NO157597B - DATA RECORDING MATERIALS, AND PROCEDURE FOR MANUFACTURING A REFLECTING MATERIAL FOR STORING OPTICAL DATA. - Google Patents

Abstract

A broadband laser recording and data storage medium for direct reading after writing, formed from a photosensitive silver halide emulsion in four steps. First, a non-saturating actinic radiation exposure is used to create latent images. A common photographic developer is used to form a medium of gray neutral density. The surface of the remaining silver halide is blurred in a water or alcohol based solution to create a very thin layer of silver precipitating cores on the surface. Finally, a single-step, negative silver diffusion transfer process is used to dissolve the unexposed and undeveloped silver halide. by the formation of silver ion complexes. These complexes are transferred by diffusion transfer to the sites of the silver precipitating cores and the filamentary silver on the surface, where the silver complexes are reduced to metallic silver both on the cores and on the filamentary silver to form a high concentration of non filamentary silver particles at the surface of a low melting temperature colloidal matrix, which is highly reflective of light and is electrically non-conductive.

Description

The invention relates to laser-recording media, and more particularly to a broadband reflective silver data-recording and storage medium.

Many types of optical recording media for laser writing have previously been developed. Some of these media need post-writing processing before they can be read, and some can be read immediately after laser writing. The media of interest here are those that have "direct read after write" capability, commonly known as "DRAW" media. Laser DRAW media now known are thin metal films in which holes can be melted, composite shiny films whose reflectivity can be locally reduced by evaporation, thin films of dyes or other coatings that can be vacuumed in a location, and dielectric materials whose refractive index can be changed at a point, causing a scattering of light when scanning with a readout laser.

The most common DRAW media are thin metal films, usually on a glass substrate. Thin metal films have several advantages: first, they can be produced for research purposes in small quantities with commercially available sputtering equipment. Secondly, they can be read either by reflection or by transmission. Thirdly, tellurium and bismuth films have relatively high recording sensitivities.

These thin metal films have enabled much research and progress to be made in the design of optical data storage systems. Until now, tellurium and amorphous mixtures thereof have been the most widely used of the metal films. However, tellurium must be produced by an expensive, batch-wise, vacuum-spray technique; it does not form any sticky coating; and it causes complications during production and with regard to the environment due to the fact that it is toxic, and since it quickly oxidizes in air must

it is encapsulated in an airtight system so that it has an acceptable archival shelf life.

What is particularly desirable about tellurium is that it has a low melting point for a metal, 450°C, and also a very low thermal conductivity of 2.4 watts per meters per degree Kelvin at 573°K. Silver metal, for example, has a melting temperature of 960°C and a thermal conductivity of 407 watts per meters per degree Kelvin at the same elevated temperature. When electrically conductive films of these two metals in the thin state are compared with respect to laser registration with short pulses of laser power, tellurium is by far the best in terms of registration sensitivity since its low thermal conductivity keeps the heat developed by the laser beam limited to a small area, and the low melting temperature makes it easier to melt the hole.

The use of an electrically conductive continuous metal film of silver as a reflective laser recording medium would be impossible for exactly the same reasons that have made tellurium accepted. These are that silver melts at more than double the temperature and has a thermal conductivity of approx. 170 times larger. despite these apparent disadvantages, non-filamentary silver can be used quite effectively if it is dispersed through a dielectric medium such as gelatin with a sufficiently high volume concentration to create a reflective surface, but also a sufficiently low volume concentration that silver -the layer is not continuous. Under these special circumstances, the laser beam only needs to melt the dielectric medium to record data on the reflective surface, and not the silver itself.

A reflective silver laser recording medium of this general type was the subject of an earlier US patent application whereby a processed black filamentary silver emulsion was converted into a reflective, non-electrically conductive recording medium by heating at a temperature in the range of 250 to 330°C in an oxygen-containing atmosphere until the surface developed a shiny reflective appearance. This laser-recording material worked effectively with visible-wavelength laser beams, but its recording sensitivity declined by a factor of approx. three for semiconductor lasers, which develop light beams near infrared at approx. 830 nm. Also, the high-temperature heating process excludes the possibility of using the plastic film substrates that are usually used for photographic films and other plastics.

Another prior US patent application discloses a reflective data storage medium derived from silver halide emulsion, and the use of a silver diffusion transfer process. No heating was required to create the reflective surface. Reflectivities of up to 24.4% were achieved for green light. However, the detection sensitivity of this material was less than that of the method described in the first US patent application which gave reflectivities of up to 17.2%.

In a third U.S. patent application, a medium was disclosed in which the recording sensitivity was greatly improved over that described in the second U.S. application. It was, however, necessary to add a reducing step at a temperature of 250°C and above to achieve the desired results. And although the registration sensitivity was very good with a green laser beam at 514 nanometers and with a red laser beam at 633 nanometers, it went back by a factor of approx. three when the laser wavelength was increased to 830 nanometers. This effect was similar to that which. was observed with materials prepared by the process of the first U.S. patent application. In the method of the third U.S. patent application, the best sensitivities were obtained with media having green reflectivities of 25.5%, although reflectivities of up to 36.6% were observed from less sensitive samples.

These last three referenced U.S. patent applications describe reflective data-storing and laser-recording media prepared from silver halide emulsions so as to form the desired reflective but non-electrically conductive surface desired for efficient laser recording. These photographic materials have further advantages in that photographic type techniques can be used either for reproducing gramophone records or for pre-registration of data or control markings on the reflective surfaces. However, these media were limited in recording sensitivity in the long wavelengths near 830 nanometers, and in achieving high reflectivity together with high sensitivity, and by needing a process with a necessary relatively high temperature, in order to be used at relatively high sensitive DRAW registration applications, which limits the choice of plastics that can be used as substrates.

The present invention provides a method for producing a reflective optical data storage medium which comprises

to expose at least part of a photosensitive medium of a

silver halide emulsion for a non-saturating level of actinic radiation,

developing said emulsion to a gray color having an optical density to red light of between 0.05 and 2.0,

forming an areal layer of silver precipitating nuclei on the surface of said developed emulsion at locations of undeveloped silver halide, and

contacting said developed and nucleated emulsion with a monobath comprising a silver halide solvent and a silver reducing agent, whereby unexposed and undeveloped silver halide forms soluble silver complexes and is transferred by diffusion transfer to said precipitating nuclei where said silver complexes is reduced to metallic silver.

Among the advantages of the preferred embodiments of the invention is the fact that the laser-recording and data-storing medium can be produced without the use of a vacuum system and on a continuous basis, and that this can be used to record low-reflective spots in a reflective field with relatively low-energy laser pulses. Another advantage is that at the long red wavelength and near-infrared wavelengths, high recording sensitivities can be achieved. The manufacturing process disclosed herein also allows the use of a greater variety of plastic substrates, and higher reflective surfaces are achieved without reducing the recording sensitivity so that there is an increase in reflective light to facilitate automatic focusing and increase the signal level when data is read.

