Thermographic camera and method for the recording and/or modification and reproduction of thermal images of a scene and/or of an object
The present invention is directed to a thermographic camera and to a method for generating thermal images with this thermographic camera as is described generically in DE 698 30 731 T2. The object of the invention is to provided a possibility for achieving an optimal signal resolution at low object temperatures in an economical manner without signal overload at high temperatures. This object is met by a method for the recording and reproduction of thermal images of a scene and/or of an object, comprising the following steps: a pixel-based thermal image frame is generated by means of IR optics and a pixel-based detector unit; a dark image is generated for implementing a non-uniformity correction (NUC) to eliminate non-uniformities arising from the detector and/or disturbance sources in the optical beam path; the existing pixel-based thermal image frame is corrected by weighting the individual pixel signals with a nonlinear characteristic line which is generated and stored beforehand for generating a corrected output thermal image; and finally, the corrected output thermal image is supplied to a display unit or to an interface for external equipment.
1. A method for recording or modifying and reproducing thermal images of a scene or of an object, comprising: generating a pixel-based thermal image frame by infrared optics and a pixel-based detector unit or retrieving a pixel-based thermal image frame from an external source, generating a dark image for non-uniformity correction to eliminate non-uniformities arising from the detector or from sources of disturbance in an optical beam path,
correcting the pixel-based thermal image frame by weighting individual pixel values with a nonlinear characteristic line generated and stored in advance to generate a corrected output thermal image, and providing the corrected output thermal image to a display unit or to an interface for external equipment, wherein the nonlinear characteristic line is calculated according to the formula:
S
out
=
S
max
1
+
1
k
-
ⅇ
-
k
·
(
1
+
S
i
n
S
max
·
k
-
ⅇ
-
k
·
S
i
n
S
max
)
,
where:
S min is a minimum signal intensity,
S max is a maximum signal intensity,
S in is an input signal intensity,
S out is an output signal intensity, and
k is a compression parameter between 0.1 and 10.
2. The method of claim 1 , further comprising generating and storing an associated nonlinear characteristic line for every pixel.
3. The method of claim 1 , wherein for 8-bit number representation (S max =255) in discrete form, the formula is:
Fi_compr
.
=
rounded
(
255
1
+
1
k
-
ⅇ
-
k
·
(
1
+
Fi
255
·
k
-
ⅇ
-
k
·
Fi
255
)
)
,
where:
Fi is a color index, and
k is the compression parameter.
4. The method of claim 1 , further comprising, after applying the characteristic line, displaying in color according to a color table,
wherein the color table
maps gray values to signals below a lower threshold,
maps color levels from yellow through orange to red to signals between the lower threshold and an upper threshold in ascending order, and
maps a red color to signals above the upper threshold.
5. A thermographic camera for the recording and reproducing thermal images of a scene or of an object, comprising: an IR objective, a pixel-based detector unit recording a pixel-based thermal image frame, and a processor unit controlling recording and correction processes, including non-uniformity correction and weighting of pixel signals with a nonlinear characteristic line of a pixel-based thermal image frame to generate a corrected output image provided to a display unit or to an interface for external equipment, wherein the nonlinear characteristic line is calculated according to the formula:
S
out
=
S
max
1
+
1
k
-
ⅇ
-
k
·
(
1
+
S
in
S
max
·
k
-
ⅇ
k
·
S
in
S
max
)
,
where:
S min is a minimum signal intensity,
S max is a maximum signal intensity
S in is an input signal intensity,
S out is an output signal intensity, and
k is a compression parameter between 0.1 and 10.
6. The camera of claim 5 , further comprising a shutter and a shutter control device for the non-uniformity correction.
7. The camera of claim 5 , wherein the pixel-based detector unit is a microbolometer focal plane array.
8. The camera of claim 7 , wherein an associated nonlinear characteristic line is stored in a storage for each microbolometer.
9. The camera of claim 5 , wherein, after the nonlinear characteristic line is applied, a color table, adjusted to the characteristic line and stored, is used for color displaying.