GB2229315A

GB2229315A - Light mediated switch

Info

Publication number: GB2229315A
Application number: GB9005695A
Authority: GB
Inventors: Peter Howson
Original assignee: Champion Spark Plug Europe SA
Current assignee: Champion Spark Plug Europe SA
Priority date: 1989-03-15
Filing date: 1990-03-14
Publication date: 1990-09-19
Anticipated expiration: 2010-03-14
Also published as: JPH02292874A; AU626391B2; ZA902005B; BE1002672A3; IT9067184A0; FR2644629B1; IT1241191B; KR900015360A; GB9005695D0; CA2012110A1; GB2229315B; ES2021503A6; GB8905910D0; DE4007979A1; IT9067184A1; FR2644629A1; AU5132690A

Description

4 0. L.

4 k i 1 Electronic switch comprising a photosensitive semiconductor The present invention relates to an electronic switch comprising a photosensitive semiconductor and a light source which, when actuated, illuminates the semiconductor and causes the latter to become conductive.

Photosensitive semiconductors are well known and need not be described in detail. The main characteristic of such semiconductors is their ability to pass from an electrically non-conductive and high resistance state to an electrically conductive and low resistance state when they are illuminated with light.

Materials presently used as or in industrial photosensitive semiconductors are for example: silicon, carbon, germanium, gallium arsenide, silicon carbide, cadmium selenide, cadmium sulphide, indium phosphide, and potassium phosphide. These prior art photosensitive semiconductors can be used in various electronic devices, for example, in electronic switches, and are generally doped with "impurities" such as, for example cobalt, copper, chromium, gold, iron, oxygen, silver, and zinc.

2 Electronic switches comprising a photosensitive semiconductor are described, for example, in the following U.S. Patents: 4,301,362; 4,347, 437; 4,438,331; 4,490,709; 4,577,114 and 4,695,733. Various configurations of switches are described in these patents. They all comprise a light source and a_ photosensitive semiconductor provided with two spaced electrodes. The semiconductors are made of various materials and the light source is generally a laser.

When, for a given semiconductor, a certain bias voltage is applied across the electrodes of a photosensitive switch, the semiconductor will not permit the electrical current to pass between the electrodes and the switch will be in its "off" position, but when the light source projects light energy on to the area of the semiconductor located between the electrodes the semiconductor becomes electrically conductive and the switch will be in its "on" position. The main advantage of switches of this type is that they can switch an electrical current on or off very rapidly.

A disadvantage of known photosensitive, switches and, in particular, of the switches disclosed in the above six U.S. patents is that they are only capable of switching electrical supplies which have voltages below a few kV, for example up to 3 -5 kV, and thus cannot be used in electrical devices where a higher voltage is required.

It is therefore a first object of the invention to provide an electronic switch activated by 4 X - 3 light that is capable of withstanding/switching high voltage, low amperage electrical currents, that is electrical supplies having a voltage of at least 30 kV and having an amperage of less than 0.2 A. It is a further object of the invention to provide a very compact and a very economical high voltage, low amperage photosensitive switch and it is also an object of the invention to provide a photosensitive switch capable of operating with a high frequency.

We have now found that certain doped cadmium sulpho-selenide semiconductor compositions have a desirable combination of properties such that when used as the semiconductor in a photosensitive switch, the latter is capable of switching high voltage, low amperage electrical supplies can be constructed compactly and cheaply and can be operated at a high frequency.

According to the present invention, there is provided an electronic switch comprising a photosensitive semiconductor and a light source which, when actuated, illuminates the semiconductor and causes the latter to become conductive, in which the photosensitive semiconductor is a sintered mixture comprising, by weight, 63 to 74% of cadmium, 16 to 24% of selenium, 8 to 14% of sulphur, 0.1 to 1,10 of chlorine, and 0.005 to 0. 1% of copper.

The semiconductor is preferably in the form of - 4 an adherent layer on an electrically insulating substrate. The semiconductor is provided with spaced electrodes to which, in use, the circuit which the switch is to control is connected. The electrodes may be two adherent. spaced electrode layers on the sintered semiconductor layer or the substrate may be provided with two adherent, spaced electrode layers and the adherent sintered semiconductor layer may be applied over the electrode layers and the substrate between them; in the latter case the electrode layers are below the semiconductor layer.

