Study of the photosensitivity of new photothermoplastic materials under conditionsthat most closely approximate the taking of pictures of earth from space

Yu. A. Cherkasov,* E. L. Aleksandrova, and M. V. Smirnov

S. I. Vavilov State Optical Institute, St. Petersburg, Russia~Submitted June 4, 1998!Opticheski� Zhurnal66, 61–65~July 1999!

An apparatus has been developed for determining the photosensitivity of photographic materialsfrom an optical test image with contrasts, frequencies, and radiances that occur whenpictures of earth are made from space. The sensitometric photosensitivity and exposure indexhave been determined for new high-resolution photothermoplastic materials with acompact structure on a rigid substrate. For various versions of the materials, these quantitiesequal 12.5–50 and 10– 40 lux21 sec21, respectively. ©1999 The Optical Society of America.@S1070-9762~99!00707-1#

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The main requirement of sensitometry is to bring texperimental conditions as close as possible to the pracconditions of picture taking~so as to provide the most adequate reproduction!, and this is satisfied fairly well in thesensitometry of silver halide photographic materials. Hoever, it is not always possible to satisfy this requirementnew types of photographic materials, including photothermplastic materials~PTPMs!, for which there are no standartest methods.1 It is fairly easy to choose a picture-takinregime for the high-quality recording of simple objectsPTPMs, for example a spatial lattice of one definite fquency, with contrast and brightness that are identical othe entire frame. However, a substantially more complex ptern of brightness distribution is observed in taking pictuof the earth from space, while the shape and size of objand their contrast is extremely varied. This has the effectthe sensitometric photosensitivity determined in terms ofstandard density for large fields can strongly differ from tpractical photosensitivity determined by the so-called exsure index from an optimum image containing various stial frequencies with a wide range of contrasts and lumnances corresponding to what is observed in taking pictuof earth from space. Therefore, Ref. 2 developed and stua special test object that contained, along with a sensitomric wedge, a set of three-bar targets of various spatialquencies, contrasts, and luminances~in all, 216 versions ofone image!. With the appropriate reduction, this test objemakes it possible to form on PTPMs an optical image wgiven parameters. The goal of this paper is to use thisobject to develop a sensitometric apparatus for testing aspace photographic materials under conditions that approas closely as possible the conditions for taking pictures ofearth from space, and to determine the sensitometric phsensitivity and exposure indices of the new high-resolutPTPMs with a compact structure on a rigid substrate.3,4

SENSITOMETRIC APPARATUS

The apparatus for studying photosensitivity under contions that approach as closely as possible the conditionsder which pictures of the earth are taken from space~Fig. 1!includes the following: test object2 1; the radiation source

628 J. Opt. Technol. 66 (7), July 1999 1070-9762/99/07062

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including illuminator 2, scatterer 3, and conversion light fiter 4; projection objective 5; shutter 6; photothermoplascamera 7, with PTPM 8 and control unit 9; test system 1and visualization system 11, with measurement of the optdensities in transmitted and reflected light.1 The test objectmakes it possible to use the objective to form an optiimage on the layer with frequencies 2 – 200 mm21, contrast0.015–0.65, and luminances of the lines on the test obvarying within 1.5 orders of magnitude. The illuminatoconsist of two 1-kW halogen incandescent lamps whcolor temperature~3200 K! is reduced by filter 4 to that othe sun~5500 K!. Moreover, illuminators were used in thform of flashlamps withTcol55600 K and drive number 24~the flash time is 1024 sec) ~not shown in Fig. 1!. The illu-minance in the plane of an image of the fields of the wedis determined by means of a secondary detector with an eof 65%. The luminance of the source is calibrated by phtographic photometry from the luminance of a photometsphere. The illuminance in the plane where the PTPMmounted is measured with a Yu-16 luxmeter. The E´ ra-27objective has a modulation transfer function close to thatthe actual Apo-Mars aerospace objective with which sentometric tests of the PTPM were carried out earlier.7 Theshutter provides delays of from 1 to 0.001 sec. The sysfor visualizing the relief-phase image formed on the surfaof the PTPM is made in the form of a GOI FP-90M darfield schlieren system,1 which works in transmitted light, anda dark-field schlieren system based on an MBS-10 optmicroscope operating in transmitted and reflected light.

