”, and as a result, tend to over-exposeestorations to be on the “safe side”. However, in so doing, theonger exposure results in generation of more heat within theooth and surrounding, exposed tissues, leading to possibleost-operative, iatrogenic complications [100,105,106]. Clini-ians will often utilize a hand-held dental curing radiometers an indicator of the potential a curing unit has for photocur-ng resin-based materials. However, these instruments haveeen shown to be inaccurate [107] and not good predictorsf clinical performance [108], and are really only meant to besed as a relative indicator of light output value over time

108].

.2. Custom, in-office exposure guide

ecently, a simple in-office test has been introduced wherebyclinician can develop a custom curing exposure guide, using

heir own light unit, any type of composite, and at any tip-o-composite distance using simple items readily available

n a common office [109]. This method relies upon the clini-ian establishing the relationship between exposure duration,nd tip distance on the thickness of cylinder of remaining,nscrapable composite made from exposing composite-filled,

( 2 0 1 1 ) 39–52 45

single-dose compules. The test also works for syringe mate-rial. The results from this procedure have been correlated withthose of a more sophisticated, laboratory test when deter-mining optimal exposure using biaxial flexural strength [109].This method allows for the determination of optimal exposuretime for any given combination of curing light and composite,thus avoiding the confusion of relying on manufacturer-recommendations for each. However, the test requires timeas well as use of an existing stock of composite material. Ifthe test is performed on a periodic basis, the clinician canalso know exactly how to adjust exposure duration of a light,found to be decreasing in output, to provide optimally effec-tive results once again. In addition, the relative performanceof a given composite when using two or more different lightunits can be determined, as well as relating the performanceof different composites when using a given curing light.

3.3. Photocuring training device and precision curingunit evaluation

A newer device, combining precise, laboratory spectral tech-nology with clinically relevant measuring conditions withinprepared dentiform teeth in a manikin head has been devel-oped (MARC (Managing Accurate Resin Curing), BlueLightanalytics inc., Halifax, Nova Scotia, Canada). This device usescosine correctors to capture emitted light from curing units,and directs the output to a calibrated spectral radiometerembedded within the manikin head. The sensors can beplaced anywhere in the dentiform, replicating a variety ofdifferent classifications of tooth preparations, locations, andpreparation depths. Output from the radiometer is fed intoa laptop computer, where custom software is used to pro-vide a myriad of real-time and accumulated data: spectralirradiance, total energy delivered over a given exposure dura-tion, and the estimated exposure duration needed to delivera specified energy dosage. Training office personnel to prop-erly perform light curing is very effective using a device suchas this. The effects of minor alteration in tip position, move-ment, or location are instantly displayed in real-time, and theultimate consequence in terms of altered energy delivered isdetermined. The unit can also be used in a “scientific mode”,whereby a calibrated, high resolution spectral irradiance plotis available for display or digital output to a spreadsheetdevice. The device has been successfully used to test literallyhundreds of clinicians in a “before training” and “after train-ing” mode, and has shown that, with effective education ofthe operator with respect to hand stabilization, tip angula-tion and position, and direct visualization the operating fieldthrough blue blocker glasses, use of the instrument providesthe optimal result in energy delivery [110]. The device can alsodiscriminate the ability of different lights to deliver adequateenergy levels between various tooth locations using this sim-ulated clinical model [111]. Accurate, recordable curing lightperformance over time is also capable of being determinedusing the same device, for without such determination, clini-cians are totally with the knowledge of the condition of their

photocuring unit [112].

The relevance of this type measurement relies on the factthat there is a critical energy level needed to optimally poly-merize dental composites [113]. In addition, the estimation of

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adequate energy delivery currently relies on the reciprocitybetween exposure duration and power density. Some liter-ature indicated that such a relationship exists [114], whileothers show evidence that it does not [115,116], and that thepolymer network developed using fast or slow delivery of ener-gies is different [117]. In addition, the spectral delivery ofenergy within the absorptive needs of the photoinitiator sys-tem present in a given composite is also not accounted for[42,48,90,118,119]. However, the device is capable of such dis-criminative capacity with software improvement, and minorhardware changes should allow in situ testing of real-timeenergy delivery to actual resin composites while curing.