The electrode layers may be formed of any suitable electrically conductive material, preferred materials being, for example, silver, indium and aluminium and resins, for example epoxy resins, loaded -with any one or more of these metals.

According to another aspect of the present invention, there is provided a method of making an electronic switch according to the invention, which comprises forming a paste of a finely divided powder mixture comprising, by weight, 35 to 55% of cadmium selenide, 35 to 55% of cadmium sulphide, 5 to 151% cadmium chloride, and 0.01 to 0.1% of copper chloride, a binder and a volatile liquid, forming a coating of ' the paste on an electrically insulating substrate and drying the coating, sintering the dried coating at a temperature of from 5400C to 8000C to form an adherent layer of photosensitive semiconductor on the substrate, providing adherent spaced electrode layers either on or below the semiconductor layer, and assembling the coated substrate with a light source.

The formation of the sintered semiconductor layer may be carried out in accordance with conventional sintering techniques which will be well known to those skilled in the art.

In order to obtain the desired k - 5 photosensitivity and electrical properties in the sintered semiconductor, the finely divided powder materials must be of the highest purity, for example of 99,999% or more, and the size of the powder particles is preferably less than 3 pm.

Suitable binders and volatile liquids for forming the paste will be known to those skilled in the art. It is generally preferred to use ethyl cellulose as the binder, but other suitable binders include, for example, linseed oil and cellulose acetate. Preferred volatile liquids are organic liquids; suitable organic liquids include, for example, turpineol, acetone and ethanol, of which the first is generally preferred. The binder is preferably present as a 10% by weight solution in the volatile liquid. The proportion of binder/volatile liquid solution in the paste will, of course, depend on the desired viscosity of the paste which will, in turn, depend on the method of coating the substrate with the paste which is used. For many purposes, a paste containing about 15% by weight of the binder/volatile liquid solution is suitable.

Suitable methods of forming a layer of the paste on the substrate include for example, screen printing, spraying, spinning and sedimentation.' Screen printing is a very precise method that produces a layer having a smooth, even surface and a thickness of 10 to 50 pm depending on the mesh of the screen; this method is generally preferred.

Suitable electrically insulating substrate materials are, for example, alumina ceramics, fused silica and "Pyrex" glass (Trade Mark). The coated substrate is then-dried to remove the volatile liquid, preferably by heating in an oven at a temperature of about 1000C for about 10 minutes.

The dried coating is then sintered in a substantially inert atmosphere at a temperature of from 1 k 6 5400C to 8000C. For this purpose the coated substrate is preferably placed in a container through which a stream of nitrogen (or other inert gas) containing a small proportion of oxygen, preferably added as air, can be passed and the container is placed in a suitable oven or furnace, preferably an electrical furnace. The proportion of air in the inert gas/air mixture is suitably 1 to 2% by volume. The inert gas/air mixture is passed through the container at a relatively slow rate, for example 2 to 6 litres per hour.

The furnace is gradually heated to the sintering temperature, for example over a period of 30 minutes, and is held at the sintering temperature for 45 to 150 minutes, preferably 45 to 120 minutes. The sintering temperature does not appear to be critical; if the temperature is much less than 5400C, the layer fails to sinter and if it is more than 8000C, the layer decomposes. Best results are obtained at sintering temperatures of from 5400C to 7000C.

The sintered layers thus obtained are firm, adherent and chemically stable. A reduction of thickness takes place during the sintering operation and the final thicknesses are generally in the range of 5 to 25 lim, depending on the initial thickness of the layer. There is also always a loss of weight during sintering and the final chemical composition of the semiconductor may vary somewhat as a function of the sintering temperature and as a function of the sintering time.

The sintered semiconductor layer is then provided with two spaced electrodes which can be made, for example, of silver, indium, or aluminium or of an epoxy res.in loaded with one of these metals. The electrodes may be formed on the semiconductor either (a) by evaporating the electrode metal on to the surface of the semiconductor, or (b) by screen printing the loaded epoxy resin on to the surface, or (c) by pressing a foil f of the metal on to the surface of the semiconductor.

Of these three methods, methods (a) and (b) are generally preferred. In both methods, the area between the electrodes is masked before applying the electrode material on to the surface of the semiconductor. Method (a) is usually followed by a short heat treatment which, when silver or aluminium is used, consists, for example, of heating the electrode layer in nitrogen at 3000C for 20 minutes. Method (b) requires that the epoxy resin is heated in air, for example, at 1500C for 35 minutes, in order to harden the layer.

