GB1592902A - Diffractive-subtractive light filter blank - Google Patents

Description

(54) DIFFRACTIVE-SUBTRACTIVE LIGHT FILTER BLANK (71) We, RCA CORPORATION, a corporation organized under the laws of the State of Delaware, United States of America, of 30 Rockefeller Plaza, City and State of New York, 10020, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention is divided on Application 15966/77 (Specification.

1,592,901).

The parent invention, of Application 15966/77 (Serial No. 1,592,901), relates to transmissive diffractive subtractive light filters and, more particularly, to the generation of a selected spatial information pattern directly thereon. The present invention relates to a filter blank for such generation thereon.

Reference is made to U.S. Patent No.

3,957,354, issued May 18, 1976 to Knop, which discloses a diffractive subtractive color filtering technique. Reference is further made to our U.S. Patent 4,062,628 issued December 13, 1977 in the name of Gale, which discloses a black-and-white diffractive-subtractive light filter. As disclosed in these references, a transmissive-subtractive light filter comprises a transmissive layer having a diffractive structure embossed as a relief pattern on a surface thereof. The zero-order output light transfer function of such a filter is determined in accordance with such parameters of a diffractive-grating structure as its spatial waveform, its line spacing and its depth, as well as the difference in index of refraction between that of the transmissive layer and that of the surroundings. More specifically, a rectangular-wave diffractive-grating structure exhibits a zero-order output light color hue that depends on and varies with the depth of the grating. Further, when the diffractive structure within a spot area comprises two crossed sinusoidal gratings of proper depth, substantially no zero-order output light emerges from the spot.

Therefore, such a spot manifests black. On the other hand, white-zero-order output light is manifested by the absence of any diffractive structure within the area of a spot; Screening may be used to achieve a gray spot by controlling the relative proportion of black-manifesting area and white-manifesting area within the spot area.

As brought out in the aforesaid references, such transmissive, diffractivesubtractive light filters may manifest pictorial information and can be mass produced by hot-pressing a diffractivesubtractive filter embossing master into the surface of a thermoplastic layer. The fabrication of such a diffractive-subtractive filter embossing master is disclosed in our copending U.K. Patent Application 14163/76 (Serial No. 1,574,910) or U.S.

Patent 4,108,660 in the name of Gale et al.

and assigned to the same assignee as the present invention. As disclosed in these specifications a recording blank is composed of a diffraction-grating structure formed on the surface of a metal substrate, the substrate surface being covered with a layer of photoresist. The photoresist layer is exposed to picture information comprising respective white and non-white manifesting regions. By developing the exposed photoresist to entirely reveal the grating portions underlying solely the whitemanifesting regions, electroplating and/or etching the revealed portions to level and obliterate the revealed grating portions, and then removing the remainder of the photoresist, an embossing master is formed.

This master, when hot-pressed into the surface of a transparent thermoplastic material, results in a transmissive, diffractive-subtractive light filter of the aforesaid picture information.

Hot-pressing transmissive diffractivesubtractive light filters (e.g., microfiche pictures) in a layer of thermoplastic material is quite inexpensive so long as the number of pressings to be made from a single master is appreciable. In this case, the "front-end cosl" of preparing the embossing master can be amortized over many hot-pressed filters. However, sometimes only a single (or very few) transmissive, diffractive subractive light filter bearing a selected spatial information pattern (which may be pictorial and/or graphic in content) is required. By way of example, in the maintenance of individual personnel records for a large number of employees, it would be desirable to store, in microfiche or microfilm form, an up-datable transmissive, diffractive-subtractive light filters of the individual personnel record histories of the respective employees. In such a case, hot pressing techniques requiring a master would be prohibitively expensive.

The parent invention is directed to a method for directly generating any selected spatial information pattern within an area of an already existent transmissive diffractivesubtractive light filter. The present invention concerns a diffractive-subtractive light filter blank which is particularly adaptable to have such a selected spatial information pattern generated thereon.

In the drawing: Figure la schematically illustrates a black-and-white transmissive, diffractive subtractive light filter prior to the generation of any selected spatial information pattern thereon, and Figure lb illustrates such a black-and-white filter subsequent to the generation of a selected spatial information pattern thereon.

