CA1049303A - Spatial frequency carrier and process of preparing same - Google Patents

Abstract

SPATIAL FREQUENCY CARRIER AND
PROCESS OF PREPARING SAME

Bruce Lee Booth Abstract of the Disclosure A transparency for reproducing preselected area delineations in colors and intensities thereof variable at will is formed by using an interference pattern formed holographically by the interference of a split laser beam.
The transparency has a plurality of mutually exclusive areas each individual area representing a variable color wherein the area or areas representing a particular variable color contains spatial frequency carriers which are oriented differently from the spatial frequency carriers of the other areas. The holographic process of preparing the transparency by separately exposing each area in a different orientation to the interference of the two said laser beams is also disclosed. The process and apparatus for utilizing the separation transparency to form predetermined images wherein the color of the individual areas and the intensity thereof forming such image may be varied at will is also disclosed.

Description

Background of the Inventlon Field of the Inventlon The present lnventlon relate~ to a transparency for reproducing area delineatlons of colored areas of a pattern on a dlsplay in an optlcal vlewer. The transparency ls made of a photopolymerlzable materlal wlth mutually excluslve areas, each lndividual area representlng a varlable color wherein each area representlng a particular variable color has encoded thereln preselected angularly oriented spatlal frequency carriers created holographically by the interference of a split laser beam.
Description of the Prlor Art In many lndustries and especlally in the textile industry a designer for styling purposes has a need for viewing different color combinations of a pattern ln which each color may be lndlvidually manlpulated at will through the entire range of color. Systems have been proposed whlch enable se-lected color combinations of a pattern to be viewed on a vlsual display. An example o~ such a system is an optical viewer which utilizes a light source and a photographic plate. me photo-graphic plate is encoded with a spatial frequency carrier for each area of the pattern, with the encoded areas diffracting llght from the light source to reproduce the pattern on a dis-play. The light is colorable and enables the pattern to be colored ln a varlety of color combinations. It is further known to use a camera to photograph the displayed pattern to make a proof.
In the past the encoded frequency carrier used in the above-described system has been prepared u~ing silver halide based systems. The technique involves focusing a Ronchi rullng on an emulsion contalning the silver halide to expose the silver

- 2 -104'~3~3 hallde followed by developlng the e~cposed sllver halide. Thls type of dlffraction grating cannot diffract more than about 7%
of the llght passed therethrough into the first order of dlf-fraction. Because of thls these dlffraction grating~ are often bleached to change the silver hallde into a transparent salt with a different index of refraction than the emulsion creating a phase dlffraction gratlng medium havlng lmproved dlffraction efflclency. me bleaching process often produces imperfectlons in the gratings which results in "nolse " when light is passed 10 through the grating. In either ca~e the resulting diffraction grating consists of lines of emulsion which are rectangular in cross-séction and usually about 10-15 microns in thickness 8Up-ported on a transparent support such as glass or organlc resln film. The rectangular cross-sectlon of the grating lines in either type grating in effect gives them a multlple frequency and produces many orders of diffraction. me use of a Ronchi grating places a practical llmit to the 2paclng of the llnes to a maxlmum ~f about 200 lines per millimeter. A thin phase grat-ing can theoretically diffract up to about 339~ of the light 20 passed therethrough into the first order of diffraction, but in practice the amount for a bleached grating 18 usually in the range of 15 to 20%. It is desirable to increase the amount of light passed into the first order of diffraction and addltion-ally to eliminate the hlgher orders since they must be blanked out.
Summary of the Invention me present inventlon relates to a transparency for reproducing area delineations of colored areas of a pattern on a display in an optical viewer and lts preparation which iæ use-30 ful in a technique in which area delineations of the coloredareas of a pattern are reproduced on a di~play. me method for l~g303 preparlng this transparency involves forming a plurality of mas~s which are severally adapted to indivldually expose to light the various areas of a pattern which areas represent indivldual colors of said pattern. mese masks are then u~ed to expo~e individually area~ of a photopolymerizable material to the lnter-ference of two laser beams to record holographically a spatial frequency carrier in each area. Each area is exposed separately at a different radial orientation to the laser i~terference 80 as to be able to diffract light in a different direction so that a plurality of radially arranged light sources having their color variable at will may be passed through the transparency and fo-cused on a display.
Brlef Descri~tion of the Drawingæ
Fig. 1 is a pattern dlvlded into mutually exclusive area delineations of independently colorable areas of the pattern.
Fig. ~ i8 a replica of Flg. 1 on whlch the area delineations are reproduced and encoded wlthin thelr boundarles by uniquely spatial frequency carriers.
Fig. 3 shows separation positive~ for each of the area delineations of Flg. 1.
Fig. 4 shows separation negatives for each of the area dellneations of Flg. 1.
Fig. 5 shows a laser beam splitter and mirror system for forming an interference pattern holographically on the transparency.
Fig. 6 shows the separation transparency as the pre~elected angularly oriented spatial frequency carrier is applied thereto stepwise for each area delineation.
Fig. 7 is a plan view of a circle containing the light sources used to illuminate the spatial frequency carrier.