It has been discovered that the silver halocenide in a photosensitive emulsion for a photographic plate or film can be exposed and developed into two optical contrast layers by a series of steps so as to create a laser-recording medium which is absorbent of laser rays both in the visible and in the near-infrared range. A partially transmitting mirror-like reflective upper layer is formed on top of an absorbent lower layer, both of which absorb light energy in the ultraviolet, visible and infrared spectrum.

This two-layer medium absorbs and spreads the heat from the laser beams that fall either in the visible wavelength or the near-infrared, and this makes this laser recording medium very broadband for the wavelength of the recording laser. The reading of data on the reflective surface is also broadband. Furthermore, the reflective surface can contain pre-registered data by creating a pattern with lower or higher reflective markings using a photographic exposure through a photo mask. This pre-registered data is also broadband in that it can be read with a visible or near-infrared laser. Laser registration is performed by melting spots in the gelatin matrix containing the reflective surface which are later read as spots with low reflectivity.

The mirror-like reflective layer consists mainly of a high-volume concentration of non-filamentary silver particles and a lower concentration of filamentary silver particles supported in a matrix of gelatin. The dark sublayer mainly consists of a moderate volume concentration of filamentary silver particles carried in a matrix of gelatin. Other colloid matrices could be used to support the silver particles.

Briefly, the two-layer medium is produced in the following way: The surface of a photosensitive medium of a fine-grained silver halide emulsion is briefly exposed to a low-to-moderate level of actinic radiation. This exposed silver halide is then developed to an optical density of typically 0.05 to 2.0, as measured by red light on a photographic densitometer. This gelatin layer containing filamentary silver particles exhibits an optical density of typically 0.05 - 0.8 for a 3 ftsn emulsion and 0.1 - 1.5 for a 6 µm emulsion. After this initial manufacturing step, the emulsion is gray in appearance,

but a large amount of the silver halide in the emulsion remains unchanged. A very thin layer of unexposed silver halide at the surface of this partially developed emulsion layer is then chemically obscured to form a very dense layer of silver precipitating nuclei at the surface. The cloudy medium is finally subjected to a negative silver diffusion transfer step whereby the silver halide in the emulsion is solvated to form soluble silver complexes. These silver complexes are deposited on the silver-precipitating cores to form a reflective layer comprising non-filamentary silver particles that form aggregates with the filamentary silver. The degree of reflectivity

on the surface can be adjusted over a range of values that depend on the ratio between the two types of silver. This same mechanism also causes some of the silver ion complex to precipitate onto the filamentary silver in the absorbing sublayer, increasing the red light optical density of this already developed sublayer by at least typically a factor of two for increased light absorption.

The final result of these two exposure/development sequences is a better reflective laser register

medium which is composed of a very thin layer of reflective, but not electrically conductive, reduced non-filamentary silver, and a much smaller amount of filamentary silver, under which lies a highly absorbent layer consisting primarily of filamentary silver in a gelatin matrix. This absorbent sublayer typically has a fine optical density to red light of between 0.2 and 3.0. The original photosensitive medium of silver halide emulsion which eventually results in the reflective laser recording medium described above is usually coated on either a plastic or a glass substrate. The reflective surface has a green light reflectivity of 44% for a typical sample.

Laser registration on this double-layer medium can be done very efficiently. The quantity of the absorbing filamentary silver particles in this reflective layer can be increased until the medium's acceptable reflectivity is reached. These filamentary particles are absorbent over a wide spectrum from ultraviolet to near-infrared, and allow a number of different lasers to be used for recording. The light energy that is not absorbed in the reflective layer is also almost completely absorbed in the sublayer, which causes a temperature rise at the interface between the reflective layer and the sublayer, and this facilitates the melting of the reflective layer. The registration is carried out by using a laser beam to melt the gelatin at a spot in the reflective layer, thereby reducing the reflectivity at the spot. Before registration, the reflectivity of the reflective layer is mirror-like, in other words incident light perpendicular to the surface will be reflected back towards its origin in a parallel line. After registration, perpendicularly incident light will be diffusely reflected due to the fact that the light that turns back towards its origin will be spread outwards in contrast to parallel return. This last effect and the increasing absorptivity at the spot lead to a reduced reflectivity. The absorbent sublayer will only be slightly penetrated by the registration process. None of the silver in either layer is melted during the recording process.

Several clearly distinct advantages are achieved by this method of manufacturing reflective laser recording media. First, it is a very sensitive laser recording medium. Second, since the surface-reflective layer retains its absorptive and reflective properties for laser light over the visible and near-infrared wavelengths, it is a very broadband laser-recording medium. Third, since different surface reflectivities can be achieved, pre-registrations can be produced using suitable photomasks to create the desired exposure patterns. Fourth, the high-recording sensitivities are achieved without resorting to a high-temperature heating process, thus permitting the use of certain commercially available plastics as substrates. Fifth, the silver halide emulsion of this invention is inexpensive because the emulsion is so thin.

With regard to the drawings:

Figure 1 is a plan view illustrating a laser-recording and data-storing medium produced by the method according to the present invention. Figure 2 is a side section illustrating a non-saturated exposure and gray development step for a photosensitive medium used in the production of a laser-recording and data-storing medium according to the present invention. Figure 3 is a side section of the chemical blurring step for creating cores on the photosensitive medium in Figure 2 in the method according to the present invention. Figure 4 is a side section of a negative diffusion transfer step from the emulsion layer to the cores on the photosensitive medium of Figure 3 in the method according to the present invention. Figure 5 is a side section illustrating a mask and exposure step for a photosensitive medium in an alternative method for producing a laser-recording and data-storing medium according to the present invention. Figure 6 is a side section illustrating the second non-saturating exposure step and gray developing step of the alternative method first illustrated in Figure 5. Figure 7 is a side section illustrating the surface chemical blurring step to create nuclei by the alternative method for the photosensitive medium in Figures 5 and 6. Figure 8 is a side section of the negative diffusion transfer step from the emulsion layer to the cores in the alternative method for the photosensitive medium in Figure 7. Figure 9 is a side section of the recording medium in Figure 4 which illustrates the procedure for laser recording.

A broadband reflective laser recording and data storage medium is formed in four main steps: an initial exposure to actinic radiation, photographic development, chemical surface obscuration or nucleation, and silver diffusion transfer. The final medium may be a disc, as shown in Figure 1, or may have other shapes.

