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Both the theoretical and the experimental problems of backgrounds are examined. We show why the current definitions of correlation length should be used with care, with attention paid to the intensity histogram of a scene. We also explain how the sub-pixel features in a measured scene affect the clutter differently for imaging and scanning systems. We measured the two-dimensional polarization of a scene and find it to compare favourably with the theoretical predictions. Finally, we show how to simulate backgrounds whose power spectrum is given, together with constraints on the image proper. This is achieved by iteratively transforming between the image plane and its Fourier conjugate, while imposing the appropriate constraints in both planes.

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The computer code LOWTRAN is widely and extensively used for the prediction of propagation of IR radiation through the atmosphere. The latest version of this code, LOWTRAN 7, is assumed to be the most elaborate and accurate one. Hence it was decided to test its validity by comparing its prediction for ground-to-space slant paths with actual transmittance measurements with the sun as a blackbody source. A good agreement between the theoretical predictions and the experimental results were obtained in the 8-12gm spectral region for all zenith angles between 90 and 60 (0 to 30 above the horizon) . In the 3-51,an spectral region some spectral discrepancy was observed though the value of the integrated measured transmittance agreed well with the predicted one. The reasons for this can be assumed to be in the new band model absorption parameters (in the 3-4pm region) and in the water continuum model (in the 4.4-5.2gm region).from EOSEAL) and a cirrus cloud model. The latest version (LOWTRAN 7 -1988)