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Flg. 8 ls a schematlc representatlon of a system for reproduclng the area dellneatlons of the pattern on the display.
Detailed Description In the process of the present invention separation negatlves are utllized in a procedure for making a repllca of a pattern. The repllca is made of a photopolymerizable material on which area dellne~tions are reproduced and encoded wlth pre-selected angularly orlented spatlal frequency carriers. m e replica i8 then used in an optlcal viewer to reproduce the area delineations on a display. m e area delineations are colored independently with selected colors.
Fig. l is a representative pattern l composed of five mutually exclu~ive area delineatlons of independently colorable areas 2, 3, 4, 5 and 6.
Po~itive and negative area ~eparation transparencies as shown in Figs. 3 and 4, respectively, are prepared by known methods for each area dellneatlon shown in Flg. l. The separa-tlon posltlves A-E of Fig. 3 are prepared first wlth each posi-tive having an opaque portion (stippling shows opaclty) which has the same shape and relative position as the corresponding area delineation of the pattern. The remainder of each positive is tranæparent. The separation negatives A-E of Flg. 4 are then produced from the posltlves. The negatives are reduced in size from the positives using a copy camera. Each negative has a transparent portion which has the same shape and relative posi-tion as the corresponding area delineation of the pattern with the rest of the negative being opaque. Transparency A of Fig.
4 shows an example of the separation~negative for area deline-ation 2 of the pattern. The portlon correspondlng to area de-lineatlon 2 ls transparent whlle the rest of the negatlve lsopaque. The positlve transparency A of Fig. 3 is slmllar but 104g303 wlth the opaque ~nd transpare~t areas reversed.
The separation negatlves A-E of Flg. 4 are used as masks to encode replica 7 shown ln Fig. 2 by the stepwise method shown ln Flg. 5 and Flg. 6. To encode the ~lrst area dellneatlon negatlve 4A is used as mask 8 which masks photopolymerlzable sheet 7. me llght from laser 9 is ~plit lnto two beams of e~-~entlally equal inten~lty of light by beam splltter lO which directs the two beams to mirrors ll and 12. The llght from mirror 12 i8 beamed to mirror 13. The light beams from mirrors ll and 13 are directed through focusing lenses 14 and 15, respec-tlvely, then through spatial filters 16 and 17, respectively, and flnally through collimatlng lenses 18 and l9, respectlvely. The spatlal frequency filters generally are 5 to 25 micron pinholes in an opaque sheet located at the focal point of focusing lenses 14 and 15 to remove noise (i.e., nonparallel light) as dlffracted by the lenses 14 or 15. me presence of such nolse 18 generally due to either lmperfections of dust on the lenses or the mirrors between the laser 9 and the focusing lenses 14 and 15. me two colllmating lenses are optional. Mirrors ll and 13 are posltioned 80 that the colllmated light beams 20 and 21 is~uing from lenses 18 and l9 produce an lnterference pattern on sheet 7 which is positloned essentlally at a 90 angle from the collimated light beam 20 issulng from lens 18. The angle ~ between the lnterfer-ing collimated beams 20 and 21 is typically about 17 to lnsure a spatlal frequency of between 500 and 750 lines per milllmeter.
me interference pattern created by the two laser beams causes a diffract~on gratlng or spatial frequency carrler to be formed in the photohardenable material. In the preferred process after a short time the photopolymerizable material is reexposed to slngle laser beam 20 to harden the remainlng material without encoding a carrler therein. Sheet 7 preferably is a layer of photopolymer mounted on a suitable support such as glass.

