A highintensity diffuse light source of ultrashort duration for reflectedlight colorphotographyP. Krehl and S. Engemann Citation: Review of Scientific Instruments 64, 1785 (1993); doi: 10.1063/1.1144011 View online: http://dx.doi.org/10.1063/1.1144011 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/64/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Ultrashort electron bunches generated with high-intensity lasers: Applications to injectors and x-raysources Appl. Phys. Lett. 83, 3888 (2003); 10.1063/1.1626016 Adjustable long duration highintensity point light source Rev. Sci. Instrum. 52, 863 (1981); 10.1063/1.1136683 SolidState, HighIntensity Monochromatic Light Sources Rev. Sci. Instrum. 30, 995 (1959); 10.1063/1.1716448 Submicrosecond HighIntensity Light Source Rev. Sci. Instrum. 30, 103 (1959); 10.1063/1.1716472 A High Intensity Short Duration Spark Light Source J. Appl. Phys. 21, 1269 (1950); 10.1063/1.1699587

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

130.18.123.11 On: Fri, 19 Dec 2014 01:20:49

A high-intensity diffuse light source of ultrashort duration for reflected-light color photography

P. Krehl and S. Engemann Ernst-Mach-lnstitut der Fmunhofer-Gesellschaft, Eckerstrasse 4, W-7800 Freiburg, Germany

(Received 9 April 1993; accepted for publication 14 April 1993)

For use in reflected-light color photography a diffuse light source has been developed which has an annular geometry (347 X 140 mm in diameter) and coaxially surrounds the optical beam of recording. The light source has a luminous peak intensity of 200 Mcd and emits a Sash with a duration of 180 ns FWHM. The principle of operation is based on a chain of nine V-shaped light-emitting gliding sparks, each consisting of at least 50 interstitial copper electrodes for the generation of micro gliding discharges. They are all connected in series and driven by a modified nine-stage Marx-surge generator. Substituting each V-shaped gliding spark by two U-shaped Xe-filled flash lamps doubles the light output as well as the flash duration.

1. INTRODUCTION

Since the earliest beginning of photography the frontal imaging of an object in reflected light has been the most widespread recording method, because it is simple in prac- tice and provides a clear view, true both in perspective and color. Reflected-light photography requires much light, de- pending on the angle of the incident light and the reflection properties of the subject surface, its color, and roughness. Additionally, a broad source is needed to give an almost shadowless effect. On the other hand, transmitted-light photography such as silhouette and schlieren methods re- quires less light, a point light source, and in the case of schlieren visualization a pretentious optical setup. They are defmitely of specific value in science for their ability to record shock waves and turbulences; however, they can only image the outline of an object.

Reflected-light photography has also been well estab- lished in high-speed photography of fast moving objects, simply by illuminating the object in a darkened room with a short-duration flash to stop motion and recording it with a still camera at opened shutter. Quite frequently an elcc- tronic flash unit is applied. This so-called single-shot method only provides good results as long as the flash light source is sufficiently intense to expose the film and short enough to produce a blurless image. However, when enter- ing submicrosecond reflected-light photography, it be- comes a great technical problem to realize a light source which delivers an extremely intense light pulse level at ultrashort flash duration.

In the case where black-and-white photos of the object are sufficient and a direct recording on film is required, this di&ulty can be eluded by using modern ultrashort pulse lasers. Alternatively, if only light sources of insufficient intensity or extended flash duration are available, it is also possible to apply low-light level image intensifying, self- shuttering electronic devices such as image converter cam- eras and microchannel plates, but the indirect exposure on 8lm is connected with a loss in resolution.

Color photography is becoming increasingly important in all phases of photography, including also the field of high-speed photography. * Advantageously, color increases

the contrast of the image. In scientific applications this facilitates the analysis, even if the colors have not been correctly transferred to the film when flash light sources with imperfect spectral distribution have been used. Be- sides classical applications in interferometry and photo- elasticity, high-speed color photography offers new prom- ising applications; for example, in ballistics to study during flight ablation effects of hypersonic projectiles and to visu- alize surface inhomogeneities of explosively formed projec- tiles, in the temporal and spatial analysis of debris emerg- ing from high-velocity impacted structures, and in thermographics of rapidly heated surfaces using thermal indicating paints.

For stop-motion photography of high-rate phenomena quite often submicrosecond flash durations are essential and in the case of high-speed color photography the flash light source should be matched spectrally to daylight to apply common color film. The latter condition can be ful- filled only by using thermal sources such as electric sparks which emit primarily a continuous spectrum. The require- ments of a high light intensity and a short duration are difficult to realize simultaneously and exclude each other to some extent, because a high-temperature plasma, large in dimensions to provide a diffuse source, cannot be turned off immediately, but rather decays with a given minimum deionization time. It is the objective of this article to re- sume this classical problem and to describe a new diffuse light source of high intensity and short duration which has been developed at the Ernst-Mach-Institut for single-shot reflected-light color photography in ballistics.

