Research Directory of the Brandenburg University of Applied Sciences

Fachhochschule Brandenburg
Brandenburg University of Applied Sciences
the image at a given spatial frequency relative to low frequencies.
For the experimental determination of the MTF
a USAF line pair standard was used. This standard consists
of a photolithography mask with line pairs. The line
pairs of each frequency are arranged in a 1 mm broad
array on the mask. These arrays are splitted by a 300 μm
wide chromium layer. The number of line pairs per millimeter
(lp/mm) is in the range 5 to 200 lp/mm, see Fig.
3.18. With a similar measurement as described above for
the single line the spatial frequency dependent contrast
of the raw signal data in the thermogram was analyzed.
Obviously the line profiles across the line pair structures
show not only the intensity modulation caused by the
line pairs but additionally a smooth raw signal change
over the whole line pair structure. The maximum raw signal
data is observed in the middle of the whole line pair
structure. This also indicates a limited MTF.
The resulting MTF (Fig. 3.18, bottom) was evaluated
according to the method described in [4]. It shows that
an accurate temperature determination from the raw signal
data becomes very difficult for small objects. For getting
accurate temperature results the camera calibration
curve should be measured using the same object size.
The spatial frequency dependent MTF has to be analyzed
in more detail. Especially the influence of the chromatic
aberration of the microscope objective due to
the large wavelength region 1.5 to 5 μm has to be investigated.
Probably, monochromatic and shorter wavelength
detection will improve the MTF results.
In this paper for object sizes below 1 mm only relative
changes of the raw signal data are used to characterize
the object properties.
10 lp/mm VIS-image 10 lp/mm IR-image 50 lp/mm VIS-image 50 lp/mm IR-image
Fig. 3.18: Determination of the spatial frequency response
(MTF) of the SC6000 with the microscope
objective. VIS- and IR- images of the 10 and 50
lp/mm structures (top).
Line profiles of the IR raw data signals across the
lines of the 10 and the 50 lp/mm structures (middle).
Spatial frequency dependent MTF determined from
the line-profile measurement (bottom).
84 Forschungsbericht Research Report 2007 – 2010