10~9303 ~ rhus, to make a ~eparation transparency of this inventlon negative A of Flg. 4 is placed ~n posltion 8 in Flg. 5 to serve as a mask and the shutter of a laser ls opened for 10 seconds to expose area 2 of sheet 7 with a spatlal frequency carrier having the orlentation indicated for area 2 of sheet 7 in Fig. 6. It is to be understood that the exposure of 10 se- -conds and other time intervals used herein are illustratlve and that the preferred time to be used in a specific lnstance may vary according to the sensitlvlty of the photopolymer or other photosensitive material used and the intensity of the llght used.
After 60 seconds the shutter of laser 9 is opened for 120 seconds with the beam 21 blocked to photopolymerize the resldual monomer remaining in area 2 of sheet 7. m en negative A is removed from sheet 7 and replaced with negative B. m e entlre assembly of 7 and 8 is then rotated 72 about axis 22 as shown in Fig. 5 whlch axls ls normal to the plane of the assembly of mask 7 and sheet 8. The shutter of laser 9 is again opened for 10 seconds using the interfering beams from the beam splltter 10 to encode exposed area 3 of sheet 7 with a spatial frequency carrier having an orientation as indlcated for area 3 of sheet 7 in Flg. 6. After 60 seconds the shutter of laser 9 is opened with beam 21 blocked for 120 seconds to photopolymerize the unphotopolymerized mate-rlal remalnlng ln area 3 of sheet 7. Negatlve B is then removed from sheet 7 and replaced with negative C. The entire assembly of 7 and 8 is then rotated an additlonal 72 about axis 22 as shown in Fig. 5. me shutter of laser 9 ls again opened for 10 seconds using interfering beams 20 and 21 from beam splitter 10 to encode area 4 of sheet 7 wlth a spatlal frequency carrier having a dlfferent orientation as indicated for area 4 of sheet 7 in Fig. 6. After 60 seconds the shutter of laser 9 ls again opened with beam 21 blocked for 120 seconds to photopolymerize the unphotopolymerized material remaining in area 4 of sheet 7.

