Data processing: measuring – calibrating – or testing – Calibration or correction system – Temperature
Reexamination Certificate
1998-10-22
2001-05-08
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Calibration or correction system
Temperature
C702S130000, C702S134000, C702S135000, C250S208100, C358S463000
Reexamination Certificate
active
06230108
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a realtime sensitivity correction method for infrared sensors and an infrared imaging system employing a realtime sensitivity correction mechanism. More particularly, the present invention relates to a sensitivity correction method which compensates for the variations in sensitivity levels among a plurality of elements that constitute an infrared sensor device, and to an infrared imaging system having a function to correct the sensitivity variations among the infrared sensor elements.
2. Description of the Related Art
Infrared imaging systems are instruments that capture infrared images, or thermographs, of a target object by sensing infrared rays radiated from its surfaces. They are utilized in various industrial fields, for example, to observe temperature distribution on target surfaces or to detect the shape of an object. Infrared imaging systems fall roughly into two categories according to the wavelengths of infrared rays that they sense; one is for 3-5 &mgr;m band, and the other is for 8-10 &mgr;m band. The 8-10 &mgr;m band systems mainly use HgCdTe sensors, while the 3-5 &mgr;m band systems use infrared sensors made of PtSi, InSn, HgCdTe, or the like. With their material composition varied, HgCdTe sensors can be applied to relatively wide wavelength ranges.
Regarding the structure of sensor devices, one-dimensional (or linear) arrays are typically used for the above purposes, while there are other types of infrared sensors, such as ones composed of discrete elements or two-dimensional arrays. One-dimensional and two-dimensional arrays contain a plurality of sensor elements, and ideally, it is desirable that all elements provide uniform responses. In reality, however, some variations in sensitivity levels inevitably exist among the elements constituting an infrared sensor. This sensitivity variation will result in non-uniform sensor outputs for a target surface having a flat temperature distribution, thus giving inaccurate target images. To improve this situation, conventional infrared imaging systems employ a sensitivity correction mechanism that compensates for the unevenness of individual sensor elements by applying appropriate data processing to the detected signals.
This kind of sensitivity correction mechanisms would properly work as long as the sensor elements keep their initial characteristics. However, since the individual elements actually vary with time, it is hard for the above correction mechanisms to maintain their long-term accuracy of sensitivity compensation.
To solve this problem, another sensitivity correction method is proposed. This method uses reference heat sources being controlled at constant temperatures. Scanning the reference heat sources, the infrared sensor outputs reference temperature detection data. This data is used to quantify the sensitivity variations among a plurality of sensor elements, allowing compensation for them to be conducted in a later stage.
FIG. 28
shows a typical conventional infrared imaging system. Infrared rays emanating from object surfaces first enter an optical system
301
, then pass through another optical system
302
having a scanning capability, and finally reach a linear infrared sensor
303
. The analog detection signal produced by the infrared sensor
303
is amplified by an amplifier
304
and fed to an analog-to-digital (A/D) converter
305
. The resultant digital detection signal is then subjected to a signal processing circuit
306
for sensitivity correction and other necessary processes. After that, a digital-to-analog (D/A) converter
307
converts the corrected signal back to an analog signal, thus allowing the captured and corrected infrared image to be displayed on a video monitor
308
.
This infrared imaging system employs two reference heat sources
310
and
311
to compensate for the sensitivity differences among sensor elements as described earlier. The reference heat sources
310
and
311
are regulated to keep their respective temperatures. Their infrared outputs are given to the infrared sensor
303
by an optical system
302
during a part of the system's scanning cycle. More specifically, the system scans the target object at regular intervals. Each scanning cycle consists of an “effective scanning period” and a “non-effective scanning period.” During the effective scanning period, the optical system
302
actually scans the target surfaces. Using the remaining time, or the non-effective scanning period, it scans the reference heat sources
310
and
311
, thus enabling the infrared sensor
303
to output detection signals for the two different reference temperatures. The signal processing circuit
306
then processes these detection signals to calculate parameters to compensate for the sensitivity variations among the sensor elements.
FIG. 29
is a diagram which shows the structure of the optical systems of FIG.
28
. To form an image of the target, the first optical system
301
comprises lenses
312
and
313
, and the second optical system
302
comprises lenses
314
and
315
. The second optical system further comprises two more lenses
316
and
316
to collect infrared rays emanating from the reference heat sources
310
and
311
, together with two reflectors
317
and
319
to direct the rays to the infrared sensor
303
. The linear infrared sensor
303
is disposed at the back of this optical system
302
in such a way that the array will be orthogonal to the direction of optical scanning. That is, the infrared imaging system scans the target both electronically (by the linear infrared sensor
303
itself) and optically (by the optical system
302
), thus achieving a two-dimensional scanning operation.
Typically, the above-described scanning operation of the optical system
302
is conducted in concert with the raster scanning operation of the video monitor
308
. In the case of interlaced video, for example, one complete picture, or frame, is obtained as a combination of two separate field scans. Here, the term “field” refers to a set of alternating lines in an interlaced video frame. In synchronization with the video monitor
308
, the optical system
302
scans odd-numbered lines in one field and then even-numbered lines in the next field. The signal processing circuit
306
joins the infrared detection signals obtained in those two scans, thereby constructing a complete infrared image of the target.
During the non-effective scanning period, the optical system
302
scans the reference heat sources
310
and
311
being regulated at constant temperatures, thus directing their infrared rays to the infrared sensor
303
. The unevenness in the infrared sensor outputs is measured in one non-effective scanning period, and this measurement result is used in the next or later effective scanning period(s) to correct the sensitivity of each individual sensor element.
The reference heat sources
310
and
311
can be implemented with Peltier effect devices, for example, with supply currents being controlled so that they will keep their respective set temperatures. While
FIG. 29
illustrates a specific arrangement where the two reference heat sources
310
and
311
are placed separately across the center line, it is also possible to place both on one side. These two reference heat sources
310
and
311
provide a high and low temperatures determined according to the range of target temperatures that the infrared sensor
303
can detect. They are controlled so that they keep a predetermined temperature difference. This means that the system can make a sensitivity correction at least at two points within the detection temperature range of sensor elements.
The video monitor
308
typically uses a cathode ray tube (CRT) to display the captured infrared images. Video signals entered to the video monitor
308
conform to the National Television System Committee (NTSC) standard, which defines an interlaced video format where each frame is composed of two fields. In the NTSC format, one frame scann
Fujitsu Limited
Helfgott & Karas P.C.
Hoff Marc S.
Vo Hien
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