US3454830A - Control system for electroluminescent image-retaining panel - Google Patents

Description

July 8, 1969 V P. w. RANBY ET AL 3,454,830

CONTROL SYSTEM FOR ELECTROLUMINESCENT IMAGE-RETAINING PANEL Filed Nov. 2, 1964 Sheet of 2 Li l w PETER W HlT TEN RANBY INVENTORS BY W Kan- ATTORNEY MALCOLM OWEN NORRIS y 8, 1969 P. w. RANBY ET AL 3,454,830

CONTROL SYSTEM FOR ELECTROLUMINESCENT IMAGE-RETAINING PANEL Filed Nov. 2, 1964 Sheet 2 of 2 /RP W PETER WHITTE'N iRANBY 'INVENTORS MALCOLM OWEN NORRIS M mm.

ATTORNEY United States Patent "cc 3,454,830 CONTROL SYSTEM FOR ELECTROLUMINESCENT IMAGE-RETAINING PANEL Peter Whitten Ranby and Malcolm Owen Norris, London, England, assignors to Thorn Electrical Industries Limited, London, England, a British company Filed Nov. 2, 1964, Ser. No. 408,253 Claims priority, application Great Britain, Nov. 18, 1963, 45,505/ 63 Int. Cl. G05f 1/10; H05b 37/02, 39/04 US. Cl. 315-311 6 Claims ABSTRACT OF THE DISCLOSURE An image-retaining electroluminescent device having electrodes with a phosphor embedded in a ceramic di electric therebetween is found to work more ellectively with a higher voltage in the dark, that is, with no image, than is required to retain the image afterward. As a consequence, a high dark voltage can be used if voltage after irradiation is reduced to prevent deterioration of the device. The higher dark voltage secures quicker response, but must be below the voltage necessary to produce illumination in the absence of irradiation. Means is provided to allow a high dark voltage, with a reduction to a lower irradiated voltage. The means can be a fixed resistance, a resistance with a positive temperature coefiicient, or one or more electron tubes, such as pentodes. The latter is the most elfective means.

The present invention relates to electroluminescent devices and constitutes an improvement in or modification of the invention claimed in British Patent N0. 966,730.

In the earlier application referred to there is claimed an electroluminescent device comprising a layer of phosphor embedded in a ceramic material disposed between two electrodes at least one of which is transparent, and means for maintaining a unidirectional potential difference between the electrodes, the nature of the phosphor and ceramic layer and the magnitude of the unidirectional potential difference being such that when the unidirectional potential difference is applied between the electrodes the layer emits substantially no light, but upon excitation of the layer by radiation or cathode rays or high energy nuclear particles which cause it to emit light, it continues to emit light for a substantial time while the said potential difference is maintained.

Devices such as are set out in the preceding paragraph usualy consist of a metal plate, preferably of iron or iron-containing metal, to which have been applied one or more vitreous enamel coatings, one of which incorporates a luminescent material or phosphor and an electrically conducting film which is substantially transparent to light and insulated from the metal base plate.

The devices with which the present invention is concerned may be constructed in any of the ways described in the earlier specification referred to.

The essential feature of devices such as have been referred to above is that if a DC. potential difiFerence be applied between the electrodes in the dark, no light is emitted from the panel, but if the panel is exposed to light or to a wide range of other electromagnetic radiations invluding X-rays and the emission from certain lasers, or to cathode rays or to high energy nuclear par- 3,454,830- Patented July 8, 1969 ticles, then after the exposure and so long as the direct potential difierence is maintained between the electrodes, the panel emits light over that portion of its surface which has been exposed to the radiation or other bombardment above mentioned, and this emission of light continues for a substantial time, for instance 20 to 30 minutes or even up to an hour or more. However, if the potential difference is removed, the emission of light from the surface is discontinued and when a direct potential difference is again applied between the electrodes, again no light is emitted until the phosphor layer has been exposed once more to some form of radiation or other bombardment as above mentioned.

Panels of this kind are useful for a number of purposes, for example for storing light patterns or messages, for use with optical lasers in setting up the option of the system, for obtaining X-ray pictures which can be studied after the X-ray source has been switched off, for recording electron traces as in certain recording cathode ray tube systems, and so on.