The test object~Fig. 2! is based on a periodic latticconsisting of alternating light and dark lines extending in thorizontal direction. Each line is 5 mm long. The widththe lines discretely increases in the vertical direction. Thare six lines of identical width: three light and three da~three periods!. These six lines form a frequency field corrsponding to one spatial frequency. The first version oftest object contains thirty-eight fields with frequencies fro4.0 to 0.29 mm21, the second contains twenty-nine with frequencies of 8.0– 0.16 mm21, and the third contains forty-twowith frequencies of 2.0– 0.47 mm21. Three versions of thetest object were required for the following reason. On ohand, the test object must contain a large range of spa

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frequencies in order to determine high resolving powerhigh contrast and optimal exposure and low resolving poat low contrast and nonoptimal exposures on one frame.the other hand, the discretization step of the spatial frequcies must be small. It follows from these two requiremethere must be a large number of frequency fractions. Tmaximization of the range of spatial frequencies andminimization of the discretization step of these frequencare mutually contradictory requirements. Therefore, the fiversion of the test object is a certain compromise betwthem. In the second version, the frequency range is increto cover a factor of 51.2, and, in the third, conversely,reducing the frequency range to cover a factor of 4.2,frequency discretization step is reduced to 2.5–5%. In eversion of the test object, each frequency field is repeathirty-six times. In this case, the contrast between the liand dark lines and their mean optical density change.mean optical density has six values. They correspond towide vertical columns into which the frequency part of ttest object is divided. Each of the indicated wide columcontains frequency fields of six different contrasts. The upand lower parts of the test object include sensitomewedges from eight fields with dimensions of 30320 mmeach.

FIG. 1. Layout of apparatus for studying the photosensitivity of PTPMs~seetext for explanation!.

FIG. 2. Test object for studying the photosensitivity of PTPMs.

629 J. Opt. Technol. 66 (7), July 1999

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Studies were carried out for PTPMs with compastructure.3,4 The PTPM is made on a rigid glass substrate,which a raster, a thermal-developing conductive electrocontacts to it, a protective layer, a CdSe injection layer 00.4 mm thick, and a thermoplastic layer 3mm thick are suc-cessively deposited. Recording was done on the PTPM fthe side of the glass substrate through the raster.

The photothermoplastic cell is made in a dust-anmoisture-proof version and has separate optical inputoptical output. Besides the PTPM, the cell containscorotron for electrostatically sensitizing the PTPM. Itmade in the form of a multifilament system with an equaling grid and counterelectrode. All the detachable parts ofsystem are sealed. The moisture is minimized by a desicplaced in special recesses of the system. Plug-and-sounits for supplying high voltage, for developing and erasicurrent pulses, and for the thermostatic control systemlocated on the housing of the cell. The photothermoplasrecording process-control unit optimizes the sensitizatiexposure, thermal development, and thermal erasuregimes, as well as keeping the temperature in the cell consat 4061 °C. A sequential photothermoplastic recording prcess is used, including sensitization of the PTPM by depiting a surface electrostatic charge, exposing it, and thmally developing it by Joule heating evolved in thconductive layer when current passes through it. The recing regimes are specified by the control unit.

RESULTS OF THE STUDY

A characteristic curve for a PTPM with a CdSe layer 0mm thick, constructed from the results of a measurementhe optical density of the fields of the photothermoplasimage in a thermal-field schlieren system, is shown in FigThe same figure shows resolvometric curves for various ctrasts of the optical image. In constructing the resolvomecurves, thirty-six values were obtained for the resolvipower of the PTPMs as a function of contrast for six zonesfrequency fields with different optical densities. The controf the optical image is defined as the product of the contof the frequency field of the test object and the correspoing value of the frequency-contrast characteristic of the

FIG. 3. Characteristic curve~I! and the corresponding resolvometric curv~II ! for PTPMs on a CdSe base:~a! perpendicular and~b! parallel orientationof the PTPM raster relative to the lines of the test object. The contrast ooptical image is indicated near the curves.

