GB1592390A - Photosensitive material for optical digital recording and high density information storage - Google Patents

Description

(54) PHOTOSENSITIVE MATERIAL FOR OPTICAL DIGITAL RECORDING AND HIGH DENSITY INFORMATION STORAGE (71) I, ELI SOLOMON JACOB, a citizen of the United States of America, of 115 East 86th Street, New York City, New York 10028, United States of America, do hereby declare the invention for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to the permanent storage of information and concerns a method of, and a recording medium for, storing an intelligence pattern using radiant energy.

Data storage systems, e.g. the storage system shown in U.S. patent No. 3,501,586 dated March 17, 1970 to James T. Russell, have been developed to permanently store data on an optically sensitive recording material by means of an intensity modulated laser beam. While laser recording equipment for this process has been developed to an advanced state, a convenient recording medium has not heretofore been found. Conventional photographic plates have been used, but are disadvantageous because they require special handling and must be "developed" before the data can be retrieved. Other media have included complex and costly chemical dyes which change colour in response to heat or light. Still other media consist of thin layers of energy absorbing material which are perforated by the pulses of laser light. However, once perforated, such media tend to wear out with continued use.

U.S. patent No. 3,715,734, dated February 6, 1973, to Jack Fajans describes a three dimensional recording medium which is a block of polymethyl methacrylate (PMMA). In this system a laser beam is expanded through a system of lenses then steeply converged to a focal point inside the block to form opaque spots by carbonizing the block itself at the beam's focal point. A very large amount of radiant energy is required to carbonize PMMA and thus an exceedingly powerful laser is required. The power requirement of the laser is further increased because of the reduction in the light intensity as the beam passes through Fajan's several optical components. By his own estimate, Fajan's system optics reduce light intensity by 50%.

According to the invention, there is provided a recording medium for permanently storing an intelligence pattern from which information can be read by exposing the medium to radiant energy at a first level of intensity and on which information is recorded by exposing the medium to radiant energy at a second level of intensity which is higher than said first level, comprising a body of solid recording material constituted by an organic substance which does not carbonize, to the extent that said organic substance becomes substantially opaque to radiant energy, upon exposure to radiant energy at said second level of intensity, said organic substance having dispersed therein particles of a reactant substance which are nonreactive when said body is exposed to said radiant energy at said first level of intensity, and chemically react exothermally, when said body is exposed to said radiant energy at said second level of intensity, to liberate sufficient heat energy so that the total amount of heat energy in said body, at the location of radiant energy contact, is sufficient for said organic substance to carbonize and, due to carbonization, become substantially opaque to radiant energy at said location.

The invention also provides a method of permanently storing an intelligence pattern comprising: converting said intelligence pattern into a sequence of pulses of radiant energy, each pulse extending over a predetermined period; applying each pulse to a body of solid recording material, said material including an organic substance which chars upon exposure to sufficient amount of heat energy; adjusting said radiant energy to an intensity sufficient to initiate an exothermic reaction of one or more reactant substances contained in said material; using heat energy from said exothermic reaction in combination with heat energy produced by radiant energy absorption to exceed said sufficient amount and thereby char said organic substance; and moving said body in relation to the path of said radiant energy between pulses thereof.

The organic substance may be, for example a synthetic plastics material interspersed with sub-micron particles of one or more reactant substances. Preferably, a pulsed laser of moderate intensity is used to provide the radiant energy for recording and, when such a pulse of moderate intensity strikes the recording medium, that volume of the medium which is in the path of the pulse absorbs the radiant energy and is heated. This heating causes the reactant substances of particles in the heated volume to react exothermally liberating sufficient additional heat energy to char the organic substance in the heated volume.

The dispersion of particles of reactant substance throughout the organic substance thus reduces the intensity of laser light needed to char the organic substance and thus the size and energy requirements of the laser are greatly decreased. By providing the particles in an active layer of organic substance supported on a substrate layer, complex lens systems which waste excessive amounts of light energy may be eliminated from the laser recording apparatus.

