CA1079552A - Black and white diffractive subtractive light filter - Google Patents
Black and white diffractive subtractive light filterInfo
- Publication number
- CA1079552A CA1079552A CA328,791A CA328791A CA1079552A CA 1079552 A CA1079552 A CA 1079552A CA 328791 A CA328791 A CA 328791A CA 1079552 A CA1079552 A CA 1079552A
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- Canada
- Prior art keywords
- zero
- peak
- diffractive
- diffraction
- manifesting
- Prior art date
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Abstract
Abstract of the Disclosure The disclosed filter provides a gray-scale color (wavelength) characteristic which is substantially independent of luminosity, and is preferably neutral.
Such a gray scale is achieved in diffractive subtractive light filter comprising: at least one region consisting of a mixture of black-manifesting subareas and white-manifesting subareas; where each black-manifesting subarea comprises a diffractive structure composed of at least one substantially sine-wave profile phase grating having a line spacing sufficient to permit the separation of substantially all higher-diffraction-order light from imaged zero-diffraction-order light, any of said phase gratings having an optical peak-to-peak amplitude of a selected value which provides a zero-diffraction-order light transmittance wavelength selectivity characteristic which exhibits a minimum zero-diffraction-order light transmittance at a wavelength within the visible wavelength spectrum; where each white-manifesting subarea comprises a substantially non-diffractive structure; and where the respective sizes of said individual subareas are sufficiently small to be substantially unresolvable in imaged zero-diffraction-order light, but are large relative to said line spacing.
Such a gray scale is achieved in diffractive subtractive light filter comprising: at least one region consisting of a mixture of black-manifesting subareas and white-manifesting subareas; where each black-manifesting subarea comprises a diffractive structure composed of at least one substantially sine-wave profile phase grating having a line spacing sufficient to permit the separation of substantially all higher-diffraction-order light from imaged zero-diffraction-order light, any of said phase gratings having an optical peak-to-peak amplitude of a selected value which provides a zero-diffraction-order light transmittance wavelength selectivity characteristic which exhibits a minimum zero-diffraction-order light transmittance at a wavelength within the visible wavelength spectrum; where each white-manifesting subarea comprises a substantially non-diffractive structure; and where the respective sizes of said individual subareas are sufficiently small to be substantially unresolvable in imaged zero-diffraction-order light, but are large relative to said line spacing.
Description
RCA 70,460 /A
~7~35S2 This invention relates to an improved diffractive subtractive filtering technique for reconstructing black-and-white images in an op~ical projector using the zero diffraction order of light transmitted through a diffractive structure in which a grating-like carrier is modulated with the image information.
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U.S~ patent 3,732,363 of W. E. Glenn, Jr., `~ issued May 8, 1973, discloses a diffractive subtra~tive filter in the form of a single, amplitude-modulated slnusoidal diffraction grating having a predetermined e spacing, which may be embossed as a relief pattern ¦ 15 in the surface of a medium such as transparent thermoplastic film. The amplitude of t~e sinusoidal grating varies from point to point in accordance with recorded pictorial or alphanumeric information. More specifically, the sinusoidal grating has a predetermined amplitude depth corresponding to all "black" points of the recorded pictorial or alphanumeric information, has a zero amplitude corresponding to all "white" points of the recorded pictorial or alphanumeric information, and has resp~ctive amplitudes greater than zero but less than the predetermined amplitude corresponding to varying degrees of "gray"
points of the recorded pictorial or alphanumeric information.
Readout of the recorded pictorial or alphanumeric information, in U.S. patent 3,732,363, may be achieved in an optical projector by illuminating the diffractive subtractive filter with white light and imaglng onto a .. ;~ -`- ' ~
.. :, '~ . ': '' 10795~Z RCA 70,460/A
1 screen only the zero diffraction order of the light which emerges from the filter. More specifically, most of the light incident on regions of the filter having the aforesaid predetermined amplitude, corresponding to "black" points, is diffracted into higher diffraction orders, so that only a small portion o the incident light corresponding to "black" points is present in the zero diffraction order I which is imaged. Therefore, these "black" points exhibit ¦ low luminosity and appear relatively black in the image.
1 10 However, light incident on the zero~ampltiude portion i of the filter, corresponding to "white" points, passes undiffracted through the filter, so that substantially ¦ all the incident light corresponding to "white" points remains within the zero diffraction order of light which is imaged. Therefore, these points exhibit high I luminosity and appear white in the image. The relative j amount of incident light diffractively subtracted from :1 ' `~ the zero diffraction order and deflected into the higher diffracticn orders by respective intermediate-amplitude region of the filter, corresponding to points of varying ~hades of "gray", is greater than it is for the "white"
points but less than it is ~or the "black" points.
Therefore, these points exhibit intermediate luminosities and appear gray in the image.
The contrast of a black-and white image obtainable with a diffractive subtractive filter of the ¦ typ~ disclosed in U.S. patent 3~732r363 on reconstruction in an optical projector depends upon the ratio between the relatively low luminosity of a "black" image point 3 to the relatively high luminosity of a "white" image point.
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1 The contrast obtainable from such a single, amplitude-modulated sinusoidal diffrac~ion grating, of the type disclosed in U.S. patent 3,732,363, is relatively poor.
However, in a paper entitled "Transmission Characteristics of Sinusoidal Phase Gratings", presented at the 1973 Annual Meeting of the Optical Society of America, Rochester, New York, C.S. Ih disclosed that zero-order contrast on reconstruction in an optical projector can be improved by the superposition of a number of sine wave gratings.
United States patent 3,957,354 issued May 18, 1976 and assigned to the ~ame assignee as the present applica-tion, discloses a diffractive subtractive color filtering technique. It is disclosed in V.S. patent 3,957,354, that the relative amount of incident light which is diffracted by a diffractive structure to higher orders is a given ~`
function of light wavelength (color of the light) which depends solely on ~1) the particular shape of the profile tsinusoidal, square wave, etc.) of the diffractive structure and ~2~ the peak-to-peak optical amplitude (i.e. physical peak-to-peak amplitude multiplied by the difference between th~ index of refraction of the diffractive structuxe and that of its surroundings) of the diffractive structure profile.
One of the profile shapes considered in Vnited States patent 3,957,354 is a sinusoid. ~ is shown that a sinusoidal profile exhibits a relatively small wavelength selectivity compared to that exhibited by a square-wave profile. Therefore, with a sinusoidal '~ `' .
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I profile, it is possible to select a particular predetermined peak-to-peak optical amplitude such that substantially all the incident light at some given wavelength in the middle of the visible spectrum (i.e. at some point in the green) is diffracted into the higher diffraction orders, and still, in this case, diffract a significant, but smaller, amount of the incident light at the opposite ends of the visible spectrum (the red end and the blue end) into the higher diffraction orders. Therefore, even when the luminosity of the light remaining in the zero diffraction order is relatively at its minimum, its color is not a neutral black or neutral dark gray, but is actually a dark shade o magenta. This is true because the relative intensity of light still remaining in the imaged zero diffraction order in both the red and in the blue portions of the visible spectrum is significantly greater than that in the green portion of the visible spectrum.
Further, in the gray-scale of zero-diffraction-order-light luminosity achieved by the approach disclosed in U.S. patent 3,732,363, (i.e. varying the relative "grayness" as a continuous function of amplitude of a single sinusoidal grating between zero and a predetermined maximum where the Iuminosity of the zero-diffraction-order light is a minimum), the color hue of the zero dlfraction order is not constant~ but varies because thë wavelength color selectivity characteristics of a sinusoidal diffractive structure~change as a function of amplitude. Thus, if "black" is manifested by a certain ~30 dark shade of magenta, "light gray" is manifested by a ~ .
~L~7955Z RCA 7 0, 4 6 O/A
1 relatively light tint of some other different color.
Therefore, a diffractive subtractive structure o~ the type disclosed in the aforesaid U.S. patent 3,732,363 is not capable of producing an essentially neutral :~
black-and-white image, when reconstructed in an optical projector.
The present invention is directed to a diffractive-subtractive filter for imaging black-and-white images which exhibits a gray-scale color ~
10 (wavelength) characteristic which is substantially `-independent of luminosity, and is preferably neutral.
This feature may be achieved by providing diffractive subtractive light filter comprising: at least one ~ ;
region consisting of a mixture of black-manifesting subareas and white-manifesting subareas; where each black-manifesting subarea comprises a diffractive ~ :-structure composed of at least one substantially sine-wave profile phase grati~g having a line spacing sufficient to permit the separation of substantially :~ :
20 all higher-diffraction-order light from imaged zero- ::
diffraction-order light, any of said phase gratings ; .
having an optical peak-to-peak ampli~ude of a selected value which provides a zero-diffraction-order light transmittance wavelength selectivity characteristic ~:~
. . .
25 which exhibits a minimum zero-diffraction-order light :~
transmittance at a wavelength within the visible :
,: , wavelength spectrum; where each white-manifesting subarea comprises a substantially non-diffractive structure; and where the respective sizes of said ::
30 individual subareas being sufficiently small to be ~ :
. :
. '. . , ' .