II. A BRIEF HISTORY

Curiously, high-speed photography started very briefly after the invention of photography and even began with the ambitious snapshot photography in reflected light: in 1851 William H. Fox-Talbot photographed a page taken from the London Times rotating at high speed on a revolving disk and being illuminated by a spark discharge from a Leyden jar.2s3 He successfully obtained a blur-free image, because the spark duration, somewhere in the microsecond regime, was very short in comparison with the event. In

1785 Rev. Sci. Instrum. 64 (7), July 1993 0034-6748/93/64(7)/ 1785/9/$6.09 @ 1993 American Institute of Physics 1785 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

130.18.123.11 On: Fri, 19 Dec 2014 01:20:49

1885 Ernst Mach, who studied the flight of supersonic pro- jectiles, recognized that an electric spark is indeed an ideal short-duration light source; however, bullet photography in reflected light would require a spark of high-intensity which was not available to him. Consequently, he favored transmitted-light methods and applied them to the study of projectiles and shock waves which now as before are used in every ballistic laboratory as standard methods of visu- alization.

Since then many laboratories have devoted themselves to the development of pulsed light sources, for a general survey on pulsed light sources the reader is referred to various review articles.3-7 Only a few constructions have been reported which can meet the requirements for submi- crosecond reflected-light color photography. Most of the reported constructions apply a fast electric pulse discharge into an electric spark or a flash lamp. Other rather exotic solutions to the problem encompass an electron bombard- ment of semiconductors, and argon bombs. In the follow- ing the practically most important methods for the gener- ation of submicrosecond light flashes will be summarized.

A. Electric spark sources

Spark light sources are now as before the most com- monly used flash sources and have been built as free sparks, partly or completely confined capillary sparks, and guided or so-called gliding sparks. They are low cost, sim- ple in construction, and very versatile to match different flash durations.

The best-known representative of a spark light source employing a free spark is probably the Fischer Nanolite.5*8 The extremely short flash duration of only 8 ns is achieved by discharging a short transmission line with a small initial charge. The light output is quite low and limits applica- tions to reflected-light photomicrography. The light inten- sity of free sparks can be increased considerably by oper- ating the spark in a rare-gas atmosphere or by increasing the pulse current, however, at the expense of an increase in flash duration.

Point light sources, mostly constructed as partly con- fined sparks in coaxial Libessart geometry, are widely used in transmitted-light photography and often self-produced by ballistic laboratories. Some are commercially available with flash durations ranging from 150 (Ref. 9) up to 300 ns (Ref. 10) with peak intensities of 2 Med. The emitted light, leaving the open capillary head-on, is much brighter than from the Nanolite; however, it is often still not suffi- cient for reflected-light photography. Again as in a free spark, an increase of the pulse current increases the light output, but also the pulse duration. On the other hand, in completely confined sparks an increase of the pulse current can cause the plasma to become opaque, resulting in no further increase in light output.* ’

Another approach to increase the light output is to use gliding sparks. Gliding sparks can either be generated along the surface of an insulator or a semiconductor and mainly differ in their trigger characteristics rather than in their light conversion efficiencies6 In comparison with a free spark, the length of a gliding spark at the same oper-

1786 Rev. Sci. Instrum., Vol. 64, No. 7, July 1993

ation voltage can be increased by a factor of 10-20. This enlarges the surface of the emitting plasma, greatly en- hances the light output level, and also improves the source diffusion. Often doped insulators are applied to realize also long sparks, or an increase in conductivity happens by it- self by electrode evaporation and surface wear. Compared to a capillary spark the plasma of a gliding spark is only semi-confined and therefore can quickly expand and cool down at the insulator surface, thus cutting flash duration Additionally, since the resistance of a gliding spark is higher than that of free or capillary sparks, it can also be matched to the characteristic impedance of the electrical pulser with a better conversion efficiency.

It is interesting here to note that as early as 1867 Au- gust Toepler” appreciated the simplicity and high light output of a gliding spark. Its geometry, linear and spatially stable in an optical experimental setup, was also ideally suited to match the schlieren knife in his famous schlieren method. Subsequently, Mach and his contemporaries pri- marily used gliding sparks to simply generate shock waves of different geometries and strengths.13 But in 1908 the construction of Toepler’s gliding spark light source was resumed and improved by Toepler’s son who photo- graphed sound waves emitted by an air spark. As a light source he used a piece of chalk which was moistened with water.14 Then nearly forgotten for half a century, the glid- ing spark, combined with a Kerr cell shutter, was redis- covered by Fiinfer” as a light sonrce for use in high-speed spark cinematography. Shortly after Schardin and his team’6r’7 developed the powerful “Defatron,” which was applied for single-shot reflected-light exposures and built in a large number. The principle involved a Pyrex tube with annular electrodes at both ends and connected to a pulse capacitor, and a central wire for triggering. Applying a high-voltage pulse to the trigger wire, it created an intense gliding discharge along the tube wall of about 1 ,us dura- tion.