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TABLE I. Photosensitivity and exposure index of five versions of PTPMs differing in the thickness of thelayer.

dCdSe,mm

Slux21 sec21

EIlux21 sec21

Rmax

with k50.2, mm21N

with MTC50.5, mm21

0.10 12.5 10 240 1300.15 18 15 180 1000.20 25 20 120 600.30 37.5 30 100 300.40 50 40 90 25

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jective being used. The data of Fig. 3a refer to a perpendlar orientation of the raster relative to the lines of the tobject, and the data of Fig. 3b refer to a parallel orientatifor which the maximum resolvable frequency is limitedthe Nyquist frequency, equal to twice the lattice period. Uing the characteristic curve for the standard density of 0above the fogging densityD0 , taken for aerial photographiplates~for the direct positive photothermoplastic process,D5Dmax20.85), the photosensitivity numbers are definedS0.85510/H ~in lux21 sec21), whereH is the exposure thacreates the standard density.5,6 It is used to determine theexposureH0 corresponding to the maximum resolving powfor various contrasts of the optical image. By comparingH0

with the exposureH that creates the standard density andusing the photosensitivity numberS0.85, the exposure indexEI5S0.85H/H0 is determined. TheS andEI values obtainedfor five versions of the PTPMs are shown in Table I. As cbe seen from this table, the photosensitivity increases f12.5 to 50 lux21 sec21 and EI increases from 10 to40 lux21 sec21 as the CdSe layer thickness goes from 0.10.4 mm. In this case, the maximum resolving power forcontrast of 0.2 decreases from 240 to 90 mm21, while thespatial frequency corresponding to a modulation-transferefficient ~MTC! of TN50.5 decreases from 130 to 25 mm21.The other general sensitometric characteristics of the PTPare determined from the characteristic curves: the phgraphic latitudeL, the contrast coefficientg, the mean gra-dient g, the maximum densityDmax, and the minimum den-sity Dmin . The general sensitometric properties of a PTPwith a CdSe layer 0.1mm thick are given in Table II. As canbe seen from Tables I and II, the resulting photosensitivvalues meet the requirements of aerospace photography~noless than 10 lux21 sec21, Ref. 2!.

An additional verification of the value obtained for thphotosensitivity is obtained by combined testing of PTP

TABLE II. General sensitometric of PTPMs with a CdSe layer 0.1mmthick.

Characteristic Value

Photosensitivity number, lux21 sec21

with D5Dmax20.8512.5

Exposure index, lux21 sec21 10Photographic latitudeL 1.3Maximum contrast coefficientgmax 1.4Maximum optical densityDmax 2.3Minimum optical densityDmin 0.3Mean gradientg 0.9

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and silver halide photographic plates. Two types of filmschosen for comparison:8 film for professional cinematography ~KN-2 negative! and aerial-photography film~type 58isopanchromatic!. The spectral sensitivity regions of thesfilms are close to that of the PTPM. This makes it possiblecompare the sensitivities of the materials using light sourwith different color temperatures, including standard sour~3200 and 5500 K!. The general sensitometric characteristof the films were first determined by the standard techniqusing standard devices. The photosensitivity numberS0.1

50.8/H of KN-2 film was 50 lux21 sec21, andS0.85510/Hof type 58 isopanchromatic film was 40 lux21 sec21. Afterthis, the photographic film and the PTPM were exposedthe new apparatus. The characteristic curves construfrom the results of the exposure are shown in Fig. 4, andresulting photosensitivity numbers are given in Table III. Acan be seen from Table III, the photosensitivity numbersthe photographic materials coincide with those determinabove, which further confirms the correctness of the phosensitivity measurements of the PTPMs.

The photosensitivity spectrum in the near IR and Uregions is given in Fig. 5~curve 1! for the PTPM studiedhere, with a thickness of the CdSe injection layer ofdCdSe

50.4mm. It also shows the photosensitivity spectraPTPMs with differentdCdSevalues. A PTPM with no injec-tion layer is sensitive in the UV region~curve2!, and PTPMswith thicker CdSe layers are sensitive in the x-ray andgranges~curves3–5!.