Another method for providing the semiconductor with two spaced electrodes consists of forming the electrodes as layers on the electrically insulating substrate, by any of the methods described above, while masking the area between the electrodes and then forming the semiconductor layer over the electrodes and the area of substrate between them.

The electronic switch according to the invention may take a number of forms. For reasons of compactness it is preferred that the semiconductor layer and the substrate should have the form of a hollow cylinder. It is further preferred that in this 1 arrangement the semiconductor layer should be on the inside of the hollow cylinder and the light source should be located on the longitudinal axis of the cylinder.

A variety of light sources may be used. It i preferred to use one or more light emitting diodes (LEDs) or a glow discharge lamp or tube giving light having a wavelength of 500 to 900 nm. Suitable -LEDs include high efficiency red LEDs with a peak emission of about 660 nm visible red light and a viewing angle of 1400, and infra-red LEDs which emit a peak emission of 830 nm and have a viewing angle of more than 300.

8 - Suitable discharge lamps are, for example neon discharge lamps and tubes.

For the better understanding of the invention, preferred embodiments of semiconductor/substrate and of switch assemblies will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a plan view of a semiconductor/ substrate embodiment, Figure 2 is a section on line II-II of Figure 1, Figure 1, Figure 3 is a section on line III-III of Figure 4 is a cross-section of a first embodiment of a cylindrical electronic switch assembly, Figure 5 is a sectional elevation on line V-V of Figure 4, Figure 6 is a cross-section of a second embodiment of a cylindrical electronic switch assembly, and Figure 7 is a sectional elevation on line VII-VII of Figure 6.

Referring to Figures 1 to 3, a semiconductor/substrate assembly comprises a sintered photosensitive semiconductor layer 1 coated on a flat electrically insulating substrate 2 and spaced electrically conducting electrodes 3A and 3B. The semiconductor layer 1 has the Cd/Se/S/C1/Cu composition referred to above.

The electronic switch shown in Figures 4 and 5 comprises an adherent sintered photosensitive semiconductor layer 11 and an insulating s ubstrate 12 the form of a hollow cylinder, and two annular electrodes 13A and 13B at each end of the cylinder. switch further comprises a light source 14 which is a discharge tube giving light having a wavelength of 500 1 VA 9 - to 900 nanometers; the light source 14 is arranged along the axis of the cylinder 11/12. The semiconductor layer 11 has the Cd/Se/S/C1/Cu composition referred to above.

The electronic switch shown in Figures 6 and 7 comprises hollow cylindrical semiconductor layer 21 and insulating substrate 22 similar to the semiconductor layer 11 and the substrate 12 of the previous embodiment, but in this embodiment the electrodes take the form of two elongated strip electrodes, 23A and 23B arranged along diametrically opposed generators of the cylinder 21. The switch assembly further comprises six light emitting diodes 24 arranged along the axis of the cylinder 21/22. The semiconductor layer 21 has the Cd/Se/S/C1/Cu composition referred to above.

The operation of the electronic switches shown in Figures 4 and 5 and Figures 6 and 7 is as follows. A bias voltage is applied across the electrodes (13A-13B, 23A-23B); no current flows as long as the light source (14,24) is not illuminated and as soon as it is illuminated the current passes.

Electronic switches as illustrated can be made very compact. The properties of the semiconductors described are such that the switches can safely handle voltage pulses of up to 30 kV or more with a spacing between the electrodes of as little as 10 or 20 mm.

In order that the invention may be more fully understood, the following examples, in which all percentages are by weight unless otherwise stated, are given by way of illustration only. Example 1 A powder mixture of the following composition lithe first composition"):

cadmium selenide cadmium sulphide cadmium chloride copper (cupric) chloride 45% 45% 9.91% 0.09% - 10 1 was formed and then dry ground in a mechanical grinder, such as a microliser, to obtain a homogeneous mixture in which all particles were less than 3 um in size.

A 10% solution of ethyl cellulose in turpineol was mixed with the powder mixture, the amount of the solution being 15% of the combined weights of the solution and the powder mixture. Mixing was continued until a smooth paste was obtained.

