Eye Protection against Lasers

C. H. Swope and C. J. Koester

From published data on threshold dosage for an observable retinal lesion, calculations were made on theattenuation required to protect the human eye against pulsed laser radiation. Several highly attenuat-ing filters were evaluated in terms of the maximum laser energy against which they provide protection.Because of their very high absorption, some of the filters were found to break or craze at relatively lowenergies. A solution to this problem which provides eye protection against an Nd-doped glass laserdelivering up to 740 J in an impact area 5 mm in diameter on the filter is described. Finally, severalsuggestions are made for protecting the eyes of personnel working with lasers.

Introduction

Because of its high intensity and narrow beamspread,the laser is a device potentially dangerous to the eye ofanyone directly exposed to its beam. It is the primarypurpose of this paper to present some findings concern-ing the nature of these dangers, the extent to whichthey exist, and how they might be avoided and pro-tected against.

Although little is known about all the possible effectsa laser beam might have on an eye, it appears that theretina is the most vulnerable part of the eye. Not onlydo the refractive media of the eye focus the nearlycollimated light of a laser to a small spot, but the retinaand the pigment epithelium provide strong absorption.The result is that, even with conventional light sources,the intensity in the spot can be so high that the tissueis actually coagulated, or at least irreparably damaged,resulting in the loss of sight in the exposed region.Should the spot fall on the macula or optic disk, one'svisual acuity can be drastically reduced in the exposedeye with just one laser flash.

Formulation of the Problem

Ham' and his colleagues at the Medical College ofVirginia have carried out extensive studies in determin-ing the threshold intensity on the retinas of rabbits thatwill produce a retinal coagulation. Figure 12 shows thatfor exposure times on the order of 150-200 use orless, the threshold energy density absorbed by the retinais independent of the duration and size of the exposure.The value that the curves approach is 0.1 cal/cm2 or0.45 J/cm 2 . Ham used a pulsed xenon arc and a carbon

The authors are with American Optical Company, South-bridge, Massachusetts.

Received 1 September 1964.This paper was presented at the 1964 spring meeting of the

Optical Society of America, Washington, D.C.

arc for light sources. Threshold studies with a rubylaser light source carried out by Campbell et al.3 atthe Columbia Presbyterian Medical Center have yieldedpreliminary results that indicate the threshold forexposure to a laser is consistent with Ham's xenon arcdata.

Although the fundamental quantity, threshold dosageabsorbed by the retina, is essentially independent ofwavelength,2 two additional pertinent parameters ofthe eye, the transmittance of the ocular media and theabsorptance by the retina, are quite strongly wave-length dependent. Figure 2 shows the value of theproduct of the transmittance of the ocular media andthe absorptance by the retina as reported by Geeraets4and co-workers in 1960. In using these figures, it mustbe kept in mind that there is not only considerable dif-ference between individuals, but also there can be agreat deal of variation from point to point in a singleeye due to variations in pigment distribution in theretina. Therefore, one can do little more than hopefor an order of magnitude approximation for thecalculations.

II-r- DOI. I.. . IO . 0 I...

Fig. 1. Threshold dose as a function of exposure time for retinalburns in rabbits. Plot of threshold dose absorbed by the rabbitretina for three different exposed areas on the retina as a function

of time. From Ham et al.2

May 1965 / Vol. 4, No. 5 / APPLIED OPTICS 523

10

5 I- 68

A

a1

400 500 000 700 o oo 900 000 1100 1200 1300 1400 1500

W-velength. mp

Fig. 2. The product (TA) vs wavelength for the human eye.Transmittance of the ocular media times the absorptance of the

pigment epithelium and choroid. Geeraets et al.4

However, for the purposes of obtaining some quanti-tative feeling for the energies involved, we can expressthe threshold in terms of a laser's characteristics byusing the data presented in Figs. 1 and 2 along with afew more approximations. If it is assumed that thelaser is pulsed, that it has a uniform energy distributionacross the beam, and that the beam has a small angularspread (), the minimum energy from the laser thatwould be required to coagulate the retina is

- = ARIt(TA)F'

where

Et = energy from laser which produces a threshold lesion,AR = area of image on the retina,It = threshold energy density on retina,(TA) = transmission of ocular media times absorption by the

retina, andF = fraction of laser's energy entering the eye.

