Patent Description:
Night vision equipment is used for many industrial and military applications. For example, such equipment may be used for enhancing the night vision of aviators, for photographing astronomical bodies and for providing night vision to soldiers or sufferers of retinitis pigmentosa (night blindness). The equipment often incorporates an image intensifier that is used to amplify low intensity light or convert non-visible light into readily viewable images. One such image intensifier is an image intensifier tube.

An image intensifier tube typically includes a photocathode with for example, a gallium arsenide (GaAs) active layer and a microchannel plate (MCP) positioned within a vacuum housing. Visible and infrared energy, for example, may impinge upon the photocathode and be absorbed in the cathode active layer, thereby resulting in generation of electron/hole pairs. The generated electrons are then emitted into the vacuum cavity and amplified by the MCP.

More specifically, when electrons exit the photocathode, the electrons are accelerated toward an input surface of the MCP by a difference in potential between the input surface of the MCP and the photocathode of approximately <NUM> to <NUM> volts depending on the MCP to cathode spacing and MCP configuration (filmed or un-filmed). As the electrons bombard the input surface of the MCP, secondary electrons are generated within the MCP. That is, the MCP may generate several hundred electrons for each electron entering the input surface. The MCP is also subjected to a difference in potential between its input surface and its output surface that is typically about <NUM>-<NUM> volts. This potential difference enables electron multiplication in the MCP.

As the multiplied electrons exit the MCP, the electrons are accelerated through the vacuum cavity toward a phosphor screen (or other anode surface) by yet another difference in potential between the phosphor screen and the output surface of the MCP. This latter potential may be on the order of approximately <NUM> - <NUM> volts.

A power supply is generally used to generate and provide the various potential differences noted above and to further provide control voltages for various components of the image intensifier tube. The power supply and intensifier tube are expected to operate under a variety of lighting conditions, including, e.g., relatively low light, relatively high light, and bright flashes. Configuring and controlling a power supply to handle all these conditions can be challenging.

A power supply according to the present invention is defined in claim <NUM>. Described herein are methods for mitigating the effects on light output from night vison equipment in the presence of a bright flash of light. In one embodiment, a method includes enabling an automatic brightness control procedure for a light intensifier having a photocathode, a microchannel plate, and an anode having a phosphor layer, the automatic brightness control procedure selecting a voltage value to be applied to the photocathode in response to light input. The method further includes sensing current being drawn by an element of the image intensifier, and when the current being drawn by the element of the image intensifier exceeds a predetermined threshold, shutting down the photocathode, disabling the automatic brightness control procedure, and storing the voltage value selected by the automatic brightness control procedure when the current exceeded the predetermined threshold. After a first predetermined period of time, the method includes applying a voltage to the photocathode in accordance with the stored voltage value, re-enabling the automatic brightness control procedure and causing the automatic brightness control procedure to select the stored voltage value as the voltage to be applied to the photocathode.

With such an approach, the automatic brightness control procedure can more quickly recover from a flash of light. The instant embodiments are particularly useful in the context of muzzle flashes from a firearm that may last no more than <NUM>-<NUM>, but might nevertheless detrimentally impact night vision equipment for, perhaps, hundreds of milliseconds. Embodiments of the invention enable the night vision equipment to recover in about <NUM>.

Like reference numerals have been used to identify like elements throughout this disclosure.

<FIG> illustrates a block diagram of a digital power supply and associated image intensifier tube in accordance with an embodiment of the present invention. Specifically, <FIG> depicts an image intensifier tube <NUM> that is powered and controlled by a digital power supply <NUM>. Intensifier tube <NUM> includes a photocathode <NUM>, a microchannel plate (MCP) <NUM> and an anode <NUM> that includes a phosphor layer <NUM>.

