Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
2001-03-15
2003-08-26
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S338100
Reexamination Certificate
active
06610984
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates generally to very sensitive thermometric instruments, known as microbolometers, which are used for the detection and measurement of radiant energy. More specifically, the present invention addresses correction of microbolometer output.
2. Related Art
Infrared detectors known as microbolometers respond to impinging infrared radiation through subtle variations in the temperature of the detector element. The detector elements include a material having a high temperature coefficient of resistance (TCR) such that these subtle variations in the temperature of the detector may be sensed. The sensing methods often employed are based on the passing of a metered electrical current through the device and measuring the resulting voltage drop. Alternatively, the temperature of the detector may be sensed by biasing the detector circuit with a known voltage and measuring the resulting current. In the simplest embodiment, the microbolometer detector is connected to a meter, and the response of the meter can be correlated to the intensity of the impinging infrared radiation.
However, in typical applications for which an image is desired, a lens is employed to focus energy onto a two-dimensional array of microbolometer detectors such that a spatially varying infrared field can be detected and converted to visible imagery using electronics and display means such as are commonly employed for visible imagery using Charge-Coupled Device (CCD) cameras. The electronics typically include a multiplexing circuit in intimate contact with the microbolometer array which converts the voltage or current variation of the many microbolometer elements to one or several multiplexed analog (e.g., voltage variation) data streams. This analog data is then converted to digital data using an analog-to-digital converter (ADC), and is then further processed to produce data for analysis or imagery on a Cathode Ray Tube (CRT) or similar video monitor.
The fact that the detection means is based on the thermal variations of the detector causes several practical problems. First, the material must be thermally isolated from surrounding matter so that a sufficiently large (e.g., several mK) temperature variation may occur as a result of the weak impinging infrared energy. Liddiard, in U.S. Pat. Nos. 4,574,263 and 5,369,280, and Higashi, et al., in U.S. Pat. No. 5,300,915 describe a microbolometer that provides thermal isolation by depositing a semiconductor material onto a pellicle, or “micro-bridge” structure that physically separates the detector from the supporting substrate. Second, the temperature of the supporting substrate must be stable so that erroneous signals are not generated from its temperature fluctuations. Experience indicates that a 15 mK variation of substrate temperature within the sampling period (or video frame rate, whichever is greater) is acceptable, but fluctuations greater than this present a significant source of system noise. Third, the output of the microbolometer varies as a result of both the impinging infrared radiation, and the absolute temperature of the substrate. In this last case, the array output may be higher or lower at different temperatures, even if that temperature is held to within the stability requirement of 15 mK. Fourth, variations in the physical construction of the microbolometer detectors result in significant variations of the output of individual microbolometer detectors within the array, and these non-uniformities must be corrected in order to obtain a low-noise image.
As a result, there exists a need for an apparatus capable of correcting the output of a microbolometer, for example, in a focal plane array (FPA), such that the effects of thermal drift are removed or eliminated.
In the particular problem of thermal variation of the substrate, microbolometer detectors are operated at a fixed temperature, typically with a stability tolerance of ±0.015° C. (i.e., 15 mK). Peltier-junction heat engines and control circuitry are commonly employed for this purpose. While this temperature stabilization scheme works well, it is not the ideal solution. For instance, the temperature stabilization system represents a significant portion of the detector package cost. Further, it is susceptible to damage from shock or vibration, and ordinarily requires tens of seconds to reach operational temperature from system start-up. Also, the temperature stabilization means is a major consumer of system power.
Since the output of the microbolometer varies as a result of impinging infrared radiation, a number of additional noise sources and undesirable effects occur. Referring now to
FIG. 7
, the microbolometer is impinged by infrared radiation from the cold shield,
608
a
, the lens,
616
a
, the dewar window,
606
a
, as well as the signal
622
a
. If the temperature of the cold shield
608
a
, lens
616
a
and dewar window
606
a
remain constant, then variation in their radiant flux also remain constant. The microbolometer output voltage or current due to variations in the signal emanating from source
618
a
may then be determined by subtracting the fixed voltage or current offset arising from the impinging radiation from the cold shield, lens and dewar window. If, however, the temperature of the cold shield
608
a
, lens
616
a
and dewar window
606
a
vary more than a few degrees Celsius, a significant source of uncertainty in the microbolometer output, termed noise, is created.
Due to limitations in the input sensitivity range of the analog-to-digital converter (ADC), these noise sources may cause under or over saturation of the ADC, resulting in the loss of sensitivity to desired signal data. The radiant flux from the source may also vary more than the input sensitivity of the ADC permits, and loss of sensitivity to desired signal data may also result.
In the particular problem of modulating the detector's sensitivity to widely varying radiant flux from the source, several methods are typically employed to maximize dynamic range. The most common method is to insert a mechanical aperture stop within the system optical path to vignette energy and effectively change the focal ratio (or “f” number). However, this method requires mechanical components that are bulky, expensive and unreliable, and require the user to manually adjust the sensitivity. A better method is to vary the duty cycle of the detector by means of changing the time that the sensor is being sampled by the system electronics. This method is a distinct improvement over the manual method, but has the disadvantage of requiring a new calibration each time the sampling time is changed. While the calibrations can be stored in digital memory, the quantity of memory required and the number of calibrations that must be performed tend to increase system size and cost.
In the particular problem of correcting for thermal variations of the cold shield
608
a,
lens
616
a
and dewar window
606
a,
the most common method is to perform frequent system calibrations to eliminate these effects. Many thermal imaging systems have built-in motorized calibration sources for this purpose. However, calibrating the system frequently reduces the availability of the system for its intended use and consumes system power. Further, the motorized calibration source (shown as
630
and
631
in
FIG. 6
) decreases the reliability of the system due to its moving parts and increases manufacturing costs.
Therefore, there exists a need for an apparatus capable of correcting the output of a microbolometer, for example, in a focal plane array (FPA), such that the effects of thermal variations in the cold shield
608
a,
lens
616
a
and dewar window
606
a
are reduced or eliminated. This apparatus should also have the ability to modulate the sensitivity of the microbolometer to large variations in the radiant flux (signal) from the source so that the microbolometer output remains within the input sensitivity range of the ADC.
SUMMARY OF THE INVENTION
It is an advantage of
Balick Steven M.
Knauth Jonathan P.
Hancock & Estabrook, LLP
Hannaher Constantine
Infrared Components Corporation
Roehrig, Jr. August E.
LandOfFree
Method and apparatus for correction of microbolometer output does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for correction of microbolometer output, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for correction of microbolometer output will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3119234