influence of some development parameters on the reflection grating structure in dichromated gelatin

Influence of some development parameters on the reflectiongrating structure in dichromated gelatin

Tuula Keinonen and Olli Salminen

We made reflection gratings by using the gelatin of Kodak 649F spectroscopic plates. The concentration of

ammonium dichromate sensitizer was varied, and reflection efficiencies of fully developed plates were

measured in different reconstructing angles. During the development process we varied the washing time,

the time interval between the washing and isopropanol baths, and the duration of the isopropanol bath. Thereflection efficiencies were measured for each processing variable. Finally, the characteristics of the gratings

were tested by varying the recording geometry.

1. Introduction

The well-known properties of dichromated gelatin(DCG) gratings are high diffraction or reflection effi-ciency, high resolution capacity, and good signal-to-noise ratio. Therefore this material is suitable forpreparing holographic optical elements. Both trans-mission typel- 7 and reflection type8-12 holograms havebeen used. Most of this work was done using thegelatin layer of Kodak 649F plates from which thesilver halides were removed. It has been observed thatthe results are not easy to repeat, and even a smallchange in environmental conditions during the pro-cessing will influence the final results.

We made our experiments by using prehardened(hard) gelatin of Kodak 649F plates. In some experi-ments plates without prehardening (soft) were alsostudied. We measured the reflection efficiency as afunction of the sensitizer concentration, the washingtime in the development, and the duration of the finalisopropanol bath. Also, the time interval between thewater wash and the isopropanol baths was varied. Thevalues of the chosen parameters can be changed easily,and these parameters influence the optical propertiesof the gratings. The reflection efficiencies for eachvariable were measured in different reconstructing an-gles to find out the maximum reflection efficiencies,the position of these maxima, and the half-height

The authors are with University of Joensuu, Physics Department,P.O. Box 111, 80101 Joensuu, Finland.

Received 5 September 1987.0003-6935/88/122573-07$02.00/0.

©0 1988 Optical Society of America.

widths of angular dependence curves. From theseresults we were able to draw some conclusions regard-ing the grating structure. Finally, we measured thecharacteristics of the formed gratings by changing therecording geometry.

11. Experimental

A. Normal Procedure for Making Holograms

The various steps of the processing procedure tomake DCG holograms from Kodak 649F plates arepresented in Table I. We did not change tempera-tures, so all the steps in Table I were done at roomtemperature and the temperature of every liquid was200C. The parameters that were varied are indicatedby an asterisk in the table. We varied only one param-eter in time and after the plates were ready the reflec-tion efficiencies were measured as a function of thereconstructing beam angle.

The atmospheric relative humidity was rather lowduring the experiments because our wintertime cli-mate is dry. The RH was found to change from 25% to40%. The values were measured with a hair hygrom-eter manufactured by Fischer (GDR).

B. Removing Silver Halides and Setting Bias Hardness

The silver halides were removed from Kodak 649Fplates by soaking them in Kodak Rapid Fixer solutionwith hardener (Table I). The concentration ratio ofsolutions A and B was 8:1 as in our previous measure-ments on transmission gratings.7 The pH value of thesolution was 4. We also used soft gelatin preparedwithout solution B. The gelatin bias hardness is animportant parameter because it influences almost ev-ery optical property of DCG holograms. 2 4 101 2

15 June 1988 / Vol. 27, No. 12 / APPLIED OPTICS 2573

C. Sensitization

The normal concentration of ammonium dichro-mate in our experiments was 5% in weight; the pH ofthe solution was 4. Like emulsion hardness, the sensi-tizer concentration also has a strong effect on the otherproperties of DCG holograms. Therefore we variedthe sensitizer concentration to determine its influenceon our procedure. The plates were immersed in thesensitizer bath for 5 min. After the bath the excesssensitizer on the back surface was removed by wipingthe surface with a rubber band. The sensitized platewas then left in the dark to dry overnight.

D. Exposure

We exposed our plates by using the 488-nm line of aSpectra-Physics model 165 Ar laser. The whole opti-cal system was on a vibration-isolated optical tablemade by NRC. The reference beam was directly onthe gelatin surface, while the object beam camethrough the glass plate to the backside of the gelatinlayer. The diameters of the beams were -1 cm. Theintensity of the object beam was slightly greater toeliminate the reflections from the glass plate. Theincident angle defined as an angle between recordingbeam direction and plate normal for both beams was250 in air. Thus both beams entered the gelatin layerat an angle of 16°, if the refractive indices of gelatinand glass are kept equal (n = 1.5).