Table I summarizes only those submicrosecond gliding spark wnfigurations which have a sufficient light output suitable for direct film recording of objects in front light. To increase the light output of a gliding spark, various methods have been performed: (i) increasing the length of the gliding spark by using a porous ceramic and impreg- nating it with an electrolyte,‘8 (ii) confining the spark by a transparent window,” (iii) operating the gliding spark in a xenon atmosphere at low pressure and splitting the dis- charge into many branches,24V25 and (iv) focusing the beam into a narrow spot by using a reflector.2o*21 The optical diffusivity of a gliding spark light source can be improved by modifying the linear spark into an S-shape geometry” or by using a multiple discharge.24,25 Of all recent gliding spark sources the Microflash” is the only one which is also commercially available. Based on the principle of the “De- fatron” but of a shorter flash duration and more handy, it was developed at MIT by Edger-ton and Killian and also widely applied by them and their team.24 Their legendary color photos, for example, of a flying bullet cutting the Jack of Hearts, of William Tell’s apple shot, or of the splash of a milk drop, are a milestone in the achievements

Diffuse light sowrw 1786 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

130.18.123.11 On: Fri, 19 Dec 2014 01:20:49

TABLE I, Comparison of various gliding spark arrangements with present results. U is the operation voltage, E the energy of flash, r the PWHM flash duration, 4 the peak light intensity, and HCP the horizontal candela light output. The pulse. current was provided either by a capacitor discharge (CD) or by a nine-stage Marx-surge generator (BAG).

Pulser YW

Gliding spark

md Ref. Type (k:) 6 SUPpofi Environment

1952’4’7 CD 22 200 Pyrex tube air, 1 bar 1954” CD 14 24.5 Ceramics air, 1 bar 1956’9 CD 22 193.6 Polyethene air, 1 bar 196do CD 16 6.4 Glass air, 1 bar

1962” CD 4 36.8 TiOl disk air, 1 bar 1963” CD 9 2.2 ceramics air, 1 bar 1971% CD 15 15.7 Glass Xe, 0.13 bar present MC 35 90.9 P.C. card Nl, 1.17 bar trsults

With bare light source. bwith specular parabolic reflector 160 mm in diameter. With tubular A.l foil reflector, 400 mm in diameter. dWith nine V-shaped gliding sparks and nine Marx sparks. With 18 U-shaped Xe-tilled flash lamps and nine Marx sparks.

Length (mm)

100.0 50.0

100.0 25.0

14.0 dia. 5.5

50.0 dia. 9X 110.0

Output data

r 4 HCP (ns) (M-=0 (cds) Re.IlXWkS

1000 “Defatron” 900 53 10 kHz strobe

loo0 S-shaped spark 400 9.v 3.6 linear spark

5o.ob linear spark 800 20.0 innular spark

40 0.1 linear spark 220 20.0 5.0 spoke-shaped sparks 180 200.0” 73.0 hybrid systemd 180 > 6oo.ff hybrid system 360 2OO.ff 131.0 hybrid system’

of high-speed color photography and have become well- known also beyond the community of high-speed photo- graphers.

B. Flash lamps

Flash lamps filled with rare gases are predominantly used to rovide flashes of millisecond and microsecond

R duration. However, it becomes very diflicult to also real- ize intense flashes of submicrosecond duration, because the afterglow of light set limits. Favorably, the afterglow of gliding sparks operated in air at atmospheric pressure is of a red color. On the other hand, rare gases commonly ap- plied in flash lamps show a strong afterglow which, shifted more into the blue part of the spectrum, increases consid- erably the effective exposure time. A short afterglow re- quires that the energy density in the plasma is kept small which is opposite of the need for a high intense source. Another condition, just as equally important for all ul- trashort duration spark light sources, is the provision of very short current pulses. This dictates the use of high voltages and small capacitors with a low internal series inductance.

To a certain extent flash lamps can also compete with gliding sparks in the submicrosecond regime. Cordin’s 500 ns flash unit,28 utilizing a small flash lamp in a reflector, provides similar data in respect to source size and peak light output as compared to the Microflash21 mentioned above.

The flash duration can be further reduced by cutting the afterglow tail, for example, by using a flash lamp with a short length and a large tube diameter, and by operating the flash lamp at low energy. Based on these principles, a 15 kHz stroboscope has been constructed.29 Using a high- efficiency short-arc xenon flash lamp, EG&G type 801 and operating at low energy of only 100 mJ, the flash duration

1787 Rev. Scl. Instrum., Vol. 64, No. 7, July 1993 Diffuse light source

could be reduced to only 150 ns. However, the low-light output limited applications to microscopic samples.

C. Semiconductor converters

Certain disk-type solid-state semiconductor targets, bombarded by a high-current density electron beam, gen- erate a super radiant, rather monochromatic light with an extremely short duration. This method of stimulated emis- sion proved to be very successful for 2 ns reflected-light photography of a bullet striking an aluminum rod.30 The effective source area amounted to 1.5 cm2 and the peak source brightness to lo9 stilb corresponding to a peak light intensity of 1.5 X IO9 cd ( I stilb= I cd/cm2), In addition, when using a mixture of various target materials, several distinguished wavelengths in the visible spectrum with greatly different decay times can be stimulated, thus pro- viding from the same event simultaneously both an instant and a streak exposure. However, this unusual method re- quires a pulsed-electron accelerator and is limited to rela- tively small objects and close distances.