Figure 2a schematically illustrates a color transmissive, diffractive-subtractive light filter prior to the generation of spatial information pattern thereon, and Figure 2b illustrates a color filter subsequent to the generation of a selected spatial information pattern thereon.

Figure 3 illustrates a preferred embodiment of the parent invention for directly generating a selected spatial information pattern within an area of a transmissive, diffractive-subtractive light filter comprised of a pre-embossed plastic sheet.

Figure 4 illustrates a modification of the arrangement shown in Figure 3, which employs a beam radiation absorber in contact with the embossed surface of the plastic sheet, and Figures 5a and 5b and Sc schematically illustrate respective first, second and third species of a pre-embossed plastic sheet on which a selected spatial information pattern may be generated, the third species embodying the present invention.

The black-and-white transmissive, diffractive-subtractive filter shown in Figure I a comprises a sheet of transparent material 100 having an index of refraction n surrounded by a medium having an index of refraction nl. Usually the surrounding medium is air having an index of refraction n1 equal to unity. Preferably the material 100 is a thermoplastic, such as polyvinylchloride (PVC). having an index of refraction n of about 1.5. Sinusoidal diffractive structure 102 is pre-embossed as a relief pattern on the upper surface of material 100. While sinusoidal diffractive structure 102 may be a single sinusoidal diffraction grating, it is preferably two crossed sinusoidal diffraction gratings of the type disclosed in the aforesaid U.S. Patent 4,062,628.

The peak-to-peak amplitude of sinusoidal relief diffractive structure 102 is typically 1.5-2.5 micrometers (,um).

Figure lb shows a selected spatial information pattern directly generated on the surface of the sheet of material 100. As shown in Figure lb, the generated pattern comprises the local obliteration of areas, such as area 104, of sinusoidal diffractive structure 102 in accordance with the spatial information represented by the pattern.

When the diffractive subtractive filter shown in Figure lb is illuminated by white light, the locally obliterated areas thereof, such as area 104, produce relatively highluminosity zero-order output light that manifests white. The remaining nonobliterated areas, which still comprises the sinusoidal diffractive structure 102 of the diffractive-subtractive filter shown in Figure lb, produce relatively low luminosity zeroorder output light that manifests black.

Local obliteration of sinusoidal diffractive structure 102 in accordance with the selected spatial information pattern may be accomplished in various ways. For instance, a pen utilizing, as an ink, a liquid having an index of refraction substantially equal to that of material 100 may be employed to write a selected spatial information pattern by filling the grooves of sinusoidal diffractive structure 102.

Preferably, this liquid ink should be fast drying to avoid any distortion of the written pattern due to capillary action by the grating lines of the diffractive structure.

Such an ink may comprise an organic substance, such as an index-matching polymer, dissolved in a volatile solvent.

Local obliteration may also be achieved by impact pressure, such as from a typewriter key, if material 100 is of a type which cold flows sufficiently under impact pressure.

However, preferably, local obliteration of sinusoidal diffractive structure 102 in accordance with a selected spatial information pattern is achieved by locally heating a diffractive-structure surface of a thermoplastic material 100 in accordance with the pattern by an amount sufficient to cause the thermoplastic material at the locally-heated surface to flow; surface tension forces then cause a levelling of the sinusoidal diffractive structure 102. For good readout contrast between the obliterated and non-obliterated areas, the peak-to-peak amplitude of the relief diffractive structure should be reduced to less than about 0.2 ,um in the obliterated areas.

The color transmissive diffractive light filters shown in Figure 2a comprises a sheet of material 200, which is preferably a thermoplastic, having a rectangular-wave diffractive structure 202 pre-embossed as a relief pattern on the upper surface thereof.

Thus, the color filter shown in Figure 2a is similar to the black-and-white filters shown in Figure la in all respects except for the substitution of a rectangular-wave diffractive structure in Figure 2a for the sinusoidal diffractive structure in Figure la.

As described in the aforesaid Patent 3,957,354, when illuminated with white light, the color diffractive subtractive filter shown in Figure 2a produces zero-order output light manifesting a color hue determined by the depth a' of rectangularwave grating 202. Local obliteration of rectangular-wave grating 202, such as obliterated area 204, shown in Figure 2b, results in white-zero-order output light.