We made gratings by using different exposure ener-gies. The energy range extended from 20 to 250 mJ/cm2 . We discovered that the optimum energy for agood grating in all our experiments was -70 mJ/cm 2 .

Table I. Processing Procedure for Making DCG Holograms from Kodak649F Plates

A. Removing silver halides and setting bias hardness.*(1) Soak plates in Kodak Rapid Fixer solution (solution A 20mliter, solution B 2.5 mliter, 60-mliter distilled water) for 15 min(pH = 4).(2) Wash in running water for 10 min (pH = 6).(3) Immerse in methyl alcohol bath.(4) Dry in vertical position.

B. Sensitization*(1) Soak plates in ammonium dichromate solution (5% per weight)for 5 min (pH = 4).(2) Remove excess solution from the glass side.(3) Dry in horizontal position overnight (relative humidity under

40%).

C. Exposure

D. Development*(1) Wash in running water for 10 min (pH = 6).*(2) Dry in air (normally skip this step).(3) Soak plates in mixture of 50% isopropanol alcohol and 50%

distilled water for 3 min.(4) Soak plates in mixture of 90% isopropanol alcohol and 10%

distilled water for 3 min.*(5) Soak plates in pure isopropanol alcohol for 10 min.(6) Dry in horizontal position in air.

Note: Asterisk indicates parameters that were varied.

(%)-hard emulsion

80

U_ 60

4 0

2 0)

La 4 0z \

2 Q

C'20 ,

0 2 4 6 8 10 (%)SENSITIZER CONCENTRATION

Fig. 1. (a) Dependence of the reflection efficiency maximum on thesensitizer concentration. (b) The angular position of the reflectionmaxima. The dashed line shows the recording angle. The results

are valid for hard emulsions.

Accordingly, every grating in this work was exposed tothis energy. Also the selected energy is relatively lowto minimize the heat developed by the absorption.Because the light intensity on the plate was 28-35 mW/cm2, the exposure times varied from 2.0 to 2.5 s.

E. Development

After exposure the ammonium dichromate was re-moved from the gelatin by washing in running water(normally for 10 min). The pH of the water was 6. Asis known the thickness of the gelatin layer increasesduring washing. To determine the total effect of thewater wash we measured the reflection efficiencies as afunction of the washing time. We also varied the timebetween the water wash and the first isopropanol bath;normally after the water bath the plates are immedi-ately soaked in isopropanol. Last, we measured theinfluence of the duration of the final isopropanol bathon the reflection efficiency.

F. Measurements After Development

The reflection efficiencies of the fully developedplates were measured in different reconstruction an-gles. The maximum reflection efficiency, its angularposition, and the half-height widths of the curves wererecorded. From these results we derived our conclu-sions about the grating structures. We defined thereflection efficiency as a ratio of intensities betweenthe first-order reflected beam and the reconstructionbeam.

Ill. Results and Discussion

The dependence of the maximum reflection efficien-cy on the sensitizer concentration for the hard emul-sion is shown in Fig. l(a). The points represent the

2574 APPLIED OPTICS / Vol. 27, No. 12 / 15 June 1988

(/)

=80

I-6

L2O0) 4

a 2 0

( )

In

zC

zC

LU

40

30

20

10

2 4 6 8 10 1%)SENSITIZER CONCENTRATION

Fig. 2. (a) Dependence of the reflection efficiency maximum on thesensitizer concentration for two sets of measurements done on dif-ferent days. (b) The angular position of the reflection maxima forthe curves in (a). The dashed line shows the recording angle. The

results are valid for soft emulsions.

average values of five series of measurements taken ondifferent days. Variations from the average value arealso shown. The variations are quite large, being typi-cal of DCG holograms, and they are due to the smallchanges in environmental conditions.

A well-known fact is that no theories are availablethat could completely describe the behavior of theDCG reflection gratings. It has been suggested byother investigators2 4'10'12 that a DCG reflection gratingis a superimposition of interference planes of differentrefractive-index modulations and fringe spacings.Thus, the maximum reflection efficiency is propor-tional to the number of interference planes with equaloptical path length separation which reflect obeyingthe Bragg condition. So, according to Fig. 1(a), thereare enough planes of equal optical path length separa-tions to produce almost an ideal reflectivity of 100% inthe 2-5% concentration range.