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1 substantially unresolvable in imaged zero-dif~raction-order light, but being large relative to said line spacing. With this arrangement, said one region exhibits a relative luminosity which is determined by the proportion of the area of said one region comprised of said black-manifesting subar~as compared to the proportion of said area of said one region comprised of white-manifesting subareas.
In the following detailed dascription of the invention, reference is made to the attached drawings in which: .
FIG. 1 is a schematic of a projector for pro- ~.
jecting only the zero diffraction order derived from a surface relief pattern transmissive diffracting medium; :-FIG. 2 is a graph showing the zero-order transmittance for visible light as a function of wavelength ~:~
for a single sinusoidal grating having a value of peak-to-peak optical amplitude which results in minimum zero-order transmittance for light having a wavelength at a point near the middle of the visible spectrum; : .
FIG. 3a, 3b, 3c and 3d, respectively, schematically show a white region, a lighter gray region, a darker gray region and a black region of a black-and-white diffractive .
subtractive filter embodying the prin~iples of the present ..
: , invention; . .
FIG. 4 is a CIE color chart showing the colori- .
metric parameters of zero order diffraction transmitted light from two sinusoidal gratings, which are angularly displaced by 90 with.respect to each other and both of ~.
which have the same optical peak-to~peak amplitude;
FIG. 5a shows respective graphs of the 1~7~S52 RCA 70,460/A
., I transmittance of respective sinusoidal gratings having respective first and second predetermined optical peak-to-peak amplitudes as a function of wavelength over the visible spectrum and Figure 5b shows the transmittance of a diffractive subtractive filter consisting of two superimposed sinusoidal gratings, which are angularly displaced by 90~ with respect to each other and which have respectively the aforesaid predetermined optical peak-~o-peak amplitudes, and Figure 6 shows tw~ respective plots for the resist removed from SHIPLEY AZ1350 photoresist as a function of light exposure, one of the plots being ~btained with a first type of development of the photoresist and the second plot being obtained with a ~15 second type of development of the photoresist.
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The type of diffractive structures, with which ; the present inventlon is concerned, are phase diffractive structures, rather than absorption diffractive l~ 20 structures. In principle, such phase diffractive I ~tructures may take the form of (1) a reflective phase ~ -diffractive structure, (2) a variation in the index of :
refraction of a transmissive medium which corresponds to the phase diffractive structure, (3) a relief pattern in the surface of a transmissive medium, in which the transmissive medium has a substantially uniform ~ , . .. .
index of refraction different from that of its surxoundings, and the relief pattern corresponds with the I phase diffractive structure or (4) a combination of some or all the aforesaid forms of phase diffractive structures.
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I However, a phase diffractive structure which takes the form of a relief pattern in the surface of a transmissive medium having a substantially uniform index of refraction different from that of its surroundings is the most practical form for a phase diffractive structure comprising a diffractive subtractive filter.
This is true because (1) this form may be inexpensively replicated by hot pressing a substantially transparent plastic material and (2) readout of this form may be compatibly accomplished with standard projectors and viewers. Therefore, in the preferred embodiment of the invention described below, it is assumed that the diffractive subtractive filter is in the form of a relief pattern embossed on the surface of a transmissive medium having a substantially uniform index of refract.ion greater than the surrounding air. However, it should be understood that the principles of the present -invention apply with equal force to all the other types of phase diffractive structures discussed above~
Referring now to Figure 1, there is shown schematically a typical example of a projector or projecting solely the zero order diffraction of diffractive subtractive filter 100 on the screen. More spe~ifically diffractive filter 100 preferably comprises a transparent sheet of material, such as embossed plastic, having a .. ~
substantially uniform predetermined index of refraction different from that of the surrounding ambient, such as air. This sheet of material includes a surface relief .
pattern of spatially distributed di.ffraction elements.
Each diffraction ele~ent has waveform profile _g_ ~795~z RCA 70,460/~
I charact~ristics and optical peak-to-peak amplitude character-istics of the type described in detail below.
Diffractive subtractive filter 100 is illuminated ', with polychromatic light from a source which preferably 5 comprises an incandescent filament 102 emitting broadband , white light. The width (i.e. diameter or largest cross-sectional dimension) of filament 102 is D, as shown in Figure 1. A pair of condenser lenses 104 and 106, each ,~, having focal length f, are situated as shown in Figure 1 10 with filament 102 located in the front focal plane of ;' ~-condenser lens 104. Therefore, the diverging light beam 108 ;
is collimated in parallel light beam 110 by condenser lens '~'' ;
104.' Condenser lens 106 converts parallel light beam , 110 into converging light beam 112.
Diffractive subtractive filter 100, which is situated as shown in relatively close proximity to condenser '~'' lens 106 and in the path of converging light beam 112, is ,-, , illumlnated by converging light beam 112. ~he light output ~from diffractive subtractive filter 100 consists of the , ,, zero diffraction order 114 and the higher diffraction orders, such as -1 order 116 and *1 order 118~ Projection lens 120, '~' which is situated solely in the path of zero diffraction :
order 114, is~effective in projecting solely the zero , , '~ diffraction order output light 114 on a screen.
Selective projection is accomplished,by the de1ection of al~l the~higher difraction order output light such as : ~
-l and ~l order output light 116 and 118 beyond the '~
aperture of projection lens 120, as shown in Figure 1.
. . .
The projector schematically shown in Figure 1 is compatible with conventional -10~
.- . - ... . ... . , ... . . ~ .. ; .. . ;.. . . . .
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I projectors and viewers, such as slide projectors and viewers and motion picture projectors.
In the readout of a zero order diffraction image, with a conventional projector of the type shown S in Figure 1, regions of the diffractive subtractive filter 100 which do not contain any diffraction grating transmit the incident light unaffected and are, $herefore, reconstructed in the image as white.
Regions in which the grating is present diffract at least some of the incident light into the first and higher diffraction orders, which fall outside the aperture of lens 120. Therefore, this diffracted light is subtracted from the zero-diffraction imaged light, so that these latter regions appear relatively dark.
~s known, the first order diffraction angle ~ is related to the grating period d such that sin a approximately equals the ratio of the wavelength to the grating period d. In orcler to ensure that none oE the first order light overlaps the zero order light in the plane of projection lens 120, the first order diffxaction angle ~ must be sufficiently large over the entire visible spectrum of between 400 nm and 700nm.
The majority of present day, commercial projectors have a projection lens apert~re of about F/2.8 and require that a grating period d~l.4~m for full compatibility (~ varying from about 17 at the blue end of the spectrum to about 30 at the red end of the spectrum)O
Given this grating period, there are in principle many grating relief profiles which can be used. Howevery it is important that the chosen profile can be readily ~ --11- , ' .. .
,. . ..
1~7~2 RCA 70,460/A
I recorded in practice. This requirement is best satisfied by a sine wave relief profile, which can be recorded as an interferencè of two plane/ coherent wavefronts (derived from a laser beam) using a photoresist S recording medium. Other profiles, such as sawtooth or triangular, cannot be easily recorded with difraction periods d approximately equal to 1.4~m.
The zero order transmittance T of a relief sine-wave profile phase ~rating is given by:
T = Jo2 (~ a) = J0 (~----); (1) where J0 is the ~ero-order Bessell function of the irst kind, a is the opthcal depth (peak-to-peak) of ~ sine wave profile/ a' is the physical depth (peak-to-peak) of a sine wave profile relief in the surface lS of a transmissive medium, ~n is the difference in refractive index between the transmissive medium and its surroundings (usually air), and ~ is the free space wavelength o~ any incident light component.
Suitable choice of a single sinusoidal profile, having an appropriate value of physical depth a', gives a transmittance which is minimum in the green and provides a relatively low luminosity over the whole visible range. Specifically, as shown by way of example in Figure 2, the zero-order transmittance for the visible spectrum of a single sinusoidal grating having a physical depth a' equal to 0.87~m in the surface of a transparent plastic, such as polyvinyl chloride, having a nominal value of index of refraction n of 1.5, provides a transmittance of more than 10%
at the blue end (400 nm~ o~ the spectrum, dropping to ~ -12- ;
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1 substantially zero at a point in the green middle (500-600 nm) portion of the spectrum and then rising again to somewhat less than 10% at the red end (700 nm) of the spectrum. In order to provide a neutral black, the zero order transmittance should be substantially flat over most of the visible (400-700 nm) spectrum.
Howevex, as can be seen from Figure 2, ~ero order transmittance of a single sinusoidal grating is not substantially flat over the visible spectrum, but exhibits a significant wavelength selectivity. In particular, the transmitted zero order light for the sinusoidal grating shown in Figure 2, when illuminated with white light, exhibits a dark magenta shade produced by the relatively large amounts of zero-order ~-transmitted light in the blue and red portions of the spectrum compared to that in the green portion of the spectrum.
. ~ ..