FIG. 4. Characteristic curves of PTPMs on a CdSe base~1!, type 58 aerialphotographic film~2!, and KN-2 negative photographic film for professionmotion-picture photography~3!.

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REPRODUCIBILITY

The reproducibility of the photosensitivity of the matrial is most important for ensuring the reproducibility of iresolving power, which depends on the position of the relution curve relative to the characteristic curve.9 The repro-ducibility of the photosensitivity was determined from threproducibility of the resolvometric curves. The results apear in Fig. 4, on which the vertical lines show the scattethe results in a series of seven measurements made on sPTPM samples. It follows from these data that the reprodibility of the characteristics of the PTPMs is high~the devia-tion from the mean value does not exceed 10%!.

CONCLUSION

An apparatus has been created for determining the ptosensitivity of photographic materials from an optical teimage with contrasts, frequencies, and luminances that owhen pictures of the earth are taken from space. The setometric sensitivity and the exposure index have been demined for new high-resolution photothermoplastic materiwith a compact structure on a rigid substrate; these eq~for various versions of the materials! 12.5–50 and10– 40 lux21 sec21, respectively. The reproducibility of thcharacteristics of the PTPMs has been studied, and itbeen shown that the deviation from the mean value doesexceed 10%. It is concluded that the resulting photosensity values meet the requirements on taking pictures ofearth from space~no less than 10 lux21 sec21).

TABLE III. Comparison of the photosensitivity numbers of five versionsPTPMs and photographic films, measured under identical exposure ctions.

Photographic material S0.1

lux21 sec21S0.85

lux21 sec21

KN-2 negative photograph film 50Type 58 aerial photography film 40PTPM 12.5-50

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The authors are grateful to A. L. Kuznetsova and Yu.Chesnokov for discussing the results. The work was pformed under grant INTAS-93-616.

*Deceased

1A. L. Kartuzhanski�, ed., Silverless Photographic Processes~Khimiya,Leningrad, 1984!, pp. 45–103.

2Yu. A. Cherkasov, Yu. V. Chesnokov, Ya. L. Ziman, E. L. AlexandrovS. P. Efimova, A. L. Kuznetsova, N. B. Zakharova, M. V. Smirnov, aA. I. Rumjantsev, ‘‘High-resolved photothermoplastic storage medialow-contrast halftone image recording,’’ Proc. SPIE3347, 101 ~1997!.

3Yu. A. Cherkasov, E. L. Aleksandrova, A. I. Rumyantsevet al., ‘‘Forma-tion kinetics of photothermoplastic relief-phase images and an analysthe possibility of implementing adaptive data recording,’’ Opt. Zh.63, No.4, 77 ~1996! @J. Opt. Technol.63, 308 ~1996!#.

4Yu. A. Cherkasov, N. B. Zakharova, and E. L. Aleksandrova, ‘‘Photothmoplastic recording of half-tone images: Technological studies ofminimization of the defects of an information medium,’’ Opt. Zh.66, No.1, 32 ~1999! @J. Opt. Technol.66, 27 ~1999!#.

5C. E. Mees and T. H. James, eds.,Theory of the Photographic Proces~Macmillan, New York, 1966; Khimiya, Leningrad, 1973, 572 pp.!.

6K. V. Chibisov,General Photography~Iskusstvo, Moscow, 1984!.7Yu. A. Cherkasov, ‘‘Prospects and problems of using photothermoplasin devices for taking pictures of the earth from space,’’ Opt. Zh.65, No. 4,82 ~1998! @J. Opt. Technol.65, 327 ~1998!#.

8Yu. N. Gorokhovski� and V. P. Baranova,Properties of Black-and-WhitePhotographic Films. Sensitometry Handbook~Nauka, Moscow, 1970!.

9H. Frieser, Photographic Information Recording~Halsted Press, NewYork, 1975; Mir, Moscow, 1978, 670 pp.!.

FIG. 5. Photosensitivity spectra of PTPMs.dCdSe50.4 ~1!, 2—0, 3—2,4—5, 5—10 mm.

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631Cherkasov et al.