Patent Application
20080179520
Not Applicable
Not Applicable
The present invention relates in general to a focal plane array, and in more particular, to a direct-view focal plane array.
Several goggle technologies have been developed to convert light or image in various infrared ranges into visible images, so as to allow the observers to see or detect images or objects in various ambient conditions. For example, night vision goggle technologies have been developed that allow imaging nighttime low light level scenes to a display for easy viewing by the human eye. Existing light is amplified and displayed on a phosphor screen without the need for high power raster scanning of a detector array or of the display. This technology is also known as image intensifying (I2) goggles. An important aspect of this technology is its light weight and low power dissipation allowing many hours of operation using only a couple commercial off-the-shelf batteries. The limitation of this technology is its inability to perform thermal imaging while maintaining the same power and weight characteristics (i.e., low power and light weight) needed by a goggle device. For example, in low light conditions (e.g., cloud cover and no ambient light), I2 goggles will not work. To make I2 goggles work in low light conditions, the wavelength response of the I2 goggle must be extended via a photocathode so that it can view thermal imagery. However, the photocathode would have to be cooled to reduce thermal generated carriers. The reason is that photocathodes used in I2 goggles are ambient temperature devices and do not have provisions for temperature control. Unfortunately, cooling of the photocathode would lead to high power dissipation and larger size and weight.
Prior art thermal imagers consume too much energy for use in a goggle. Thermal imaging systems create images of heat patterns and are useful even when there is no ambient light. There are many types of detectors that are used to detect thermal infrared light that occurs with greater than approximately 3.0 microns in wavelength such as solid state photovoltaic or photoconductive infrared detectors. Unfortunately, these detectors require cooling systems such as cryogenic closed cycle types and thermoelectric types which all require too much power to be feasible for use in a goggle. Uncooled bolometer type detectors do not require any cooling nor thermal stabilization systems. Unfortunately, even these types of detectors when coupled to focal plane array readout electronics, digitizing electronics, drive electronics, digital signal processing electronics, and display electronics require too much power to be feasible for use in a goggle. State of the art in low power camera design has produced an imaging module that only requires approximately 1 W of electrical power to operate, and this does not include the display. Unfortunately, even these imaging modules would use up the batteries for night vision goggles in less than one hour. Excessive power requirements of the systems discussed above require special batteries that increase the cost, size and weight of the systems.
In summary, the prior art focal plane array based systems have the desired functionality, but require too much power and are too large for man-portable goggle type applications. Therefore, there is a substantial need in the art to provide a focal plane array based system that provides the desired functionality, packaged in a small size and operates at low power.
A direct-view focal plane array operative to detect infrared and convert the infrared light into a visible image is provided. With the direct-view focal plane array, an infrared object or spatial image can be viewed directly by human eyes or a conventional visible focal plane array. The direct-view focal plane array as provided eliminates the requirement of multiplexing and de-multiplexing processes, digitization, high-speed digital data manipulation and operations, and signal conditioning on a continuous basis. By the direct-view focal plane array, a direct path from the detector to the display element is established via a configurable digitally set analog circuit that controls gain and level for non-uniformity correction (NUC). Configurable pixels, switchable to be used for symbology (by way of example and not limitation, text on the display) or for displaying the detector signal, are formed. The ability to add functionality by adding layers allowing additional capability to multiplex images on demand for capture and transmission of images of interest is enabled. The direct-view focal plane further allows for image processing functions in the layers such as interpolation where one detector influences many higher resolution display pixels. The enabling technology may be characterized as the ability to form circuitry on layers that can be stacked while having interconnection at the pixel level that results in a vertically integrated circuit. By way of example and not limitation, there may be a detector at the input of this vertically integrated circuit, and a display element at the output of the vertically integrated circuit thereby forming one pixel. The vertically integrated pixels may be arranged in a two dimensional mosaic array organized in a rectilinear fashion, or some other pattern such as an interleaved or honeycomb pattern that determines an active electro-optical focal plane on both sides of the array. The individual pixels functions may be controlled using circuitry located at the periphery of this active pixellated region.
The direct-view focal plane array may comprise a detector layer, an amplification layer, a non-uniformity correction layer and a display layer. A layer is defined in this text in terms of functionality and also in terms of physical circuitry. One or more layers of functionality may reside in one layer, or level, of physical circuitry. The detector layer is operative to detect an infrared image with a raw image pixel response x and convert the infrared image into an electrical signal. The electrical signal is then fed into the amplification layer for pre-amplification to enable detector noise limited performance and the non-uniformity layer for offset and gain correction. In the case where a simple two-point non-uniformity correction scheme is used, an offset correction coefficient b and a gain correction coefficient m are input to the non-uniformity layer, such that the raw image pixel response x can be transformed into a corrected pixel response y by the relationship of y=mx+b. Thereby, the true scene content in the image captured by the detector layer will not be masked by the fixed pattern while being displayed by the display layer. Other methods of fixed pattern noise suppression known in the art or developed in the future may also be implemented as well.