The best recording medium for originally recording a sinusoidal relief pattern i5 a positive ~20 ~ photoresist, such as 5HIPLEY ~Z1350. By proper development of this photoresist (described in detail below), high resolution recording of a sinusoidal pattern can be achieved with fair linearity. However, because complete linearity cannot be obtained, some distortion of the sine-wave depth profila occurs. When such a photoresist origlnal recording is used to ultimately derive embossed plastic replicas, in a manner known in the art, the final embossing stage introduces additional distortion ln the~sine-wave profile. -The effect of these distortions is that the predicted ~ -13-, i . ' '~
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1 theoretical luminous density in zero-order is not realized in practice in the embossed thermoplastic replicas. For example, a single sine-wave grating with an optical depth a of 450 nm, provides a theoretical zero-order density for the filter of 2.05. However, due to the aforesaid distortions, such an embossed single sine-wave grating exhibits a typical density of only about 1.3. That is, in practice, the contrast exhibited by an embossed black and-white di~fractive subtractive filter comprising a single sine-wave grating is relatively poor~
The zero order contrast obtainable from a black-and-white diffractive subtractive filter may be increased by superimposing at least two angularly-displaced sine-wave gratings. For example, two crossed sine-wave gratings with an optical depth a of 450 nm exhibit a theoretical zero-order density of 3.59 (rather than the 2.05 zero-order density of a single sine-wave grating) but, in practice, a typical zero-order density obtained for an embossed replica of two crossed sine wave gratings is only about 1.8 (rather than the 1.3 value of a single sine wave grating). Thus, in practice, it i5 necessary to use at least two superimposed gratings to obtain good ~25 contrast, although in theory a single grating would suffice~ However, undesired wavelength (color) selectivity of a single sine~wave gxating discussed above in connection with Figure 2, is substantially increased when two cross~d sine-wave gratings with the same optical amplitude a are employed, so that there is -~ -14-' "' ~795SZ RCA 70,460/A ~ -1 produced a "muddy" black with observable color, rather than a desirable neutral "black". Further, the unwanted zero-order color is not constant, but varies with the value of the optical depth a o~ the gratings, which is most undesirable in a gray scale. In typical projectors and viewing environments this zero-order color is readily observable at luminous densities of about l.8 such as those obtained for embossed replicas of two crossed sine wave gratings. The observed color follows the general color sequence as a function of the grating optical amplitudes a as described ~n detail below in connection with Figure 4.
Referring now to Figures 3a, 3b, 3c and 3d, there is schematically shown a black-and-white diffractive subtractive filter incorporating tWQ ~ -crossed sine-wave gratings which exhibit a substantially non-wavelength selective gray scale. Specifically, Figure 3a schematically shows a "white" manifesting ; region 300 in which no diffraction structures are ~20 present. Therefore, substantially all light incident ; on white region 300 is undiffracted and, therefore, remains in the imaged zero diffraction order. "Lighter-gray"~manifesting region 302, shown in Figure 3b, and "darker-gray" manifesting region 304, shown in Figure 3c, ~ of a diffractive subtractive filter axe both divided into sub areas covered by diffractive structure (such as the subareas covered by diffractive structures 302a in Figure 3b and by d1ffractive structures 304a in ~ Figure 3c) and by subareas left uncovered by such 30 ~ diffractive structure (such as subareas 302b in Figure 3b ~ ;
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1 and 304b in Figure 3c). The only material structural difference between "lighter-gray" region 302 and "darker-gray" region 304 is that the proportion of the overall ~ -area of "darker-gray" region 304 covered by diffractive 5 structure subareas 304a is greater than the proportion of the overall area of "lighter-gray" regi.on 302 covered by diffractive-structure subareas 302a. In the case of any "all-black" manifesting region 306 of a subtractive diffractive filter, shown in Figure 3dl the entire 10 area thereof is covered by a diffractive structure 306a. Except for their respective sizes, all of diffractive structures 302a, 304a and 306a are substantially identical.
Specifically, each one of diffractive structures 302a, 304a and 306a comprises two superimposed crossed sinusoidal gratings, which are angularly displaced from each other by substantially 90~
. Although the respective optical depths al and a2 and .
; the corresponding physical depths a'l and a'2 of the ~20 two crossed sinusoidal gratings may be equal to each other, they preferably have different values selected . to provlde a neutral "black", as described below. In ~. i any case, the respective optical depths of the sinusoidal gratings are selected to provide substantially a minimum ~:
zero-order luminosity~for each respective subarea covered ..
hy any of diffractive structures 302a, 304a and 306a. .-;:-: ~ Further, the respective sizes of each individual -~:
one of the diffractive-structure subareas 302a and 304a and the~ respective sizes of each individual one of 3 non-diffractive subareas 302b and 304~ of each of ~ 16-,''.
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1 lighter and darker gray regions 302 and 304 should be sufficiently small to be individually substantially unresolvable in the zero order diffraction image.
However, each individual one of these diffractive structures should still be large with respect to the line spacing d of its constituent sinusoidal gratings.
Reference is now made to Figure 4, which is a plot line 400 on a CIE color chart showing the wavelength (color) selectivity of the zero order diffraction light derived from two superimposed crossed sinusoidal gratings having equal optical depths (al=a2) as a function of the value of such optical depths. In Figure 4, the effective white color W of the light source illuminating the diffractive structure IS is assumed to be 3200K. Plot line 400 in Figure 4 shows only the chromaticity characteristics of the zero diffraction order. ~ substantially neutral gray scale over the entire l~minosity~range from white to black would require tha~ the chromaticity remain in the nei~hborhood of poi~t W, such as point Z, for values of optical depths of the two crossed sinusoidal gratings from 0 to abou~ 450 nm. However, as plot line 400 indicates, the chromaticity, of two crossed sinusoidal gratings of ~qual optical depth varies widely as a function of the value of optical depth and occupies "color-mani~esting" points (shown in Table I below~
remote from poin~s W and Z over most of its length.
Therefore, a gray scale provided by crossed sine wave .
gratings having the same optical depth is not neutral, but manifests an observable color at each point thereof.
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I Further, this observable color is not constant over the gray-scale range from black to white, but varies with the value of optical depth, as indicated by the shape and position of plot line 400 within the CIE color chart shown in Figure 4, as shown in Table I.
TABLE I
optical depth (al=a2) color manifested 350 nm red-brown 410 nm magenta 450 nm blue The zero order transmittance T of two crossed sine- ;.
wave gratings as a function of wavelength is T = J0 (- al) J0 (- a2) ~2) By properly selecting different optical depths al and a2 for each respective one of the two crossed sine-wave :~ gratings, a low luminosity, substantially neutral black is obtainable. Specifically, using equation (1) above, Figure 5a separately shows a first graph 500 of the zero-order . ~ ~ transmittance as a function of wavelength over the visible ~20 spectrum of a sinusoidal grating having an optical depth a of 790 nm and a similar graph 502 of a sinusoidal grating having an optical depth a2 of 450 nm. As shown by graphs 500 and 502, ~he zero order transmittance as a function of wavelength for both a sinusoidal grating having an optical depth al of 790 nm and a sinusoidal grating having an optical depth a2 of 450 nm ranges fxom zero to about 15%. . .
- However, a~sinusoidal grating ha~ing an optical depth of 7q:Q.nm exhibits zero transmitt~nc~:to ~zero-order light at a wav~length of about 450 nm,,while a slnusoldal grating having an RCA 70,460/A
~L~79~
1 optical depth a2 of 450 nm exhibits a zero transmittance for zero-order light at a wavelength of nearly 600 nm.
Using e~uation 2, Figure 5b shows a graph of two crossed sine-wave gratings, one of which has an optical depth al of 790 nm and the other of which has an optical depth a2 of 450 nm. Tha zero diffraction order transmittance of graph 504 of Figure 5b is the product o~ the zero order transmittance of the individual sine-wave gratings at each wavelength in the visible spectrum. As can be seen from graph gO4, the zero-order transmittance of the two crossed sine wave gratings having diffexent optical depths is relatively small and is relatively independent of wavelength over most of the visible spectrum, as compared to the zero-order transmittance of the individual ~15 sine-wave gratings shown in graph 500 and 502. Therefore, the di~ractive subtractive filter having a zero-order tran~mittance characteristic shown in graph 504 provides a relatively high density, neutral black transfer function, whose effective "color" in the CIE chart of Figure 4 is at point Z. ;
The particular value of 790 nm for the optical depth al of one of the crossed sinusoidal gratings and the par~icular value 450 nm fsr the optical depth a2 f .
the other of the two crossed sine wave gratings is only 25~ illustrative. All that is required to obtain a more neutral "black", than that obtainable by the prior art, is that the respective values of the optical depths, such as a1 and a2, of at least two superimposed, angularly-displaced sinusoidal gratings be selected so that they all exhibit a zero-order transmittance oharacteristic, ~ -19~
: ~"
~L~7955z RCA 7 0, 4 6 0 /~
1 similar to graphs 500 and 502, which goes through a minimum within the visible spectrum, and that the respective zero order transmittance minimum for each selected optical depth occurs at a separate and distinct wavelength in the visible spectrum. Preferably, the respective optical dep~hs of two sine-wave gratings should differ from each other by at laast 100 nm, to provide a more-or-less optimum resultant zero-order transmittance characteristic, similar to that shown by graphs 504, which exhibits a substantially neutral chromaticity point in the color chart of Figure 4 in the neighborhood of points W and Z.
