Lead Sulfide (PbS) in thin film form was prepared by thermal evaporation in order to evaluate its optical characteristics . The absorption coefficient of lead sulfide in the 3-1 1 microns band of the infrared spectrum was calculated under smooth and rough surface conditions , The calculated and measured absorption coefficient was in a good agreement with that published by Semiletov . These results were an orderof magnitude higher than that published in the handbook of optical constants of solids and referred to Scanlon. I IntroductionMany attempts [2,3,4,5,6,] were made to calculate and measure the optical constants of Lead sulfide (PbS) in its bulk and thin film form in order to evaluate its characteristics to be used as an infrared detector .The detector performed from this material had a very good detectivity in room temperature environment and the detectivity increases by cooling to 77k to meet all detection , searching and tracking applications . Different methods were used to improve PbS in thin film form also, different approaches were used to calculate the optical constants of PbS depending on the spectral region of measurement or calculation .The results of measurements and calculation are classified into 2 groups : 1-The optical constants measured and calculated by Dixon[4] , Zemel, Jensen[3] , Scanlon[9] and these results 2-The optical constants measured and calculated by Semiletov[ IO] which is one order of magnitude higher than that published by the first group. A theoretical comparative study was performed between the two groups and a comparison between the optical constants calculated in this work is introduced. Fig.( 1) shows the refractive index ; absorption coefficient variations with wavelength reported by several workers, are referenced in handbook of optical constants by Palik [7].. I1 Theoretical ApproachesA basic phenomena associated with the transmission spectrum of films is the existence of the interference fringing that causes the variation of the transmission between a maxima and a minima as a function of wavelength for different thickness of the layers .For a film arrangement as illustrated in Fig.(%) where the is deposited on a smooth substrate whose thickness is much larger than that of the film so that in practice the planes are parallel so that all interference effects due to substrate are destroyed ; there are two approaches for analyzing the transmission spectrum of the film .The first approach is to consider the film as a uniform one with thickness uniformly distributed over the substrate surface , In this case if the film refractive index(n) is complex such that air (n,=l) Substrate (s) Fig. (2) Film arrangement. n=n-i k (1) where k.. . . extinction coefficientThe transmission spectrum is given by the well known relation :

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The increased requirements on ranging precision in many military, industrial and astronomical applications illustrate the importance of the Laser ranging systems (called Laser radar) as a modern ranging system. In this paper, the authors study the main features of laser ranging systems in its two main modes, the pulsed mode and the continuous wave subcarrier mode (CWSM). The main advantages of each mode are discussed then proved out by experimental data given by the satellite ranging system Helwan and an experimental setup built on the bases of CWSM. These results show good agreement with that expected theoretically.