D. Explosive light sources

Argon bombs are primarily constructed as flash light sources of microsecond duration and useful for high-speed cinematography to illuminate a full scene. For single-shot photography the flash duration it too long, but can be re- duced by decreasing the thickness of the shock-compressed argon layer. Using a thin-layered argon bomb with a charge of 4.8 g and a diameter of 1.3 cm, Bagley3* realized an extremely short flash with a duration of 10 ns and a luminosity of about 8 X lo7 lumen. However, improving the diffusivity as well as the light output by extending the size of the argon layer would lead to impractically large charges with tremendous hazards to personnel and shock interferences with experimental arrangements. Therefore, the use of explosive light sources has been limited mostly to

1787 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

130.18.123.11 On: Fri, 19 Dec 2014 01:20:49

special experiments when high explosives are used anyway. Argon bombs have been applied successfully also in reflected-light color photography, since they approximate daylight quite closely, for example in the study of the det- onation mechanism of explosives using a rotating mirror camera32 or of projectiles and shaped charges using a ballistic-synchro-streak method.33

111. INSTRUMENT DESIGN

A. Physical concept

Compared to previous gliding spark light sources (cf. Table I), the present construction34 applies a large number of individual small gliding sparks which, all connected in a series, are operated simultaneously via a fast modified Marx-surge generator. This new concept of a light source allows to better meet the requirements for ultrafast reflected-light color photography than previously reported light sources. Specified in more detail, it offers the follow- ing advantages:

(i) an increase in light ou~ut level by using many light emitting gliding sparks, incorporating also the Marx switching gaps into the generation of light, applying a pe- riodically damped discharge of high ringing frequency with a relatively large current reversal, and focusing the light by using a reflector or a Fresnef lens;

(ii) a decrease in frash duration bj subdividing each gliding path into many individual micro gliding sparks, reinforcing micro plasma cooling by using interstitial cop- per electrodes, reducing the effective afterglow by operat- ing all sparks in a N2-atmosphere, driving all micro sparks simultaneously, and minimizing the effective inductance in the discharge circuit by providing a compact puiser con- struction;

(iii) an improvement in the optical quality of colorpho- tos by increasing the diffusivity of the light source by pro- viding hundreds of light emitting sparks distributed over a large source area, and providing a real frontal illumination of the object by arranging all sparks symmetrically around the optical beam of recording.

B. Electrical circuit and gliding spark setup

The concept of driving simultaneously a large number of micro gliding sparks requires a series connection of all sparks and the application of a high-voltage pulse of some 100 kV. However, since the gliding sparks are operated in a gas atmosphere, this high voltage would immediately cause a breakdown at the chain terminals or between indi- vidual sparks. Therefore, the chain of gliding sparks was split up into nine equal links, each constructed as a V-shaped gliding spark and incorporated into a stage of a modified nine-stage Marx-surge generator. This circuit modification limits the peak voltage across each link to the maximum charging voltage of 35 kV, thus avoiding a high output voltage of 3 15 kV as in the case of a standard Marx- surge generator.

The electrical circuit is shown in Fig. 1. Each stage includes a series connection of two pulse capacitors Co= 33 nF,35 two charging resistors R = 2 MR,36 a Marx switching

1788 Rev. Scl. Instrum., Vol. 64, No. 7, July 1993

r I’ , r ‘1 ’ c: i : ,: * tr qr]-rt,l !a zpq’? 1’1: iz!

FIG. 1. Electrical circuit of the modified nine-stage Marx-surge genera- tor. Its load, a chain of light emitting gliding sparks, is equally split up and distributed among each stage, thus avoiding a high output voltage. The bold lines mark the low-inductance path of discharge.

gap, and a V-shaped gliding spark. Together with a Marx gap each V-shaped gliding spark forms a light-emitting hybrid spark ensemble. Each Marx switching gap is con- structed as a free spark gap with spherical brass electrodes 10 mm in diameter and a gap length of 12 mm% only the spark gap in the last stage was adjusted to a spacing of 3 mm. Each V-shaped gliding spark is fabricated from two strips of a printed circuit board,37 thus facilitating to match

:, I ; r” C,>,,;Fr-;‘l J l.l$

r [,,IlY II i I “Ii 3.f j ip~r* io,42mml

1

ia) - ?Omm

- ‘- 2.54mm ( L

id FIG. 2. One leg of a Y-shaped gliding spark: (a) Schematic. Applying a high voltage at both terminals produces a large number of individual micro gliding sparks between neighboring copper lugs. (b) Formation of a single discharge path, compare Figs. 4, 8, and 10, and (c) branching into multiple discharges. Note also the closely integrated hlarx switching gap.

Diffuse light s-mr~e 1788 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

130.18.123.11 On: Fri, 19 Dec 2014 01:20:49

FIG. 3. AutoCAD-generated exploded view of the optical experimental setup for single-shot reflected-light photography in ballistics. The projectile is photographed by a 35-mm still camera with 90 mm focus length right through the gliding spark light source. For color photography it is useful to further enhance the light diffusion by inserting a tubular reflector between flash source and subject.

the total resistance of all gliding sparks to the characteris- tic impedance of the generator. The strip geometry is shown in Fig. 2(a). The copper soldering lugs act as in- terelectrodes for the micro gliding sparks, each V arrange- ment contains at least 50 micro sparks.

The Marx-surge generator is charged up to a maxi- mum voltage of U,= 35 kV using a high-voltage power supply with a current reading in small steps of only 10 FA.~* This proved to be very helpful in the detection of corona losses in the discharge chamber during the charging cycle. They have to absolutely be avoided because they prevent a full charging of the last capacitors in the line, thus leading to serious trigger problems.