Digital power supply (or simply "power supply") <NUM> includes a battery <NUM>, or other energy source, that supplies power to be used by the power supply <NUM> and that is delivered to the intensifier tube <NUM>. The power supply <NUM> further includes a central processing unit (CPU) <NUM> and memory <NUM>, which stores, among other things, control logic <NUM> and state variables <NUM> (discussed further below). Battery <NUM> supplies power for each of the control voltages V1, V2, and V3, which are respectively applied to components of the intensifier tube <NUM>. The values of these control voltages may be set by CPU <NUM> in accordance with instructions received from control logic <NUM>.

In one possible implementation, CPU <NUM> controls circuitry controls the application of voltages V1, V2, V3 to the photocathode <NUM>, MCP <NUM> and anode <NUM>, respectively. An operational amplifier <NUM> is configured to sense current I3 flowing in anode <NUM>. Current I3 is representative of the brightness of the light <NUM> being received at photocathode <NUM> only where V1 and V2 are not being modified to control the output brightness of the phosphor screen. A value of current I3 can be used by control logic <NUM> and CPU <NUM> to, for example, adjust the value of V1 or V2 (e.g., higher V1 or V2 for higher brightness, and lower V1 or V2 for lower brightness).

<FIG> is a circuit diagram of a switch configuration <NUM> that may be used to control the application of a voltage to the photocathode <NUM> of the intensifier tube <NUM> in accordance with an embodiment of the present invention. One advantage of using a digital power supply <NUM> is the ability not only to switch various voltages on or off , but also to manipulate the waveform(s) of, e.g., the photocathode voltage V1 and/or other control voltages. In this regard, <FIG> depicts the connection of the photocathode <NUM> to the V1 supply voltage. As shown, the photocathode <NUM> connection is placed between two high voltage transistors <NUM>, <NUM> which can isolate the photocathode <NUM> from the two control voltages. In one possible implementation, presented here, the off state of the photocathode <NUM> is the MCP voltage V2 minus an offset (e.g., <NUM> volts) to ensure the photocathode <NUM> experiences a hard reset or reverse bias state.

In operation of the switch configuration <NUM> of <FIG>, both gate drives (gate drive <NUM>, gate drive <NUM>) are controlled such that they are not on at the same time, otherwise the photocathode supply voltage V1 would be shorted to the MCP supply voltage V2. The circuit allows the photocathode <NUM> to be supplied with a gated photocathode voltage V1' that is set to the supply cathode voltage V1 by turning on gate drive <NUM>. As long as transistor <NUM> is on, the photocathode voltage is fixed. If gate drive <NUM> is off, the gated photocathode voltage V1' floats. The cycling of the gate drive <NUM> signal to transistor <NUM> may be referred to as the "update frequency" or "re-fresh rate" of the intensifier tube <NUM>. An update frequency parameter or re-fresh rate parameter may be stored as one of the state variables <NUM> and used by CPU <NUM> to operate the intensifier tube <NUM>. Opening gate drive <NUM> pulls the gated photocathode voltage V1' to V2 - 15V, or reverse biases the photocathode <NUM>. This stops any photocathode current from reaching the MCP <NUM>, effectively shutting off an output of the intensifier tube <NUM>.

As noted, an image intensifier and associated power supply that applies the several control voltages are expected to operate under a broad range of conditions, including bright flashes in a dark scene. As further noted, the intensifier tube <NUM> applies gain via the MCP <NUM> and corresponding relatively high V2 in low light scenes. A bright flash from, e.g., a muzzle of a firearm, when such gain is applied, can overwhelm, i.e., saturate, the anode current sense operational amplifier <NUM> causing the intensifier scene to go dark (i.e., the control voltages may be turned down/off in response) until the operational amplifier <NUM> comes out of saturation, and the control algorithm can regain control. During this potentially "dark" time, the intensifier tube <NUM> is either at peak output brightness or is totally shutoff, in an attempt to protect itself. Either state leaves the user of the night vision equipment at a disadvantage.

