Radiant energy – Infrared-to-visible imaging
Reexamination Certificate
1999-08-16
2002-03-05
Hannaher, Constantine (Department: 2878)
Radiant energy
Infrared-to-visible imaging
C250S252100, C250S339090, C250S339040
Reexamination Certificate
active
06353223
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an infrared camera with offset compensation.
2. Description of the Related Art
FIG. 9
is a block diagram relating to a conventional infrared camera. The drawing shows an subject M, an infrared optical system
1
, an image pickup element formed on the imaging surface of the infrared optical system
1
, an element temperature monitor
3
thermally connected to the image pickup element
2
, a shutter
4
provided between the infrared optical system
1
and the image pickup element
2
, a bias power source
5
connected to the image pickup element
2
, a driver circuit
6
connected to the image pickup element
2
, pre-positioned amplifying circuit
7
connected to the image pickup element
2
, a display processing circuit
8
connected to the pre-positioned amplifying circuit
7
, an element temperature stabilizing means
9
thermally connected to the image pickup element
2
, a timing generation circuit
10
connected to the shutter
4
, the driver circuit
6
, and the display processing circuit
8
, a body
11
, an offset compensation execution switch
12
provided in the outside of the body
11
and connected to the timing generation circuit
10
. The display processing circuit
8
comprises an A/D converter
13
, an addition averaging circuit
14
, a frame memory
15
, a subtraction circuit
16
, and a D/A converter
17
. Also shown in
FIG. 1
are a reference voltage power source
18
, a differential amplifying circuit
19
connected to the element temperature monitor
3
and the reference voltage power source
18
, a vacant element package
20
accommodating the image pickup element
2
, the element temperature monitor
3
, and the element temperature stabilizing means
9
, and an infrared window
21
typically made of germanium, leaving a vacant space enclosed by the vacant element package
20
and the infrared window
21
.
FIG. 10
shows an example structure of the image pickup element
2
which is, for the sake of brevity of explanation, comprised of 3×3 elements. The drawing shows infrared detectors
22
to
30
, transistors
31
to
48
, capacitors
49
to
51
, a vertical scanning circuit
52
, and a horizontal scanning circuit
53
. The infrared detectors
22
through
30
are microbolometers having a hollow structure, as described in Japanese Patent Laid-open No. Hei 7-509057, which are made of vanadium oxide or titanic oxide for reducing thermal conductance with respect to the surrounding so that heat quantity of absorbed infrared can be efficiently converted into an increase of temperature of the detector thereby achieving high sensitivity.
In operation, the reference voltage power source
18
outputs a reference voltage corresponding to an operation temperature of the image pickup element
2
to the differential amplifying circuit
19
. The differential amplifying circuit
19
compares the supplied output and an output from the element temperature monitor
3
to feed back a power corresponding to the difference between the outputs to the element temperature stabilizing means
9
for stabilizing the operation temperature of the image pickup element
2
.
Next, the bias power source
5
supplies bias voltage Vb and gate voltage Vg to the transistors
46
through
48
and the driver circuit
6
sends a driving clock to the vertical scanning circuit
52
according to a timing generated by the timing generation circuit
10
for selection of a row of infrared detectors. In response to the clock, the vertical scanning circuit
52
renders the transistors
31
through
33
conductive for a predetermined period, whereby a bias current defined according to gate voltage Vg is caused to flow into the infrared detectors
22
to
24
, so that the voltage corresponding to the respective resistance values will be caused at the infrared detectors
22
through
24
.
Subsequently, when a sample-hold clock is applied, the transistors
40
to
42
are made conductive so that the voltage according to the resistance values of the infrared detectors
22
to
24
is temporarily stored in the capacitors
49
to
51
. Then, after shutting off the transistors
40
to
42
, the horizontal scanning circuit
53
sequentially makes the transistors
43
through
45
conductive so as to output voltage according to the resistance values of the infrared detectors
22
through
24
.
Thereafter, the vertical scanning circuit
52
selects the row of infrared detectors
25
through
27
so that the voltage corresponding to the resistance values thereof will be output in the same procedure as that is applied to the infrared detectors
22
through
24
.
While repeating the above procedure, voltages corresponding to the resistance values of the infrared detectors
22
through
30
which constitute the image pickup element
2
are sequentially output and, after being amplified in the pre-positioned amplifying circuit
7
, are supplied to the A/D converter circuit
13
. Then, the timing generation circuit
10
sends a signal for closing the shutter so that the shutter is closed.
After the shutter was closed and consistent infrared were introduced into the infrared detectors
22
through
30
, the timing generation circuit
10
sends a signal to the display processing circuit
8
, for obtaining offset compensation data for the first time. Then, the A/D converter circuit
13
converts an output from the pre-positioned amplifying circuit
7
into a digital signal. Further, an addition average is obtained for every infrared detector in the addition averaging circuit
14
so that variation of the resistance values of the infrared detectors
22
through
30
, in other words, offset variation, is stored in the frame memory
15
.
Then, the shutter
4
is opened, and infrared radiation emitting from the direction of subject M is collected in the infrared optical system
1
. The converged infrared radiation then passes through the infrared window
21
to form an image on the infrared detectors
22
through
30
. This causes a slight increase of the temperatures of the infrared detectors
22
through
30
by an order of a few mK according to the strength of the collected infrared radiation. As a result, the respective resistance values of the infrared detectors are changed from those before the shutter
4
was opened.
Outputs from the infrared detectors
22
through
30
are then amplified in the pre-positioned amplifying circuit
7
and converted into digital signals in the A/D converter circuit
13
, similar to when offset compensation data is obtained. Then, the data stored in the frame memory
15
is subtracted from the digital signals for every pixel in the subtraction circuit
16
to remove fixed pattern noise due to offset variation of the infrared detectors, and the result is converted into an analogue video signal in the D/A converting circuit
17
before being output.
Here, a change in the inside temperature of the body
11
due to heat generation of an electric circuit or a change of ambient temperature may change an output voltage of the reference voltage power source
18
, characteristics of the element temperature stabilizing means
9
, the amount of heat discharged from the image pickup element
2
, or the amount of infrared radiation from the infrared optical system
1
, resulting in a slight change to an operation temperature of the image pickup element
2
. Accordingly, the resistant values of the infrared detectors
22
through
30
are changed for every pixel. Because the amount of change of the resistance value differs for every pixel, offset variation of an output is changed from that at the time when offset compensation data was first obtained, leaving outstanding fixed pattern noise in a video signal. In such a case, offset compensation data is obtained again by operating the offset compensation switch
12
to restore the image.
While the compensation operation described above as being applied to a non-cooling type of infrared camera whose image pickup element
2
has a two-dimensional array of microbo
Burns Doane , Swecker, Mathis LLP
Gagliardi Albert
Hannaher Constantine
Mitsubishi Denki & Kabushiki Kaisha
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