Degree of coherence in the image of a quasi-monochromatic source.

A method is described for measuring the modulus of the degree of coherence in a plane illuminated directly or through an optical system, by a quasi-monochromatic source which may be coherent, partially coherent, or incoherent. A birefringent element is placed after the plane in which the degree of coherence is to be measured and an image of this plane is formed by means of a corrected lens. Two sheared images of the plane interfere and the visibility of the interference phenomenon gives the modulus of the degree of coherence. The method is applicable when the degree of coherence is a function of the separation of two points and not of their positions. Experimental results are given.