Figure 1(b) shows the angular position of the reflec-tion maxima of Fig. 1(a). The dotted line representsthe initial recording angle. From the reconstructionangles we can estimate the values of the optical pathlengths between the planes reflecting according to theBragg condition. When the optical path lengths be-tween successive interference planes become longer, itmust mean that the reconstruction angle reduces be-cause the same laser light is used for recording andreconstruction. Accordingly, from Fig. 1(b) we canobserve that the reconstruction angle of the reflectionmaxima is shifted from blue to red when the sensitizerconcentration increases. During the sensitizationprocess the dichromate penetrates into the gelatin andthe thickness of the sensitized layer increases withincreasing dichromate concentration. When the ex-

(%)C

,:4

< 3

I-LDl"I2

C

- 1

U1C

0 2 4 6 8 10 (/)SENSITIZER CONCENTRATION

Fig. 3. Noise-to-signal ratio as a function of the sensitizer concen-tration for both hard and soft emulsions.

posed plate is washed the dichromate is carried awayby water. However, according to Fig. 1(b) the gratingstructure of the fully developed plate (i.e., optical pathlength distribution of the interference planes) dependson the thickness of the sensitized layer. From thefigure it is also seen that the reconstruction wavelengthis 488 nm when the dichromate concentration is be-tween 5 and 6%.

The maximum reflection efficiency and its angularposition as a function of sensitizer concentration forthe soft emulsion are seen in Fig. 2. Two differentcurves measured on separate days are displayed. As arule the variations of the optical properties for softemulsions are greater than for hard emulsions. Thegelatin thickness varies in a complicated manner whenrelative humidity, temperature, and also pH of theprocessing solutions are changed. Soft gelatin is moresensitive to these changes than hard gelatin becauseunhardened gelatin dissolves during the sensitizationand development processes. The amount of dissolvedgelatin depends on the environment.0'l2 When thewet emulsion loses gelatin, its thickness is reduced;also according to Fig. 2(b) dry soft gelatin layers haveshorter optical path lengths between interferenceplanes than hard layers. From the same figure it isseen that the reconstruction wavelength is shiftedfrom blue to red as in Fig. 1(b) when the sensitizationconcentration increases.

We also measured the noise-to-signal ratio (NSR) asa function of the concentration for soft and hard emul-sions. The noise was measured by reading the averageintensity around the diffracted beam; typical curvesare presented in Fig. 3. The noise for hard emulsion is-0.5%, being almost independent of the concentration.For soft emulsion the noise increases as the concentra-tion increases. A possible explanation is that the dis-solved gelatin causes deviations from the sinusoidalindex modulation; this in turn causes the distortion.

During the water wash following the exposure, thedichromate dissolves away and the gelatin layer swellsstrongly. It is known that at this stage the watertemperature, the pH value, and the duration of thewater bath influence the final grating structure.10'12

We measured the last mentioned dependence, i.e., thereflection efficiency as a function of washing time; theresults for hard emulsions are seen in Fig. 4. The


soft emulsion0

a

I I1 I

bl I

A oft

hard

C)

LnzC)LU

(%)Li

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Lu' 0zC

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(0)

3129

,, 27, z25

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(0)

I

D503

29

0

Ij 30°U_ :

5 10 15 20 25 (minI

WASHING TIME

Fig. 4. Dependence of the reflection efficiency maximum on thewashing time after exposure. (b) The angular position of the reflec-tion maxima. (c) The half-height widths of the reflection efficiencyvs reconstruction angle curves. The results are valid for hard emul-

sion.

maximum reflection efficiency [Fig. 4(a)] increasesduring the first minutes of the water wash; but thereconstruction angle [Fig. 4(b)] remains almost con-stant, being slightly shifted to blue. The lower diffrac-tion efficiencies in the beginning of the water washmay be caused by gelatin that has not yet swelledenough, indicating that dichromate is still inside thegelatin.

We have also displayed the half-height width of thereflection curves of Fig. 4(a) in Fig. 4(c). The half-height width is a measure of the optical path lengthdistribution between successive interference planes.If the band is broad, there are other interference planespresent with slightly different optical path lengths. Inother words, the broader the band, the broader thespectral bandwidth. Following this reasoning we mayconclude that the maximum spectral bandwidth isachieved when washing time is longer than 10 min.