With devices of the kind above discussed, which will be referred to hereinafter for convenience as I.R.P.s (image retaining panels), it is found to be important that the polarity of the applied direct potential difference should be such that the iron or iron-containing base plate should be negative relative to the transparent electrode. Reversal of the applied polarity can be useful for quickly erasing an image retained on the panel, but the potential must be of the correct polarity when the panel is used to retain an image or pattern.

It has been found that when I.R.P.s are connected to a source of direct potential difference in the dark, 2. small current (called the dark current) flows between the electrodes. This current increases in magnitude when the panel is exposed to exciting radiation and gradually builds up to a maximum. After exposure, the panel continues to emit light where it has been exposed but the current between the electrodes gradually decreases in magnitude.

The brightness of the light emitted from an I.R.P. follows a similar curve to the current and the current taken by the panel can therefore be used as a convenient means of measuring the behaviour of the panel. For example the brightness of the glowing image retained on the surface of an I.R.P. depends on the current, so that if the voltage applied to the panel is increased then the current taken by the panel is increased and so the brightness of the glowing image is increased. However, there is a limit to the voltage which can be applied to the panel without causing electrical breakdown. Moreover the maximum voltage which can be applied to the panel when not excited by radiation (and consequently in a condition of low conductivity) is different from the maximum voltage which can he applied without causing breakdown when the panel is irradiated and therefore in a more highly conducting state.

We have discovered that there are considerable advantages in operating these panels in such a manner that the applied direct voltage can be varied. For example if an I.R.P. be operated at a given voltage during the excitation by radiation in order to obtain a visible image, then after the source of excitation is removed the applied voltage can advantageously be increased so that the brightness of the image is increased. If the panel had been operated at the higher voltage while it was being irradiated, the higher conductivity at this stage might have caused either overheating or electrical breakdown.

Furthermore we have found that the unexcited (that is the unirradiated) panel can have a higher direct voltage applied to it than that normally applied for satisfactory operation during excitation or when emitting light. Now the speed of response of the panel to radiation, i.e. the sensitivity of the panel, is greater at the higher applied voltage, but if this voltage were to be applied continuously after excitation then the panel might overheat or be overloaded. It is important to make use of this property in order to increase the speed of response of these panels, and one of the objects of the invention is to provide means whereby this can be done.

According to the present invention there is provided an electroluminescent device comprising a layer of phosphor embedded in a ceramic material disposed between two electrodes at least one of which is transparent, and means for maintaining a unidirectional potential difference between the electrodes, the nature of the phosphor and ceramic layer and the magnitude of the unidirectional potential difference being such that when the unidirectional potential difference is applied between the electrodes the layer emits substantially no light, but upon excitation of the layer by radiation or cathode rays or high energy nuclear particles which cause it to emit light, it continues to emit light for a substantial time while the said potential difference is maintained, wherein means are provided for varying the said unidirectional potential difference as a function of the reciprocal of the current flowing between said electrodes.

Further according to the present invention there is provided an electroluminescent device comprising a layer of phosphor embedded in a ceramic material disposed between two electrodes at least one of which is transparent, and means for maintaining a unidirectional potential difference between the electrodes, the nature of the phosphor and ceramic layer and the magnitude of the unidirectional potential difference being such that when the unidirectional potential difference is applied between the electrodes the layer emits substantially no light, but upon excitation of the layer by radiation or cathode rays or high energy nuclear particles which cause it to emit light, it continues to emit light for a substantial time while the said potential difference is maintained, wherein the circuit between the electrodes including the said means for maintaining a unidirectional potential difference between the electrodes includes series-connected resistive means.

The invention will be described by way of example with reference to the accompanying drawings, which show circuit diagrams of a number of different embodiments of the invention.

Referring to FIG. 1, there is shown a circuit including an I.R.P., a source of unidirectional potential difference in the form of a battery B and a series resistor R. When the panel is excited and when thus the current increases, an increasing proportion of the applied voltage from B is dropped across the resistor R. By proper selection of the value of the resistor R the variation in the voltage across the I.R.P. can be made suitable.

As shown in FIG. 2 the resistor R of FIG. 1 may be replaced by a device such as a filament lamp L whose resistance increases with the current therethrough. The filament of a normal incandescent electric lamp has a resistance which increases many times as its current rises from zero to its normal working value.