The paste was applied to the surface of a high density 96% alumina ceramic substrate of the kind used for the production of thick film circuits, by silk screen printing. The substrate had the dimensions. 5cm x 3.5cm x 0.05 cm and the paste was printed to cover a rectangular area of 5cm x 3cm. A 305 mesh screen material was used which gave a layer of paste approximately 15 um in thickness.

The coated substrate was placed in an oven and heated to 1000C for 10 minutes to evaporate the turpineol.

A paste similar to that described above but having the following composition of inorganic components Ilthe second composition"):

cadmium selenide 35% cadmium sulphide 55% cadmium chloride 9.991/70 copper chloride 0.01% was silk screen-printed to form two strips, 3mm in with and 4.5 cm in length, which overlapped the long edges of the previously printed layer by 1.5 mm.

The purpose of these strips was to provide better electrical contact between the sintered semiconductor layer and the indium electrodes (see below); it has been found that compositions containing more than 45% cadmium sulphide and a relatively low proportion of copper give better electrical contact than compositions containing 45% or more of cadmium selenide and a higher proportion of copper.

0 The coated substrate was returned to the oven and heated to 1000C for 10 minutes to evaporate the turpineol from the printed strips.

The coated substrate was then placed in a Pyrex glass container having a top cover of the same material. Gas could be introduced into one end -of the container through a Pyrex glass tube and could be vented from the opposite end through a gap between the container and its top cover. This assembly was placed in an electrically heated furnace.

Prior to heating, pure nitrogen was first passed through the container at a flow rate of 4.5 litres per hour for a period of 20 minutes to purge out the bulk of the air that was initially present. The furnace was then heated to 5800C over a period of 50 minutes while continuing to pass pure nitrogen. Once a temperature of 5800C has been attained, a small proportion of air, that is 1% by volume, was added to the nitrogen and flow of this mixture was continued at 4.5 litres per hour. The temperature of 5800C was maintained for 60 minutes and at the end of this period, the furnace was turned off and allowed to cool over a period of 60 to 90 minutes. The flow of the nitrogen/air mixture is continued until the furnace had cooled to below ISOOC. Thereafter air could be blown through the furnace to accelerate cooling.

The sintered layer obtained was firm, adherent and chemically stable. There was an approximately 40"70 reduction in the thickness of the layer. Electron microscope studies of the surface of the sintered material showed that the latter consisted primarily of large grains of about 9 pm fused together at the grain boundaries.

The chemical composition of the sintered first composition was as follows: cadmium selenium sulphur chlorine 68. 4% 20.2% 11.06% 0.3% copper 0.04% Indium was then evaporated onto the margina portions of the long sides of the substrate to form electrodes. For this purpose a metal mask was positioned to cover all of the first sintered composition and the lmm wide marginal zones of the second composition on either side of the first composition. Indium was then evaporated using a conventional metal evaporation apparatus to coat the masked substrate. The mask was then carefully removed; the thickness of the indium coating was 0.5 r M. The coated substrate was placed in an oven and heated to 1600C for 15 minutes to cause fusion of the indium to the surface of the sintered second composition.

The final product accordingly had a band of the first composition about 27mm wide (between the long sides of the substrate), a strip of the second composition about lmm wide on either side of it, and an indium electrode strip about 3 mm wide on the othe,r side of the second composition and extending to the long edges of the substrate.

To form the switch, six light emitting diodes (LEDs) of the visible light type were mounted on a piece of printed circuit board measuring 5cm x 3. 5cm in two r.ows of three running across the width of the board. Each row was positioned with its centre at 1.75cm from each end and the LEDs w.ere arranged to be equally spaced along the length of the row. Each row of three LEDs was connected electrically in series and the two rows were connected in parallel to form a common drive connection. The array of LEDs was mounted over the - 13 1 surface of the semiconductor on the substrate with a spacing of 3mm from the surface to the tip of each LED. The assembly of the LEDs board and the coated substrate were mounted in a rigid plastics frame that electrically insulated the coated substrate from the LEDs. Electrical connections were made to the indium electrodes on each edge of the substrate. The whole assembly was then immersed in transformer oil to prevent electrical flashover from occurring across the surface of the semiconductor layer.

The LEDs used were high efficiency red diodes with a peak emission of 660 nm visible red light and a viewing angle of 1400. Their light output for a current of 20 ma was 200 mcd (0.2 candela); for pulsed current operation, the light output for 100 ma pulses was typically 1 candela at 250C.