AR, the area of the laser illuminated spot on the retina,and F, the fraction of the light entering the eye, aredetermined by parameters of the laser and by thecircumstances under which the potential victim viewsthe laser. The area, An, has a lower limit of about4.6 X 10-5 mm2 corresponding to the diffraction limitof the eye.5

By way of illustration, two bad but very possiblesituations can be considered. In the first, it is assumedthat someone looks directly at the end of a fiber laseror at the junction of a diode laser. Then, if the limitingaperture is the pupil of the eye, the value of Et is in-dependent of the distance from the eye to the laser.For example, if the laser is a pulsed Nd-doped glassfiber laser of 1-mm diam emitting at 1.06 A and havingaOof 50, thenE, = 1.5 X 10-3J. Thiskindofsituationis not extreme, and even an 0.5-J output from such adevice is not difficult to obtain. For a second example,it is assumed that all of the energy from a ruby laseremitting at 694 my with a 0.50 beamspread enters theeye (that is, F = 1) and that the eye is focused forinfinity. E then is about 10-4 J. That is to say that,

under the assumed circumstances, if the laser had anoutput of more than 10-4 J, then the retina would becoagulated, the coagulated area being about 1.7 X 10-2mm2 . In this case, if the laser had had an output of1 J an attenuation of 40 dB would be needed just toreduce the energy density to threshold, with no marginof safety.

AttenuatorsIn considering methods for obtaining this degree of

attenuation, certain subtle but important factors mustbe kept in mind. It is very desirable that, while thelaser light is attenuated, the remaining portion of thevisible spectrum is attenuated as little as possible.N/loreover, if the attenuator is to be worn over the eyes,then it must be of reasonable weight and size. Althoughthese points may seem trivial, it must be emphasizedthat any laser eye protection is effective only when it isused, and that it must not hinder or fatigue the wearerto such an extent that it increases his chances for amore serious accident with equipment, such as high-voltage power supplies, coolants, etc., usually associatedwith laser operation. A shutter, such as a very fastacting photochromic system, would make an idealattenuator, but none has been reported capable ofclosing in the very short durations of laser pulse risetimes and capable of being activated by the laser lightitself. Although reflectors should not be dismissedaltogether as possible protective devices, they are,however, difficult to fabricate with a reflectance as highas 99.9% at one wavelength at normal incidence.Even this reflectance would not provide adequateprotection in most cases. It is even more difficult toachieve these high reflectances for all angles of in-cidence-another requirement that must be met inmost applications. For these reasons the authors haveconcentrated their efforts on finding suitable absorbers.

The transmittances of thin samples of many filterglasses were measured, and the transmittances for thestandard safety lens thickness of 3.5 mm were calcu-

14

12

> 0 HSP-134-B----- __ __I- 569) 570

Z 9782

-J

U a- ~ ~ / "

400 500 600 700 800 900 1000 1100WAVELENGTH (mp)

Fig. 3. Optical density vs wavelength. Four experimentallaser protective glasses: American Optical HSP-134-B, 569, and

570, and Corning 9782. Each 3.5-mm thick.

524 APPLIED OPTICS / Vol. 4, No. 5 / May 1965

I I I I a - I -

0 _ I

o r.Ruby. 694 , >

I .# .!A ,!A Pita ALA AIA NINA ..IA A d~~~~~~~~~~~~~~~~~~~~~~

4

2

24

22

20

)I-

14

C 1212

010

0 8

400 600 800 1000 1200WAVELENGTH (mp)

Fig. 4. Optical density vs wavelength. Two commerciallyavailable laser protective glasses: Schott BG-18, 4 mm thick;

Schott BG-18 + BG-38, each 2 mm thick.

lated using Bouguer's law. Figures 3 and 4 are opticaldensity curves for some promising absorbers. From thestandpoint of protection against lasers with outputs inthe red and infrared while having good visible trans-mittance, glass BG-18 seems to be the best filter glassthat has been found to date.6

In order to evaluate a filter glass in terms of theamount of protection it provides, a quantity calledSafe Laser Output (SLO) can be defined. The relationbetween threshold energy and SLO is given by

SLO - T-S (2)

where

SLO = safe laser output,Et = threshold energy from laser,T= transmittance of laser eye protection at the laser's

wavelength, andS = safety factor.

SLO, then, is the maximum output of the laser injoules for which a wearer of a filter with transmittanceT can be considered protected. With a safety factorof 10, the values of SLO have been calculated forseveral filters under the conditions of the second situationdescribed above where the eye is focused for infinityand the entire laser beam enters the eye. Under thesecircumstances SLO is a function of 02. Thus, in orderto present the SLO values for a certain filter glass for arange of 0, a master curve with a transparent overlayas shown in Fig. 5 has been employed. To use it, thetransparent overlay is placed on the master with themarker placed at the appropriate 0 and the value ofSLO read opposite the appropriate value for thewavelength.