Once the operational amplifier <NUM> comes out of saturation, in one embodiment, the control circuitry, e.g., in the form of an "automatic brightness control" procedure, takes a finite amount of time to adjust the MCP voltage V2, photocathode voltage V1, and the photocathode gating duty factor (or update frequency or modulation mode), to bring the intensifier gain and output brightness back into a controlled state. This may take a period of time on the order of <NUM> to <NUM>. For example, the MCP <NUM> may take hundreds of milliseconds to respond to a change in its supplied voltage V2.

A common situation with time frames and brightness levels which send the operational amplifier <NUM> into saturation is the firing of a <NUM> caliber machine gun where the muzzle flash, lasting only <NUM>-<NUM>, spaced approximately <NUM> apart, overwhelms the circuitry of the device. In such a case, the user must pause from firing to allow the night vision equipment to recover, and then again view the scene.

Embodiments of the present invention address this issue by leveraging the speed of the digitally controlled power supply <NUM> to decrease the flash response time of the intensifier tube to less than about <NUM>.

In an embodiment of the invention, once a flash (or any bright light) occurs that saturates the anode current (I3) sense operational amplifier <NUM>, control logic <NUM> is configured to freeze or separately store the previously "in control state variables" (e.g., V1, V2, V3, and/or update frequency/re-fresh rate) as part of state variables <NUM>.

Once the state variables are frozen or separately stored, the photocathode voltage V1 is immediately turned off using, e.g., the switching configuration <NUM> shown in <FIG>, under the control of CPU <NUM>. This suppresses the effects of the flash.

The automatic brightness control procedure is also disabled at this time, for a period of time, such that the control voltages are not further altered. Without such a step, all of the control parameters would be pushed to their extreme values in an attempt to dim the intensifier tube in response to the bright light.

After a short time period, e.g., on the order of <NUM>-<NUM> (which may be referred to as the "shutter pulse duration"), the photocathode <NUM> is turned back on by applying its previously known "in control state," i.e., the most recent voltage V1, and other state variables <NUM> stored/frozen at the time of the detected bright light/flash. This allows the photocathode <NUM> to again start being responsive to the light conditions in the scene. However, the control logic <NUM> still does not act on the output of operational amplifier <NUM> for a total of about <NUM> (referred to as the "shutter flash delay") as the level of anode current I3, as a result of a flash, causes the operational amplifier <NUM> to still be saturated for that length of time, and as such, the output of operational amplifier <NUM> may not reliably represent the current light conditions. Under a muzzle flash scenario, the overall scene, after the <NUM>-<NUM> delay, should again be dark and the prior state (stored/frozen) state variables <NUM> should be applicable, and consequently, are used again as soon as the automatic brightness control procedure is allowed to restart. As noted, the automatic brightness control procedure may be re-enabled after a total delay of about <NUM> inclusive of the <NUM>-<NUM> shutter pulse duration, a time period that allows the I3 current to decay and the operational amplifier <NUM> to come out of saturation.

If the operational amplifier <NUM> is still in saturation after the shutter flash delay of <NUM>, this suggests that the overall scene brightness has changed and the automatic brightness control procedure should be allowed to adjust the control voltages accordingly, without necessarily using the stored state variables <NUM>.

<FIG> is a state diagram depicting a series of operations for mitigating the effects of a bright flash in accordance with an embodiment of the present invention. At <NUM>, an automatic brightness control (ABC) procedure operates to maintain an appropriate level of brightness for a user of the night vison equipment. The ABC may be operating as part of, e.g., control logic <NUM> in combination with CPU <NUM> (i.e., digital control), or may function as an analog process, or a combination thereof. The ABC may be considered a type of automatic gain control, which may operate, e.g., linearly from extremely low light conditions to some threshold level of light <NUM> (such that, e.g., a <NUM>% increase in input light results in a <NUM>% increase in brightness of the phosphor layer <NUM> of the anode <NUM>), and beyond that threshold of light, as a governor that maintains a predetermined level of brightness from the phosphor layer regardless of the input light level. As will be appreciated by those skilled in the art, the embodiments described herein provide a particular reaction to a particular kind of light event or condition, namely a flash of light, which cannot normally be handled quickly enough by the ABC. For instance, the ABC may control the voltage to the MCP <NUM>, but even if the voltage to the MCP <NUM> were quickly turned off, it may take on the order of hundreds of milliseconds for the MCP <NUM> to react in the manner desired to reduce the output brightness of the intensifier tube <NUM>.