Although the present invention may be practiced with a diffractive structure composed of more than two crossed sine-wave gratings, all having different optical depths, there is little advantage in doing so. This is true because the mixed highex diffraction orders can then amerge closer to the zero order so that they tend to be collected by the projection lens, leading to a ~further reduction in filter density and contrast. For example, if three gratings angularly displaced by 120 and all having equal periods of d are employed, the (1, 1, 1) diffraction order is parallel to the zero-order direction.
This can only be avoided if, in addition to choosing optimum ~ orientations, one or more of the grating periods is reduced by about a factor of two or more. This reduction in qrating period is however, undesirable from the recording viewpoint. Similar considerations hold for combinations of more than three gratings. Therefore, although more than two superimposed angularly-displaced gratings may be used, two crossed sine-wave gratings with :` ::
,, : . . . .: .- . -: . .... . . . . . .. . .
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1 90 orientation to each other is to be preferred.
One technique for recording an original diffractive structure, for use in deriving a diffractive subtractive filter embodying the features of the present invention, will now be described. Specifically, the recording medium used is SHIPLEY AZ1350 positive photoresist which exhibits different photoresist development characteristics for respective developers AZ1350 and AZ303, both of which are manu$actured by the Shipley Co., Inc., Newton, Mass. The measured development characteristics of these developers for an exposing wavelength ~=436 nm are shown in Figure 6. As shown by plot 600, a 6 second developement of AZ1350 positive photoresist in AZ303 developer at 1:4 dilution in water gives an approximately ..
linear development characteristic of resist removed with respect to the amount of exposure which the photoresist has previously undergone. However, graph 602 shows th~t a 30 second development of AZ1350 positive photoresist in AZ1350 developer at 1:1 dilution in water provides a steeper, nonlinear development characteristic, which requires higher exposures, but does not attack the photoresist at unexposed areas thereof. The detailed form of these curves is a ~unction of tha recording wavelength and development time, but the general features remain.
Similar results may alss be found for other positive photoresists with suitable developersO
In making an original diffractive subtractive ~ilter, an AZ1350 photoresist film is first spun-coated onto a suitable substrate, such as glass or Plexiglass ~ .-0 to a film thickness of at least 2.5~m, to thereby enable ~955i;~
RCA 70,460 I a deep wave profile to be formed. The photoresist isexposed first to a sinusoidal grating pattern of proper line spacing d r formed by a conventional two beam interference arrangement of light from a HeCd ~aser.
The amount of exposure is selected in accordance with graph 600 to provide for a preselected physical depth a'l ~which is equal to ~n times the optical depth al~ ;
to be achieved on development for the first of the two sine wave gratings. The photoresist-coated substrate is then rotated through 90~ and the second exposure is made of the second sine-wave grating. These kwo exposures may have a ratio of 16:9 to obtain respective grating depths which provide an optimum neutral zero order black, as described above. The photoresist is then developed in the 1:4 AZ303 developer for about 6 ~ ~-seconds to give a crossed sine-wave grating structure, wherein each of the two sine-wave gratings has a different .
proper grating depth. Development progress may be monitored by measuring the zero-order transmittance in ~ the red (e.g. using a HeNe laser). Optimum depth for use in making polyvinyl chloride replicas is reached shortly after a minimum is reached in the red zero-order transmission (measured for the dry resist layer in air) or when the red zero-order transmission falls to about 27~ (measured for the resist layer in water). The ; resulting resist grating blank is still light sensitive and, therefore, must be stored and handled in the dark or under ye1low or red safe-lights. -~
The resulting resist grating blank, as so far described, may be used in this form, ~i desired, as -.
.
9552 RCA 70, 460 /A
1 a master recording for deriving an embossed replica diffractive subtractive filter in a thermoplastic, such as polyvinyl chloride. Such a diffractive subtractive filter, which would provide a substantially uniform neutral "all-black" image, o~ the type shown in Figure 3d, can be useful in itself. For instance, such an embossed "all black" subtractive diffractive filter might be used as a diffraction grating substrate for produciny the type of recording medium blanks described in U.S. patents 3,669,673 and 3,743~507 of Ih et al. Therafore, such a neutral "all-black"
subtractive diffractive filter is contemplated by the present invention.
Howeverl in most cases, the resulting photoresist 15~ grating blank, which comprises two crossed sine-wave gratings each having a different physical and optical depth, is not used directly in this form to derive diffractive subtractive filter replicas. Instead, ~; the resulting photoresist grating blank is image-exposed ~2~ ~ to object-information light derived from a positive transparency. In particular, the positive transparency may be either imaged or contact printed onto the photoresist grating blank using either HeCd laser light or ,. :.
incoherent light from an ultraviolet lamp. ~he object information in the positive transparency is preferably ;~ already i~n ~h-e "h~ t~ne" ~orm. In this case, the gray scale, of the type shown in Figures 3a, 3b, 3c and 3d, i! . :`
inherently derived in the image exposure, without .
the use of any additional screening. This approach 3 gives the best results. However, if the objPCt 1079 SSZ RCA 70,46~A
.
1 information in the positive transparency is not in "half-tone" form, the object information light from the positive transparency can be passed through an appropriate screen, similar to the screens used to provide half-tones in the printing art, before being incident on the photoresist grating blank, and the "half-tone" form generated in tha subsequent development.
The image-exposed photoresist grating blank is then high-gamma developed by AZl350 developer, discussed in connection with Figure 6, to completely remove during development all areas and subareas of the photoresist grating blank which have been exposed to any light during image exposure. However, all areas and subareas of the photoresist blank which remained unexposed during image exposure are substantially unaffected during ~ -development by AZl350 developer. This is true because, as shown in Figure 6, the photores1st exposures re~uired when using AZl350 developer are co~siderably ~ higher than those required when using AZ303 developer, -~
zo as shown in Figure 6. Therefore, the prior grating exposure alone does not lead to any significant photoresist removal during the later AZ1350 development of the object information image. This fact was confirmed experimentally by immersing gratings in a photoresist, recorded as above, for 30 seconds in l:l AZ1350 developer. It was found, ~n this case, that no change in the zero-order denqity or color was observable.
-~ S1nce all those areas or subareas of the photo- -resist ~rating blank which received~any light during the seco~d object-information image exposure are completely ~- -2~-':
- ~ : . . . . . ~.. - , . : -~07955Z RCA 70,460 /A
1 removed during the AZ1350 development, the entire cxossed sine-wave grating structure is removed from these particular exposed areas and subareas, leaving revealed corresponding ~lat substrate areas and subareasO These flat substrate areas and subareas define the white areas in the reconstructed picture, while the remaining areas and subareas define the blac:k areas in the reconstructed picture. The final step is to produce a metal master from the resulting original photoresist recording for hot-pressing embossed replicas into thermoplastic sheets. Using this technique, high quality zero-order-diffraction images have been recorded with full, neutral gray scale and a black density of approximately l.8.
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~7~35S2 This invention relates to an improved diffractive subtractive filtering technique for reconstructing black-and-white images in an op~ical projector using the zero diffraction order of light transmitted through a diffractive structure in which a grating-like carrier is modulated with the image information.
~, .
U.S~ patent 3,732,363 of W. E. Glenn, Jr., `~ issued May 8, 1973, discloses a diffractive subtra~tive filter in the form of a single, amplitude-modulated slnusoidal diffraction grating having a predetermined e spacing, which may be embossed as a relief pattern ¦ 15 in the surface of a medium such as transparent thermoplastic film. The amplitude of t~e sinusoidal grating varies from point to point in accordance with recorded pictorial or alphanumeric information. More specifically, the sinusoidal grating has a predetermined amplitude depth corresponding to all "black" points of the recorded pictorial or alphanumeric information, has a zero amplitude corresponding to all "white" points of the recorded pictorial or alphanumeric information, and has resp~ctive amplitudes greater than zero but less than the predetermined amplitude corresponding to varying degrees of "gray"
points of the recorded pictorial or alphanumeric information.
Readout of the recorded pictorial or alphanumeric information, in U.S. patent 3,732,363, may be achieved in an optical projector by illuminating the diffractive subtractive filter with white light and imaglng onto a .. ;~ -`- ' ~
.. :, '~ . ': '' 10795~Z RCA 70,460/A
1 screen only the zero diffraction order of the light which emerges from the filter. More specifically, most of the light incident on regions of the filter having the aforesaid predetermined amplitude, corresponding to "black" points, is diffracted into higher diffraction orders, so that only a small portion o the incident light corresponding to "black" points is present in the zero diffraction order I which is imaged. Therefore, these "black" points exhibit ¦ low luminosity and appear relatively black in the image.