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~ ~ ~93UD3 Negatl~e C is then removed from sheet 7 and replaced with negative D. The assembly 7 and 8 ls again rotated an additional 72 about axls 22. The shutter of laser 9 is again opened for 10 seconds using the interferlng beams 20 and 21 from beam splitter 10 to encode exposed area 5 of ~heet 7 wlth a spatial frequency carrier havlng the new orientation as indicated for area 5 of sheet 7 in Fig. 6. After 60 seconds the shutter of laser 9 i~ then opened with beam 21 blocked again for 120 seconds to photopolymerize the unphotopolymerized material remainlng in ~o area 5 of sheet 7. Negatlve D is then removed from sheet 12.
Flnally æheet 7 is then rotated an addition~l 72 about axls 22.
The shutter of laser 9 is again opened for 10 seconds uslng in-terferlng beams 20 and 21 from beam splitter 10 to encode area 6 of sheet 7 with ~ spatial frequency carrier having the different orientat~on as indicated for area 6 of sheet 7 in Fig. 6. After 60 seconds the ahutter of laser 9 is then opened wlth beam 21 blocked to photopolymerlze the unphotopolymerized material re-maining in area 6 of sheet 7. It should be noted that after the encoding of each of areas 2 to 5 of sheet 7 with the spatial fre-quency carrier, the remaining unphotopolymerized materlal ln eacharea was photopolymerized in such a way that no additional car-rier encodement took place. Thiæ meant that when area 6 was exposed to the spatial freguency carrler encodement step no mask was necessary and, thus, the need to prepare negative E was elim-lnated. If thls photopolymerizatlon of the unphotopolymerlzed material remaining after each of the spatial frequency carrier encodement steps is not done, it is then necessQry to use nega-tive E as mask 8 in the last step. Negative E is also neces6ary when using a silver halide emulsion. The remaining unphotopoly-merlzed material in each of the masked areas can then be photo-polymerized all at once by exposure of whole sheet 7 to ordinary lighto It is to be understood that prior to completion of the 1~49;303 last spatlal carrier encodement step that sheet 7 of photopoly-merlzable material ls to be malntained in the dark or in the presence of safe lights to protect ~rom exposure to wavelengths of light which will initiate photopolymerization except for the ~pecific exposure steps ~et forth above. Obviously a greater or fewer number of area delineations can be used. Because there are flve areas used ln the foregoing de~cription~ the angular orientation of the spatial frequency carriers are separated by essentially uniform 72 lntervals, although lf a greater number of areas are used, the intervals will be less. In the past using bleached silver halide spatial frequency carrlers prepared using Ronchi rulings, lt has been necessary to maintain a minl-mum of about 72 between the radial orientatlon of each of the spatial frequency carrlers thus limiting the number of area delineation6 on a transparency, The transparencies of the pre-sent invention function without cross talk between areas when the radial orientation between each of the spatial frequency carriers i8 as small as 20 thus enabling a transparency to have up to eighteen area delineations encoded thereon. This is due princlpally to the ellmlnation of the higher orders of diffrac-tlon whlch are difflcult to mask from several pattern areas.
After the transparency 19 produced, lt can be removed from the support and mounted between thln glass slldes using optlcal cement.
In order to reconstruct the combined image of the recorded designs in the compound grating and dlsplay each lmage area in varlous lnfinitely varlable colors for vlewlng, a sult-able display system wlll be described with reference to Figs. 7 and 8. In order to relmage the lnformation recorded on the five area delineatlon transparency sheet 7 prepared above, flve light source positions 23, 24, 25, 26, 27 are posltioned equidistant around the circumference of an imaginary circle 28 as shown ln ~1049303 Flg. 7 standing in a plane parallel to transparency ~heet 7 and display image 29 as shown in Fig. 8. There are three lights in each of these five posltions. The innermo~t light of each posi-tlon is ~et up with blue light filters 30, 31, 32, 33, and 34 and the llght sources are directed toward the transparency sheet 7 positloned ln space with the surface normal passing through the center of the imaginary circle contalning the llghts. The angular displacement of the lights from the normal is determined by the Bragg angle whlch is a function of the blue wavelength transmitted by the blue filters 30, 31, 32, 33, and 34 and the spatlal frequency encoded in transparency sheet 7. The equation for determining the Bragg angle is sin (0/2) = 1 /2d where ~/2 is the Bragg angle, ~, is the wavelength of light ln mm and l/d s the number of lines per mm encoded in the ~patial fre-quency carrier. me middle light of each of the five light source posltlons is arranged radially displaced further from the center of the imaginary circle 28 they are contained within than the innermost circle of lights. All five middle light sources are set up with green filters 35, 36, 37, 38 and 39. Again the dl~tance the light sources are displaced from the center of the imaglnary circle 28 is determined by the Bragg angle, A third light of each of the five light source posltions is arranged radially displaced further from the center of the imaginary circle 28 they are contalned within than the middle lights. All five of these outer llght sources are set up with red filters 40, 41, 42, 43 and 44. Agaln the distance the light sources are dis-placed from the center of the imaginary circle 28 ls determined by the Bragg angle. Each position of three lights i~ arranged around the circle spaced 72 from its neighborlng positlon of three lights.
Fig, 8 is a schematic representation of a side v~ew of - : .
,, . ~ , . .