The present invention applies with equal force both the black-and-white diffractive subtractive filters, of the type shown in Figures la and lb, and to color diffractive subtractive filters, shown in Figures 2a and 2b. When the material 100 or 200 is a thermoplastic surrounded by air, and the thermoplastic has an index of refraction of about 1.5 (which is usually the case), the depth a' has a certain value which is normally between 1 and 2.5 . Preferably, the line spacing d of any diffractivestructure is no greater than 2 Mm, and is usually about 1.5 ,xtm (1.4 ,um and 1.7 ,um being used in practice).

Referring to Figure 3, there is shown an arrangement for directly generating, with very high resolution, a selected spatial information pattern on a pre-embossed plastic sheet (of the type described above) by locally heating the diffractive structure thereof with a signal-modulated scanning focused laser beam. As known in the laser recording art, the signal-modulated scanning focused laser-beam source 300 includes a laser for generating a beam of light wave energy at a predetermined wavelength in the ultraviolet, visible or infrared spectrum. Source 300 further includes a modulator, such as an electrooptic crystal, for intensity-modulating this laser beam in accordance with an information signal. Suitable deflection means, such as moving mirrors, and an imaging lens are included in source 300 to raster-scan diffractive-structure surface 302 of pre-embossed plastic sheet 304 with a signal-modulated scanning focused laser beam 306.

Preferably, pre-embossed plastic sheet 304, at least at surface 302, is absorbtive of light wave energy at the laser wavelength, so that efficient heating takes place. Since most thermoplastic material, such as PVC, is absorbtive at ultraviolet wavelengths (A < 300 nanometers), the wavelength of beam 306 may be in the ultra-violet. In this case, a plastic sheet which is transparent over the visible spectrum still will be highly absorbent of the ultraviolet wave energy in beam 306. However, a source of ultraviolet light wave energy not only requires an ultraviolet laser but requires that optical means, such as lenses, thereof be made of ultraviolet transmissive materials. This substantially increases the cost of such a source compared to a visible-light laser source. On the other hand, steps must be taken to ensure that visible-light wave energy is efficiently absorbed at surface 302 so as to heat the plastic thereat to its flow point.

One way of accomplishing this efficient heating, when the filter is composed of a clear plastic, is shown in Figure 4. In this case, surface 302 is placed against the surface of beam radiation absorber 400 and plastic 304 is illuminated from the rear by writing laser beam 402 having a wavelength in the visible spectrum and focused onto the surface of absorber 400. Beam radiation absorber 400 is made of a material which is highly absorbtive at least at the wavelength of beam 402 and has a low thermal conductivity. (As an example, the absorber may be made of glass colored to absorb at the wavelength of beam 402.) One of the shortcomings of the arrangement shown in Figure 4 is that the heat must flow by conduction from beam radiation absorber 400 to surface 302, rather than being generated thereat. Another approach permitting visible light to be employed as the writing beam, which avoids the need for beam radiation absorber 400, is to place a dye which absorbs the visible wavelength of writing laser beam either within at least the surface portion of plastics 304 or on one surface 302 itself. For efficient heating about l0-20V of the light should be absorbed in the vicinity (i.e. within about 2 ,um) of surface 302. A higher absorption is not desirable since the writing efficiency is not significantly increased, bl t the net absorption averaged over all visible wavelengths is increased and leads to additional attenuation of the zero-order output light during readout, thereby unnecessarily reducing the brightness of the projected image.

Specifically, as shown in Figure 5a, the entire plastics filter 304, which normally comprises a sheet having a thickness in the order of 100--200 ,um, may be bulk dyed.

However, since only the dye in the vicinity of surface 302 contributes to the surface heating, the remaining dye again causes an unnecessary reduction in the projected image brightness.

A second approach, shown in Figure Sb, is to employ clear plastics substrate 304 and to merely add a thin coating of dye to diffractive-structure surface 302. Such a coating would normally having a thickness of only between 100--200 nm. While such a dye coating efficiently absorbs the writing beam light, the underlying plastics layer forming the diffractive-structure would be heated to its flow point only indirectly by conduction from the dye coating itself. For short, intense light pulses a considerable temperature difference could then arise between the dye coating and the underlying plastics, leading to problems with dye overheating and vaporization.