1 10 However, light incident on the zero~ampltiude portion i of the filter, corresponding to "white" points, passes undiffracted through the filter, so that substantially ¦ all the incident light corresponding to "white" points remains within the zero diffraction order of light which is imaged. Therefore, these points exhibit high I luminosity and appear white in the image. The relative j amount of incident light diffractively subtracted from :1 ' `~ the zero diffraction order and deflected into the higher diffracticn orders by respective intermediate-amplitude region of the filter, corresponding to points of varying ~hades of "gray", is greater than it is for the "white"
points but less than it is ~or the "black" points.
Therefore, these points exhibit intermediate luminosities and appear gray in the image.
The contrast of a black-and white image obtainable with a diffractive subtractive filter of the ¦ typ~ disclosed in U.S. patent 3~732r363 on reconstruction in an optical projector depends upon the ratio between the relatively low luminosity of a "black" image point 3 to the relatively high luminosity of a "white" image point.
':
1079SS~
RCA 70,460 /A
1 The contrast obtainable from such a single, amplitude-modulated sinusoidal diffrac~ion grating, of the type disclosed in U.S. patent 3,732,363, is relatively poor.
However, in a paper entitled "Transmission Characteristics of Sinusoidal Phase Gratings", presented at the 1973 Annual Meeting of the Optical Society of America, Rochester, New York, C.S. Ih disclosed that zero-order contrast on reconstruction in an optical projector can be improved by the superposition of a number of sine wave gratings.
United States patent 3,957,354 issued May 18, 1976 and assigned to the ~ame assignee as the present applica-tion, discloses a diffractive subtractive color filtering technique. It is disclosed in V.S. patent 3,957,354, that the relative amount of incident light which is diffracted by a diffractive structure to higher orders is a given ~`
function of light wavelength (color of the light) which depends solely on ~1) the particular shape of the profile tsinusoidal, square wave, etc.) of the diffractive structure and ~2~ the peak-to-peak optical amplitude (i.e. physical peak-to-peak amplitude multiplied by the difference between th~ index of refraction of the diffractive structuxe and that of its surroundings) of the diffractive structure profile.
One of the profile shapes considered in Vnited States patent 3,957,354 is a sinusoid. ~ is shown that a sinusoidal profile exhibits a relatively small wavelength selectivity compared to that exhibited by a square-wave profile. Therefore, with a sinusoidal '~ `' .
' ~'': '~'`
~79`~
RCA 70,460/A
I profile, it is possible to select a particular predetermined peak-to-peak optical amplitude such that substantially all the incident light at some given wavelength in the middle of the visible spectrum (i.e. at some point in the green) is diffracted into the higher diffraction orders, and still, in this case, diffract a significant, but smaller, amount of the incident light at the opposite ends of the visible spectrum (the red end and the blue end) into the higher diffraction orders. Therefore, even when the luminosity of the light remaining in the zero diffraction order is relatively at its minimum, its color is not a neutral black or neutral dark gray, but is actually a dark shade o magenta. This is true because the relative intensity of light still remaining in the imaged zero diffraction order in both the red and in the blue portions of the visible spectrum is significantly greater than that in the green portion of the visible spectrum.
Further, in the gray-scale of zero-diffraction-order-light luminosity achieved by the approach disclosed in U.S. patent 3,732,363, (i.e. varying the relative "grayness" as a continuous function of amplitude of a single sinusoidal grating between zero and a predetermined maximum where the Iuminosity of the zero-diffraction-order light is a minimum), the color hue of the zero dlfraction order is not constant~ but varies because thë wavelength color selectivity characteristics of a sinusoidal diffractive structure~change as a function of amplitude. Thus, if "black" is manifested by a certain ~30 dark shade of magenta, "light gray" is manifested by a ~ .
~L~7955Z RCA 7 0, 4 6 O/A
1 relatively light tint of some other different color.
Therefore, a diffractive subtractive structure o~ the type disclosed in the aforesaid U.S. patent 3,732,363 is not capable of producing an essentially neutral :~
black-and-white image, when reconstructed in an optical projector.
The present invention is directed to a diffractive-subtractive filter for imaging black-and-white images which exhibits a gray-scale color ~
10 (wavelength) characteristic which is substantially `-independent of luminosity, and is preferably neutral.
This feature may be achieved by providing diffractive subtractive light filter comprising: at least one ~ ;
region consisting of a mixture of black-manifesting subareas and white-manifesting subareas; where each black-manifesting subarea comprises a diffractive ~ :-structure composed of at least one substantially sine-wave profile phase grati~g having a line spacing sufficient to permit the separation of substantially :~ :
20 all higher-diffraction-order light from imaged zero- ::
diffraction-order light, any of said phase gratings ; .
having an optical peak-to-peak ampli~ude of a selected value which provides a zero-diffraction-order light transmittance wavelength selectivity characteristic ~:~
. . .
25 which exhibits a minimum zero-diffraction-order light :~
transmittance at a wavelength within the visible :
,: , wavelength spectrum; where each white-manifesting subarea comprises a substantially non-diffractive structure; and where the respective sizes of said ::
30 individual subareas being sufficiently small to be ~ :
. :
. '. . , ' .
, lC~7955i2 RCA 7 0, 4 6 O/A
1 substantially unresolvable in imaged zero-dif~raction-order light, but being large relative to said line spacing. With this arrangement, said one region exhibits a relative luminosity which is determined by the proportion of the area of said one region comprised of said black-manifesting subar~as compared to the proportion of said area of said one region comprised of white-manifesting subareas.
In the following detailed dascription of the invention, reference is made to the attached drawings in which: .
FIG. 1 is a schematic of a projector for pro- ~.
jecting only the zero diffraction order derived from a surface relief pattern transmissive diffracting medium; :-FIG. 2 is a graph showing the zero-order transmittance for visible light as a function of wavelength ~:~
for a single sinusoidal grating having a value of peak-to-peak optical amplitude which results in minimum zero-order transmittance for light having a wavelength at a point near the middle of the visible spectrum; : .
FIG. 3a, 3b, 3c and 3d, respectively, schematically show a white region, a lighter gray region, a darker gray region and a black region of a black-and-white diffractive .
subtractive filter embodying the prin~iples of the present ..
: , invention; . .
FIG. 4 is a CIE color chart showing the colori- .
metric parameters of zero order diffraction transmitted light from two sinusoidal gratings, which are angularly displaced by 90 with.respect to each other and both of ~.
which have the same optical peak-to~peak amplitude;
FIG. 5a shows respective graphs of the 1~7~S52 RCA 70,460/A
., I transmittance of respective sinusoidal gratings having respective first and second predetermined optical peak-to-peak amplitudes as a function of wavelength over the visible spectrum and Figure 5b shows the transmittance of a diffractive subtractive filter consisting of two superimposed sinusoidal gratings, which are angularly displaced by 90~ with respect to each other and which have respectively the aforesaid predetermined optical peak-~o-peak amplitudes, and Figure 6 shows tw~ respective plots for the resist removed from SHIPLEY AZ1350 photoresist as a function of light exposure, one of the plots being ~btained with a first type of development of the photoresist and the second plot being obtained with a ~15 second type of development of the photoresist.
.
The type of diffractive structures, with which ; the present inventlon is concerned, are phase diffractive structures, rather than absorption diffractive l~ 20 structures. In principle, such phase diffractive I ~tructures may take the form of (1) a reflective phase ~ -diffractive structure, (2) a variation in the index of :
refraction of a transmissive medium which corresponds to the phase diffractive structure, (3) a relief pattern in the surface of a transmissive medium, in which the transmissive medium has a substantially uniform ~ , . .. .
index of refraction different from that of its surxoundings, and the relief pattern corresponds with the I phase diffractive structure or (4) a combination of some or all the aforesaid forms of phase diffractive structures.
-` ~8-' ~9 55Z RCA 70,460/A
I However, a phase diffractive structure which takes the form of a relief pattern in the surface of a transmissive medium having a substantially uniform index of refraction different from that of its surroundings is the most practical form for a phase diffractive structure comprising a diffractive subtractive filter.
This is true because (1) this form may be inexpensively replicated by hot pressing a substantially transparent plastic material and (2) readout of this form may be compatibly accomplished with standard projectors and viewers. Therefore, in the preferred embodiment of the invention described below, it is assumed that the diffractive subtractive filter is in the form of a relief pattern embossed on the surface of a transmissive medium having a substantially uniform index of refract.ion greater than the surrounding air. However, it should be understood that the principles of the present -invention apply with equal force to all the other types of phase diffractive structures discussed above~
Referring now to Figure 1, there is shown schematically a typical example of a projector or projecting solely the zero order diffraction of diffractive subtractive filter 100 on the screen. More spe~ifically diffractive filter 100 preferably comprises a transparent sheet of material, such as embossed plastic, having a .. ~
substantially uniform predetermined index of refraction different from that of the surrounding ambient, such as air. This sheet of material includes a surface relief .
pattern of spatially distributed di.ffraction elements.
Each diffraction ele~ent has waveform profile _g_ ~795~z RCA 70,460/~
I charact~ristics and optical peak-to-peak amplitude character-istics of the type described in detail below.