According to a preferred form of the invention, there is provided a recording medium for permanently storing an intelligence pattern, from which information is read by exposure to radiant energy at a first level of intensity and on which information is recorded by exposure to radiant energy at a second level of intensity which is higher than said first level, which recording medium comprises: a metallic substrate having a smooth, optically reflective upper surface; a uniform layer of recording material over said substrate, said material being made of an organic substance selected from the class of plastics consisting of polymers containing furfuryl alcohol and polymers containing vinyl chloride which carbonize to the extent that said organic substance becomes substantially opaque to radiant energy upon exposure to a predetermined minimum amount of heat energy, and partially absorb said radiant energy in an amount insufficient to produce heat energy of said predetermined minimum amount when exposed to radiant energy at said second level of intensity; dispersed in said organic substance, particles of reducing and inorganic oxidizing agents which are nonreactive when exposed to the amount of heat energy produced in said organic substance when said body is exposed to radiant energy at said first level of intensity, during the reading of information from said recording medium and chemically react exothermally when exposed to the amount of energy produced in said organic substance when said body is exposed to radiant energy at said second level of intensity, during the recording of information on said recording medium and, due to the exothermic reaction, liberate sufficient heat energy for the total amount of heat energy in said layer, at the location of radiant energy contact, to exceed said predetermined minimum amount so that said organic substance carbonizes and, due to carbonization, becomes substantially opaque to radiant energy at said location; and a substantially transparent protective layer of plastics material over said layer of recording material, said protective layer and substrate formed to seal each side of said layer of recording material.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: FIGURE 1 is a block diagram of an analog to digital to optical recording system capable of producing a permanent record on the recording medium of the present invention; and FIGURE 2 is a vertical cross-section through a recording medium embodying the present invention.

Figure 1 shows a typical device for recording information by means of a pulsed laser. The system includes a recorder unit 10 having its input connected to an audio visual analog signal source 12, such as a microphone or television camera. This analog input signal 23 is applied to the input of an analog-to-digital signal converter 24 provided in the recorder unit 10 and which produces a digitally encoded electrical output signal 26. The output of the analog-todigital signal converter 24 may be directly connected to an electrical optical digital signal recorder 28 through an amplifier 29 if it is desired to record the digital signal in real time simultaneously as it is generated. However, it may be desirable to temporarily store the digital signal 26 on the magnetic tape or other memory device of a digital computer 30 and to record such signals later at a more convenient time.

The electrical optical digital signal recorder 28 converts the digital electrical signal into a digital light signal and records such light signal by scanning a pulsed laser light beam 40 of small spot size on a photosensitive recording medium to produce a track of digitally encoded spots of less than about 10 microns in diameter. The spots are light opaque and correspond to either 0 or 1 bits of a binary code. The recording medium is supported, in a fashion which may be conventional, for movement in a path perpendicular to the optical axis and is mechanically coupled to a recording medium positioning mechanism 44 adapted to move the medium between pulses, to produce the track of digitally encoded spots. The recording medium positioning mechanism 44 may include a drive motor (not shown) which is energized selectively in response to signals transmitted by the optical digital signal recorder 28.

The recording medium 42 of the present invention includes a body of recording material consituted by an organic substance, such as a plastics material, which readily chars when exposed to a sufficient amount of heat energy. Interspersed in the organic substance are particles of one or more reactant substances which are stable when exposed to laser light of a first level of intensity, but which react exothermally when exposed to laser light at a second, higher level of intensity.

In Figure 2, the recording medium 42 is shown as an active layer 50 coated on a smooth, optically reflective upper surface 52 of a metal substrate 54. Such an active layer preferably will be no more than six (6) microns thick. A substantially transparent protective layer 56 of a plastics material is coated over the active layer 50 so that the protective layer 56 and substrate 54 seal opposite sides of the active layer from the atmosphere.