The best approach is an embodiment of the invention shown in Figure Sc, and is to employ an embossed plastics 304 which comprises an underlying clear substrate layer of plastics 500 (which may have a thickness in the order of about 100--200 Mm) covered by a thin dye plastics layer 502 having a thickness in the order of 1--2 jtm.

The surface of thin layer 502 defines diffractive-structure surface 302. The structure shown in Figure Sc provides a diffractive-subtractive light filter blank that is particularly adapted to have a selected spatial information pattern generated thereon by given visible radiation incident thereon. The radiation is absorbed directly in that portion of the filter blank which is required to be heated and flowed, and the net absorption of the filter during readout is minimized.

A diffractive-subtractive filter blank, of the type shown in Figure Sc, may be prepared as follows: A nickel master of the required relief structure (usually the crossed-sinewave structure disclosed in the aforesaid U.S.

Patent 4,062,628 is spin-coated or roller coated with a thin (approximately 2 ,um) layer of a solution of PVC and dye in a solvent, a typical solution composition (by weight) is: 30 parts THF (tetrahydrofuran) 10 parts toluene Solvent 10% PVC 0.05% dye (Examples of dyes that may be used include oil-soluble yellow and Fluorescein.) A sheet of clear PVC film (approximately 150 ,um thick) is then heat sealed onto the dyed PVC layer. Upon cooling, the composite PVC film is peeled of the nickel master to yield the structure shown in Figure 5c.

Alternative techniques to provide the structure in Figure Sc include roller coating a clear PVC film with a thin dyed-layer of PVC followed by an embossing of the surface relief structure or alternatively by multiple solvent casting techniques. The concentration of dye in layer 502 is sufficiently high that most of the writing energy is absorbed exactly where it is required in the immediate vicinity of surface 302. On the other hand, due to the thinness of layer 502, the net attenuation of readout light averaged over the visible spectrum is still small, so that a relatively bright zeroorder image still may be obtained.

The color of the dye is the complement of the absorbed color. Thus, a yellow dye absorbs blue light; a cyan dye absorbs red light, and a magenta dye absorbs green light. In general, the absorption characteristics of the dye should be matched to the wavelength of the laser being used. In particular, yellow dye operates well with a He-Cd or Argon laser emitting blue light in generating a selected spatial information pattern on a black-andwhite diffractive-subtractive filter. In this case, curing read out white areas will be manifested as yellow, which still has a high contrast with respect to black. If the approach shown in Figure Sb is employed, it is possible to remove the surface dye (by washing in a suitable solvent) after the selected spatial information pattern has been generated, so that white areas will be truly white on read out.

Dye absorbers can also be used to sensitise the plastic to writing wavelengths outside the visible spectrum, for example the 300--400 nm ultraviolet region or the 7001000 near infrared region (for writing with a solid state injection laser with an emission wavelength of about 900 nm).

By utilizing high peak-power, short pulses of laser light, very high resolution spatial information patterns may be generated and the recording sensitivity may be increased.

By way of example, utilizing an Argon laser, (wavelength of 488 nm) focused by a 20x objective, to illuminate a yellow-dyed surface layer with pulses of light, recorded spots of about 3 mum (about two grating lines) diameter were achieved utilizing pulses of approximately 60 nanoseconds in duration at an exposure level equivalent to about 150 mJ/cm2. Longer pulses result in a larger volume of plastic being heated because the heat diffuses further into the bulk plastic, and thus require more energy to bring the plastic surface to a given temperature. Over a range of pulse durations t having a value of from less than 10-7 seconds up to 10-2 seconds, the required exposure energy was found to vary approximately as t06.

Local-heating may be achieved by other means than source 300. For example, rather.

than using a modulated scanning laser beam to write a spatial information pattern, contact printing may be employed. In this case the source could be an ultraviolet point flash tube point source, collimated by a spherical lens, or, alternatively, a linear flashtube source focused by a cylindrical lens to a high intensity line image through which the plastic is translated. Further, projection, rather than contact printing, can be utilized to locally heat the plastic in accordance with a pattern.