Figure 5 displays the corresponding relationshipsfor soft emulsions. There are two sets of measure-ments seen in Fig. 5(a). Two points from the other setdo not match the curve; this is typical of soft emul-sions. Results may vary considerably even during thesame measuring process. However, with this type ofemulsion we can achieve almost ideal reflection effi-ciencies.

The optical path lengths corresponding to the reflec-tion maxima become smaller when the washing time isincreased [Fig. 5(b)]. Thus the reconstruction wave-length moves from red to blue. The probable cause isthe dichromate which is not fully dissolved in the water

(%)

C) _oU0

U- U'i X6

.~ _

L LU

L . 4 o

zC (0)

I 1)

-, ,3 (A In

-U)

(0O)

60 C3

D5 0

I-

,I4 0

LI30

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5 10 15 20 25 (min)WASHING TIME

Fig. 5. (a) Dependence of the reflection efficiency maximum on thewashing time after exposure for two different sets of measurements.(b) The angular position of the reflection maxima. (c) The half-height widths of the reflection efficiency vs reconstruction angle

curves. The results are valid for soft emulsion.

wash. When the washing time is increased and thedichromate is completely dissolved, the optical pathlengths become shorter (the gelatin shrinks more).The initial recording angle is achieved when the wash-ing time is normal, i.e, 10 min. The half-height widthof the reflection efficiency remains almost constant[Fig. 5(c)]. The widths are greater than those ob-tained for hard emulsions. Because the reflection effi-ciencies can be very high, the number of interferenceplanes must be, large, suggesting that the structure ofthe soft gelatin gratings is fragile and thus sensitive tosmall changes in the development process.

Typical noise-to-signal ratios as a function of thewashing time for both hard and soft emulsions arepresented in Fig. 6. The ratio is better for hard emul-sions except when the washing time exceeds 25 min.In addition, the unexposed parts of soft emulsions turna milky color. Our observations that the signal-to-noise ratio is better for hard emulsions are in agree-ment with the observations of other investigators.4' 10"12

The probability to get a certain type of grating is alsobetter for hard emulsions. For these reasons duringthe following steps we studied only hard emulsions.

Normally, the exposed and washed gelatin plates areimmediately immersed in isopropanol alcohol. Wedetermined the influence of the time interval betweenwater wash and isopropanol bath. The results areseen in Fig. 7. The reflection maxima decrease as thetime interval increases [Fig. 7(a)]. This behavior


0

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00

00

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Fig. 6. Noise-to-signal ratio as a function of the washing time forsoft and hard emulsions.

(0/)

Z >80 _UL

LU~ 60U- U-U U-

ZL 40 hard emulsion

C) o ( °1 _o <-

I I

U

=L b

10 20 30 40 (minITIME INTERVAL BETWEENWATER AND ISOPROPANOL BATHS

Fig. 7. (a) Dependence of the reflection efficiency maximum on thetime interval between water and isopropanol baths. (&) The angu-lar position of the reflection maxima. (c) The half-height widths ofthe reflection efficiency vs reconstruction angle curves. The results

are valid for hard emulsions.

could be predicted, because the gelatin starts to dryfirst on its surface, being still wet on the glass side.Drying without any dehydration causes the indexmodulation to diminish starting from the gelatin sur-face. Accordingly the reflection efficiency goes downwhen the time interval proceeds. Optical path lengthsbetween the interference planes become longer whenthe time interval is increased [Fig. 7(b)]. This mustmean that the interference planes near the glass platehave longer fringe spacings and are responsible forreflections. Figure 7(c) also supports this suggestionbecause, according to the figure, the number of inter-ference planes decreases.

The last property we measured concerning the de-velopment process was the dependence of the reflec-

z 80

L-160

U _

_) 4

LU

U_o - 30_m 20 o <10_oz (0o)

30

LU 25_ 20

2 0-

a hard emulsion

b

I I . I I

C-

0I I I I I I

5 10 15 20 25 (min)DRYING TIME IN ISOPROPANOL

Fig. 8. (a) Dependence of the reflection efficiency maximum on thedrying time in isopropanol. (b) The angular position of the reflec-tion maxima. (c) The half-height widths of the reflection efficiencyvs reconstruction angle curves. The results are valid for hard emul-

sions.

tion efficiency on the duration of the final 100% isopro-panol bath. The results are seen in Fig. 8. There areonly small variations in the reflection efficiency curves[Fig. 8(a)], meaning that the dehydration is alreadycomplete after 5 min. Only a slight decrease in thehalf-height width curve [Fig. 8(c)] indicates that thegrating structure becomes more homogeneous during aprolonged isopropanol bath.