The organic substance selected for use as the matrix material of the active layer should be of the type which undergoes a "charring" reaction when heated, e.g. plastics material such as polymers based on furfuryl alcohol or vinyl chloride. If furfuryl alcohol polymers are used, suitable material for the active layer may be formed by dispersing sub-micron particles of the reactant substance or substances in furfuryl alcohol prior to the addition of a catalyst to accelerate polymerization. The combined plastics material and reactant particles are formed into an active layer of uniform thickness by conventional means.

The size and spacing of reactant particles in the active layer will affect the quality of the recording material. For example, if vinylidene chloride is selected as the plastics matrix material and reactant particles of 0.3 to 0.6 microns in diameter are dispersed therein, a spacing of one particle per cubic micron of vinylindene chloride is suitable for most types of reactant substances. By experimentation, it is easy to determine which particle size and spacing will be the moxt advantageous for any given combination of matrix and reactant materials.

The reactant substances may be of several types including thermite type mixtures, match compositions, delay fuse mixtures, flare compositions, and simple explosives. Preferably, the reactant substance will comprise such substances which react exothermally when heated to 300"C.

Thermite type mixtures usually contain a reducing element which is a metal or an easily oxidized non-metal. In the present invention, the reducing agent could be the charable plastics matrix material of the recording material. An oxidizing agent is also included which is typically a metal oxide or salt rich in oxygen.

Match compositions likewise contain oxidizing and reducing agents. Suitable examples include red phosphorus or phosphorus sesquisulfide (P4S3) as the reducing agent with potassium chlorate (KClO3) as the oxidizing agent.

In delay fuse compositions zirconium and amorphous boron are typical reducing agents and barium chromate (BaCrO4) is a suitable oxidizing agent. Reactant particles of delay fuse materials could, for example, include any of the fuel-oxidizer combinations shown in Table I.

TABLE I Composition Reducing Agent (wt. 9 Oxidizing Agent (Wt. 9 1 Zr 22 BaCrO4 78 2 Zr 28 BaCrO4 72 3 Zr 35 BaCrO4 65 4 Zr 40 BaCrO4 60 5 B 5 BaCrO4 95 6 B 10 BaCrO4 90 7 B 15 BaCrO4 85 Flare compositions contain magnesium or aluminum metal as a reducing agent. These metals are susceptible to laser ignition for oxidation by a wide variety of oxidizing agents.

Explosive compositions include compositions containing compounds having excess reso nance energy or compositions containing the fuel-oxidizer combination within a single molecule, for example lead styphnate.

The substrate 52 comprises a material which can be treated to form an optically reflective surface. Metals such as low carbon steel, aluminium, and brass, may be polished to provide a suitable surface. Other materials such as metalized ceramics, or metalized plastics can be used in some circumstances. The protective layer 56 may be made of any material which is substantially transparent to laser light when bonded to the matrix material.

To record on the described medium embodying the present invention a pulse of laser light 40a is beamed normally toward the protective layer 56 of the medium 42a. The pulse passes through the transparent protective layer 56 and enters the active layer 50 where it is partially absorbed heating a spot in the layer. Light not absorbed by the active layer 50 is reflected from the surface 52 back through the heated spot so that an additional fraction of the light energy may be absorbed.

Each pulse is adjusted to have a predetermined duration so that it produces an amount of radiant energy below the amount needed to carbonize the plastics matrix material of the recording material when the reactant substance or substances are absent. However, each pulse contains an amount of radiant energy which is above the predetermined minimum amount needed to initiate an exothermic reaction of the one or more reactant substances contained in the plastics matrix material of the recording material. The exothermic reaction thus initiated will increase the heat energy in the plastics matrix material to about the amount of heat required to carbonize the plastics matrix material causing a carbonized spot to be produced in the plastics matrix material.

If laser light having an intensity of 100 millijoules per pulse and a wave length of about 5300A is focused into a one cubic micron spot in a matrix material with a specific heat of about 0.5- calories per gram, a cubic micron of the medium would rise almost 500"C in temperature assuming total absorption of the light. Total absorption is not however possible, in practice, and a realistic absorption of about 50% of the incident light would create a temperature rise insufficient to char the plastics matrix material.