104g303 a system for reproducing the area delineatlons of transparency 7 on display 29. For clarity only one light source position lndicated generally at 23 is shown. Each light source posltlon comprises three llght bulbs 45, 46 and 47 equlpped with a col-llmating lens 48~ 49 and 50, respectively. The three individual lights 45, 46 and 47 are also equipped with blue filter 30, green filter 35 and red fllter 40, respectlvely. Each llght 45, 46 and 47 i8 al~o equlpped with means not shown to vary the lntenslty thereof. Sultable means can be a rheostat or varlac to control the current fed to each light or a diaphragm or neu-tral density filter lnterposed between the individual llght bulbs and thelr colllmatlng lens. By varying the relatlve amountæ of red, green and blue llght the color produced by light source posltlon 23 can be lnfinltely varled at wlll. The area dellne-atlon of transparency 7 whlch is to cooperate wlth llght source positlon 23 i8 rotatlonally oriented 80 that light from light source position 23 will be diffracted to lens 51 and focused on display 29. E~ch of the three indlvidual light sources which make up light source positlon 23 are radially displaced from axis 52 by an amount determlned by the Bragg angle whlch i9 fixed by the spatial frequency encoded in transparency 7 and the wavelength of the light transmitted by filters 30, 35 and 40.
The llght from light source position 23 which is not lncldent on the area delineation whlch is oriented to cooperate with llght source posltlon 23 will ~ot be diffracted because of the dlf-ference in the orientation of the spatial frequency carrier.
Zero order light, i.e., llght which passes through transparency 7 without belng diffracted, 18 blocked by stop 53. In the same way light from remaining light source positions 24, 25, 26 and 27 is directed through transparency 7 focusing lens 51, and the aperture of stop 53 to reproduce ~rea delineations 2, 39 4, 5 and 6 on display 29. It should be recalled that higher order 104g303 light i8 not present ln the preferred embodiment uslng photo-polymer~ and thus need not be blocked.
An alternative method of recording a pattern on a grating and reimaglng a pattern on the screen may be accompllshed by uslng converging llght both ln recordlng and recon~tructlon in place of the colllmated light used in the above example. Another alternative i9 to substltute diverging light for both the record-ing of the pattern and recon~tructlon o~ the lmage. Another option which may be employed is to substltute one xenon or other whlte light ~or the tungsten llghts. InsteAd of employing three separate llght sources for each of the flve positi~ns, a slngle xenon llght source can be used in con~unctlon wlth elther a series of mirrors or optlcal fibers in order to transmit light ~rom the ~ingle source to the flve equl~paced posltlons on the clrcumference of each of the three llght clrcles.
The invention may be practlced u~ing a dlfferent num-ber of masks other than the illustrated four to record dlfferent patterns; e,g., 2-20 different masks. Also the number of color fllters may be varled; e.g., from 2-6.
Transparency 7 preferably ls prepared by casting a photopolymerizable composltion on a glass plate or other support followed by the ~equentlal exposure process descrlbed above.
Sultable photopolymerlzable materlals are dlsclosed in U.S.
Patent ~o. 3 658 526 lssued April 25, 1972, to Haugh. Generally these photopolymerlzable composltlons contain (l) at least one additlon polymerlzable, nongaseous, ethylenlcally unsaturated monomeric compound having a boiling point above 100C at normal atmospherlc pressure and being capable of forming a high polymer by free radical initlated, chain-propagating addition polymeri-zation, and (2) a free radlcal generating polymerlzatlon initi-ator or system activatable by actinic radiation. In addition to components (l) and (2), the photopolymerizable material can lO~g303 contain an organic binder, a plastlclzer for the binder, a thermal polymerizatlon inhlbltor, and a chain transfer agent or polymerlzatlon accelerator and other components.
A ~ultable compositlon for use herein is 5.2 g of cellulose acetate butyrate (ca. 17% butyryl, ca. 29.5% acetyl, and ca. 1.5~ hydroxyl: having a viscosity o~ 117 poises as de-termined by ASTM Method D-1343-~4T in solution as Formula A, ASTM Method D-871-54T); 5.8 g triethyleneglycol diacrylate;
o.6 g 2-0-chlorophenyl-4,5-di(m-methoxyphenyl)imidazolyl dimer;
0.3 g (3,4-dimethoxyphenyl)-bis(2-methyl-4-diethylaminophenyl)-methane; 0.02 g N-phenylglycine, 0.2 ml ethylene bis(2-oxyethyl)-acetate; and 80 g of A solvent which is 10 weight percent 1,2-dichloroethane and 90 weight percent dichloromethane. The above composition is prepared by mixing and then cast on a glass plate, and air dried in the dark to glve a coating welght of 5 mg/cm2.
The photopolymerizable materlal when exposed properly can give a transparency having a carrier having spatial frequency o~ up to about 3,000 lines per millimeter. m e number of llnes per millimeter obtained in the transparency ls a function of the wavelength of the light produced by laser 9 and angle ~ (Fig. 5) between the reference beam 20 and split beam 21 when forming the interference pattern on tran~parency 7. The preferred range is from 500 to 750 lines per milllmeter. This preferred frequency range can be obtained using an argon laser (4880 A) and an angle between reference beam 20 and split beam 21 of about 17.
The tran~parency encoded wlth the spatlal frequency carrier of the present invention is generally from 50 to 100 microns in thickness. This compares with about 10-15 microns in thickness whlch i8 about the typical optimum with the silver halide diffraction gratings prepared using either the Ronchi ruling or the holographic process. m e use of laser interfer-ence beams to create the spatial frequency carrier results in the carrier belng formed of transparent wavelike regions of photopolymer embedded ln transparent photopolymer havlng a some-what different refractive index than the refractlve lndex of the wavelike regions me~e wavelike regions are disposed thr~ugh the thickness of the transparency 7 at a small angle to the nor-mal to the surface being the angle which ls essentlally the bisector o~ the angle of the two beam~ 20, 21 used to create them. me fact that it is transparent enables practically 100~
llght transmission by the transparency. The fact that the trans-parency ls a thick grating enables it to diffract about 98-99%
of the transmitted light into the +l order of diffraction. In practice about 50 to 60% of the light is diffracted and the re-malnder pas~ed straight through. Because of this the intensity of the llght reaching the display is from 2.5 to 10 times greater when using the transparencv of the present invention than when using thin gratings using a silver hallde emulsion or bleached silver halide emulsion said thin gratings being formed using the Ronchi ruling system. Silver halide gratings produced by the Ronchi ruling method are generally limited in resolution in the type of display device described herein to an upper frequency of about 200 lines per milllmeter. However, in the case of the transparency of the present invention a practical range for the spatial frequency has been found to be from 500 to 750 lines per millimeter. Thls results in a much larger Bragg angle, thus larger angular separation of the wavelengths, which enables the display device to use individual lights as the light source directly without requiring the use of mlrrors or fiber optics.
At this spatial frequency for a transparency 40 microns thick the acceptance angle for vislble light is about +2.5. For thicker gratings the acceptance angle can be as small as +1.
Because of the laser beam interference pattern used to holo-graphically expose the photosensitive material the pro~ess of .