To test the resolution properties of our gratings wemeasured the reflection efficiency as a function of therecording angle; results are seen in Fig. 9. The plateswere developed in the normal way. The reflectionefficiency [Fig. 9(a)] in the 5150-6100-lines/mm grat-ing period range stays almost constant at -80%. Thereading angle as a function of the recording angle [Fig.9(b)] shows that the reconstructing wavelength shiftedtoward red except at smaller angles, where it staysconstant at -5. The half-height width curve [Fig.9(c)] together with this constancy of the reconstructionangle shows that the grating structure must be almostindependent of the recording angle when this is small-er than 150 in air. If we believe that the resolution isthe reason for this constancy, the resolution maximummust be over 6100 lines/mm.

By rotating the plate we could tilt the interferenceplanes inside the emulsion. We measured the reflec-tion efficiency maximum as a function of this interfer-ence plane slant, as shown in Fig. 10(a). As before wehave also displayed the reconstruction angle of themaxima [Fig. 10(b)] and the half-height width [Fig.10(c)]. The reflection efficiency maxima are almost


. . . . . .

. .

. . . . .

(%)0 0

° 0 0 0

.L LU

Lu 20 a hard emulsion

(0)

bC, 40

30LU

60 00

X_ _ -J 4 0 0< -

In 2 0

10 20 30 40 50 (0)

RECORDING ANGLE

Fig. 9. (a) Dependence of the reflection efficiency maximum on therecording angle. The plates were developed in the normal wayaccording to Table I. (b) The angular position of the reflectionefficiency maxima. The straight line shows the theoretical value.(c) The half-height widths of the reflection efficiency vs reconstruc-

tion angle curves. The results are valid for hard emulsions.

independent of the slant factor. The position of themaxima shows a shift toward red which seems to be thetendency of our gratings in other experiments as well.The half-height width is almost constant, indicatingthat the grating structure does not change when theslant of the interference planes is <300.

IV. Summary

This work was done to solve some problems in thelarge and interesting area of processing DCG reflectiongratings. During the measurements it was found thatthe results could vary considerably if measured ondifferent days. The reason for this was the smallchanges in temperature and relative humidity. Ac-cordingly, if the measurements were taken during ashort period they behave more logically.

Reflection efficiencies above 90% were available byusing a prehardened gelatin. With soft emulsions wecould gain almost an ideal efficiency of 100%. Howev-er, it was noted that the noise-to-signal ratio of hardgelatin gratings was better. Also the hard layers didnot have the milky appearance of the unexposed partsof the soft layers. By varying the sensitization concen-tration it was noted that high reflection efficiencieswere achieved when the concentration was -5%. Alsothe reconstruction angles were then near the recordingangle. A prolonged washing time after exposure

1%)

CE L>__ 8 0,_ Z

LUL

_ 60U-

LU ( )-J

0 40

C; 30

I-i

I:' 2 0

LA= 10

Cr

= ( 0)

I ° 30

-JI< I 2 01

LU= -

0 10 20 30 40 ( 0)ROTATING ANGLE

Fig. 10. (a) Reflection efficiency maxima as a function of the rotat-ing angle. The rotating angle is a measure of the plate deviationfrom its initial value (both beams entering at an angle of 250).Interference plane tilt (the upper numbers in the horizontal scale)gives an angle between plate surface and interference planes. (b)The angular position of the reflection efficiency maxima. (c) Thehalf-height widths of the reflection efficiency vs reconstruction an-

gle curves. The results are valid for hard emulsions.

changed the distribution of interference planes. Bestreflection efficiencies for hard emulsions wereachieved when the washing time was 10 min or longer.At the same time the spectral bandwidth was at itsmaximum.

The final 100% isopropanol bath completely dehy-drated hard and exposed emulsions within 5 min.

By chaiiging the recording geometry it was foundthat the maximum resolution of the prehardened grat-ings is more than 6100 lines/mm.

