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

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<ul><li><p>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 </p><p> 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:</p><p>130.18.123.11 On: Fri, 19 Dec 2014 01:20:49</p><p>http://scitation.aip.org/content/aip/journal/rsi?ver=pdfcovhttp://oasc12039.247realmedia.com/RealMedia/ads/click_lx.ads/www.aip.org/pt/adcenter/pdfcover_test/L-37/1945292795/x01/AIP-PT/NorCal_RSIArticleDL-121714/AIP_Banner9NXPowerLite.jpg/47344656396c504a5a37344142416b75?xhttp://scitation.aip.org/search?value1=P.+Krehl&amp;option1=authorhttp://scitation.aip.org/search?value1=S.+Engemann&amp;option1=authorhttp://scitation.aip.org/content/aip/journal/rsi?ver=pdfcovhttp://dx.doi.org/10.1063/1.1144011http://scitation.aip.org/content/aip/journal/rsi/64/7?ver=pdfcovhttp://scitation.aip.org/content/aip?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/apl/83/19/10.1063/1.1626016?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/apl/83/19/10.1063/1.1626016?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/rsi/52/6/10.1063/1.1136683?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/rsi/30/11/10.1063/1.1716448?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/rsi/30/2/10.1063/1.1716472?ver=pdfcovhttp://scitation.aip.org/content/aip/journal/jap/21/12/10.1063/1.1699587?ver=pdfcov</p></li><li><p>A high-intensity diffuse light source of ultrashort duration for reflected-light color photography </p><p>P. Krehl and S. Engemann Ernst-Mach-lnstitut der Fmunhofer-Gesellschaft, Eckerstrasse 4, W-7800 Freiburg, Germany </p><p>(Received 9 April 1993; accepted for publication 14 April 1993) </p><p>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. </p><p>1. INTRODUCTION </p><p>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. </p><p>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. </p><p>In the case where black-and-white photos of the object are sufficient and a direct recording on film is required, this di&amp;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. </p><p>Color photography is becoming increasingly important in all phases of photography, including also the field of high-speed photography. * Advantageously, color increases </p><p>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. </p><p>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. </p><p>II. A BRIEF HISTORY </p><p>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 </p><p>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:</p><p>130.18.123.11 On: Fri, 19 Dec 2014 01:20:49</p></li><li><p>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. </p><p>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. </p><p>A. Electric spark sources </p><p>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. </p><p>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. </p><p>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.* </p><p>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- </p><p>1786 Rev. Sci. Instrum., Vol. 64, No. 7, July 1993 </p><p>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. </p><p>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 Toeplers gliding spark light source was resumed and improved by Toeplers 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 team6r7 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. </p><p>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 Tells apple shot, or of the splash of a milk drop, are a milestone in the achievements </p><p>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...</p></li></ul>