As such, if at <NUM>, excessive (above a predetermined threshold) screen current (i.e., anode current I3) is detected by control logic <NUM>, the state of the process proceeds to <NUM>. At <NUM>, control logic <NUM> shuts down the photocathode by turning off its control voltage V1, stops the operation of the ABC (to avoid the control voltages being potentially incorrectly adjusted in response to the light event), and freezes or stores the then-current control voltages and any photocathode re-fresh rate or update frequency parameters.

At <NUM>, after a predetermined period of time (the shutter pulse delay) of e.g., <NUM>-<NUM>, the state of the process proceeds to <NUM>, where the control logic <NUM> and CPU <NUM> turn on the photocathode by reapplying the stored control voltage and re-fresh rate.

The process is then delayed, at <NUM>, by a second predetermined period of time (the shutter flash delay), and at <NUM>, the ABC is turned back on. If it was determined at <NUM>, or during the shutter flash delay of <NUM>, that excessive current is not being drawn, this is indicative that the light event was just a flash, and the ABC is re-enabled using the stored values previously used. On the other hand, if at <NUM>, or during the shutter flash delay of <NUM>, it was determined that excessive current was being drawn, this is indicative that the light event was not limited to a flash, but might, in fact, be an overall light level change. In this scenario, the ABC is re-enabled, but permitted to select control voltages autonomously. From <NUM>, the process proceeds back to <NUM> where the intensifier tube operates under normal conditions.

<FIG> is flow chart depicting a series of operations for mitigating the effects of a bright flash in accordance with an embodiment of the present invention. At <NUM>, an operation includes enabling an automatic brightness control procedure for an image intensifier tube having a photocathode, a microchannel plate, and an anode having a phosphor layer, the automatic brightness control procedure automatically selecting a voltage to be applied to the photocathode responsive to light input to the photocathode. At <NUM>, an operation is configured to sense current being drawn by an element of the image intensifier. At <NUM>, when the current being drawn by the element of the image intensifier tube exceeds a predetermined threshold, an operation is configured to shut down the photocathode, disable the automatic brightness control procedure, and store, as a stored voltage value, a value of a voltage that had been selected by the automatic brightness control procedure when the current exceeded the predetermined threshold. At <NUM>, after a first predetermined period of time (e.g., about <NUM>), an operation is configured to apply a voltage to the photocathode in accordance with the stored voltage value. Finally, at <NUM>, an operation is configured to re-enable the automatic brightness control and cause the automatic brightness control procedure to select the stored voltage value as the voltage to be applied to the photocathode.

It is noted that the anode current I3 has been the current relied upon to detect a quick increase in light level. However, those skilled in the art will appreciate that current being drawn by the photocathode or MCP could also be used to trigger the flash recover methodology described herein.

In sum, the embodiments described herein provide faster flash response time for an image intensifier by using a digital shutter made possible by storing the last known "good state" and re-applying those settings after a suitable delay. The embodiments described herein allow the power supply to react more quickly to step changes in light level for all background light levels.

Claim 1:
A power supply for an image intensifier of a night vision device, the power supply comprising:
a battery (<NUM>);
a memory (<NUM>); and
a processor (<NUM>),
wherein the processor is configured to: in response to current drawn by an anode of the image intensifier, turn off a switch via which a voltage is supplied to a photocathode of the image intensifier; store, as a stored voltage value, a value of the voltage in the memory; after a first predetermined period of time, turn on the switch and re-apply a voltage to the photocathode in accordance with the stored voltage value; and
enable an automatic brightness control procedure using the stored voltage value.