IL33887A - Automatic brightness control system - Google Patents

Description

Automatic br ghtness control system IH BR ATIOIAL SM RD ELECTRIC CORPORATION Gi 32184 A. W. Hoover- 1 Background of the Invention Field of the Invention This invention relates to image intensifier tubes and particularly to an improved system for controlling the brightness of a phos-phor display screen to extend the useful range of light which can be detected and amplified.

Description of the Prior Art A presently known image intensifier system utilizes an image intensifier tube, a voltage multiplier applying a direct accelerating voltage between the input photocathode and output phosphor screen and stepped voltages to two additional stages therebetween, an oscillatoi: supplying alternating voltages to the multiplier, and a direct current source, such as a battery, supplying current to the oscillator. A current limiting resistor is connected between the multiplier and a second stage anode lead of the tube. As light input and current through the tube increase, the voltage drop across the resistor increases to limit the accelerating voltage across the output intensifier stage and the resultant brightness of the display screen. This process continues at higher light intensities until there is no longer sufficient voltage across the output stage to accelerate electrons to an energy level which can excite the phosphor. At this point the output stage ceases to function and the display disappears.

The range of light intensities in which satisfactory operation occurs is considerably smaller than that desired in many applications and the effective use of the system is substantially reduced. Without such a limiting resistor, the output current and phosphor display brightness continually increase with input current and light over an extended range until saturation occurs. However, the display becomes excessively bright and is difficult to observe by eye, definition of objects becomes poor and, over an extended period, the high intensities A. W. Hoover-1 may cause gaseous emission from the phosphor which can damage the photocathode and limit tube life. Other brightness control systems have utilized external photodetectors and servo controlled iris arrangements which are more complex, bulky and slow acting.

Summary of the Invention It is therefore the primary object of the present invention to provide an improved efficient system for automatically controlling the brightness of an image intensifier display while having the capability of operating over an extended dynamic range of light levels.

This effect is achieved by utilizing a current limiting device connected between the direct current source and the oscillator supplying the alternating voltage to the multiplier which is connected to the image intensifier tube. As light intensity on the input photo-cathode and corresponding brightness and output current of the phosphor display screen increase, the current demand of the intensifier stages also increases. This is reflected to the oscillator and direct current source which supplies the increased current, with the current limiter having no effect at lower levels. However, upon reaching a predetermined level, the current limiter becomes effective to change the impedance relations of the circuit and supply a relatively constant current to the oscillator which in turn limits the voltage applied to the multiplier and intensifier. A maximum brightness level is then maintained which is readily observable until the voltage becomes insufficient to excite the phosphor. The image intensifier is thus usable over a much wider range of input light levels. The details of the invention will be more fully understood and other objects and advantages will become apparent from the following description and accompanying drawings.

Brief Description of the Drawings Fig» 1 is a schematic representation of the novel system, and A. W. Hoover-1 Fig. 2 is a plot showing relative response characteristics of the intensifier with and without the prior art and present limiters.

Description of the Preferred Embodiment As shown in Fig. 1, a three stage image intensifier tube 10 in-eludes a curved photocathode 12 on a fiber optic plate 13 at one end. The photocathode, which may be at ground potential, emits electrons in accordance with the intensity of light projected thereon. The light input may be in the form of visible or invisible radiations from a discrete source of light or an image of a paticular scene. The elec-trons are focused onto a facing curved phosphor coated anode 14 by an anode cone 16 of the first stage of the image intensifier. The phosphor coating forms a screen on one side of an interstage fiber optic plate 18 which couples the phosphorescent light to the photocathode coating 20 on the other side of plate 18 which is the input electrode of the second stage of the image intensifier. A similar fiber optic plate having a phosphor anode and photocathode coatings respectively on opposite sides thereof couples light from the second to third stages, with like anode cones focusing electrons onto the second stage anode and third stage phosphor coated anode 22, the latter being the output display screen.

In the actual unit, each stage is formed separately, with the cathode and anode on the concave curved portions of the fiber optic plates at opposite ends. The interstage optic coupling plate 18 is formed of two portions positioned back to back and sealed together. A similar type of cascaded image tube utilizing intermediate fiber optic partitions with phosphor and photoemissive coatings on opposite sides is described in U.S. Patent No. 3,321,658 issued May 23, 1967.

Stepped accelerating and focusing voltages are supplied to respective anode and cathode electrodes between each stage by a voltage multiplier network 24 of a known configuration. Typical direct A. . Hoover- 1 voltages for the tube may be 8 to 14 kllovolts on the first stage anode and second stage photocathode which are connected together to the same voltage multiplier tap, 26-30 Kv on the second anode and third photo-cathode and 39-42 Kv on the output anode screen. Resistor R, connected in series with the second stage anode and third stage cathode and to the voltage multiplier, represents the current limiting resistor as utilized in the prior art system.