4. Current methods and findings in curingunit characterization

4.1. Hand held curing radiometers

With wide ranging claims of curing light performance toutedby manufacturers, it becomes of great importance to be ableto distinguish the true operating characteristics of curinglights so that valid evaluation of those claims can be made[120]. Unfortunately, many published articles rely on the valid-ity devices called “hand-held curing radiometers” to provideaccurate data from which performance among lights is cor-related to some parameter measuring polymerization extent:depth of cure, flexural strength, hardness, etc. Great discrep-ancies among measurement of light unit output have beenfound using such hand-held dental curing radiometers, val-idating that they are not considered reliable indicators inranking the potential for depths of cure among lights [108].The hand held radiometers are really just a photometers cal-ibrated in radiometric units [30,107]. A typical device consistsof a detector port on which light from the emitting end of theunit is directly placed. A drawback of this type unit is thatthe detector must be smaller than the diameter of the tip inorder for the meter to work correctly [120]. Thus, the unit isnot exposed to the total emitted power, and is assuming thatemissions are homogeneously distributed over the emittingface end. Secondly, there is no way to discriminate among thespectral emission of lights, as the unit only responds to allradiation passed to the photodiode through whatever restric-tive bandpass filter it uses. This limitation means that notonly is the distribution of power with respect to frequency isnot known, but the unit may also not be responding to wave-lengths outside of the bandpass which might be present. Inaddition, these type devices only provide an indication of exi-tance irradiance (the emitting tip-end value), which is not anindicator of light performance when held at a distance fromthe tooth [121,122]. Thus, these devices are really not meantto report accurate data to characterize light unit emission, butare instead designed to operate as a method to evaluate theperiodic performance of a curing light over time for purposesof detecting output changes [107,123].

4.2. Conventional irradiance measurement

True characterization of the emissions from dental curinglights has been provided in the literature since the introduc-

7 ( 2 0 1 1 ) 39–52

tion of the devices [17]. With more current technology, theseprocesses have become less expensive and provide higherresolution, and respond much faster than the older types ofinstruments [124]. A well equipped light measurement facilitywould first determine the total power output of the unit mea-sured using a well calibrated thermopile. These instrumentsare black body absorbing plates with layers of thermocou-ples that respond to absorption of radiant energy in a linearmanner over a very wide range of frequencies. For accuratemeasurement, all radiation from the exiting port must fall onan area larger than the beam. Because dental curing lightshave a very wide difference with respect to beam diversionand increasing tip distance, this means that the tip end needsto be held as close to the detector plane as physically possible,without touching it [122]. Unfortunately, when following ISO106500-1 testing standard for determining exitance radiationof curing lights, the tip ends are held at a distance from thedetector plate [125]. Depending on the distance between thetop plane of the thermopile to the depth where the black plateis located, the tip will be held at varying distances, and willthus provide inaccurate results for light power emission [126].Therefore a wide diameter, shallow-welled thermopile is bestfor evaluation of dental curing lights. In addition, it is opti-mal to have at least two independent, calibrated thermopilesystems so that the tolerance of calibration measurementscan be checked. However, a thermopile will provide accurateindication of the emitted power, but without respect to itsspectral distribution: measuring the radiant flux (mW). Know-ing this value, the area of the emitting end over which lightis being projected is measured accurately using a microscopeand is divided into the total power to derive the irradianceor exitance irradiance (more casually referred to as “powerdensity”): mW/cm2.

4.3. Spectral irradiance

As mentioned previously, the thermopile does not discrimi-nate with respect to frequency over which it measures power:it only measures total power. Spectroradiometers are used toevaluate dental curing radiometers and measure power gen-erated over the visible spectrum. The spectral distributionof dental curing lights has been determined since UV unitswere introduced [17,21]. Early technology required large andvery expensive equipment. However, advancements in the sci-ence have allowed very precise measurements to be accuratelymade using only a modest investment. These instrumentsare commonly referred to as “hand-held spectroradiometers”(model USB2000 + RAD, Ocean Optics, Dunedin, FL).