It is not essential that the present invention be restricted to blanks for the direct generation of spatial information patterns within an area of transmissive diffractive-subtractive light filter for the explicit purpose of utilizing the spatialinformation bearing filter itself to derive a zero-order image. The spatial-information bearing filter may be a master recording from which a hot-pressing stamper may be derived.

WHAT WE CLAIM IS:- 1. A diffractive-subtractive light filter blank adapted to have a selected spatial information pattern generated thereon by given radiation incident thereon; said blank comprising: a substrate layer composed of a material which is transparent and substantially nonabsorptive with respect to said given radiation, and a thermoplastic film coating on a side of said substrate layer, said film being composed of a material which is absorptive with respect to said given radiation, the outer surface of said film being preembossed with a certain diffractive structure to form a predetermined diffractive-subtractive filter.

2. The blank defined in Claim 1, wherein said substrate layer is composed of a given thermoplastic material which is inherently transparent and substantially nonabsorptive with respect to said given radiation and wherein said thermoplastic film is absorptive with respect to said given radiation.

3. The blank defined in Claim 2 comprising an absorptive material in said thermoplastic material which is absorptive with respect to said given radiation, wherein said given radiation is colored light having a wavelength distribution within a given portion of the visible spectrum, and wherein said absorptive material is a color dye which is absorptive to light wavelengths within said given portion of the visible spectrum, but is substantially transparent to light wavelengths within the remainder of the visible spectrum.

4. The blank defined in Claim 1, wherein said film has a thickness of about one to two micrometers.

5. The blank defined in Claim 4, wherein said substrate has a thickness of at least onehundred micrometers.

6. The blank defined in Claim 4, wherein said certain diffractive structure comprises a diffraction grating having a line spacing no greater than two micrometers.

7. A diffractive-subtractive light filter blank substantially as hereinbefore described with reference to Figure Sc.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. a range of pulse durations t having a value of from less than 10-7 seconds up to 10-2 seconds, the required exposure energy was found to vary approximately as t06. Local-heating may be achieved by other means than source 300. For example, rather. than using a modulated scanning laser beam to write a spatial information pattern, contact printing may be employed. In this case the source could be an ultraviolet point flash tube point source, collimated by a spherical lens, or, alternatively, a linear flashtube source focused by a cylindrical lens to a high intensity line image through which the plastic is translated. Further, projection, rather than contact printing, can be utilized to locally heat the plastic in accordance with a pattern. It is not essential that the present invention be restricted to blanks for the direct generation of spatial information patterns within an area of transmissive diffractive-subtractive light filter for the explicit purpose of utilizing the spatialinformation bearing filter itself to derive a zero-order image. The spatial-information bearing filter may be a master recording from which a hot-pressing stamper may be derived. WHAT WE CLAIM IS:-

1. A diffractive-subtractive light filter blank adapted to have a selected spatial information pattern generated thereon by given radiation incident thereon; said blank comprising: a substrate layer composed of a material which is transparent and substantially nonabsorptive with respect to said given radiation, and a thermoplastic film coating on a side of said substrate layer, said film being composed of a material which is absorptive with respect to said given radiation, the outer surface of said film being preembossed with a certain diffractive structure to form a predetermined diffractive-subtractive filter.

2. The blank defined in Claim 1, wherein said substrate layer is composed of a given thermoplastic material which is inherently transparent and substantially nonabsorptive with respect to said given radiation and wherein said thermoplastic film is absorptive with respect to said given radiation.

3. The blank defined in Claim 2 comprising an absorptive material in said thermoplastic material which is absorptive with respect to said given radiation, wherein said given radiation is colored light having a wavelength distribution within a given portion of the visible spectrum, and wherein said absorptive material is a color dye which is absorptive to light wavelengths within said given portion of the visible spectrum, but is substantially transparent to light wavelengths within the remainder of the visible spectrum.

4. The blank defined in Claim 1, wherein said film has a thickness of about one to two micrometers.

5. The blank defined in Claim 4, wherein said substrate has a thickness of at least onehundred micrometers.

6. The blank defined in Claim 4, wherein said certain diffractive structure comprises a diffraction grating having a line spacing no greater than two micrometers.

7. A diffractive-subtractive light filter blank substantially as hereinbefore described with reference to Figure Sc.