The electrical properties of the switch were as follows: Dark Conditions:

The following properties were recorded with the switch immersed in transformer oil at a temperature of 250C and in total darkness.

For a DC voltage of 30 kV applied across the semiconductor contacts, a current of less than 2PA,was recorded; this corresponds to a switch dark resistance of more than 15000 mega ohms.

Electrical breakdown of the semiconductor occurred at a DC voltage of 3540 kV from tests carried out on several samples.

The switch could withstand high voltage pulses of the type generated by a petrol engine ignition coil to a magnitude of at least 35 kV and pulse repetition rate of 200 per second. Illuminated Conditions:

The following properties were recorded with the switch immersed in transformer oil at a temperature t of 250C and the following illumination conditions. Diode Forward Current, current pulses of 3 ms duration and magnitudes greater than 0.15, at pulse repetition frequency 50 per second. Time response Test A 500 volt DC bias was applied across the semiconductor layer contacts and the above forward current conditions applied to the diodes causing pulses of current to be generated in the 500 volt bias circuit. The characteristics of these pulses were as follows:

peak current recorded; time from minimum current to maximum current where zero time marks the beginning of the illumination pulse time for the current to fall to 1% of the peak value following termination of the diode current pulse estimated minimum resistance 18.5 mA 1.4 ms 1.7 ms 27,000 ohms Maximum Ignition Pulse Current Capability Test condition An ignition coil H.T. output was connected in series with the electrodes of the semiconductor layer so that the switch represents the only impedance to current in the circuit. The ignition coil was arranged to produce a pulse of ignition current at a point in time that coincided with the centre of the time duration of the LED forward current pulse, i. e. the ignition current pulse occurs at the point where the pulse operated switch is in a state of least resistance and conducts on every pulse.

The basic waveform of the current pulse produced by this circuit was that of a rise to peak 1 1 current in a period of 75 ps and then a progressive decay to zero current over a time period of 1.5 ms at a pulse repetition rate of 50 per second.

Peak currents are varied by changing the ignition coil primary current and to a greater extent by changing the type of ignition coil.

pulsed peak current that was sustained for a time period greater than two hours with no signs of damage to the switch 70 ma estimated energy of the ignition pulse power dissipated by the layer at 25 C pulsed peak current that caused damage the semiconductor layer after 10 mins estimated energy of the ignition pulse power dissipated by the layer of 25 C mJ 2.2 W ma 93 mJ 4. 65 W Example 2

A sintered semiconductor layer was formed on an alumina substrate as described in Example 1, but with the following variations. The inorganic materials composition was as follows:

cadmium selenide cadmium sulphide cadmium chloride copper chloride 35% 55% 9.95% 0.05% 1 Only this composition was used (i.e. a second inorganics composition was not used in this example).

The substrate was of the same material and of the same size as in Example 1, but the paste was screen printed to form a rectangle of 3cm x 2cm.

In the sintering stage, the percentage of air in the nitrogen/air mixture was 2% by volume and the sintering temperature was 5400C.

Indium was evaporated on to the sintered semiconductor to form contact layers running across the width of the substrate and each extending from a short edge of the substrate to overlap the semiconductor layer by 1.5 mm.

The finished coated substrate was, therefore, generally of the kind illustrated in Figures 1,2 and 3 and comprised a semiconductor layer approximately 2.0 cm in width and 2.7cm in length with indium contacts running across the width of the layer and overlapping the edge of the latter by a distance of 1.5 mm at each end.

This coated substrate was assembled with two neon glow discharge tubes as the light source. These were commercially available neon indicator lamps. The two neon tubes were positioned with the length of each tube arranged to pass across the width of the semiconductor and spaced so that the centre axis of the tubes is positioned 1 cm from the electrodes and 1 cm above the surface of the semiconductor. A small flat reflector was positioned above the neon tubes to increase the illumination of the surface.

Each of the neon tubes was connected in series to the other and the current waveform used to drive the neon tubes was the same as that in Example 1 except that the peak current was limited to 20 mA. Electrical Properties: Dark Conditions The following properties were recorded with this switch immersed in transformer oil at a temperature of 250C and in total darkness.

For a DC voltage of 30 Kv applied across the semiconductor layer contacts a current of less than 1FA was recorded, equivalent to a switch dark resistance more than 30,000 mega ohms.

Electrical breakdown of the semiconductor t 17 - material occurred at a DC voltage of 37 to 39 kV from tests conducted on several samples.