A. Initial exposure

The first step in the manufacturing process according to the present invention is exposure of a photosensitive silver halide medium to actinic radiation. This initial exposure is non-saturating, leaving as much as 2/3 or more of the photosensitive silver halide unactivated. Referring to Figure 2, the photosensitive silver halide medium is preferably a fine-grained silver halide photoplate, such as is used to produce semiconductor photomasks. The initial exposure can be obtained from a weak source or from a very short exposure to a moderate source of actinic radiation. Actinic radiation is the general term that describes any exposure that creates a latent image. Latent image is the term that describes the activation of unexposed silver halide.

The initial exposure results in the formation of a latent image which, when developed photographically, forms a medium of gray color having an optical density of 0.05 - 2.0. This gray medium plays an important role in the performance of the broadband reflective laser recording medium of this invention. Clear gelatin absorbs very little light energy and thus a laser-recording material using gelatin will not be very sensitive. The present invention relates to the discovery that black filamentary silver particles can be formed to create a light-absorbing medium while a large part of the silver halide remains unactivated. This remaining silver halide is then used to create the reflective surface according to the present invention.

The exposure of the photosensitive silver halide medium can be at uniform intensity over the surface of the medium, as illustrated in Figure 2. This will give a uniform density of the latent image within the photosensitive medium, which upon photographic development results in a uniform gray tone or optical density throughout the medium.

An alternative to a uniform exposure and thus a uniform density of latent images is a patterned exposure with variable intensity, as illustrated in Figure 5. For example, the exposure of the photosensitive silver halide medium can be composed of alternating concentric bands of actinic radiation with high and low intensity over the surface of the photo-sensitive medium, by changing the intensity of the exposure in an alternating manner, by means of a screen mask 14 having two gradations of transmittance for actinic radiation, the density of latent images inside the photo-sensitive medium will vary in relation to the intensity of the exposure level. By patterning this differentiated exposure so that concentric or spiral bands of higher and lower density latent images are formed, it is possible to create a light-absorbing layer which can then be used in further processing to create a servo track pattern with two different reflectivities.

This original gray layer becomes an absorbent sublayer in the final product, which is covered with a very thin layer of reflective reduced silver. The combination of a reflective silver coating over a light-absorbing underlayer is in turn supported by a carrier substrate 19 . This carrier substrate can either be glass or certain plastic or ceramic materials. It is not necessary that this carrier substrate be transparent either to the exposing actinic radiation or to the radiation produced by the optical readout direction.

It is also clear that the combination of reflective silver coating over absorbent underlayer can be placed on both sides of such a carrier substrate. It is, for example, possible and practical to use a photographic plate which has photosensitive silver halide material deposited on its opposite sides. The fact that the photosensitive material is finally obtained in silver-coated over an absorbent substrate covering opposite sides of the carrier substrate has no detrimental effect on the usability of the final product and in fact provides the double data storage -capacity.

The photosensitive silver halide medium discussed above may be a commercially available black and white photographic plate or a black and white film product, such as a film strip with or without a gelatin overcoat. Photographic plates used for semiconductor photomasks or holographic registrations do not have any coating. The smaller the grain size in the silver halide emulsion, the greater the recording resolution of the final product obtained using this invention. The emulsion grain size should be less than 5 - 10% of the recording hole size to give the best results. As shown in the examples that follow, commercially available high resolution silver halide emulsion photographic plates used in the formation of semiconductor integrated circuits are particularly useful in the practice of this invention. These photographic plates primarily have grain sizes below 0.05 µm and will provide non-filamentary silver particles in the high resolution reflective layer formed in the final process step. The silver halide in such plates is held in a colloidal matrix, usually gelatin. But the invention is in no way limited to these photographic plates nor is it limited to the use of only commercially available photosensitive silver halide materials. Any photosensitive silver halide emulsion with grain sizes primarily below 0.05 µm can be used in the practice of the present invention for high resolution laser recording. For lower resolution recordings, silver halide grains can

size be greater than 0.05

For the purposes of this patent application, the term "silver halide emulsion" means a silver halide emulsion without any gelatin overcoat, unless an overcoat is specified.

It is clear that there are many different initial exposures that can be used in accordance with this invention. The two given here, a uniform exposure and an exposure through a mask of different transmissions for actinic radiation, are only two extensive examples. Examples of several different variations of this original exposure step can be imagined.

B. Photographic development

The second step of the present invention relates to the photographic development of the latent images formed by the original exposure. This produces a gelatin layer containing filamentary silver particles with a gray optical density, as measured by red light in a photographic densiometer, of typically between 0.05 and 0.8 for a 3 µm thick emulsion and between 0.1 and 1.5 for a 6 / wn thick emulsion. This development creates a gray layer on the surface of which a reflective layer will be formed. The gray layer becomes the absorbing sublayer, which in recording/playback looks like a neutral density filter when the original gray layer is in the highest range for optical densities, and has a reddish color when the original gray layers have lower optical density . The starting gray layer has an initial optical density of typically 0.05 - 2.0, measured with red light, although for some applications a range of 0.1 - 0.8 may be preferred.

Reference is now made to Figures 2, 3, 6 and 7, where the black spots in photosensitive medium 11 illustrate the formation of filamentary silver as a result of development. The volume concentration of activated silver halide determines the volume concentration of filamentary silver. The volume concentration of filamentary silver, in turn, determines the optical density of the absorbing sublayer. The extent of development also affects the optical density.

When the non-filamentary silver particles are distributed in the gelatin, light transmitted through the gelatin will give the appearance of a reddish color due to the scattering effect of the thin non-filamentary silver particles. When filamentary silver particles are distributed in the gelatin, light passing through the gelatin gives the appearance of a gray color, as in a neutral density filter. When both filamentary and non-filamentary silver particles are distributed in the gelatin, the color of the light transmitted through the gelatin can range from reddish to a neutral gray to reddish-gray, depending on the concentrations of the two types of silver. The absorption created by the filamentary silver with a gray appearance is very broad-band from the visible to the near-infrared. in addition to this absorptivity, it is of the non-filamentary silver that adds to the absorptivity in the green and blue wavelengths, but has a smaller effect at red and infrared wavelengths.

Photographic development is the process of converting the activated silver halide comprising the latent image into filamentary silver. This transformation is actually a reduction of the activated silver to form a black filamentary silver. The development process is well known in the photographic industry. Generally, the manufacturers of commercially available silver halide photosensitive materials that can be used in accordance with this invention have suggested developing agents.

In addition to the relatively simple interplay between exposure time and development, one must consider the desired optical density of the absorbent sublayer formed by this development step. It must be remembered that the product formed by the disclosed invention is a broadband optical laser recording and data storage medium. As such, there are many possible uses for the product according to the invention. The specifications of one consumer may not be the same as those of another, and this requires flexibility with regard to the sensitivity of the medium.

