Aodulation 
same pixel 
directions 
TF can be 
amples for 
haracterise 
)ptics. The 
bilities for 
line-signal 
  
Reinhard Schuster 
similar to that in figure 7a. The shape of the curve reflects the intensity drop across the FoV, known as the cos“ law, but 
the high frequency variation is caused by the variation of responsivity (photo response non-uniformity - PRNU) of the 
line-pixels. It seems useful to correct the signals pixel-wise to obtain constant signals with homogenous illumination. 
The correction can be done very simply online by subtraction (dark signal) or multiplication (PRNU and limb 
shadowing) by the digital part of the electronics. An alternative solution would be off-line correction during the image 
processing work. In both cases it is necessary to determine the radiometric correction values in a separate calibration 
process. After this correction the line signal must be free from PRNU and limb shadowing (figure 7c). 
  
  
  
  
Figure 7. Line-signal: a-uncorrected, b-PRNU-correction values, c-corrected 
Normally CCD camera systems are sensitive in a relatively wide spectral band. Therefore it is not possible to determine 
the spectral behaviour of radiance of an unknown source. Nevertheless the absolute calibration can be done to an 
averaged value given by the convolution of the normalised responsivity with the spectral characteristics of the source. 
This absolute calibration is a part of the calibration program of the DLR facility. To proceed the following steps are 
necessary: 
l. Measurements of the irradiance in front of the camera optics with separate detector dependent on wavelength. 
2. Measurement of the spectral responsivity of the pixels and normalisation with the spectral behaviour of the 
facility. 
Measurement of an absolute calibrated source (radiometric sphere) covering the total FoV of the camera. 
4. Calculation of the absolute radiometric correction values. 
>> 
The method of absolute calibration was tested with a spaceborne camera system and gave a Noise Equivalent Radiation 
(NER) of 30-70 10*W/m3sr with a resolution of 7 10^W/m?sr. The performance of the PRNU correction was better than 
0.2%. Figure 8 shows some sample results. 
RADI. LB2 BDSS_UW 
E+6 
6.00 150 | | 
  
  
  
  
  
  
  
  
  
T 1 uorum T T SE 
400 500 600 700 800 900 400 500 600 700 800 900 
Figure 8. Spectral responsivity of (a) a pixel, (b) radiance of the radiometric W/m?sr 
3 PHOTOGRAMMETRIC CALIBRATION FACILITIES AT LH SYSTEMS 
The main optomechanical parts of the ADS40 are produced by LH Systems in Heerbrugg, Switzerland. Consequently, 
new instruments were developed for dual use at this location: to test lenses during the manufacturing process and to 
perform the final calibration of the integrated camera head at system level. 
3.1 Geometric Calibration 
Geometric calibration of the ADS40 includes the quantitative determination of the image quality and of the registration 
geometry. To speed up measurement, both criteria should be measured simultaneously. This is accomplished by a coded 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part Bl. Amsterdam 2000. 291