Triggering of the generator occurs with a triggered spark gap,39 R,=20 Ma and CT=250 pF. To minimize during discharge noise interferences into the ground wire and to permit the incorporation of a miniature Rogowski coila with grounded shielding for current measurements, the discharge path, marked by bold lines in Fig. 1, is grounded by a resistor RG= 100 kfi.

C. Mechanlcal construction

The Marx-surge generator is housed in a nylon body and immersed in transformer oil. The important output characteristics of the light source, such as flash duration and peak intensity, are essentially the same for operating the gliding sparks in air or nitrogen. However, the use of nitrogen avoids the generation of toxic ozone which, caused by the some hundreds of micro sparks, would be released in a large quantity. All sparks are housed in a Nz-pressurized annular discharge chamber with a diameter

1789 Rev. Sci. Instrum., Vol. 64, No. 7, July 1993

of 347X 140 mm and operated at about 0.2 bar above at- mospheric pressure. The Plexiglas window in front of the discharge chamber reduces the acoustic noise emission as well as provides for all Marx switching gaps a sealed, of the test site independent gas atmosphere, thus ensuring a reli- able triggering.

The light source can be mounted on an optical trian- gular bench. A part-sectioned drawing of the light source is shown in Fig. 3. As depicted in Fig. 1 each stage contains a series connection of two capacitors C,, thus forming the Marx capacitor C=Cd2. They are cylindrical and have a double bushing configuration. To minimize the effective inductance in a series connection, a coaxial current return as illustrated in Fig. 4(b) has been provided. Compared to a simple series connection by a strap, Fig. 4(a), this re- duces the full width at half maximum (FWHM) flash du- ration by about 30 ns.

For convenience, each combination of a V-shaped glid- ing spark and a Marx switching gap is mounted on an interchangeable support plate as shown in Fig. 5. Alto- gether nine in number, they are uniform in construction. Figure 6 shows a photo of the light source taken at the moment of flashing.

D. Optlcal setup

The light source has been primarily designed to front- illuminate objects with diameters up to 0.5 m at a relatively close distance to the source. At EM1 specific fields of ap- plications are, for example, supersonic projectiles in free flight and penetration phenomena. Two examples of opti- cal setups are shown in Fig. 7.

Diffuse light source 1789 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

130.18.123.11 On: Fri, 19 Dec 2014 01:20:49

r- cooxiol shield

FIG. 4. ExampIes of a current return in the series connection of the two pulse capacitors C,: (a) using a simple strap, and (b) reducing the loop inductance by using a coaxial return.

Aiming at an illumination as uniform as possible over an area ranging up to 2ooO cm2 excludes the use of high- efficiency specular reflectors, but rather requires the apph- cation of a diffuse reflector. A good compromise in respect of simplicity in construction, low costs, and practicability in a ballistic environment, is the use of a cylindrical reflec- tor which, simply made from cardboard, is covered in the inside by a glossy aluminum foiL4’ An example of such an optical setup is shown in Fig. 7 (a). Besides, it increases the diffusivity of the light source and the fidelity of color re- production because also beams of light s2 coming from the reflector contribute and illuminate the object under a

FIG. 5. View of an interchangeable hybrid spark support. Altogether nine in number and uniform in construction, each support carries a free spark serving as a Marx switching gap and a V-shaped gliding spark. They are both connected in series, thus flashing simultaneously.

1790 Rev. Sci. Instrum., Vol. 64, NO. 7, July 1993

FIG. 6. Photo of the light source during Bashing. The nine light emitting Y-shaped gliding sparks and their corresponding Marx switching gaps are clearly visible.

larger angle of incidence than merely beams s1 being emit- ted directly from the gliding sparks. The reflector factor p, which is the ratio of light with the reflector to that received without the reflector, depends on the tubus length L and the distance x from the reflector exit. For comparison pur- poses light focusing experiments have also been performed with a plastic Fresnel lens, the optical arrangement is shown in Fig. 7(b). The Fresnel lens,42 379 mm in diam- eter and with a focal length of 400 mm, was turned out to

l ight source cardboard tubus

object x~ ,,..>; \\’ 1 &,y\’

0 \ . .’ I .\

_- Fresnel lens

f--“yIypj p----,

FIG. 7. Light output arrangements for reflected-light photography with (a) a cylindrical reflector, compare Figs. 8, 10, and 4, and (b) an annular Fresnel lens.

Diffuse light tBxnT%? lf90 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

130.18.123.11 On: Fri, 19 Dec 2014 01:20:49

an annulus for transmission of the optical beam of record- ing and positioned at a distance y from the light source.