The principal role of the initial actinic exposure step is to create a latent image through the silver halide emulsion ion layer, which can be produced photographically as a gray layer created by filamentary silver particles in the gelatin matrix. No fixing step is used after development since the remaining silver halide is used to create the desired reflective surface. The stronger the original exposure and the stronger the development, the darker the gray color of the media and the less silver halide is left behind to create the reflective layer. Thus, if a light-shielding mask 14 is used to create a pattern of latent images, and if this is followed by another moderate exposure, the developed plate will have areas with different gray densities. When such a medium is processed to create a reflective surface, the reflectivity will vary in inverse proportion to the gray density of the medium. The actinic exposure step can therefore play the important role of pre-patterning or pre-registering the reflective laser-recording medium and also creating the absorbent sub-layer necessary for efficient laser-recording. The developing step is intended to transfer the patterned exposure to a patterned gray layer that has areas of different gray densities.

The preferred initial optical density falls within a wide range for many reasons. If, for example, a high surface reflectivity of 60% is required, then the initial density will be below 0.5, since the higher the initial density, the lower the final reflectivity. This high reflectivity may be necessary to achieve a high signal-to-noise ratio or light automatic focusing. And along these same lines, if two or more reflectivities are necessary to pre-register servo tracks or such, the high reflectivities will require lower original gray densities.

As another example, it can be mentioned that if a high registration sensitivity is desired, the optical density of the sub-layer should be higher than the minimum.

The third example involving the optical density assumes that this is not critical with regard to registration sensitivity. From a theoretical point of view, an optical density of 1 indicates an absorption of 90% of the laser beam, and an optical density of 2 indicates an absorption of 99%. Difficulties in exposure and processing to achieve these two optical densities would be great, but the effect on sensitivity would be small, i.e. only approx. 10%.

The two purposes of the absorbent sublayer will be discussed below in section E.

It is important to keep the unexposed and undeveloped silver halide in this state between the photographic development step and the next step. For this reason, the developing step and the chemical surface fogging step are carried out in the absence of light or by using a safety light, so that the amount of actinic radiation is kept to a minimum. Moreover, it is necessary to limit the exposure of the medium to actinic radiation at the transition from the original exposure to the photographic development. This is obviously because a carefully controlled original exposure would be destroyed by exposure to sporadic actinic radiation at the transition.

C. Chemical surface fogging

Reference is now made to figures 3 and 7. After the photosensitive silver halide medium has been exposed and developed to an optical density of 0.05 - 2.0, the surface of the photosensitive medium is then blurred. Blurring, or nucleation, is the process by which silver-precipitating nuclei are created. These cores form a thin layer where the silver in silver ion complexes can be reduced to metallic silver and absorbed. Essentially, all blurring takes place to create an area where transported silver-ion complexes can form aggregates and be reduced to reflective silver. The nuclei formed by blurring are indicated by the sign + in Figures 3 and 7.

It is clear from the figures that the photosensitive medium 11 must be penetrated by the obscuring agent in order to create a surface layer of cores. In the case of photosensitive silver halide material, gelatin is usually used as the suspending colloidal medium for the silver halide emulsion. Thus, in order to form a layer of silver-precipitating nuclei at the surface of the medium, it is necessary to slightly penetrate the silver halide emulsion. It is well known that when silver halide emulsions absorb water, the emulsions swell. This swelling results in a rapid and deep penetration of the opacifier in any water-based solution into the emulsion. This is not desirable as it will create a thick layer of cores, which will result in a thicker, less reflective surface.

When a photographic plate or other photosensitive medium without a gelatin overcoat is used, it is found that the use of a water-miscible alcohol, for example methanol, almost completely eliminates the swelling and thus reduces the penetration of the obscurant to a minimum. This weak penetration gives a thin and very dense core layer, which after the blurring step becomes the desired thin, highly reflective data-recording medium. When the photosensitive medium contains a gelatin overcoat, usually also called a supercoat, it is preferred to use a water- or alcohol-based blurring solution in order for the blurring solution to penetrate the overcoating and create nuclei at the surface of the silver halide emulsion.

It must again be remembered that the purpose of this invention is to prescribe a method for producing a reflective laser recording medium. The medium obtained by this invention has a very thin reflective silver surface layer, preferably a small fraction of a / m, covering a much thicker dark sublayer having an optical density of 0.2 - 3.0. This reflective silver surface is created by transporting silver ion complexes from the silver halide in the sublayer to the silver precipitating cores in the surface layer, and then reducing the silver complexes formed at the cores to reflective non-filamentary silver particles. It is clear that the most effective localization for the silver-precipitating nuclei is at the surface of the photo-sensitive silver halide medium.

It is desirable to limit the penetration of the obscuring agent to as close to the surface of the photosensitive agent as is possible and practical. In this regard, methanol or any other water-miscible alcohol is particularly useful when the photosensitive medium used is an uncoated photographic plate. However, any aqueous solution of a reducing agent will penetrate most commercially available photosensitive materials, thus creating a depth-wise decreasing concentration of silver-precipitating nuclei. Generally, the blurring agents are used in a bath and • penetrate the entire medium. In the method according to the present invention, however, the depth of penetration is regulated by the choice of solvents and time. It is important that the emulsion has uniform dryness before immersion in the blurring agent to prevent varying degrees of penetration of the blurring agent.

The purpose is to create a very thin, dense layer of cores.

When there is no gelatin overlay on top of the photographic emulsion, methanol is a very useful carrier for the opacifier since it penetrates the gelatin much more slowly than water, and therefore its penetration can be limited and controlled. When a gelatin overcoat is present, the opacifier must be able to penetrate the overcoat to reach the photosensitive silver halide without penetrating deeply into the silver halide emulsion. Water or alcohol can be used for this purpose since they are effective in penetrating the overcoat.

An obfuscating agent is a highly active reducing agent. Any of the hundreds of photographic developers available are reducing agents that could theoretically be made active enough to blur silver halide with correct adjustments of concentration and pH. All would have a certain solubility in methanol, but it would be a question of whether sufficient developer could be dissolved to become active while at the same time sufficient antioxidant was dissolved to protect the developer from air oxidation. Of the simplest compounds that could be active reducing agents and soluble in methanol, two examples are hydrazine and hydroxylamine. Both are high pH blurring agents. However, both are silver halide solvents which could partially dissolve the surface of the silver halide being blurred, which would be an undesirable effect. Borohydride is an example of a practical compound for the method of this invention, and it is very active in reducing silver halide, it is not oxidized by air and has no properties as a silver halide solvent. Borohydrides of lithium, sodium, potassium, cesium and rubidium could be useful.

In accordance with the above limitation, the depth-wise penetration of the obscurant is kept at typically less than 10% of the depth of the photosensitive medium. The competing factors, the penetration and duration of exposure to the obscurant interact to determine the factors at the obscuring stage. With limited depth penetration of the nucleating agent to typically 5 or 10% of the depth of the emulsion, or one f. m or less, the final reflective silver layer will typically occupy approximately the top 5 or 10% of the medium, and the gray sublayer will occupy the remaining 90%. Typically, the photosensitive medium is less than 15 µm thick.

