Abstract: On the base of spatial-temporal analogy in properties of the optical fields the equivalence of temporal and spatial photodetector averaging of light interference field is shown. In the low-coherence interferometry the interferential signal is vanished when optical path difference becomes more than light coherence length l$-c$/ approximately equals $lambda$+2$//$Delta$lambda@. This value determines the accuracy of the testing of the reflecting layer location. The analogous vanishing of the interferential signal can be realized by a spatial averaging of the interference field in the case of the sufficiently wide receiving photodetector aperture. For such averaging Michelson interferometer with a laser beam focussed on the object surface can be used. In this case interferential signal is vanished when the surface displacement is more than a focussed beam parameter $delta@($Delta@z) equals $lambda@/(NA)$+2$/, where NA is a numerical aperture of the objective. With NA approximately equals 1 only a few oscillations of interferential signal is observed. It is corresponding to advanced achievements of the low-coherence interferometry. The experimental results show a high testing accuracy of the position of smooth and rough surfaces, the thickness of transparent coating attached to the rough surface and multilayered scattering objects. Thus, interferometers with a sharply focussed probing beam can be consider as a good alternative for the low-coherence interferometry and can be used in coherent optical tomography. !18
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