A 50% absorption would, however, be sufficient to initiate an exothermic reaction between reactant substances interspersed in the matrix material. For example, interspersed reactant mixtures would ignite at temperatures in the range of 200-300"C. depending on the size, spacing and environment of the reactive particles. Thus, such reactive particles can easily be ignited by a laser having the above-described characteristics. Once ignited, the thermite particles produce heat pulses of 300-950 calories per gram which raise the temperature at the spot of laser excitation in the active layer to between 3,000 and 5,000"C. When raised to this elevated temperature, the plastics matrix material chars leaving a darkened spot as a permanent record of the laser light pulse.

Combustion does not propagate from the spot of laser excitation because the reactant particles constitute a discontinuous phase in a plastics matrix material which will not sustain combustion in the absence of a source of ignition. After ignition, heat in the active layer rapidly diffuses into the surrounding environment, but is insufficient to ignite reactant particles outside the area of laser excitation. The presence of a metal substrate further aids in preventing the uncontrolled propagation of the exothermic reaction by acting as a heat sink to draw off excess heat from the reaction.

Data recorded as charred spots on the recording medium is read by directing a beam of low intensity laser light toward the protective layer of the recording medium. This playback light beam must be of lower intensity than the recording beam so that the active layer absorbs insufficient energy to initiate an exothermic reaction between the remaining particles of reactive substances. If the playback light beam encounters a charred spot in the active layer, it is absorbed or scattered. If, however, the playback beam does not encounter a charred spot, it is reflected from the reflective surface of the substrate to a photosensitive detector apparatus (not shown) which converts the reflected light into an electrical digital signal. Alternatively, after the data are recorded, the reflective metal substrate might be stripped off the active layer and protective layer. For playback, the light would then be transmitted through the recording medium to a detector placed on the opposite side of the record from the scanner. If it is possible to provide sufficient laser energy, or if the active layer is sufficiently sensitive, a transparent substrate without reflector could be used.

Again, the data would then be played back by transmitting light through the recording medium rather than by reflection.

WHAT I CLAIM IS: 1. A recording medium for permanently storing an intelligence pattern from which information can be read by exposing the medium to radiant energy at a first level of intensity and on which information is recorded by exposing the medium to radiant energy at a second level of intensity which is higher than said first level, comprising a body of solid recording material constituted by an organic substance which does not carbonize, to the extent that said organic substance becomes substantially opaque to radiant energy, upon exposure to radiant energy at said second level of intensity, said organic substance having dispersed therein particles of a reactant substance which (a) are nonreactive when said body is exposed to said radiant energy at said first level of intensity, and (b) chemically react exothermally, when said body is exposed to said radiant energy at said second level of intensity, to liberate sufficient heat energy so that the total amount of heat energy in said body, at the location of radiant energy contact is sufficient for said organic substance to carbonize and, due to

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

Claims (24)