To analyze light emission, a device needs to be utilized thatcaptures all radiant output from the emitting end. One typedevice is an integrating sphere, which consists of an entranceport into which the emitting end of a curing light is placed,and within which light reflects multiple times off of a highlyreflective, diffusing film. A fiber optic cable is fitted to an exitport of the sphere, and connects to a small spectroradiometer.This device then separates the input light into its spectral com-

ponents using a grating, and projects that beam onto a fixed,multiple element diode array. The more diodes in the array,the smaller the entrance slit size, the more finely the spectrumcan be resolved. The array output is analyzed using software,

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here the contents of each pixel captured over a specific timerame are determined and displayed on screen as a plot ofmitted power (photon count) with respect to wavelength.hese systems must be calibrated against a highly accurate

ight source that typically resides inside of the sphere, so thathe sphere, connecting cables, and spectral radiometer are allalibrated as a single instrument. Any change in tightening orabling will negate the calibration. This type system is bulky,s the sphere is typically 6 in in diameter, and requires a ded-cated, computer-controlled power supply for the calibrationamp. A limitation of the device is that it cannot determineariation in power or wavelength values across the emittingip, as the internal reflections average out all such differences,roviding only a single, value. The output measure from usinguch equipment is the spectral radiance: mW/nm.

A simpler experimental system involves use of a cosineorrector (CC-3, Ocean Optics) connected to a fiber optic cable,hich again is attached to the spectroradiometer. The cosine

orrector is a lambertian diffuser (typically opaline glass,pectralon®, or Teflon) that captures light from all incidentngles within 180 degrees from the plane on its exposed side.ypically, the optical input diameter of the corrector is approx-mately 4 mm, so it does not capture all emitted power from

tip larger in size, and assumes a uniform distribution ofoth irradiance and wavelength across the curing unit tip face.owever, by providing a known capture area, the resulting

pectral distribution can be reported using irradiance units,ot just power. The entire system (cosine corrector, and spec-rometer, must also be calibrated using a traceable source thats capable of providing uniform light of known quantity acrosshe cosine corrector face. However, once calibrated, the instru-

ent is portable, as it consists only of a small cosine corrector,fiber optic cable, and palm-sized spectral radiometer, and

asily operates from a laptop computer. Data generated usinghis type setup provides the spectral irradiance of a light cur-ng unit: mW/nm/cm2.

.4. Determining beam irradiance inhomogeneity

eam irradiance mapping (intensity contour generation) is notew to the dental field, as large discrepancies were detected

n the UV curing lights [21] and as early as 1985 for visibleight models [127]. Recent work [124,128,129] has developed

method for observing the uniformity of light across themitting end of contemporary light curing units. The instru-entation needed for such work is adapted from analysis of

aser beams, where knowledge of how the laser interfacesith a surface is essential to its performance: if the beamower is higher in the core or higher in the periphery. Theasic setup of such an instrument consists of a CCD camerapon which the beam is targeted. The system is configuredifferently for analyzing dental curing light beam uniformity,s there must be a translucent target present and a lens-ng/iris system to adjust for focusing and saturation of theetector. The distance between the camera and target is fixedo that accurate dimensional measurements can be made of

he beam image at that plane. The target material is criti-al to obtaining accurate results, in that the material muste opaque enough to block imaging the tip through the tar-et, but translucent enough to provide sufficient light to pass

( 2 0 1 1 ) 39–52 47

and make an image on the camera side. A target material thatseems to perform quite well for tip-end imaging is 1500 gritground glass (DG100X100-1500, Thorlabs, Newton, NJ). First,the total power emitted from the tip end is measured using acalibrated system: either a thermopile or integrating sphere.Next, the emitting end of the curing unit is placed in directcontact with the target glass. It is essential to obtain accuratedistinction from a signal and background noise, so a systemmust be used that provides uniform baseline values for allpixels when no signal is present (UltraCalTM, Ophir-Spiricon,Logan, UT). The camera-side image is then analyzed usingsoftware and displayed on the computer screen. The lens aper-ture must be adjusted so that the maximal power observed bythe camera is set just under the saturation limit of the detec-tor, providing full dynamic range to scale the result. The totalmeasured power obtained using the thermopile is enteredinto software, and the computer distributes that value acrossthe pixels within the beam image, and color renders differentvalue ranges. By providing accurate dimensional informationat the target plane, the result is a calibrated, color-coded imageof the beam irradiance dispersion across the emitting face.