The switch could withstand high voltage pulses of the type generated by an ignition coil to a magnitude of at least 32 kV and pulse repetition rate of 200 per second. Illuminated Conditions Switch properties were recorded with the switch immersed in transformer oil at a temperature of 250C and illumination conditions as follows:

neon current, current pulses of 3 ms duration and magnitudes greater than 0.02, at pulse repetition frequency 50 per second. Time response Test A 500 volt DC bias applied across the semiconductor layer contacts and the above forward current conditions applied to the neon tubes to cause pulses of current to be generated in the 500 volt bias circuit. The characteristics of these pulses were as follows: peak current recorded; 12.5 mA time from minimum current to maximum current where zero time marks the beginning.of the illumination pulse time for the current to fall to 1% of the peak value following termination of the diode current pulse estimated minimum resistance Maximum Ignition Pulse Capability Test condition An ignition coil H.T. output was connected in series with the electrodes of the semiconductor layer so that the switch represents the only impedance to current in the circuit. The ignition coil was arranged 1 2 ms 3. 7 ms 40,000 ohms to produce a pulse of ignition current at a point in time that coincided with the centre of the time duration of the neon current pulse, i.e. the ignition current pulse occurs at the point where the pulse operated switch is in a state of least resistance and conducts on every pulse.

The basic waveform of the current pulse produced by this circuit was that of a rise to peak current in a period 75 ps and then a progressive decay to zero current over a time period of 1.5 ms at a pulse repetition rate of 50 per second.

pulsed peak current that was sustained for a time period greater than two hours with no signs of damage to the switch estimated energy of the ignition power dissipated by the layer at pulsed peak current that caused the photoconductive layer after C damage to 10 mins estimated energy of the ignition pulse power dissipated by the layer at 25 C 11 ma 54 mJ 2.7 W ma 96 mJ 4.80 W 1 k

Claims

CLAIMS:

1. A electronic switch comprising a photosensitive semiconductor and a light source which, when actuated, illuminates the semiconductor and causes the latter to become conductive, in which the photosensitive semiconductor is a sintered mixture comprising, by weight, 63 to 74% of cadmium, 16 to 24% of selenium, 8 to 14% of sulphur, 0.1 to 1% of chlorine, and 0.005 to 0.1% of copper.

2. A switch according to claim 1, in which the semiconductor is in the form of an adherent layer on an electrically insulating substrate.

3. A switch according to claim 2, which comprises two adherent, spaced electrode layers on the sintered semiconductor layer.

4. A switch according to claim 1, which comprises an electrically insulating substrate, two adherent, spaced electrode layers on the substrate, and an adherent layer of the sintered semiconductor comp6sition overlying the electrode layers and the substrate between them.

5. A switch according to claim 3 or 4, in which the electrode layers are formed of silver, indium or aluminium or of an epoxy resin loaded with silver, indium or aluminium.

6. A switch according to any of claims 2 to 5, in which the adherent semiconductor layer and the substrate have the form of a hollow cylinder.

1 -

7. A switch according to claim 6, in which the light source is located on the longitudinal axis of the cylinder.

8. A switch according to any of claims 1 to 7, in which the light source is one or more light emitting diodes.

9. A switch according to any of claims 1 to 7, in which the light source is a glow discharge lamp or tube giving light having a wavelength of 500 to 900 nanometers.

10. A method of making an electronic switch according to claim 1, which comprises forming a paste of a finely divided powder mixture comprising, by weight, 35 to 55% of cadmium selenide, 35 to 55% of cadmium sulphide, 5 to 15% of cadmium chloride, and 0.01 to 0.1% of copper chloride, a binder and a volatile liquid, forming a coating of the paste on an electrically insulating substrate and drying the coating, sintering the dried coating at a temperature of from 5400C to 8000C to form an adherent layer of photosensitive semiconductor on the substrate, providing adherent, spaced electrode layers either on or below the semiconductor layer, and assembling the coated substrate with a light source.

PablUhad 1990 at The Patent 0Mce. State House.66?1 Righ Holborn. London WC1R 4TP. Purther copies maybe obtainedfrom The Patent OfficeWes Branch, St Mary Cray. Orpington, Kent BR5 3RD. Printed by Multiplex technIques ltd. St Mary Cray. Kent, Con. 1.137