The reflection properties of the hard emulsions didnot change when the interference planes were tilted upto 300.

We wish to thank Pertti Ketolainen for helpful dis-cussions during this work and also for critically readingthe manuscript. We are also grateful to Pertti Silfstenfor his technical assistance.

References1. B. J. Chang and C. D. Leonard, "Dichromated Gelatin for the

Fabrication of Holographic Optical Elements," Appl. Opt. 18,2407 (1979).

2. R. W. Evans, "The Development of Dichromated Gelatin forHolographic Optical Element Applications," Proc. Soc. Photo-Opt. Instrum. Eng. 523, 302 (1985).

3. D. Meyerhofer, "Phase Holograms in Dichromated Gelatin,"RCA Rev. 33, III (1972).


- hard emulsion

_ 0

00-a

I I I I I I

0 6,2 12,4 18,2 23,5 28,2I I I I ,

. . . . . . . .

4. H. Owen, "Holographic Optical Elements in Dichromated Gela-tin," Proc. Soc. Photo-Opt. Instrum. Eng. 523, 296 (1985).

5. R. D. Rallison, "Holographic Optical Elements (HOEs) in Dich-romated Gelatin (DCG): Progress," Proc. Soc. Photo-Opt. In-strum. Eng. 523, 292 (1985).

6. S. Sjollnder, "Dichromated Gelatin and Light Sensitivity," J.Imaging Sci 30, no 7, 151 (1986).

7. T. Keinonen and 0. Salminen, "Dichromated Gelatin as a Holo-graphical Recording Material. I. Transmission Gratings,"Publication of U. Joensuu, Series B1, No. 23 (1982).

8. S. Sjolinder, "Bandwidth in Dichromated Gelatin HolographicFilters," Opt. Acta 31, 1001 (1984).

9. D. J. Coleman and J. R. Magarinos, "Controlled Shifting of theSpectral Response of Reflection Holograms," Appl. Opt. 20,2600 (1981).

10. R. A. Cullen, "Some Characteristics of and Measurements onDichromated Gelatin Reflection Holograms," Proc. Soc. Photo-Opt. Instrum. Eng. 369, 647 (1983).

11. Y. Ishii and K. Murata, "Flat-Field Linearized Scans with Re-flection Dichromated Gelatin Holographic Gratings," Appl.Opt. 23, 1999 (1984).

12. S. P. McGrew, "Color Control in Dichromated Gelatin Reflec-tion Holograms," Proc. Soc. Photo-Opt. Instrum. Eng. 215, 24

NASA continued from page 2553

ing to an estimate, a highly reliable, compact joint could be built forless than $1 per channel and could accommodate one or severalchannels, each with a 1-MHz bandwidth. The concept could beused in the transfer of high-speed data to rotating antennas or acrossthe joints of robots and manipulators in automated manufacturing.

The concept calls for a row of laser or light-emitting-diode trans-mitters on the outside member of a cylindrical joint and a series ofcorresponding photodiode receivers on the inner member (see Fig.6). The receivers would be arranged as a stack of rings. A transmit-ter would continually beam light at its receiver ring as the latterrotates. Instead of facing each other across the gap, the transmit-ting and receiving diodes could be located away from the gap, con-nected to the transmission points and the receiving rings by opticalfibers. The joint could be made as long as necessary to accommo-date the requisite number of transmitter/receiver pairs for eachchannel. For bidirectional data flow, a combination of transmitters

and receivers would be mounted on both sides of the joint. Therewould be no mechanical contact between transmitters and receiversand, therefore, no wear.

In a variation of the concept (also shown in the figure), transmit-ters would be arrayed along a radius of a flat disk, and receiver ringswould be arrayed along a pole surrounded by a conical mirror. Themirror would reflect light from the lasers-the only suitable trans-mitters in this case-into the receiving photodiodes.

This work was done by Fred J. Becker of McDonnell DouglasCorp. for Johnson Space Center. Refer to MSC-21182.

Adjustable, audible continuity tester for delicate circuitsAn adjustable, audible electrical-continuity tester gives an audi-

ble indication, making it unnecessary for the user to look directly at

0 oX

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useful when one of the members must have a flat surface.

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small circuit board.continued on page 2594


9

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influence of some development parameters on the reflection grating structure in dichromated gelatin

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