The chemical surface-obscuring or nucleating step can be eliminated by incorporating a thin layer of silver precipitation nucleating agent. This is common practice in silver diffusion transfer processes. In Chapter 16, "Diffusion Transfer and Monobaths" in T.H. James: The Theory of the Photo-graphic Process, 4th edition, a number of effective nucleating materials are mentioned which have been incorporated into a silver-precipitating layer, including copper, silver, silver sulphide, selenium, cadmium sulphide, lead sulphide and mercury sulphide . When a reflective surface is the aim, rather than a black surface, it is important that round crystals of silver are formed, and not the filamentary type that leads to a black surface. Pure silver particles of a roundish shape would be preferred for this nucleating layer since sulphides tend to form filamentary silver, leading to a low reflectivity surface.

D. Silver diffusion transfer to nuclei

After a thin layer of silver-precipitating nuclei has formed on the surface of the photosensitive silver halide medium, the final step of the method according to the present invention entails transporting the silver in the remaining silver halide to the silver-precipitating nuclei and by means of the silver complexes there reduce the silver. This process is usually carried out by placing the nucleated photosensitive medium in a monobath. This monobath contains both a silver halide solvent and a silver reducing agent. This step is also performed in the dark or using a safety light until the silver diffusion transfer is complete.

The two elements in this monobath, a silver halide solvent and a silver reducing agent, comprise a silver diffusion transport and reduction system. The silver halide solvent acts on the silver halide in the photosensitive medium to produce mobile silver ion complexes. These free silver complexes are transported inside the photosensitive medium to and through the surface of the medium. These silver complexes are then subjected to reduction, and metallic silver is formed on the silver cores and on the filamentary silver at the surface. This is indicated by the clustered spots on the reflective layer 17

in Figures 4 and 8.

The reflective layer formed in this step is electrically non-conductive and has low thermal conductivity and can be patterned photographically. These last two properties are strongly desired for laser-recording media. The complexed silver ions are created by reacting a suitable silver solvent and the silver halide which is left undisturbed in the emulsion. A developing or reducing agent must be included in this solution to allow the complexed silver ions to be precipitated on the core layer. This combination of developer and silver-complexing solvent in one solution is called a monobath solution. Preferred monobath compositions for highly reflective surfaces include a developer which may be characterized as having low activity. The specific type of developer chosen appears to be less critical than the level of activity as determined by developer concentration and pH.

The developer should have a sufficient redox potential to cause silver ion reduction and absorption or agglomeration on silver cores. The developer concentration and pH of the monobath should not cause the growth of filamentary silver, which gives a black low-reflectance appearance. The developed silver particles should have a geometric shape, such as a spherical or hexagonal shape, which, when concentrated, gives a well-reflective surface.

Developers having preferred properties are well known in the industry, and almost any photographic developer can be made to work by choosing the concentration, pH and silver complexing agent so that there is no chemical reaction between the developer and the complexing agent. forming agent. It is well known that photographic developers need an antioxidant to preserve them. The following are typical combinations of developer/antioxidant that can be used together with a sodium thiocyanate (NaSCN) solvent/complexing agent.

The preferred solvents/silver complexing agents, which must be compatible with the developer, are thus mixed, in proportions which promote a complete diffusion-transfer process within a reasonably short time, such as a few minutes. Such silver complexing agents in practical volume concentrations should be able to dissolve substantially all of the silver halide in a fine-grained emulsion in just a few minutes. The solvent should not be reacted with the developing silver grains to dissolve them or to form silver sulphide as this tends to create non-reflective silver. The solvent should be such that the specific rate of reduction of its silver complexes at the silver core layer is suitably rapid, even in the presence of low activity developers, which is preferred to avoid creating low-reflective black filamentary silver in the initial development of the latent surface image.

The following chemicals act as silver halide solvents and silver complexing agents in dissolution-physical development. They are grouped approximately in relation to their speed during dissolution-physical development, i.e. the amount of silver that is deposited per time unit on precipitating nuclei, when used with a developing agent consisting of p-methyl-aminophenol-ascorbic acid.

Most active

Thiocyanates (ammonium, potassium, sodium etc.)

Thiosulphates (ammonium, potassium, sodium etc.) Ammonium hyd rok s yd

Moderately active

0-picoline-(3-phenylethyl bromide

Ethylenediamine

1- aminophenol-furan

n-butylamine

2- aminophenol-thiophene

Isopropylamine

Much less active

Hydroxylamine-sulphate

Potassium chloride

Potassium bromide

Triethylamine

Sodium sulphite

From the above, it can be seen that the thiocyanates and ammonium hydroxide are among the most active solvents/complexing agents. While almost all developers suitable for solution-physical development can be made to work by the silver diffusion transfer process of the present invention at the proper concentration and pH, not all solvents/complexing agents will work within the desired short development time or in the right way. For example, the thiosulfate salts, which are the most common silver halide solvents used in photographs and in Polaroid-Land's black and white snapshot diffusion-transfer process, do not work in this process for two reasons. Their complexed silver ions are so stable that a strong reducing agent is required to precipitate silver on the cores, and this strong reducing or developing agent will have an undesirable effect on developing low-reflective black filamentary silver. They have another undesirable effect, which is also exhibited by the thiourea solvent, namely that they form black, low-reflective silver sulphide with the developing silver grains. On the other hand, black silver is a desired result of Polaroid-Land's black and white process. Sodium cyanide, although an excellent silver halide solvent, is not recommended because it is also an excellent solvent for metallic silver and thus would etch away the image formed. It is also about 50 times as toxic as sodium thiocyanate, which is a common photo-

graphic reagent.

Also, if the solvent concentration is too low, the solvent would not be able to convert the silver halide into a silver complex within a short process time, and if the reducing agent is too weak, the unexposed, undeveloped silver halide would pass past the silver precipitating nuclei and cause much of the silver complex to dissolve rather than be precipitated. The process by which the silver complex is reduced at the silver-precipitating cores and increases the size of

the cores, is called resolution-physical-development.

It is important to note that in solution-physical development, as used here, the silver particles do not grow as filamentary silver as in direct or chemical development, but instead grow roughly equally in all directions, and this results in a developed image which is composed of compact, round particles. As the particles grow, a transition to hexagonal shape is often observed. If the emulsion that is developed has an extremely high density with silver cores to build on and there is sufficient silver halide material to be dissolved, then eventually the spheres will grow until they come into contact with other spheres, and aggregates of several spheres or hexagons.