**WARNING** start of CLMS field may overlap end of DESC **. To record on the described medium embodying the present invention a pulse of laser light 40a is beamed normally toward the protective layer 56 of the medium 42a. The pulse passes through the transparent protective layer 56 and enters the active layer 50 where it is partially absorbed heating a spot in the layer. Light not absorbed by the active layer 50 is reflected from the surface 52 back through the heated spot so that an additional fraction of the light energy may be absorbed. Each pulse is adjusted to have a predetermined duration so that it produces an amount of radiant energy below the amount needed to carbonize the plastics matrix material of the recording material when the reactant substance or substances are absent. However, each pulse contains an amount of radiant energy which is above the predetermined minimum amount needed to initiate an exothermic reaction of the one or more reactant substances contained in the plastics matrix material of the recording material. The exothermic reaction thus initiated will increase the heat energy in the plastics matrix material to about the amount of heat required to carbonize the plastics matrix material causing a carbonized spot to be produced in the plastics matrix material. If laser light having an intensity of 100 millijoules per pulse and a wave length of about 5300A is focused into a one cubic micron spot in a matrix material with a specific heat of about 0.5- calories per gram, a cubic micron of the medium would rise almost 500"C in temperature assuming total absorption of the light. Total absorption is not however possible, in practice, and a realistic absorption of about 50% of the incident light would create a temperature rise insufficient to char the plastics matrix material. A 50% absorption would, however, be sufficient to initiate an exothermic reaction between reactant substances interspersed in the matrix material. For example, interspersed reactant mixtures would ignite at temperatures in the range of 200-300"C. depending on the size, spacing and environment of the reactive particles. Thus, such reactive particles can easily be ignited by a laser having the above-described characteristics. Once ignited, the thermite particles produce heat pulses of 300-950 calories per gram which raise the temperature at the spot of laser excitation in the active layer to between 3,000 and 5,000"C. When raised to this elevated temperature, the plastics matrix material chars leaving a darkened spot as a permanent record of the laser light pulse. Combustion does not propagate from the spot of laser excitation because the reactant particles constitute a discontinuous phase in a plastics matrix material which will not sustain combustion in the absence of a source of ignition. After ignition, heat in the active layer rapidly diffuses into the surrounding environment, but is insufficient to ignite reactant particles outside the area of laser excitation. The presence of a metal substrate further aids in preventing the uncontrolled propagation of the exothermic reaction by acting as a heat sink to draw off excess heat from the reaction. Data recorded as charred spots on the recording medium is read by directing a beam of low intensity laser light toward the protective layer of the recording medium. This playback light beam must be of lower intensity than the recording beam so that the active layer absorbs insufficient energy to initiate an exothermic reaction between the remaining particles of reactive substances. If the playback light beam encounters a charred spot in the active layer, it is absorbed or scattered. If, however, the playback beam does not encounter a charred spot, it is reflected from the reflective surface of the substrate to a photosensitive detector apparatus (not shown) which converts the reflected light into an electrical digital signal. Alternatively, after the data are recorded, the reflective metal substrate might be stripped off the active layer and protective layer. For playback, the light would then be transmitted through the recording medium to a detector placed on the opposite side of the record from the scanner. If it is possible to provide sufficient laser energy, or if the active layer is sufficiently sensitive, a transparent substrate without reflector could be used. Again, the data would then be played back by transmitting light through the recording medium rather than by reflection. WHAT I CLAIM IS:

1. A recording medium for permanently storing an intelligence pattern from which information can be read by exposing the medium to radiant energy at a first level of intensity and on which information is recorded by exposing the medium to radiant energy at a second level of intensity which is higher than said first level, comprising a body of solid recording material constituted by an organic substance which does not carbonize, to the extent that said organic substance becomes substantially opaque to radiant energy, upon exposure to radiant energy at said second level of intensity, said organic substance having dispersed therein particles of a reactant substance which (a) are nonreactive when said body is exposed to said radiant energy at said first level of intensity, and (b) chemically react exothermally, when said body is exposed to said radiant energy at said second level of intensity, to liberate sufficient heat energy so that the total amount of heat energy in said body, at the location of radiant energy contact is sufficient for said organic substance to carbonize and, due to

carbonization, become substantially opaque to radiant energy at said location.

2. A medium according to claim 1 wherein said body comprises an active layer.

3. A medium according to claim 2 further comprising a substrate layer having a smooth, optically reflective upper surface positioned against said active layer.

4. A medium according to claim 3 wherein said active layer is no more than six (6) microns thick.

5. A medium according to claim 3 or 4 further comprising a substantially transparent protective plastics layer positioned against said active layer opposite said substrate layer.

6. A medium according to claim 3, 4 or 5 wherein said substrate layer comprises a metal composition selected from the class consisting of low carbon steel, aluminum and brass.

7. A medium according to any preceding claim wherein said reactant substance comprises sub-micron particles dispersed within said recording material.

8. A medium according to claim 7 wherein said particles are between 0.3 and 0.6 microns in diameter, and about one said particle is included per cubic micron of said organic substance.

9. A medium according to any preceding claim wherein said reactant substance comprises substances which react exothermally when heated to 3000C.