Diffractive subtractive filter 100 is illuminated ', with polychromatic light from a source which preferably 5 comprises an incandescent filament 102 emitting broadband , white light. The width (i.e. diameter or largest cross-sectional dimension) of filament 102 is D, as shown in Figure 1. A pair of condenser lenses 104 and 106, each ,~, having focal length f, are situated as shown in Figure 1 10 with filament 102 located in the front focal plane of ;' ~-condenser lens 104. Therefore, the diverging light beam 108 ;
is collimated in parallel light beam 110 by condenser lens '~'' ;
104.' Condenser lens 106 converts parallel light beam , 110 into converging light beam 112.
Diffractive subtractive filter 100, which is situated as shown in relatively close proximity to condenser '~'' lens 106 and in the path of converging light beam 112, is ,-, , illumlnated by converging light beam 112. ~he light output ~from diffractive subtractive filter 100 consists of the , ,, zero diffraction order 114 and the higher diffraction orders, such as -1 order 116 and *1 order 118~ Projection lens 120, '~' which is situated solely in the path of zero diffraction :
order 114, is~effective in projecting solely the zero , , '~ diffraction order output light 114 on a screen.
Selective projection is accomplished,by the de1ection of al~l the~higher difraction order output light such as : ~
-l and ~l order output light 116 and 118 beyond the '~
aperture of projection lens 120, as shown in Figure 1.
. . .
The projector schematically shown in Figure 1 is compatible with conventional -10~
.- . - ... . ... . , ... . . ~ .. ; .. . ;.. . . . .
~ RCA 70~460/A
I projectors and viewers, such as slide projectors and viewers and motion picture projectors.
In the readout of a zero order diffraction image, with a conventional projector of the type shown S in Figure 1, regions of the diffractive subtractive filter 100 which do not contain any diffraction grating transmit the incident light unaffected and are, $herefore, reconstructed in the image as white.
Regions in which the grating is present diffract at least some of the incident light into the first and higher diffraction orders, which fall outside the aperture of lens 120. Therefore, this diffracted light is subtracted from the zero-diffraction imaged light, so that these latter regions appear relatively dark.
~s known, the first order diffraction angle ~ is related to the grating period d such that sin a approximately equals the ratio of the wavelength to the grating period d. In orcler to ensure that none oE the first order light overlaps the zero order light in the plane of projection lens 120, the first order diffxaction angle ~ must be sufficiently large over the entire visible spectrum of between 400 nm and 700nm.
The majority of present day, commercial projectors have a projection lens apert~re of about F/2.8 and require that a grating period d~l.4~m for full compatibility (~ varying from about 17 at the blue end of the spectrum to about 30 at the red end of the spectrum)O
Given this grating period, there are in principle many grating relief profiles which can be used. Howevery it is important that the chosen profile can be readily ~ --11- , ' .. .
,. . ..
1~7~2 RCA 70,460/A
I recorded in practice. This requirement is best satisfied by a sine wave relief profile, which can be recorded as an interferencè of two plane/ coherent wavefronts (derived from a laser beam) using a photoresist S recording medium. Other profiles, such as sawtooth or triangular, cannot be easily recorded with difraction periods d approximately equal to 1.4~m.
The zero order transmittance T of a relief sine-wave profile phase ~rating is given by:
T = Jo2 (~ a) = J0 (~----); (1) where J0 is the ~ero-order Bessell function of the irst kind, a is the opthcal depth (peak-to-peak) of ~ sine wave profile/ a' is the physical depth (peak-to-peak) of a sine wave profile relief in the surface lS of a transmissive medium, ~n is the difference in refractive index between the transmissive medium and its surroundings (usually air), and ~ is the free space wavelength o~ any incident light component.
Suitable choice of a single sinusoidal profile, having an appropriate value of physical depth a', gives a transmittance which is minimum in the green and provides a relatively low luminosity over the whole visible range. Specifically, as shown by way of example in Figure 2, the zero-order transmittance for the visible spectrum of a single sinusoidal grating having a physical depth a' equal to 0.87~m in the surface of a transparent plastic, such as polyvinyl chloride, having a nominal value of index of refraction n of 1.5, provides a transmittance of more than 10%
at the blue end (400 nm~ o~ the spectrum, dropping to ~ -12- ;
,~, ',"' .'.
~79~ RCA 70,460/A
1 substantially zero at a point in the green middle (500-600 nm) portion of the spectrum and then rising again to somewhat less than 10% at the red end (700 nm) of the spectrum. In order to provide a neutral black, the zero order transmittance should be substantially flat over most of the visible (400-700 nm) spectrum.
Howevex, as can be seen from Figure 2, ~ero order transmittance of a single sinusoidal grating is not substantially flat over the visible spectrum, but exhibits a significant wavelength selectivity. In particular, the transmitted zero order light for the sinusoidal grating shown in Figure 2, when illuminated with white light, exhibits a dark magenta shade produced by the relatively large amounts of zero-order ~-transmitted light in the blue and red portions of the spectrum compared to that in the green portion of the spectrum.
. ~ ..
The best recording medium for originally recording a sinusoidal relief pattern i5 a positive ~20 ~ photoresist, such as 5HIPLEY ~Z1350. By proper development of this photoresist (described in detail below), high resolution recording of a sinusoidal pattern can be achieved with fair linearity. However, because complete linearity cannot be obtained, some distortion of the sine-wave depth profila occurs. When such a photoresist origlnal recording is used to ultimately derive embossed plastic replicas, in a manner known in the art, the final embossing stage introduces additional distortion ln the~sine-wave profile. -The effect of these distortions is that the predicted ~ -13-, i . ' '~
1079SS2 RCA 70,460/A
1 theoretical luminous density in zero-order is not realized in practice in the embossed thermoplastic replicas. For example, a single sine-wave grating with an optical depth a of 450 nm, provides a theoretical zero-order density for the filter of 2.05. However, due to the aforesaid distortions, such an embossed single sine-wave grating exhibits a typical density of only about 1.3. That is, in practice, the contrast exhibited by an embossed black and-white di~fractive subtractive filter comprising a single sine-wave grating is relatively poor~
The zero order contrast obtainable from a black-and-white diffractive subtractive filter may be increased by superimposing at least two angularly-displaced sine-wave gratings. For example, two crossed sine-wave gratings with an optical depth a of 450 nm exhibit a theoretical zero-order density of 3.59 (rather than the 2.05 zero-order density of a single sine-wave grating) but, in practice, a typical zero-order density obtained for an embossed replica of two crossed sine wave gratings is only about 1.8 (rather than the 1.3 value of a single sine wave grating). Thus, in practice, it i5 necessary to use at least two superimposed gratings to obtain good ~25 contrast, although in theory a single grating would suffice~ However, undesired wavelength (color) selectivity of a single sine~wave gxating discussed above in connection with Figure 2, is substantially increased when two cross~d sine-wave gratings with the same optical amplitude a are employed, so that there is -~ -14-' "' ~795SZ RCA 70,460/A ~ -1 produced a "muddy" black with observable color, rather than a desirable neutral "black". Further, the unwanted zero-order color is not constant, but varies with the value of the optical depth a o~ the gratings, which is most undesirable in a gray scale. In typical projectors and viewing environments this zero-order color is readily observable at luminous densities of about l.8 such as those obtained for embossed replicas of two crossed sine wave gratings. The observed color follows the general color sequence as a function of the grating optical amplitudes a as described ~n detail below in connection with Figure 4.
Referring now to Figures 3a, 3b, 3c and 3d, there is schematically shown a black-and-white diffractive subtractive filter incorporating tWQ ~ -crossed sine-wave gratings which exhibit a substantially non-wavelength selective gray scale. Specifically, Figure 3a schematically shows a "white" manifesting ; region 300 in which no diffraction structures are ~20 present. Therefore, substantially all light incident ; on white region 300 is undiffracted and, therefore, remains in the imaged zero diffraction order. "Lighter-gray"~manifesting region 302, shown in Figure 3b, and "darker-gray" manifesting region 304, shown in Figure 3c, ~ of a diffractive subtractive filter axe both divided into sub areas covered by diffractive structure (such as the subareas covered by diffractive structures 302a in Figure 3b and by d1ffractive structures 304a in ~ Figure 3c) and by subareas left uncovered by such 30 ~ diffractive structure (such as subareas 302b in Figure 3b ~ ;
- ~. .
~ -15-'.
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.
1 and 304b in Figure 3c). The only material structural difference between "lighter-gray" region 302 and "darker-gray" region 304 is that the proportion of the overall ~ -area of "darker-gray" region 304 covered by diffractive 5 structure subareas 304a is greater than the proportion of the overall area of "lighter-gray" regi.on 302 covered by diffractive-structure subareas 302a. In the case of any "all-black" manifesting region 306 of a subtractive diffractive filter, shown in Figure 3dl the entire 10 area thereof is covered by a diffractive structure 306a. Except for their respective sizes, all of diffractive structures 302a, 304a and 306a are substantially identical.