During the initial exposure and development steps, a gray layer is formed. The black filamentary silver that the layer comprises is present throughout the photosensitive medium. Thus, the reflective layer formed in the final step, the diffusion transfer, contains some filamentary silver. The filamentary silver is important in determining the absorptivity of the reflective layer. It is typical that the volume concentration of silver in the reflective layer exceeds the average concentration in the lower layer by a ratio of at least 3:1. Typically, the volume concentration of silver particles in the reflective layer is a minimum of 20% and a maximum of 50%. Filamentary silver in the reflective layer may be present in an amount as small as 1% and in an amount as large as 50% of the total amount of silver in the reflective layer.

Moreover, in the preparation of the gray layer, as the second step in the formation of this broadband reflective recording medium, it must be remembered that the original optical density of this layer can fall anywhere within the range of 0.05 - 2.0. The optical density of the gray layer is increased by the silver diffusion transfer step, whereby more silver is deposited onto the filamentary silver on this layer, as shown by the splashes present in Figures 4, 8 and 9, increasing the density to between maybe 0.2 and 3.0 in the final product. For most applications, however, the initial density would still fall within the range of approx. 0.1 - 0.8 for optical density. All the above optical densities are measured with red light.

In the second set of drawings, numbers 5 to 8, a mask is used in the original exposure, thus creating areas of greater and lesser optical density during development. In the final step of this invention, a reflective layer with differing reflectivities, 17a and 17b, is formed by negative diffusion transfer.

In Figure 5, the photosensitive medium 11 is exposed through mask 14 to a non-saturating level of actinic radiation from source 13. The mask 14 has a transmissive area 14b and an absorbable area 14a, which creates a pattern at the surface of the photosensitive medium with two different intensities of actinic radiation. Then, in Figure 6, the mask has been removed and the same photosensitive medium 11 is exposed to a uniform level of actinic radiation. The cumulative effect of these successive exposures is to form a latent image of at least two different densities while less than half of the photosensitive silver halide present in the photosensitive medium is activated. In this example, the density of the filamentary silver formed during development in the area 11a is less than the density in the area 11b. Figure 7 shows the formation of chemical surface obscuration of a thin layer of silver-precipitating cores. In Figure 8, a reflective layer is formed by negative diffusion transfer as discussed immediately above.

The reflective layer in Figure 8 has two different reflectivities, shown at 17a and 17b. The reflectivity at 17a is greater than that at 17b for two clearly distinct reasons. First, the area lia contains a greater concentration of unexposed, undeveloped silver halide or 11b does. There is thus more immediately available silver ion complex in the lia when the photosensitive medium is exposed to the monobath. Since the concentration of the silver-precipitating nuclei is area-wise constant, the only relevant factor in the formation of reflective silver is the availability of silver ion complexes. Since there are more complexes available in lia than in 11b, there will thus be more silver that is aggregated and then reduced in 17a than in 17b.

Secondly, as will be remembered from the section on photographic development, filamentary silver is present throughout the developed photosensitive medium. in figures 6, 7 and 8, the density of filamentary silver in 11b is greater than in lia. The concentration of filamentary silver particles is present in the reflective layer in substantially the same concentration or density as it is in the rest of the photosensitive medium immediately below the reflective layer. Thus, the amount of filamentary silver present in the reflective layer 17a is less than that in 17b, and as would be expected, the resulting reflectivity in 17b is reduced in comparison to 17a, due to the higher absorbability of the filamentary particles-* -ler in 17b. It should be remembered that the quantity of the reflective silver present in the reflective layer is substantially greater than the amount of absorbing filamentary silver, but where the density of the filamentary silver varies in an areal manner, its incorporation into the reflective layer has an estimate on local reflectivity. The result of the silver diffusion transport and reduction step is a very thin layer of reflective silver particles over an absorbent substrate. This very thin reflective layer is easily distorted, melted or made holes in with a laser, and is thus suitable for laser registration. Reflectivity between fluff and 75% can be achieved. The usual high thermal and electrical conductivity of silver is not present since the silver particles are not in contact. The deg-ge layer is electrically non-conductive. Another disadvantage of using silver is its price. In this invention, however, very little silver is used both in the formation of the reflective silver coating on the opaque substrate or substrate and in that there is very little silver to begin with in the photosensitive silver halide medium. This invention thus has the advantage of being cheap and of using commercially available materials and also of being applicable to well-known photochemistry.

E. Laser registration

Reference is now made to Figure 9 which is an illustration of recording on a broadband reflective laser recording medium according to this invention. The reflective layer 17 has been hollowed out with a laser beam which forms a shallow crater 23 which is a hole in the reflective layer. It is important to note that the penetration of the laser beam is very shallow, and barely penetrates the reflective layer. The reflective gelatin matrix incorporating the reflective silver is melted, causing a loss of reflectivity where the melting takes place.

Before registration, the reflectivity of the reflective layer is specular, in other words incident light perpendicular to the surface will be reflected back towards its origin in a parallel line. After registration, light falling perpendicularly on a crater or hole will be diffusely reflected due to the fact that the light returned towards the origin will be scattered and not parallel. This last effect and the increasing absorptivity of the stain lead to a reduced reflectivity. Crater 23 may be 1 / f* m in diameter or less, although for some purposes a larger crater may be desired. For example, in analogue recording, ovals with a length of more than 1 µm are sometimes used.

When laser registration is performed, the higher the absorptivity of the surface, the easier it is to melt craters that appear as holes in the surface. If, however, the reflected light is used for reading the recorded data or for automatic focusing, then a high reflectivity is desired. The incorporation of varying amounts of filamentary silver along with the non-filamentary silver in the reflective layers provides an important means of adjusting the reflectivity to the desired level. The filamentary silver particles are also absorbable even near-infrared wavelengths, which results in these media becoming broadband laser-recording media.

The absorbent sublayer, 21, which is the result of the exposure and development steps, serves two purposes in laser recording. First, the filamentary silver absorbs the light energy in the recording laser beam and converts it into heat. The reflective layer according to this invention is partially permeable, especially in the near-infrared range where semiconductor lasers work. Thus, light energy that has passed through the reflective layer is absorbed in the absorbent sublayer immediately below the reflective surface that it reaches. The light energy is converted into heat, and this causes a rise in temperature in the lower layer and the reflective layer in contact with it. This in turn makes it easier to melt the reflective layer by raising its temperature. The conversion of light energy into heat by the filamentary silver has a synergistic effect on recording, because as the temperature of the reflective layer increases, the temperature rise necessary to reach the melting point of the reflective gelatin matrix will decrease. A similar effect can be achieved by adding to the emulsion materials that are absorbable at the wavelength of the recording laser, for example by coloring the gelatin.