the present invention can be used to create sllver hallde transparencie~ in wh~ch the ma~or portion of the light dlffracted thereby ls ln the +l order of diffraction.
In addltion to being u~eful in the textile indu~try for fiber and fabric producers, converters, and prlnters, the reproduction of colored areas of a pattern on a display screen in an optical vlewer, wlth the displayed colored pattern recorded on a transportable proof, is also useful for designing, viewing, and merchandising wall and floor coverings, advertising layouts, commerclal packaging, interior decoration, art and textile edu~
catlon, architecture, paper products (e.g., greeting cards, wrap-ping paper, party supplies), paint induætry products, automotive styling, building construction, and the like.

Claims (8)

What is claimed is:

1. A process of preparing a transparency having a plurality of mutually exclusive areas, each area containing differently oriented spatial frequency carriers comprising (1) covering a layer of photosensitive material with a mask, (2) mounting said masked layer of photosensitive material in a holder where two split beams from a laser interfere in the region exposed by the mask, (3) activating both laser beams for a period of time sufficient to produce an interference diffraction grating in said exposed photosensitive material, (4) rotating said layer of photosensitive material about an axis normal to the plane of said layer of photosensitive material to a position at least about 15° from each position where previously exposed, (5) replacing the mask with another mask adapted to expose an area of said layer of photosensitive material which is mutually exclusive of the area exposed in (3) above, (6) repeating (3) and (4) (7) repeating (5) and (6) for each additional area to be encoded on the transparency except the last area, and (8) repeating (3) in the last area,

2. The process of Claim 1 wherein the photo-sensitive material is a photopolymerizable material.

3. The process of Claim 2 wherein the photo-polymerizable material comprises a photopolymerizable monomer.

4. The process of Claim 3 wherein in each repetition of step (3) after the two laser beams are deactivated, one of the laser beams is reactivated to polymerize the remaining unpolymerized monomer.

5. The process of Claim 4 wherein the last area or areas to be simultaneously encoded are exposed to the two split laser beams as in step (3) in the absence of a mask as in step (1).

6. The process of Claim 3 wherein a mask is used in step (8) and after all areas have been encoded by action of both laser beams, the whole sheet is exposed to light to polymerize the remaining unpolymerized monomer in each of the areas.

7. A film-like transparency from about 50 to about 200 microns in thickness having a plurality of mutually exclusive areas, each individual area containing differently oriented spatial frequency carriers formed of a photo-polymerized organic resin, said spatial frequency carriers having a spacing of from 200 to 2,000 lines per millimeter.

8. The transparency of Claim 7 wherein the spatial frequency carriers have a spacing of from 500 to 750 lines per millimeter.