7

For recording, a 35mm reflex camera with a mechan- ical shutter43 is applied which, projecting its 90 mm tele- photo lens*( into the central aperture of the case of the light source, allows imaging of an object within a visual angle of 20”. Since the light-emitting sparks surround the beam of recording “coaxially,” reflected-light photography is accomplished in an ideal manner. However, it should be noted here that a camera with a mechanical shutter is ab- solutely required, because modern microcomputer- controlled cameras malfunction in the strong electromag- netic noise during the discharge of the light source. To operate the mechanical shutter in a harsh ballistic environ- ment also from a safe distance, a remote-controlled electric activator, individually adjustable to trigger any camera in respect to its required release force and displacement, was constructed by one of US.‘~

6

5 8 >- 4

n .0 U, = 17SkV C, q 15nF

c g2 f .: 3 z=

zm2 2 4; Ff 341

0 500 TIME t [nsl

1

0

IV. LIGHT OUTPUT MEASUREMENT

6- 7 C = 45nF U, = 17SkV

n=8 T : 245ns

The light-time profiles 4(t) have been recorded with an ultrafast photodetector with an S-20 surface.46 The peak light intensity Jd of the diffuse gliding spark light source has been measured at a large distance rd from the light source to approach a point source and compared at a dis- tance rp to the peak light intensity of two commercially available Fint light sources9P10 which both provide a peak intensity 4P =2 Med. Applying the well-known inverse- square law and taking the peak related photodetector volt- ages, UP at r,=2.3 m and ud=U,=l.5 V at rd=23.5 m, yields for the peak light intensity of the diffuse source in candlepower

8 I z b- iii- p .$ z= s2 P .& 2 f- L 35

0 L-4 0

&= (&/Up) (rd/rp)2&,z2~ Med. The light output, which is responsible for the film expo- sure, was computed as the integral of the measured light- time profile over the entire flash duration.

FIG. 8. Effects of the operation parameters of an n-stage Marx-surge generator on the light-time profiles 4(i) and the pulse duration T at FWHM: (a) Increasing the pulse capacitor C in each stage increases the peak light intensity, but at the expense of an increase of r, and (b) increasing the number of stages n increases the peak light intensity, and even favorably decreases 7.

V. RESULTS

A. Gliding spark light source 7. Profofype sfudles

In the initial stage of development a feasability study was carried out to what extent a Marx-surge generator is suitable for driving the necessary high-voltage short- duration current pulse through all gliding sparks. The pro- totype was already a modified Marx-surge generator ac- cording to Fig. 1. However, for a parametric testing it was provided with a variable number of stages (I-8) and a stepwise variable Marx capacity (C= Co, 2Co, 3Co) by using a parallel connection of up to three capacitors Co= 15 nF. The pulser, charged up to a maximum voltage of only 17.5 kV, was still air insulated. The gliding spark in each stage corresponded to one leg of the V-shaped ar- rangement used later on.

flash duration by 20%, however, at the expense of a reduc- tion in peak intensity to f, and in light output even to f. Fortunately, the flash duration decreases with increasing number of stages, Fig. 8(b), and is even accompanied by an increase in peak light intensity. This specific behavior highly qualifies the Marx circuit as an ideal pulser for driv- ing the long chain of gliding sparks.

2. Present performance characteristics

For the example of the prototype the typical light out- put characteristics are depicted in Fig. 8. Aiming at a short flash duration requires the use of small capacitors. As il- lustrated in Fig. 8(a), a reduction of C to f reduces the

The example of Fig. 8 shows that a small Marx capac- itor and a large number of stages are required to realize a high-intense short-duration light source. In addition, a high voltage is favorable to increase the light intensity. Therefore, in the present construction the effective Marx capacitor was chosen to be only 16.5 nF and the charging voltage doubled to 35 kV. The number of stages was in- creased only slightly from eight to nine to still ensure a reliable triggering. The resulting time profiles of current and light intensity are shown in Fig. 9. The peak in light emission is somewhat delayed in regard to the current peak, a typical behavior which has also been observed pre- viously in the operation of a flash tube.47 However, surpris- ingly and contrary to flash lamps,48 the optimum perfor-

1791 Rev. Sci. Instrum., Vol. 64, No. 7, July 1993 Diffuse light source 1791 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

130.18.123.11 On: Fri, 19 Dec 2014 01:20:49

0 200 1000 2000 TIME t Ins)

FIG. 9. Typical wave forms of luminous intensity # t) and current I(t) for a gliding spark setup operated in air at 1 bar. Note that the peak light intensity is emitted closely at zero passage of the driving current.

mance in regard to a short flash duration at f and f peak as well as to a high peak intensity is not obtained in a criti- cally damped pulse, but rather at a large current reversal of nearly 60%. This is probably due to the different spectral character of the tail which for a gliding spark is more in the red region.

To obtain a short flash duration, the duration of the first half cycle of the current pulse should be as short as possible. Note that the discharge current, Fig. 9, is not periodically damped, but rather exhibits nonperiodic zero passages. Computer simulations for a simple resonant cir- cuit with a time-variant load resistance showed that this typical phenomenon with a decreasing time between zero current passages can be modeled by an increase in the load resistance at the end of discharge.

The effective resistance of the full chain of gliding sparks and the total inductance of the discharge circuit, R and I., respectively, are time dependent and cannot be de- termined directly from the current-time profile of Fig. 9. Taking in addition current and voltage data of a single V-shaped gliding spark yielded an average value of 2.2 R, thus resulting in I2~20 G for the total chain of gliding sparks. Approximately, the n-stage Marx pulser can be modeled by a simple resonant circuit with an effective ca- pacitor C”=C/n= 1.833 nF, a time-invariant load resis- tance R and an outer time-invariant inductance L. Assum- ing a constant ringing frequency of 2/e, 8=230 ns being the time of the first half cycle of I(t), yields for the total inductance L=2.9 PH and for the characteristic imped- ance Z= J( L/c*) z-40 a. Aiming at a short flash dura- tion, it is useful to operate the circuit at a periodically damped mode (Z>R) rather than at critical damping (Z =R/2).