10. A medium according to any preceding claim wherein said reactant substance comprises a reducing agent, and an oxidizing agent exothermally reactive with said reducing agent when ignited.

11. - A medium according to claim 10 wherein said reducing agent comprises said organic substance.

12. A medium according to claim 10 wherein said oxidizing agent is selected from the class consisting of metal oxides, oxygen-containing salts and mixtures thereof.

13. A medium according to claim 10 wherein: said reducing agent is selected from the class consisting of red phosphorus, phosphorus sesquisulfide (P4S3) and mixtures thereof; and said oxidizing agent comprises potassium chlorate.

14. A medium according to claim 10 wherein: said reducing agent is selected from the group consisting of amorphous boron, zirconium, and mixtures thereof; and said oxidizing agent comprises barium chromate.

15. A medium according to claim 10 wherein said reducing agent is selected from the glass of metals consisting of magnesium, aluminum, and mixtures thereof.

16. A medium according to any one of claims 1 to 8 wherein said reactant substance comprises a single explosive substance.

17. A medium according to claim 17 wherein said explosive substance comprises lead styphnate.

18. A medium according to any preceding claim wherein said organic substance is selected from the class of plastics consisting of polymers containing furfuryl alcohol and polymers containing vinyl chloride.

19. The medium of claim 1 wherein said reactant substance comprises a substance which chemically reacts exothermally when exposed to laser light of a predetermined level of intensity.

20. A recording medium for permanently storing an intelligence pattern, from which information is read by exposure to radiant energy at a first level of intensity and on which information is recorded by exposure to radiant energy at a second level of intensity which is higher than said first level, which recording medium comprises: a metallic substrate having a smooth, optically reflective upper surface; a uniform layer of recording material over said substrate, said material being made of an organic substance selected from the class of plastics consisting of polymers containing furfuryl alcohol and polymers containing vinyl chloride which (a) carbonize to the extent that said organic substance becomes substantially opaque to radiant energy upon exposure to a predetermined minimum amount of heat energy, and (b) partially absorb said radiant energy in an amount insufficient to produce heat energy of said predetermined minimum amount when exposed to radiant energy at said second level of intensity; dispersed in said organic substance, particles of reducing and inorganic oxidizing agents which (a) are nonreactive when exposed to the amount of heat energy produced in said organic substance when said body is exposed to radiant energy at said first level of intensity, during the reading of information from said recording medium, and (b) chemically react exothermally when exposed to the amount of energy produced in said organic substance when said body is exposed to radiant energy at said second level of intensity, during the recording of information on said recording medium and, due to the exothermic reaction, liberate sufficient heat energy for the total amount of heat energy in said layer, at the location of radiant energy contact, to exceed said predetermined minimum amount so that said organic substance carbonizes and, due to carbonization, becomes substantially opaque to radiant energy at said location; and a substantially transparent protective layer of plastics material over said layer of recording material, said protective layer and substrate formed to seal each side of said layer of recording material.

21. A medium according to claim 20 wherein said organic substance comprises said reducing agent.

22. A method of permanently storing an intelligence pattern comprising: converting said intelligence pattern into a sequence of pulses of radiant energy, each pulse extending over a predetermined period; applying each pulse to a body of solid recording material, said material including an organic substance which chars upon exposure to sufficient amount of heat energy; adjusting said radiant energy to an intensity sufficient to initiate an exothermic reaction of one or more reactant substances contained in said material; using heat energy from said exothermic reaction in combination with heat energy produced by radiant energy absorption to exceed said sufficient amount and thereby char said organic substance; and moving said body in relation to the Dath of said radiant energy between pulses thereof.

23. A method according to claim 22 further comprising backing said recording medium with a metallic material during said applying, said metallic material acting as a mirror to reflect radiant energy back into said medium and as a heat sink to receive excess heat energy and thereby limit the propagation of said exothermic reaction.

24. A recording medium permanently storing an intelligence pattern produced with laser light, substantially as hereinbefore described with reference to the accompanying drawings.