A pentode or a beam power tetrode valve has a characteristic relating its anode current to its anode voltage whose slope approximates closely to that required, and such a valve can be incorporated as shown at V with an I.R.P. as indicated in FIG. 3. The cathode heater terminals are indicated at H--H. This circuit has the ad vantage over one containing a resistor that the value of the limiting current can be adjusted by adjusting the control grid voltage using for example a potential divider P in parallel with a source of direct potential difference B Even better control can be obtained as shown in FIG. 4 by using two pentodes V and V (or two beam power tetrodes or a combination of the two). Suitable connections for these valves are indicated in FIG. 4 and do not require further explanation. The two valves may of course be incorporated within a single envelope.

In FIG. 5 there is shown a circuit arrangement including a triode V connected in series with a pentode V (01' a beam power tetrode) and in this way both the voltage and the current are readily controlled. In this case also the valves can of course be combined in a single envelope.

As indicated in FIG. 6 the resistor R of FIG. 1 may be arranged to be normally short-circuited by means of a fuse F, this fuse being of such value that it will blow when the panel is in its maximum conducting state.

As indicated in FIG. 7 the fuse may be replaced by a relay S which may be arranged to be manually operated by means of a. switch SW. Alternatively as shown in FIG. 8 the relay S may be arranged to be operated automatically with the aid of a triode valve V The triode V may be replaced by a thyratron valve.

Instead of using batteries to provide the control grid voltage on the valve in FIG. 3 a variable resistor R may be connected in series with the cathode, as shown in FIG. 9. Circuits similar to those of FIGS. 4 and 5 but incorporating this series cathode resistor are shown in FIGS. 10 and 11 respectively.

It will be understood that only some of the possible ways by which the desired characteristics can be obtained have been described. Equivalent results can be obtained by using other types of resistors or circuits equivalent to resistors.

We claim:

1. An electroluminescent device comprising a layer including phosphor material embedded in ceramic material, an electrode on each side of said layer, means for maintaining a unidirectional potential difference between said electrodes, the nature of the phosphor and ceramic layer and the magnitude of the unidirectional potential difference being such that when the unidirectional potential difference is applied between the electrodes the layer emits substantially no light, but upon excitation of the layer by radiation or cathode rays or high energy nuclear particles which cause it to emit light it continues to emit light for a substantial time while the said potential difference is maintained, and means for varying said unidirectional potential difference as a function of the reciprocal of the current flowing between said electrodes.

2. An electroluminescent device comprising a layer including phosphor material embedded in ceramic material, an electrode on each side of said layer, means for maintaining a unidirectional potential difference between said electrodes, the nature of the phosphor and ceramic layer and the magnitude of the unidirectional potential difference being such that when the unidirectional potential difference is applied between the electrodes the layer emits substantially no light, but upon excitation of the layer by radiation or cathode rays or high energy nuclear particles which cause it to emit light it continues to emit light for a substantial time while the said potential difference is maintained, a circuit connecting said electrodes, and a resistive means connected in said circuit in series with said means for maintaining the unidirectional potental difference.

3. A device according to claim 2, wherein said resistive means includes the anode-cathode path of an electron discharge tube.

4. A device according to claim 2, comprising manually operable means for varying the resistance of said resistive means.

5. A device according to claim 2, wherein said resistive means comprises current-sensitive resistive means, the resistance of said resistive means increasing with increase in the current therethrough.

6. An electroluminescent device comprising a layer including phosphor material embedded in ceramic material, an electrode on each side of said layer, means for maintaining a unidirectional potential dilference between said electrodes, the nature of the phosphor and ceramic layer and the magnitude of the unidirectional potential ditference being such that when the unidirectional potential difference is applied between the electrodes the layer emits substantially no light, but upon excitation of the layer by radiation or cathode rays or high energy nuclear particles which cause it to emit light it continues to emit light for a substantial time while the said potential difference is maintained, a circuit connecting said electrodes, a resistive means connected in said circuit in series with said means means responsive to current flowing between said electrodes to vary automatically the resistance of said resistive means.

References Cited UNITED STATES PATENTS 2,985,795 5/1961 Bird 3l5307 X 3,215,847 11/1965 Ranby et al. 3l5-l50 2,149,080 2/1939 Wollf 32322 10 JAMES W. LAWRENCE, Primary Examiner.

PALMER C. DEMEO, Assistant Examiner.

US. Cl. X.R.

for maintaining the unidirectional potential difference, and 15 313-408; 3 l5-150, 307

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