An oscillator 26 of a known type provides an alternating voltage, which raay for example be about 2800 volts peak to peak, to the voltage multiplier which in turn converts and steps this voltage up to the direct voltages applied to the tube. The oscillator is connected to a source of direct current 28 which may be a battery, of for example 6.75 volts DC, with an intermediate current limiting circuit 30 connected between the battery and oscillator. During operation, as the light projected onto the input photocathode 12 increases, electron emission and current through the tube increase correspondingly in a relatively linear manner at low light levels. The light input-output response characteristics for various conditions are shown in Fig. 2. The increased current demand is reflected to the oscillator which effectively has a reduced input impedance and the additional current is furnished by the direct current source throught the current limiter.

The current limiter operates in a known manner with current normally flowing from the positive terminal of the battery through a unidirectional conducting diode 32 and an emitter resistor 34, through the emitter-collector path of transistor 36 connected in a common base configuration and to the oscillator. A small steady current also flows through the emitter-base path of the transistor, with a small voltage drop across these two electrodes maintaining the transistor in a forward conducting state. Another current path through a zener diode 38 provides a constant reference voltage which determines the point at A. W. Hoover- 1 which the limiting action occurs. A typical value for this voltage may be 2.7 volts. When the voltage drop across the emitter resistor 34 and the base-emitter resistance exceeds the reference voltage, limiting action occurs wherein the impedance of the transistor in-creases, with a larger voltage drop developed thereacross and a lesser voltage appearing across the load and the decreasing oscillator impedance. The series impedances thus tend to compensate and the net effect is that the battery looks into a relatively constant impedance which is supplied with a constant current. A base resistor 39 completes the circuit to the other terminal of the battery which also connects to the other terminal of the oscillator. A large bypass capacitor 40 of about 10 microfarads across the output terminals of the limiter provides required low impedance and improved operation for the oscillator.

As the light level and current demand continue to increase, a greater portion of the battery voltage is dropped across the current limiter and a smaller voltage appears at the oscillator terminals. This in turn results in a smaller voltage being supplied to the stages of the image intensifier, with the process continuing until there is insufficient voltage available to excite the phosphor display screen of the tube. At this point the image is lost and the intensifier blanks out. The result of the current limiter action is that the image intensifier is usable over a much wider range of input light levels while maintaining a reasonable level for observation on the output display screen after amplification. This is shown in Fig. 2 wherein the present image intensifier with automatic brightness control, indicated as curve A, ceases to function at an input light level of approximately 10 foot candles, as plotted on a logarithmic scale, while the prior intensifier utilizing a current limiting resistor, indicated as curve B, ceases to function at an input light level of A. W. Hoover- 1 -3 about 2 to 5 x 10 foot candles. In addition, the peak output level of the present novel system is held to a reasonable limit of about 160 foot lamberts as compared to a peak value of 640 foot lamberts for the intensifier without any limiting device, as shown in curve C. It is thus apparent that the present invention provides a novel image intensifier automatic brightness control system which is capable of substantially improved operation over an extended range of light levels.

Claims (7)

A. W. Hoover-1 WHAT IS CLAIMED IS:

1. An image brightness control system comprising: an image intensifier tube having a photocathode input electrode emitting electrons in response to the intensity of light applied thereto and a phosphor display screen output electrode providing an image display brightness and an output current in accordance with electron impingement thereon, a voltage multiplier applying a direct accelerating voltage between said input and output electrodes, an oscillator circuit applying an alternating voltage to said voltage multiplier, means supplying direct current to said oscillator in accordance with said output current, and direct current limiting means connected between said direct current supply means and said oscillator to supply a relatively constant current to said oscillator above a predetermined light intensity and output current level.

2. The system of Claim 1 wherein the input light intensity and output current and display brightness response characteristics are relatively linear in the range below said predetermined level.

3. The system of Claim 1 wherein said tube includes three cascaded image intensifier stages and stepped connections between said voltage multiplier and said stages, each stage including a photocathode input electrode and phosphor screen output electrode, and first and second fiber optic means coupling light from the first and second stage phosphor screens to the second and third stage photocathodes respectively, each said fiber optic means including a phosphor screen and photocathode coatings on opposite sides thereof, each of said coatings on o osite sides havin a common ste ed connection to said multiplier. A. W. Hoover- 1

4. The system of Claim 2 wherein the input impedance of said oscillator decreases with increasing current therethrough and the input impedance of said limiter above said predetermined level increases with increasing current therethrough to provide a relatively constant total impedance above said level.

5. The system of Claim 3 including a focusing anode extending from each phosphor screen toward each photocathode .

6. The system of Claim 4 wherein said limiting means includes a common base connected transistor having an emitter electrode and emitter resistor connected in series with one terminal of said direct current supply means and a collector electrode connected to a terminal of said oscillator, a zener diode connected between said one terminal and the base electrode to establish a reference voltage for said predetermined level, and a resistor having one end connected to said base electrode, the other terminal of said direct current supply means being connected to the other end of said resistor and the other terminal of said oscillator.

7. The system of Claim 6 including a by-pass capacitor connected across said current limiter providing a low impedance for said oscillator. EG: cm January 27, 1969 For DR.