4.5. Importance of beam imaging to curing lightcharacterization

Beam imaging is a necessary component for characterizingoutput from a curing light. Often, it is very difficult to deter-mine the exact area over which light is being emitted from theunit end. This situation is especially true for LED curing lightsthat have chips located at the distal end of the handpiece, andutilize no lensing system, but instead, have the emitters onlybehind a clear plastic plate, or behind nothing at all. Unlessthe precise emission area is known, determination of validirradiance values are not possible to calculate. When deter-mining the irradiance of curing lights utilizing glass bundledfiber optic guides, many researchers measure the diameter ofthe bundle and assume that all exiting light is actively dis-persed across the entire bundle end. Using the beam profiler,recent research has identified specific incidences where theactive, lighted area of a fiber optic bundle is less than that ofthe actual bundle area itself [124]. Without such knowledge,large errors in irradiance values assigned to such a light wouldoccur.

A measure of the uniformity of power distribution withinthe beam is called the “top hat factor”, or THF. In this calcula-tion, software determines the weighted average of each pixelmaking up the beam image, and relates it to the maximumresponse seen. Thus, a number from unity (representing per-fectly uniform distribution) to zero is presented, with lowervalues indicating less uniformity [124].

The extremely important aspect of such type analysis isthat a histogram of the irradiance distribution as a functionof pixel values occupying the area on the beam face can becalculated [124]. When such procedures are performed, it isseen that many lights display very wide ranges of irradiancevalues, while others seem to be more narrow-ranged [124].The

critical point being made is that conventional methods for irra-diance determination assign a single, average value (using anintegrating sphere or cosine corrector) to a curing light. How-ever, what should be reported is the histogram of irradiance

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distribution, as it is not normally distributed and truly indi-cates the range of output values the unit presents. Providingsuch information gives a much better comprehension of thequality of a unit than does presentation of a single, averagedvalue. In addition, the effect of irradiance distribution whendifferent types of light guides are used on the same unit bodycan be identified and quantified [124]. The clinical relevanceof such differences in irradiance is pointed out by the factthat localized areas of irradiance value deviation at the tipend have been precisely matched to the mapping of hardnessvalues on the surface photocured by that unit, as well as ata depth of 2 mm [129]. These differences could affect the rateof localized curing, and of the resulting polymerization stressdevelopment, which could also affect the integrity of the resin-tooth interfacial bond. In addition, the differences could affectlocalized conversion values and wear resistance, leading to adifferential in restoration surface wear over time [130].

4.6. Nonuniformity of wavelength distribution withinthe beam

The beam analysis system can also be used to measure thehomogeneity of wavelength dispersion across the beam tip[131]. For units having broad banded sources (QTH and PAClights), there is no difference, as all portions of light in thebeam contain all output wavelengths. However, in the thirdgeneration LEDs, the off axis arrangement of the differentcolor chips is reproduced in the forward beam image, result-ing in localized areas of radiation occupied by only one chips’wavelength: very little frequency mixing within the beam[130]. Such a condition may result in disproportionate utiliza-tion of resin systems containing multiple photoinitiators, eachrequiring different wavelengths for activation, not only at thetop, irradiated surface, but also within the depths of the com-posite. Again, conventional methods of curing light analysiswould have not detected such differences, however with thesenew analytical techniques, clinically relevant characteristicsare detected and quantified, and valid distinctions among lightunits can be made.

4.7. Curing tip movement

Knowing that discrepancies in both irradiance and wavelengthmay exist in a light beam obviates the reaction to move thebeam during exposure in order to better distribute values.Large discrepancies were noted in the beam profiles for theearly UV light [21], and as a result, the clinician was advisedto move the tip during exposure to purposefully average outlocalized diffrerences and hopefully create a more uniformlypolymerized restoration [1]. Later work was done in this areausing large diameter and smaller diameter QTH light guides[132]. It was found that circular movements, as opposed tofixed, or overlapping exposures resulted in a much poorerdistribution of composite hardness, and that large and smalldiameter light guides provided the best results when used ina rigidly fixed position. Similar, but contemporary work per-

formed using LED lights also confirmed that tip movement(either fast or slow circular or rastering motion) resulted inlower overall hardness at both the top, irradiated surface,as well as at a 2 mm depth [133]. However, in that study,

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all exposures lasted the same amount of time: 20 s. It wascontemplated that, if longer exposure duration had beenapplied when moving the tip, hardness values would havebeen greater better due to the increased energy applied to eachlocation.