The flash duration at the i and f peaks is 180 and 250 ns, respectively. The total trigger delay, including the delay in the Marx pulser itself and the internal delay in the 30 kV trigger pulse generator, was measured to 5 ps with a jitter of 0.2 ps. The electrode erosion in each micro gliding spark starts primarily at the corners of the copper foil lugs, Fig. 10, but remains small even after hundreds of flashes. The reproducibility of the light-time profiles is excellent, for example, deviations in peak intensity are below 5%.

1792 Rev. Ski. Instrum., Vol. 64, No. 7, July 1993

(W

FIG. 10. Electron micrographs of the micro gliding spark path: (a) fresh PC board, and (b) erosion and melting of copper lugs and partial detach- ment of foil comers after hundreds of flashes.

B. Beam focusing setups

The two reflector setups, Figs. 7(a) and 7(b), have also been tested. In the arrangement of Fig. 7(a) the re- flector factor reaches in axial direction an optimum value of p=3.4 at a distance n=: L/2. In terms of film exposure this gain in light output allows (if to reduce the camera aperture by approximately one and a half steps, thus im- proving the field of depth, or (ii) to reduce the film sensi- tivity by about 5 DIN, thus increasing the resolution. The first measure is interesting for reflected-light photography of projectiles in free flight when the trajectory is not locally stable. The second measure can be useful in the case of small objects when an image magnification subsequently becomes necessary.

For a Fresnel lens setup, Fig. 7 (b), the resulting gain in light output at a close distance to the light source is lower than from a cylindrical reflector, Fig, 7 (a), A Fresnel lens setup becomes more effective at a large dis- tance from the light source. However, it is more expensive than a simple cylindrical cardboard reflector and even re- quires particular shielding in the case of shock-loaded ob- jects or in an explosive environment.

C. Flash lamp light source

Tests have also been performed with flash lamps. Sub- stituting each V-shaped gliding spark by a series connec- tion of two U-shaped Xe-filled flash lamps,49 each having

DiffuaDg light source 1792 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

130.18.123.11 On: Fri, 19 Dec 2014 01:20:49

U, = 35kV I n -9 c q 16SnF

0 200 1000 2000 TIME t Ins1

FIG. 11. Comparison of light-time proliles for a gliding spar! setup (nine V-shaped gliding sparks and nine Marx switching gaps: 9=200 Mcd, r= 180 ns), and for a U-shape$ Xc-f&d flash lamp setup (18 flash lamps and 9 Marx switching gaps: 4=zoO Mcd, 7=364l ns).

an effective arc length of 52 mm and an inside diameter of 4 mm, yielded optimum matching conditions to the Marx pulser characteristics in regard both to flash duration and luminous peak intensity. Figure 11 compares the light-time profile of such a flash lamp setup with a V-shaped gliding spark setup. The flash duration at f and f peak is 360 and 580 ns, respectively. Note that the peak luminous intensity in both cases is pretty much the same, but that the light output, increased by a factor of about 2, is achieved at the expense of doubling the flash duration. Nevertheless, the increased flash duration of 360 ns FWHM of such a flash lamp light source is still quite short and might still be tolerable for many practical applications when the level of light output becomes the crucial parameter.

In conclusion, compared to previously reported submi- crosecond flash light sources the concept of a modified Marx-surge pulser can provide the most intense light pulses for both a gliding spark and a flash lamp setup.

The new diffuse light source will be commercially available through EMI.

ACKNOWLEDGMENTS

S. Yabuto, a visiting graduate student on an IAESTE grant from the University of Tokyo, Department of Aero- nautics, assisted in the initial stage of the development of the prototype pulser. The instrument was manufactured in the EMI machine shop by P. Murr and his colleagues.

‘A. S. Dub&k, The Phorogmphic Recording of High-Speed Processes (Wiley, New York, 1981), pp. 387-395.

‘W. H. Fox-Talbot, C. R. Seances Acad. Sci. Paris 32, 911 (1851). ‘W G. Hyxer, Engineering and Sciendfc High-Speed Photogmphy

(h&milhn New York, 1962), pp. 299-304. ‘ht. P. Va&kov and A. A. Mak, Sov. Phys. Usp. 66, 137 (1958). ‘F. Friingel, High-Qeed P&e Technology (Academic, New York and

London, 1965), Vol. 2, pp. 1-92. ‘K. Vollrath and G. Thomer, Kuunzeitphysik (Springer, Wien, 1967),

pp. 108-l 10. ‘M. Hugenschmidt and K. Vollrath, Light Sources and Recording Meth-

t&, in Vol. 18-B of Methods of Experimental Physics, edited by R. J. Emrich (Academic, New York, 1981), pp. 687-753.

‘Nanolite Model KGK, Impulstechnology Association e.V. Hamburg, Institut fti allgemeine Imp&-Forschung, W-2000 Hamburg 56, Ger-

9E: Light Source Model 5401, Cordin, Salt Lake City, UT 84119. loPoint Light Source Model 1188, Marco Scientific, Santa Clara, CA

95051.