Filter BreakdownIt turns out that, in some cases, at energy densities far

below those which the SLO curve would indicate thefilter should give eye protection, the filter crazes orbreaks. For example, it was found that, when a pieceof felt-polished BG-18 was placed in an Nd: glass laserbeam, it crazed at about 5.5 J in an area with a 5-mmdiam. For this reason it was felt that, by placing asecond filter with a lower absorption coefficient in frontof the strong filter, the combination would have anover-all higher threshold for crazing, and secondly,that should the front filter break, the strong filterwould remain intact to absorb any remaining radiationand to prevent the broken pieces of the front filter fromentering the eyes. Specifically, suppose one wants toprevent a plate of BG-18 from breaking when the unitis struck in a 5-mm spot by a 900-J Nd: glass laserbeam. This sets the transmittance of the front filterat about 6.1 X 10-s at 1.06 ,. A reasonable thicknesswould be 2 mm. Schott glass BG-38 has very nearlythe desired transmittance characteristics. Figure 3is the optical density curve for the combination BG-18+ BG-38. It was found that the felt-polished BG-38glass crazes at an energy of about 9.5 J in an area 5 mmin diameter, thus raising the over-all threshold fordamaging the unit by a factor of 1.7. However, thereally important factor is the protection afforded theeye. This was raised by two orders of magnitude, forwhen the combination was irradiated, the BG-18 filterwas left intact at energies in a 5-mm spot up to 740 J.Goggles employing the combined filter plates witha clear plastic plate between them and the wearer'seyes for added protection from breakage have beenfabricated and are presently commercially available.

Incidentally, it was also established that pitch-polishing the filters raised the threshold 50% over thatfor felt-polished filters.

The type of crazing glass BG-18 undergoes whenirradiated by the glass laser is seen in Fig. 6. Thephotographic effect seen here is obtained by using aHe-Ne laser and illuminating only part of the field.The type of crazing which arose because of a tension

MASTER

WAVELENGTH (m")

OVERLAY

o A ELENGTH ('W)

WAVELENGTH (mO)j

Fig. 5. Method of finding SLO for various values of (0).

May 1965 / Vol. 4, No. 5 / APPLIED OPTICS 525

l

I

Fig. 6. Photograph of the surface of BG-18 irradiated by 5.5J in a 5-mm spot using a glass Nd-doped laser.

Fig. 7. Photograph of the surface of BG-38 irradiated by 9.5 Jin a 5-mm spot using a glass Nd-doped laser. Note the fringe

patterns. Compare with Fig. 6.

produced at the surface should be contrasted with thepattern seen in Fig. 7 where the lower absorptioncoefficient glass BG-38 was used. Here the crazedarea was under compression before the craze developed,and the glass in theouter most layer buckled awayfrom the glass beneath, giving rise to the fringe pat-terns. The other set of rather wide parallel fringes is

probably due to interference between the front and backsurfaces of the glass sample.

SummaryOf the many hazards associated with operating a

powerful laser, we have attempted to describe here themost subtle and, hence, perhaps the second greatest ofthese hazards (the first being the electrical hazardsassociated with the pumping source). It is possiblewith lasers emitting in the infrared to produce manysmall lesions without the victim being aware of it untilthe loss of sight becomes noticeable. Therefore, thefollowing recommendations should be observed:

(1) Never look into a laser beam either direct orreflected.

(2) Wear laser eye protective glasses or goggles.(3) Contain the laser beam as much as possible by

using light traps.(4) Be examined periodically by an ophthalmologist

trained in photocoagulation.(5) Avoid the use of highly reflecting surfaces as much

as possible.

The authors wish to acknowledge gratefully theassistance of N. Brandt, D. LaMarre, W. Shiner, andW. Smith.

References1. W. T. Ham, Jr., H. Wiesinger, F. H. Schmidt, R. C. Williams,

R. S. Ruffin, M. C. Shaffer, and D. Guerry, III, Am. J.Ophthal. 46, 700 (1958).

2. W. T. Ham, Jr., R. C. Williams, W. J. Geeraets, R. S. Ruffin,and H. A. Mueller, Acta Ophthal. Suppl. 76, 60 (1963).

3. C. J. Campbell, K. S. Noyori, and M. C. Rittler, privatecommunication.

4. W. J. Geeraets, R. C. Williams, G. Chan, W. T. Ham, Jr.,D. Guerry, III, and F. H. Schmidt, Archives of Ophthal-mology 64, 606 (1960).

5. H. H. Emsley, Visual Optics (Hatton Press Ltd., London,1939), 2nd ed., p. 48.

6. H. W. Straub, "Protection of the human eye from laserradiation", Rept. of Harry Diamond Labs., Army MaterielCommand, 10 July 1963.

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