Radiant energy – Calibration or standardization methods
Reexamination Certificate
2002-03-27
2004-05-18
Gagliardi, Albert (Department: 2878)
Radiant energy
Calibration or standardization methods
C250S332000
Reexamination Certificate
active
06737639
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to infrared detection systems. More specifically, the present invention relates to systems and methods for providing dynamic range control and non-uniformity calibration of staring infrared imaging systems.
2. Description of the Related Art
Infrared imaging systems have achieved a wide level of deployment in industrial, commercial and military applications. The leading edge of technological development has typically been in the military arena as the ability to generate infrared images has many advantages in both tactical and strategic applications. This is particularly true in airborne systems.
Forward looking infrared (hereinafter “FLIR”) systems have been used in airborne applications for many years. Such systems typically employ a focal plane array (“FPA”) of infrared image sensing pixels that are coupled with an optical system which views a scene through an entrance pupil of the FLIR system. The FPA is typically housed within a cooled dewar vessel that serves to reduce thermal noise and improve the signal to noise ratio of the system. Infrared energy from a viewed scene is directed to and focused on the FPA through an optical system that may include both refractive and reflective optical components. The optical components in the FLIR system are located between the entrance pupil and a cold aperture of the dewar. Thus, the optical system operates at ambient temperatures that may fluctuate during operation of the FLIR system. Therefore, the optical components themselves are a source of noise to the FLIR systems.
Another limitation on the optical performance of a typical FLIR system is due to the fact that the optical components do not have perfect transmissivity.
As infrared energy is incident upon the FPA, it is integrated over time to produce frames of a video image. In a typical application, with real-time video performance, integration times are on the order of 10 to 20 milliseconds. The video frames generated are conditioned and provided to a video display for viewing by the operator of the FLIR system. The output of each pixel of the FPA is an analog signal proportional to the amount of light energy incident upon that pixel over the integration time period. For a variety of reasons, the output signals from all the pixels in the FPA are not identical, even given a uniform scene.
In typical applications, the analog signal output from the FPA are digitized and changed to a standard video format, such as the EIA RS-170 video standard. One of the principle functions of the FLIR system is to provide a detailed display of the infrared energy emitted from a scene to the pilot. The range of infrared energy levels emitted is very large. These energy levels can be thought of as visible light brightness, especially at the point where the pixel appears on the FLIR display screen for viewing. For example, a FLIR system could be directed to view the ground scene on a cold winter night at northern latitudes where the ground temperature is below zero degrees Fahrenheit. As an alternative, the scene could be a hot desert with battlefield emissions sources, such as fire and explosions. Since staring sensors are DC coupled devices which monitor infrared energy levels (photon flux) varying exponentially with temperature, it should be appreciated that given a range of sensor output levels as they are correlated to scene energy levels, there are problems and opportunities to adjust the sensor dynamic range to accommodate the scene's dynamic range. Sensor inputs that are too low result in noisy output while inputs that exceed the sensor's dynamic range result in saturated video.
It is known in the art to employ multiple dynamic range maps between ranges of scene infrared energy levels to the sensor levels. Sensor brightness levels are referred to as “bucket fill” levels by those skilled in the art. Bucket fill level, or ‘BF’ for short, is defined herein as the ratio of the sensor's digital output (digitized but not yet processed as video) to the maximum digital output of an analog-to-digital converter. For example, in a relatively warm environment, a first range of bucket fill (BF) values will be mapped to a range of scene energy levels. On the other hand, in the case of a relatively cool scene, a lower range of bucket fill (BF) levels are mapped to scene energy.
Changing the dynamic range of the sensor is equivalent to changing the gain (a combination of gain state and/or integration time) and will result in a remapping of BF for the scene. If one switches the sensor to a lower dynamic range (higher gain) while viewing the relatively cool scene, then the BF will rise in proportion to the gain.
In another aspect of prior FLIR systems, it is understood that the FPA sensor array and related control circuitry and sensor circuitry must be calibrated in order to assign correction factors for pixel to pixel responsivity equalization, and pixel to pixel level equalization. Responsivity is defined as gain and is usually expressed in delta mV of sensor output for a 1 Kelvin change in scene temperature (assuming blackbody scene).
As noted above, given a change in uniform scene energy level, not every pixel in the FPA will yield the same change in output signal level. This discrepancy is corrected for during calibration by directing the entrance pupil of the FLIR system toward a uniform thermal body (known to the art as a thermal reference source “TRS”) and then calibrating the pixel to pixel responsivity using a responsivity equalization (“RE”) process. This operation is typically accomplished using specialized hardware adapted to this particular function. The RE calibration is a two-point calibration because gain must be calculated by looking at the difference of two uniform scenes- first near the lower temperature/BF limit of the dynamic range, and a second near the higher temperature/BF limit of the dynamic range. Having this calibration and given a uniform change in scene temperature, the change in sensor brightness levels will be the same for all pixels. The resultant RE set of correction coefficients are gain multipliers for each pixel. One RE set may be used to cover multiple dynamic ranges.
The responsivity equalization eliminates the detector cold shield effect (center pixels will be: brighter without this calibration) as well as FPA pixel to pixel gain variation.
In addition to the RE calibration, typically pixel to pixel level equalization (“LE”) calibration is employed for each dynamic range. The LE serves to equalize the level factor applied to each pixel in the FPA, and this function is also provided by specialized hardware, and requires that the entrance pupil of the FLIR system be directed to the TRS so that a scene of uniform energy emission is to be viewed during the calibration process. The appropriate RE set for the dynamic range must be used during LE calibration and the TRS temperature must be adjusted to achieve a BF between the high and low BF values of the RE calibration (which produces the RE set being used). A single LE calibration for each dynamic range is typically employed. The resultant LE set of coefficients are level adders for each pixel. The level equalization is required to eliminate fixed patterns created by the optics as well as FPA pixel to pixel level variation. The combination of RE and LE calibrations are required to achieve good uniformity for uniform scenes that (in combination with the optics temperature) produce BF values between the low and high BF values of the RE calibration.
During the course of operation of a FLIR system, even though the system may have been calibrated prior to operation, the scene and/or optics temperature can change. The change frequently pushes toward a bucket fill limit of the previously selected dynamic range, and it therefore becomes necessary to change the sensor's dynamic range (i.e., change detector gain equally across all pixels) so that the scene is properly mapped to the sensor output (i.e., not too low to prevent noisy
Alkov Leonard A.
Gagliardi Albert
Gunther John E.
Lenzen, Jr. Glenn H.
Raytheon Company
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