1793 Rev. W. Instrum., Vol. 64, No. 7, July 1993 Diffuse light source

” P. Krehl and J. B. Hagelweide, Rev. Sci. Instrum. 52, 863 ( 198 1). “A. Toepler, Poggendorffs Annalen 131, 183 (1867). l3 P. Krehl, Shock Waves 1, 3 (1990). I’M. Toepler, Ann. Phys. 27, 1043 (1908). “E. Fiinfer, Technical Report No. 9/46, DEFA-LRSTA, Institut ftir

Ballistik, Saint Louis, France, 1946. “H. Schardin and E. Fiinfer, Z. Angew. Phys. 4, 185 (1952). 17P. Fayolle and P. Naslin, J. Sot. Motion Pit. Television Eng. 60, 603

(1953). “H. Luy and R. S&de, in Proceedings of the 2nd Inrernational Congress

on High Speed Photography, Paris, France, 1954, edited by P. Naslin and J. Vivie (Dunod, Paris, 1956), p. 59.

l9 E. P. Tawil, in Proceedings of the 3rd International Congress on High Speed Photagmphy, London, Great Britain, 1956, edited by R. B. Col- lins (Butterworths, London, 1957), p. 9.

20H. E. Edgerton, J. Tredwell, and K. W. Cooper, in Proceedings of the 5th International Congress on High Speed Photogmphy, Washington D.C., USA, 1960, edited by J. S. Courtney-Pratt and B. E. Cantab (Society of Motion Picictures and Television Engineers, New York, 1962), p. 29.

” Microflash Model 549, EG&G, Electra-Optics and Electronic Compo- nents Division, Salem, MA 01970.

=J. C. Moden, G. W. Reece, and S. Pooley, in Proceedings of the 6th International Congress on High Speed Photogmphy, The Hague/ Scheveningen, Holland, 1962, edited by J. G. A. Graaf and P. Tegelaar (Tjeenk Willink & Zoon, Haarlem, 1963), p. 158.

“S I Andreev and M. P. Vanyukov, in Proceedings of the 6th Interna- , . tional Congress on High Speed Photography, The Hague/Scheveningen, Holland, 1962, edited by J. G. A. Graaf and P. Tegelaar (Tjeenk Willink & Zoon, Haarlem, 1963), p. 166.

“M. Schwertl and A. Stenzel, Report No. RT 12/71, Deutach- Franzasisches Forschungsinstitut, ISL, Saint Louis, France, 1971.

“M. Schwertl and A. Stenzel, in Proceedings of the 10th International Congress on High Speed Photography, Nice, France, 1972, edited by E. Laviron (ANRT, Paris, 1972), p. 335.

26H. E. Edgerton and J. R. Killian, Jr., Moments of Vision (MIT Press, Cambridge, MA, 1979).

27H. E. Edgerton, Electronic Flash, Srrobe (MIT Press, Cambridge, MA, 1979), pp. 131-133.

“Model 5403, Cordin, Salt Lake City, UT 84119. tgPrivate communication, High-Speed-Photosysteme, W-2COO Wedel,

Germany. “J. L. Brewster, J. P. Barbour, F. M. Charbonnier, and F. J. Grund-

hauser, in Proceedings of the 9th International Congress on High Speed Photography, Denver, 1970, edited by W. G. Hyzer and W. G. Chace (Society of Motion Pictures and Television Engineers, New York, 1970), p. 303.

“C. H. Bagley, Rev. Sci. Instrum. 30, 103 (1959). “M. Sultanov and R. L. Jameson, J. Sot. Motion Pit. Television Eng. 69,

113 (1960). “M. Held, Sprengstoffe Pyrotechnik 28, 10 (1991). “P. Krehl, German Patent Application P 4223457.3 (July 1992). 35Type KUR 416, LCC, Courbevoie Cedex, France. 36Type M G 815, Caddock Europe BV, W-8000 Miinchen, Germany. 37Type Roth RE 323, Fa. Biirklin, W-8ooO Miinchen, Germany. “Type HCN 140-35CU0, FUG, W-8200 Rosenheim, Germany. 39Type GP 70, EG&G, Salem, MA 01970. QType PIM-220-$&a& Physics International Company, San Leandro,

CA 94577. “Adhesive aluminum foil, Alkor Kunststoffe, W-8OCXl Miinchen 71, Ger-

=Y. 42Type 399615, Spindler & Hoyer, W-3400 Giittingen, Germany. 43Minolta SRT 303, Minolta Camera Co., Osaka, Japan. 44 ATX Macro 90 mm, Tokina, Japan. 45S. Engemann, EM1 Report No. T l/93 (Ernst-Mach-Institut, Freiburg,

1993). &Model 620, Optics Technology, Palo Alto, CA 94304. 47C. R Amolds, P. J. Rolls, and J. C. J. Stewart, AppZied Photography,

edited by D. A. Spencer (Focal Press, London, 1971), p. 325. @mashlamp Applications Manual, EG&G, SaIem, MA 01970. 49Type 050-822-1, Vijlkner Electronic, W-3300 Braunschwkg, Germany.

1793 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

130.18.123.11 On: Fri, 19 Dec 2014 01:20:49