Specifically, each one of diffractive structures 302a, 304a and 306a comprises two superimposed crossed sinusoidal gratings, which are angularly displaced from each other by substantially 90~
. Although the respective optical depths al and a2 and .
; the corresponding physical depths a'l and a'2 of the ~20 two crossed sinusoidal gratings may be equal to each other, they preferably have different values selected . to provlde a neutral "black", as described below. In ~. i any case, the respective optical depths of the sinusoidal gratings are selected to provide substantially a minimum ~:
zero-order luminosity~for each respective subarea covered ..
hy any of diffractive structures 302a, 304a and 306a. .-;:-: ~ Further, the respective sizes of each individual -~:
one of the diffractive-structure subareas 302a and 304a and the~ respective sizes of each individual one of 3 non-diffractive subareas 302b and 304~ of each of ~ 16-,''.
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~Q7~355Z
1 lighter and darker gray regions 302 and 304 should be sufficiently small to be individually substantially unresolvable in the zero order diffraction image.
However, each individual one of these diffractive structures should still be large with respect to the line spacing d of its constituent sinusoidal gratings.
Reference is now made to Figure 4, which is a plot line 400 on a CIE color chart showing the wavelength (color) selectivity of the zero order diffraction light derived from two superimposed crossed sinusoidal gratings having equal optical depths (al=a2) as a function of the value of such optical depths. In Figure 4, the effective white color W of the light source illuminating the diffractive structure IS is assumed to be 3200K. Plot line 400 in Figure 4 shows only the chromaticity characteristics of the zero diffraction order. ~ substantially neutral gray scale over the entire l~minosity~range from white to black would require tha~ the chromaticity remain in the nei~hborhood of poi~t W, such as point Z, for values of optical depths of the two crossed sinusoidal gratings from 0 to abou~ 450 nm. However, as plot line 400 indicates, the chromaticity, of two crossed sinusoidal gratings of ~qual optical depth varies widely as a function of the value of optical depth and occupies "color-mani~esting" points (shown in Table I below~
remote from poin~s W and Z over most of its length.
Therefore, a gray scale provided by crossed sine wave .
gratings having the same optical depth is not neutral, but manifests an observable color at each point thereof.
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~07~5;Z
I Further, this observable color is not constant over the gray-scale range from black to white, but varies with the value of optical depth, as indicated by the shape and position of plot line 400 within the CIE color chart shown in Figure 4, as shown in Table I.
TABLE I
optical depth (al=a2) color manifested 350 nm red-brown 410 nm magenta 450 nm blue The zero order transmittance T of two crossed sine- ;.
wave gratings as a function of wavelength is T = J0 (- al) J0 (- a2) ~2) By properly selecting different optical depths al and a2 for each respective one of the two crossed sine-wave :~ gratings, a low luminosity, substantially neutral black is obtainable. Specifically, using equation (1) above, Figure 5a separately shows a first graph 500 of the zero-order . ~ ~ transmittance as a function of wavelength over the visible ~20 spectrum of a sinusoidal grating having an optical depth a of 790 nm and a similar graph 502 of a sinusoidal grating having an optical depth a2 of 450 nm. As shown by graphs 500 and 502, ~he zero order transmittance as a function of wavelength for both a sinusoidal grating having an optical depth al of 790 nm and a sinusoidal grating having an optical depth a2 of 450 nm ranges fxom zero to about 15%. . .
- However, a~sinusoidal grating ha~ing an optical depth of 7q:Q.nm exhibits zero transmitt~nc~:to ~zero-order light at a wav~length of about 450 nm,,while a slnusoldal grating having an RCA 70,460/A
~L~79~
1 optical depth a2 of 450 nm exhibits a zero transmittance for zero-order light at a wavelength of nearly 600 nm.
Using e~uation 2, Figure 5b shows a graph of two crossed sine-wave gratings, one of which has an optical depth al of 790 nm and the other of which has an optical depth a2 of 450 nm. Tha zero diffraction order transmittance of graph 504 of Figure 5b is the product o~ the zero order transmittance of the individual sine-wave gratings at each wavelength in the visible spectrum. As can be seen from graph gO4, the zero-order transmittance of the two crossed sine wave gratings having diffexent optical depths is relatively small and is relatively independent of wavelength over most of the visible spectrum, as compared to the zero-order transmittance of the individual ~15 sine-wave gratings shown in graph 500 and 502. Therefore, the di~ractive subtractive filter having a zero-order tran~mittance characteristic shown in graph 504 provides a relatively high density, neutral black transfer function, whose effective "color" in the CIE chart of Figure 4 is at point Z. ;
The particular value of 790 nm for the optical depth al of one of the crossed sinusoidal gratings and the par~icular value 450 nm fsr the optical depth a2 f .
the other of the two crossed sine wave gratings is only 25~ illustrative. All that is required to obtain a more neutral "black", than that obtainable by the prior art, is that the respective values of the optical depths, such as a1 and a2, of at least two superimposed, angularly-displaced sinusoidal gratings be selected so that they all exhibit a zero-order transmittance oharacteristic, ~ -19~
: ~"
~L~7955z RCA 7 0, 4 6 0 /~
1 similar to graphs 500 and 502, which goes through a minimum within the visible spectrum, and that the respective zero order transmittance minimum for each selected optical depth occurs at a separate and distinct wavelength in the visible spectrum. Preferably, the respective optical dep~hs of two sine-wave gratings should differ from each other by at laast 100 nm, to provide a more-or-less optimum resultant zero-order transmittance characteristic, similar to that shown by graphs 504, which exhibits a substantially neutral chromaticity point in the color chart of Figure 4 in the neighborhood of points W and Z.
Although the present invention may be practiced with a diffractive structure composed of more than two crossed sine-wave gratings, all having different optical depths, there is little advantage in doing so. This is true because the mixed highex diffraction orders can then amerge closer to the zero order so that they tend to be collected by the projection lens, leading to a ~further reduction in filter density and contrast. For example, if three gratings angularly displaced by 120 and all having equal periods of d are employed, the (1, 1, 1) diffraction order is parallel to the zero-order direction.
This can only be avoided if, in addition to choosing optimum ~ orientations, one or more of the grating periods is reduced by about a factor of two or more. This reduction in qrating period is however, undesirable from the recording viewpoint. Similar considerations hold for combinations of more than three gratings. Therefore, although more than two superimposed angularly-displaced gratings may be used, two crossed sine-wave gratings with :` ::
,, : . . . .: .- . -: . .... . . . . . .. . .
" 1079S52 RCA 70,460/A
1 90 orientation to each other is to be preferred.
One technique for recording an original diffractive structure, for use in deriving a diffractive subtractive filter embodying the features of the present invention, will now be described. Specifically, the recording medium used is SHIPLEY AZ1350 positive photoresist which exhibits different photoresist development characteristics for respective developers AZ1350 and AZ303, both of which are manu$actured by the Shipley Co., Inc., Newton, Mass. The measured development characteristics of these developers for an exposing wavelength ~=436 nm are shown in Figure 6. As shown by plot 600, a 6 second developement of AZ1350 positive photoresist in AZ303 developer at 1:4 dilution in water gives an approximately ..
linear development characteristic of resist removed with respect to the amount of exposure which the photoresist has previously undergone. However, graph 602 shows th~t a 30 second development of AZ1350 positive photoresist in AZ1350 developer at 1:1 dilution in water provides a steeper, nonlinear development characteristic, which requires higher exposures, but does not attack the photoresist at unexposed areas thereof. The detailed form of these curves is a ~unction of tha recording wavelength and development time, but the general features remain.
Similar results may alss be found for other positive photoresists with suitable developersO
In making an original diffractive subtractive ~ilter, an AZ1350 photoresist film is first spun-coated onto a suitable substrate, such as glass or Plexiglass ~ .-0 to a film thickness of at least 2.5~m, to thereby enable ~955i;~
RCA 70,460 I a deep wave profile to be formed. The photoresist isexposed first to a sinusoidal grating pattern of proper line spacing d r formed by a conventional two beam interference arrangement of light from a HeCd ~aser.
The amount of exposure is selected in accordance with graph 600 to provide for a preselected physical depth a'l ~which is equal to ~n times the optical depth al~ ;
to be achieved on development for the first of the two sine wave gratings. The photoresist-coated substrate is then rotated through 90~ and the second exposure is made of the second sine-wave grating. These kwo exposures may have a ratio of 16:9 to obtain respective grating depths which provide an optimum neutral zero order black, as described above. The photoresist is then developed in the 1:4 AZ303 developer for about 6 ~ ~-seconds to give a crossed sine-wave grating structure, wherein each of the two sine-wave gratings has a different .
proper grating depth. Development progress may be monitored by measuring the zero-order transmittance in ~ the red (e.g. using a HeNe laser). Optimum depth for use in making polyvinyl chloride replicas is reached shortly after a minimum is reached in the red zero-order transmission (measured for the dry resist layer in air) or when the red zero-order transmission falls to about 27~ (measured for the resist layer in water). The ; resulting resist grating blank is still light sensitive and, therefore, must be stored and handled in the dark or under ye1low or red safe-lights. -~
The resulting resist grating blank, as so far described, may be used in this form, ~i desired, as -.