The direct-view focal plane array may include a two-dimensional array of pixels, and each pixel may include a detector unit, an amplification circuit, a gain-correction circuit, and a display unit. The detector unit may be operative to detect an infrared optical signal with a raw image pixel response x and convert the infrared signal into an electrical signal. The amplification circuitry may be operative to amplify the electrical signal and to offset the raw image pixel response x by an offset correction coefficient b. The gain correction circuitry is operative to provide a gain correction coefficient m to the raw image pixel response x. The display unit may be operative to convert the amplified and corrected electrical signal into a visible signal with a corrected pixel response y, wherein y=mx+b. Preferably, both the offset and gain correction values m and b are analog to reduce the circuit complexity to perform non-uniformity correction. Digital signals are also contemplated. The amplification circuitry in the preferred embodiment may comprise a digital-to-analog converter to convert a predetermined digital offset correction value into the analog offset correction coefficient b, and the gain correction circuitry comprises a digital-to-analog converter to convert a predetermined digital gain correction value into the analog gain correction coefficient m. The display unit includes a light-emitting device such as an organic light-emitting diode (OLED) unit or a field-emission display (FED) unit. The focal plane array may further comprise a buffer to temporarily store the electrical signal before being fed to the amplification circuitry. The amplification circuitry and the gain correction circuitry can be integrated on a single or multiple layers. In addition, the detector unit, the amplification circuitry, the gain correction circuitry, and the display unit may be in electrical communication with each other and integrated by vertical integrated sensor array technology.
It is contemplated to multiplex small groups of detectors with small groups of corresponding display elements to reduce the number of interconnects between each layer. For example, the FPA may be organized into smaller arrays, such as 2×2 subarrays, (that may be multiplexed at a relatively low frequency as compared to a conventional FPA, but high enough that the transitions between pixels is transparent to the human eye) through one interconnect for each subarray in the FPA, to the corresponding 2×2 subarray of display elements. In this way, only one interconnect is required for the subarray as compared to four interconnects in this case. Multiplexing the small subarrays will add some complexity and power dissipation to the circuitry.
The focal plane array can be used in a goggle to provide direct view for a user or an observer under various ambient conditions. The goggle includes an objective optic for focusing infrared light onto the focal plane array. The focal plane array then converts the infrared optical signal into a visible optical signal for the user to observe. There may also be an optical element on the visible side of the focal plane array to enable easier viewing by the observer. The direct view focal plane array may also be used with a normal visible camcorder or digital camera to view a scene in the infrared in conjunction with a re-imaging optics chain with the direct view FPA at the intermediate focal plane of the optical system.
The direct-view focal plane array focal plane array can be operated with a very low power and used as an infrared, or thermal, goggle, a LLL (low light level) SWIR (shortwave infrared) goggle, or a LLL (low light level) visible goggle, depending on the specific type of detector array as selected. The direct-view focal plane array may be in the form of a multi-layer semiconductor circuit. On one side of the multi-layer semiconductor circuit may be an array of detectors, and on the other side may be an array of low power display elements.
These and other features and advantages of the various embodiments disclosed herein will be better understood with respect to the following description and drawings, in which like numbers refer to like parts throughout, and in which:
As discussed above, most of the currently available prior art focal plane array systems that have the desired functionality require too much power for man-portable goggle type applications. The functions that consume significant power are listed in Table I and outlined for the FPA alone in the functional signal flow diagram as shown in
To resolve the power consumption issue, the continuous operation of the circuitry that provides the high-speed switching and the high-bandwidth amplification outlined in
The direct-view focal plane array 10 as shown in
The circuit for each pixel as shown in
The output of the amplifier 42 is input to unit 14a. The non-uniformity correction unit may comprise offset correction 14a and gain correction 16a. The non uniformity correction unit may comprise an offset stage, here shown as a summing amplifier 46 with the negative input connected with two resistors 44 and 45, where the resistor 44 is connected to the buffer 42 allowing the voltage signal to be input to the offset unit 16a, and the resistor 45 is fed with an analog offset value converted from a predetermined digital offset value converted by a digital-to-analog converter 43. The positive input of the amplifier 46 is grounded in this embodiment and a multiplier 49 is used for gain correction. A predetermined digital gain value is converted to an analog gain value by a digital-to-analog converter 48 and input to the multiplier 49 allowing the gain to be corrected according to the digital gain value. The output of the multiplier 49, that is, the non-uniformity correction circuit unit 16a is then connected to the display unit 18a for displaying the image captured by the detector unit 12a in the visible wavelength range, such that the observer or user can obtain a direct view of the infrared image as shown in
The offset and gain corrections as described above are the coefficients used for non-uniformity correction. As understood, there is very low contrast in the infrared wavelength range or at low light levels in the visible wavelength range, so that any slight difference in the responses between pixels to the same input stimulus will cause a fixed pattern to appear in the image that masks the true scene content. This is called fixed pattern noise. The response curve of each pixel is modeled as a linear function of output pixel response over the input optical power incident on the pixel. By adjusting the offset level and the gain level for each pixel, all of the apparent pixel responses can be made to look exactly alike. In the prior art, the output of a focal plane array is directly applied to a digitizer and all of the pixel responses are fed to a digital signal processor with all of the non-uniformities, and the corrected image is calculated digitally using stored calibration coefficients for each pixel. These operations must be performed on every pixel for every frame. In contrast, in the embodiment as shown in
In an aspect of the focal plane array discussed herein, the same may be electrically connected to peripheral circuitry. The peripheral circuitry may be connected to certain pixels for use in symbology such as text, numbers, stop sign, left turn, etc. The peripheral circuitry may be operative to send a high or low voltage based on the symbol to be displayed. By way of example and not limitation, inputs of certain pixels may be connected to a high or low voltage rail instead of the detector.
In an aspect of the focal plane array discussed herein, the same may be optionally be in communication with peripheral circuitry. The peripheral circuitry may receive the information provided by the outputs of the pixels and produce a time-division multiplexed video signal for viewing by another user(s).
The above description is given by way of example, and not limitation. Given the above disclosure, one skilled in the art could devise variations that are within the scope and spirit of the invention disclosed herein. Further, the various features of the embodiments disclosed herein can be used alone, or in varying combinations with each other and are not intended to be limited to the specific combination described herein. Thus, the scope of the claims is not to be limited by the illustrated embodiments.