5. Future developments

With respect to curing light characterization, the methodsoutlined above should be considered for incorporation intonational as well as international standards, as existing stan-dards and evaluation protocols are obviously deficient. Thephysical properties of resins cured with various lights havebeen found to be inferior, although all units passed the ISOstandard for measuring depth of cure, conforming that thestandard method is not truly discriminating in characteriz-ing curing light performance [134]. In the near future, curinglight manufacturers, and the clinical offices using light emit-ting devices will more than likely be held responsible formonitoring the radiant output from such units as a resultof regulatory agency requirements. Such requirements arealready being adopted in Europe for nearly all classes of prod-ucts that emit any sort of electromagnetic radiation [135,136].However, because of the well documented retinal damage oflight within the spectral range currently used for photoinitia-tion causes, curing lights may be helped to an even higher levelof scrutiny. Therefore, clinical offices may have to either payfor professional, certified services to periodically provide char-acterization of their lights (as they currently are required to dofor their X-ray machines), or purchase a qualified device thathas been certified and worthy of such measurements. With thealarmingly poor performance and maintenance noted frommultiple studies assessing curing light status in private prac-tice [137,138], accountability of the clinician to provide optimalpolymerization performance should, and hopefully will, berequired.

Future studies investigating material properties of resinsresulting from photopolymerization by dental curing lightsshould provide better documentation and description relatedto the true output characteristics of the lights used in the studyso that better correlation can be made between resulting prop-erties and exposure to radiation. The research communitymust revisit current standardized methods of determiningperformance of restorative resins (such as determining depthsof cure), and devise more appropriate testing that accounts fordiscrepancies in localized irradiance and wavelength discrep-ancy. Currently, only a 4-mm diameter composite specimen isused for this purpose. Clearly, any great off-axis distributionof either irradiance or applied wavelength may significantlyalter the performance of a light, relative to one where thedistributions have resulted in a high concentration of lightin the center of the tip. To this end, it would be the bur-den of manufacturers to develop lights having more uniformoutput characteristics. Internationally accepted standards forallowing specified amounts of deviance in beam characteris-

tics could be developed, permitting more valid comparison ofresin specimens photocured with such units.

With world energy sources dwindling and the need forenergy efficient use of power, the application of light emit-

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ing diodes in medicine and dentistry will continue to grow.lready, use of light for diagnostic and therapeutic purposesas become wide spread. However, again, as the LED indus-ry makes advances in light-generating capabilities, the dentaleld will typically be the first to incorporate any new tech-ology into clinical practice. Of specific interest is the use ofrganic light emitting diodes (OLEDs). These products allowxtremely thin and flexible video displays to be made, but theurrent technology is such that output levels remain muchower than those of conventional LED chips. However, it isot unreasonable to envision impression trays with walls andoors lined with these emitting films, designed to evenly irra-iate all surfaces of a photo-curable impression material. Inddition, use of quadrant type trays for the simultaneous andide area illumination of teeth for photocuring of sealants or

ight-enhanced vital bleaching or veneer bonding would be areat advantage, as would be the application of thin, adaptablelms for photocuring resins retaining orthodontic brackets.

Another area where tremendous advances have been madeith incorporation of state-of-the-art lighting technology inedical therapy has been the field of quantum dots. These

ubstances are semiconductors with conducting characteris-ics closely related to the size and shape of their individualrystals. Smaller crystals display a larger band gap. Thus, ashe difference in energy between the highest valence bandnd lowest conducting band increases, more energy is neededo excite the dot, but also, more energy is released when therystal is back in its resting state. Such technology allowsuorescence to occur at shorter wavelengths than those ofxcitation. This condition would allow red light exposureo result in emission of blue light, which might be usedor photoinitiation. If such dots were incorporated into ahotopolymerizable resin composite, it might be possible tonable the entire mass to release light within itself, resultingn a “curing from within”, which is an aspect always dreamedf, but never realized for dental applications.

. In summary

t is truly a remarkable time in dentistry. So many recent tech-ical advancements have become clinical realities and areow necessary tools for the contemporary practice of den-istry, it is difficult to imagine how techniques could get much

ore automated. However, the beauty of our profession ishat, despite the complexity of instruments and tools used,he dentist is still the health care provider who spends the

ost time in direct contact with a non-sedated patient. Thus,entists tend to build loyal patient bases, because the practicef the profession remains a one-on-one, individualized treat-ent, and not a remote, robotic, impersonal procedure. This

s truly a great time for dentistry.

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