.
9552 RCA 70, 460 /A
1 a master recording for deriving an embossed replica diffractive subtractive filter in a thermoplastic, such as polyvinyl chloride. Such a diffractive subtractive filter, which would provide a substantially uniform neutral "all-black" image, o~ the type shown in Figure 3d, can be useful in itself. For instance, such an embossed "all black" subtractive diffractive filter might be used as a diffraction grating substrate for produciny the type of recording medium blanks described in U.S. patents 3,669,673 and 3,743~507 of Ih et al. Therafore, such a neutral "all-black"
subtractive diffractive filter is contemplated by the present invention.
Howeverl in most cases, the resulting photoresist 15~ grating blank, which comprises two crossed sine-wave gratings each having a different physical and optical depth, is not used directly in this form to derive diffractive subtractive filter replicas. Instead, ~; the resulting photoresist grating blank is image-exposed ~2~ ~ to object-information light derived from a positive transparency. In particular, the positive transparency may be either imaged or contact printed onto the photoresist grating blank using either HeCd laser light or ,. :.
incoherent light from an ultraviolet lamp. ~he object information in the positive transparency is preferably ;~ already i~n ~h-e "h~ t~ne" ~orm. In this case, the gray scale, of the type shown in Figures 3a, 3b, 3c and 3d, i! . :`
inherently derived in the image exposure, without .
the use of any additional screening. This approach 3 gives the best results. However, if the objPCt 1079 SSZ RCA 70,46~A
.
1 information in the positive transparency is not in "half-tone" form, the object information light from the positive transparency can be passed through an appropriate screen, similar to the screens used to provide half-tones in the printing art, before being incident on the photoresist grating blank, and the "half-tone" form generated in tha subsequent development.
The image-exposed photoresist grating blank is then high-gamma developed by AZl350 developer, discussed in connection with Figure 6, to completely remove during development all areas and subareas of the photoresist grating blank which have been exposed to any light during image exposure. However, all areas and subareas of the photoresist blank which remained unexposed during image exposure are substantially unaffected during ~ -development by AZl350 developer. This is true because, as shown in Figure 6, the photores1st exposures re~uired when using AZl350 developer are co~siderably ~ higher than those required when using AZ303 developer, -~
zo as shown in Figure 6. Therefore, the prior grating exposure alone does not lead to any significant photoresist removal during the later AZ1350 development of the object information image. This fact was confirmed experimentally by immersing gratings in a photoresist, recorded as above, for 30 seconds in l:l AZ1350 developer. It was found, ~n this case, that no change in the zero-order denqity or color was observable.
-~ S1nce all those areas or subareas of the photo- -resist ~rating blank which received~any light during the seco~d object-information image exposure are completely ~- -2~-':
- ~ : . . . . . ~.. - , . : -~07955Z RCA 70,460 /A
1 removed during the AZ1350 development, the entire cxossed sine-wave grating structure is removed from these particular exposed areas and subareas, leaving revealed corresponding ~lat substrate areas and subareasO These flat substrate areas and subareas define the white areas in the reconstructed picture, while the remaining areas and subareas define the blac:k areas in the reconstructed picture. The final step is to produce a metal master from the resulting original photoresist recording for hot-pressing embossed replicas into thermoplastic sheets. Using this technique, high quality zero-order-diffraction images have been recorded with full, neutral gray scale and a black density of approximately l.8.
; ::' ` ~
~
;~
~ .
:
~, ' " ' ; `~
- ....
..
.:
Claims (7)
1. A diffractive subtractive light filter exhibiting a gray scale, said filter comprising:
a) at least one region consisting of a mixture of black-manifesting subareas and white-manifesting subareas b) each black-manifesting subarea comprising a diffractive structure composed of at least one substantially sine-wave profile phase grating having a line spacing sufficient to permit the separation of substantially all higher-diffraction-order light from imaged zero-diffraction-order light, any of said phase gratings having an optical peak-to-peak amplitude of a selected value which provides a zero-diffraction-order light transmittance wavelength selectivity characteristic which exhibits a minimum zero-diffraction-order light transmittance at a wavelength within the visible wavelength spectrum, c) each white-manifesting subarea comprising a substantially non-diffractive structure, d) the respective sizes of said individual subareas being sufficiently small to be substantially unresolvable in imaged zero-diffraction-order light, but being large relative to said line spacing, e) whereby said one region exhibits a relative luminosity which is determined by the proportion of the area of said one region comprised of said black-manifesting subareas compared to the proportion of said area of said one region comprised of white-manifesting subareas.
a) at least one region consisting of a mixture of black-manifesting subareas and white-manifesting subareas b) each black-manifesting subarea comprising a diffractive structure composed of at least one substantially sine-wave profile phase grating having a line spacing sufficient to permit the separation of substantially all higher-diffraction-order light from imaged zero-diffraction-order light, any of said phase gratings having an optical peak-to-peak amplitude of a selected value which provides a zero-diffraction-order light transmittance wavelength selectivity characteristic which exhibits a minimum zero-diffraction-order light transmittance at a wavelength within the visible wavelength spectrum, c) each white-manifesting subarea comprising a substantially non-diffractive structure, d) the respective sizes of said individual subareas being sufficiently small to be substantially unresolvable in imaged zero-diffraction-order light, but being large relative to said line spacing, e) whereby said one region exhibits a relative luminosity which is determined by the proportion of the area of said one region comprised of said black-manifesting subareas compared to the proportion of said area of said one region comprised of white-manifesting subareas.
2. The diffractive subtractive light filter defined in Claim 1, wherein each diffractive structure of a black-manifesting subarea is composed of at least two superimposed, angularly spaced, substantially sine-wave profile phase gratings, and wherein the optical peak-to-peak amplitude of each of said phase gratings has a sufficiently different selected value to render the wavelength selectivity, and hence the color selectivity, of the zero-diffraction-order-light transmittance of said diffractive structure over the visible wavelength spectrum smaller than that which could be obtained if the optical peak-to-peak amplitude of each of said phase gratings all had substantially the same selected value.
3. The diffractive subtractive light filter defined in Claim 2, wherein said diffractive structure consists of two superimposed sine-wave profile phase gratings which are angularly spaced by substantially ninety degrees, and wherein the respective optical peak-to-peak amplitudes of said two phase gratings differ by at least one hundred nanometers.
4. The diffractive subtractive light filter defined in Claim 1, wherein said diffractive subtractive filter comprises a substantially transparent medium exhibiting a given difference in index of refraction with respect to its surroundings, wherein any black-manifesting subarea diffractive structure comprises a relief pattern of any phase grating composing that diffractive structure embossed in a surface of said medium with the peak-to-peak physical amplitude of each embossed phase grating being equal to the product of its selected optical peak-to-peak amplitude and said given difference in index of refraction, and wherein any white-manifesting subarea comprises a substantially flat portion of said surface of said medium.
5. The diffractive subtractive light filter defined in claim 4, wherein each diffractive structure of a black-manifesting subarea is composed of at least two superimposed, angularly spaced, substantially sine-wave profile phase gratings, and wherein the optical peak-to-peak amplitude of each of said phase gratings has a sufficiently different selected value to render the wavelength selectivity, and hence the color selectivity, of the zero-diffraction-order-light transmittance of said diffractive structure over the visible wavelength spectrum smaller than that which could be obtained if the optical peak-to-peak amplitude of each of said phase gratings all had substantially the same selected value.
6. The diffractive subtractive light filter defined in claim 5, wherein said diffractive structure consists of two superimposed sine-wave profile phase gratings which are angularly spaced by substantially ninety degrees, and wherein the respective optical peak-to-peak amplitudes of said two phase gratings differ by at least one hundred nanometers.
7. The diffractive subtractive light filter defined in Claim 6, wherein the optical peak-to-peak amplitude of a first of said two gratings is substantially 450 nanometers and the optical peak-to-peak amplitude of a second of said two gratings, is substantially 790 nanometers.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA328,791A CA1079552A (en) | 1976-01-19 | 1979-05-31 | Black and white diffractive subtractive light filter |
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB1917/76A GB1538342A (en) | 1976-01-19 | 1976-01-19 | Structure and recording for diffractive relief images for zero-order read-out in black-and-white |
| CA269,267A CA1071447A (en) | 1976-01-19 | 1977-01-06 | Black and white diffractive subtractive light filter |
| CA328,791A CA1079552A (en) | 1976-01-19 | 1979-05-31 | Black and white diffractive subtractive light filter |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1079552A true CA1079552A (en) | 1980-06-17 |
Family
ID=27164855
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA328,791A Expired CA1079552A (en) | 1976-01-19 | 1979-05-31 | Black and white diffractive subtractive light filter |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1079552A (en) |
-
1979
- 1979-05-31 CA CA328,791A patent/CA1079552A/en not_active Expired
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