Secondly, gelatin is a good heat insulator. Thin metal layers, such as tellurium, which are used for laser registration, have a higher degree of thermal conductivity than the gelatin layer containing either the reflective or the filamentary silver. Thus, heat that is spread in the reflective layer will not flow quickly into the substrate, and thus heat energy is conserved and the registration process becomes efficient. The broadband laser recording and data storage medium of this invention has the advantage of metal-like reflectivity while avoiding the disadvantages of high thermal conductivity and high melting temperature associated with the use of a metal film. In addition, gelatin melts at a temperature of approx. 350°c, compared to 450° for tellurium, which is the most common laser recording material used today.

Example 1

This example illustrates the relationship between the level of initial light exposure and the final reflectivity of the surface layer. If the first exposure is intense, then, the emulsion photographic plate will develop dark gray to black, leaving no silver halide to form the reflective layer. For the same reason, a weak initial exposure will result in a final highly reflective surface. To illustrate this point, a commercial Agfa-Gevaert Millimask HD emulsion, 4.5 µm thick, containing a shielding dye, was exposed on a Mark VII sensitometer manufactured by Edgerton, Germeshausen & Grier, Inc. The plate was placed emulsion side down and exposed with the instrument's tungsten light source through a step block with steps in optical density units of 0.1. The photo plate

-2

was exposed for 10 seconds throughout the step block. The photographic plate was developed for 4 minutes in a developer with the following composition: 36.9 g sodium sulfite, 7.9 g hydroquinone, 0.52 g phenidone, 7.4 g potassium hydroxide, 2.7 g potassium bromide, 0.07 g benzotriazole and water added up to a volume of 1 litre.

After developing, washing and drying, the resulting neutral optical density was measured with red light on a Macbeth densitometer Model TR527. The optical densities were as follows:

A similar plate, as described previously, was prepared and immersed in a surface blurring solution of 0.5 g per square meter. liter of KBH4 and 0.5 g per liters of NaOH in methanol for 15 seconds to create silver nuclei on the surface. This nucleated plate was then washed for 1 minute and placed in a monobath developer solution containing: 10 g sodium sulfite, 2.5 g ascorbic acid, 0.25 g Elon (Kodak developer), 2.0 g sodium hydroxide, 125 g sodium thiocyanate and water added up to a volume of 1 litre. After immersion for 2 minutes in the monobath, the photographic plate was analyzed for reflectivity and optical density. The resulting optical densities were measured with a Macbeth densi-

tometer Model TR 527, and the reflectivities were measured with a system comprising a He-Ne laser and an International Light, Inc. Model IL 710A research photometer. The light at 633 nanometers reflected from the sample was compared to an aluminum mirror with 92% reflection. The results were as follows:

This example shows that by changing the original exposure level as done above using a

step-block as a mask, it is possible to form an optical data-storing medium with areas of reduced reflectivity where higher actinic exposure has taken place and areas of higher reflectivity where actinic exposure has taken place with lower intensity.

Example 2

The first part of this example shows that only a low initial actinic exposure is necessary to create the necessary amount of filamentary-type silver in the surface layer and sublayer. The volume concentration of the filamentary silver in the final media is determined by two effects. The first is the initial exposure and development, and the second is the immersion of the photographic plate in the monobath. The first effect creates a given number of filamentary silver particles, while the second effect results in the intensification of the build-up of filaments by the metallic deposition of silver from the silver complex created when the monobath acts on the silver halide.

A photographic plate coated with a commercial Konishiroku emulsion, 3/ µm thick, with an anti-overshine backing and containing no shielding dye, was exposed on a Mark VII sensitometer manufactured by Edgerton, Germeshausen & Grier, Inc. The photographic plate was placed with emulsion -side down and exposed with the instrument's tungsten light source through a step block with optical density units of 0.1. Actinic radiation exposure was 2 x 10 seconds. The exposed photographic plate was then developed for 4 minutes and was not fixed. The anti-radiation backing is removed in this process. The composition of the developer was as follows: 36.9 g of sodium sulphite,

7.9 g of hydroquinone, 0.52 g of phenidone, 7.4 g of potassium hydroxide, 2.7 g of potassium bromide, 0.07 g of benzotriazole and water added up to a volume of 1 liter.

After washing and drying, the resulting neutral optical densities were measured in red light on a Macbeth densitometer, Model TR 527. Each step in the step block is defined by a number. The step identification and accompanying optical density are indicated in the two columns on the left:

This exposed and developed step block was then immersed for 3 minutes in a monobath solution comprising: 10 g sodium sulfite, 2.5 g ascorbic acid, 0.25 g Elon (Kodak developer), 2.0 g sodium hydroxide, 60 g sodium thiocyanate and water added up to a volume of 1 litre. Note the significant increase in optical density compared to the original optical density.

The second part of this example illustrates how the original gray density is in relation to the final reflectivity for the Konishiroku photographic plate coated with an ST emulsion, 3/tm thick. Another sample was exposed and developed identically to the procedure described above. Before inserting the sample into the monobath solution for 3 minutes, however, it was carefully dried, and then immersed for 15 seconds in a blurring solution comprising: 0.15 g KBH4 and 0.6 g NaOCH3 and methanol added until a volume of 1 liter. The photographic plate was then carefully washed before and after immersion in the monobath.

The reflectivities were then measured for the more reflective step counts at 633 nanometers using a DR2J micro-flectometer system manufactured by Gamma Scientific Inc. The results were as follows:

We thus see that stage 10 had an initial absorbency of 0.20 and a final reflectivity of 48% at 633 nanometers. For higher initial absorbance, the final reflectivity was lower and vice versa.

Example 3

This example shows that the reflective laser registration performed by the method according to this invention allows registration at lower laser powers than any of the previous media described by the same inventors in previous patent applications, and also that this high sensitivity is achieved in connection with a reflectivity of at least 44%, which is considerably higher than that previously achieved. Moreover, the reflectivity remains at a relatively high level up to near-infrared.

A 19.35 cm <2> photographic plate coated with a commercial Konishiroku ST emulsion, 3 µm thick, containing no shielding dye, was exposed for 1.0 seconds at 3580 nanoamp/cm on an Ultratech contact printer, Model CP210 . This is approximately equivalent to 10 lumens/9.29 dnr 2 for 10 seconds. It was developed in a developer solution for 5 minutes and not fixed. The developer was composed of: 36.9 g sodium sulfite, 7.9 g hydroquinone . The photographic plate was then dipped for 15 seconds in a solution of 0.15 g KBH^ and 0.6 g NaOCH^ and with methanol added to a volume of 1 liter. The photographic plate was then washed in water for 10 minutes and not dried. To to achieve silver diffusion transfer, the photographic plate was then immersed for 3 minutes in a monobath developing solution comprising: 10 g sodium sulfite, 2.5 g ascorbic acid, 0.25 g Elon (Kodak developer), 2.0 g sodium hydroxide, 60 g of sodium thiocyanate and water added to a volume of 1 L. The final washing in water was carried out for 10 minutes, followed by air oven drying for 2 0 minutes at 40°c.

The resulting mirror-like coating on the glass substrate is a broadband reflective laser-recording and data-storing medium. Laser registration was performed with an argon laser using the green line at 514 nanometers. The laser beam diameter was approximately 0.8 µm at the surface of the medium, and pulse lengths of 100 nanoseconds were used. Tests were carried out to register low-reflective spots with the laser beam by melting the reflective surface of the gelatin. Then the reflectivity of the hole was compared to the reflectivity of an adjacent area. A comparison of these reflectivities leads to the measurement of a relative contrast ratio. This process was repeated for 60 holes at a given laser power level. (The samples according to the known technique were tested with 32 holes). The relative contrast ratio was determined by averaging results from the 60 holes. Furthermore, the statistical distribution of the relative contrast ratio for the 60 holes was calculated, and an a - 1 sigma distribution was calculated. The resulting data are listed as sample 4 in Table 1. For comparison with the performance of prior art media, the performances of samples 1, 2 and 3, also shown in Table 1, are listed as milestones for the prior art. Note that for all media the relative contrast ratio decreases with decreasing laser power. It should be understood that when the laser power drops to near the lower levels, the recorded hole becomes smaller and smaller. Thus, when the 0.8 /m laser beam is aimed at e.g. a 0.6/«m hole, some light is reflected back from the undisturbed reflective area around the hole. This is important to understand when assessing the data shown in table 1.

Acceptable registrations can also be defined as those that give the smallest -/sigma distribution as a percentage of the average relative contrast ratio. Thus we see that for the previously known example 2, the material is unusable at 2.8 milliwatts since the distribution of -834 is in reality greater than the relative contrast of 640. If we set a freely chosen registration sensitivity limit at the power level where e.g. the sigma distribution is not more than 20% of the relative contrast ratio, then one could arrive at a means for comparing the laser registration sensitivities of the three previously known samples 1, 2 and 3 and that of sample 4. this definition, sample 1 would have a minimal power requirement of approx. 2.2 milliwatts, sample 2 would have a minimum power requirement of more than 15.4 milliwatts, sample 3 would have a minimum power requirement of 1.7 milliwatts and sample 4, using the method of the present invention, would have a minimum power requirement of 1 milliwatt and is thus the most sensitive of the group. In addition, sample 4 has a reflectivity of at least 44%, compared to 17%, 21% and 25.5% respectively for samples 1, 2 and 3. This is important when the reflected signal is used such as in automatic focusing and when maximizing signal -to-noise ratio. The reflectivity of sample 4, prepared by the method according to the present invention, maintains suitable reflectivities over a considerable range. The reflectivities of this sample were measured with a DR2J microreflectometer system manufactured by Gamma Scientific Inc.

The light absorbance of several samples was compared to determine if sample 4 is more broadband in absorbance. Since sample 2 has a much lower detection sensitivity than the others, it was omitted from the comparison. For visible measurements, a Macbeth densitometer, Model TR 527, was used, and for the near-infrared absorbability measurements, a Beckman DK-2 spectrophotometer was used. The results were as follows:

The absorbability properties of sample 4 are clearly more broad-band, that is, more uniform over the spectral range of interest. Over the spectrum shown, the optical densities for samples 1 and 3 vary by approx. a factor of 4 compared to a factor of 2 for sample 4. Also, the optical density in the near-infrared region is considerably higher for sample 4 than for samples 1 and 3, and this makes it more suitable for recording with a near-infrared semiconductor- diode laser, which typically operates at 780 nanometers and 830 nanometers.

Claims (13)

1. Reflective optical data-storing and recording medium, characterized in that it comprises a colloidal matrix sublayer (11) which primarily has filamentary silver particles in it which have an optical density for red light of between 0.2 and 3.0 and which can absorb visible and infrared light, and a reflective surface layer (17) primarily of non-filamentary individual silver particles in said colloidal matrix, where the reflective surface layer is placed on top of said sublayer, which surface layer has maximum particle dimensions primarily below 0.05 µm, where some of the individual silver particles are aggregated with similar particles, and with a volume concentration of silver particles that is greater in said surface area than in said colloidal matrix sublayer, where the reflective surface layer has at least one area that has substantially uniform reflectivity, and where the uniform reflectivity is between 10 and 75%. 2. Data recording medium according to claim 1, characterized in that the volume concentration of silver in the reflective surface layer (17) exceeds the lowest volume concentration within said colloidal matrix sub-layer by a ratio of at least 3:1. 3. Data-recording medium as specified in claim 1 or 2, characterized in that the thickness of the reflective surface (17) is less than 1 µm thick. 4. Data recording medium as stated in claim 1, 2 or 3, characterized in that the volume concentration of silver particles in the reflective layer (17) is at least 20% and at most 50%. 5. Data-recording medium as stated in any one of claims 1 to 4, characterized in that the colloidal matrix layer (11) is photographic gelatin of the type used in the production of silver halide emulsions. 6. Data-recording medium as stated in any one of claims 1 to 5, characterized in that the reflective surface layer (17) is electrically non-conductive. 7. Method for producing a reflective optical data-storing and recording medium as stated in claims 1 to 6, characterized by (a) so as to expose' and afterwards develop a photosensitive layer (11) comprising silver halide grains with a size smaller than 1/ 10 of the size of the data recording units, that the exposed areas acquire a gray color having an optical density between 0.05 and 2.0 for red light, by (b) forming a layer of silver-precipitating nuclei on the surface of the photosensitive layer (11) by chemical activation, or by incorporating a surface layer of core-forming material, by (c) forming in said photosensitive layer soluble silver ion complexes by means of a developer/solvent monobath, said complexes being transported by diffusion to the silver precipitating cores, and by (d) precipitating reflective silver from the silver ion complexes on the silver precipitating cores. 8. Method as stated in claim 7, characterized in that the exposure of the photosensitive layer (11) is carried out by directing actinic radiation with uniform intensity over the surface of the photosensitive layer (11). 9. Method as stated in claim 7, characterized in that the exposure of the photosensitive layer (11) includes creating a pattern of two or more levels of non-saturating actinic radiation over the surface of the photosensitive layer (11). 10. Method as stated in claim 7, 8 or 9, characterized in that the formation of a layer of silver-precipitating cores is carried out by bringing a surface of said photosensitive silver halide layer (11) into contact with a blurring agent. 11. Method as stated in claim 10, characterized in that a blurring agent is used which comprises a solution of borohydride anion. 12. Method as stated in any of the preceding claims 7-11, characterized in that the gray color is developed to an optical density of between 0.1 and 0.8 for red light. 13. Method as stated in any of the preceding claims 7-12, characterized in that the layer of silver-precipitating cores is formed by providing a silver-precipitating layer on the surface of the silver halide layer (11).