In this part, several methods for quantitative measurement based on the optical double diffraction of the phase distribution in material are described. 1) When a sharp cut-off amplitude filter (i.e., mask) is scanned at a constant velocity in Fraunhofer diffraction plane of the specimen observed, the density distribution of photographic film obtained by photographing the image of a specimen corresponds to the distribution of phase gradient in material. 2) When a linear graded amplitude filter (optical wedge) is inserted in the diffraction plane, the phase distribution can be calculated by the photometric result of the image intensity distribution. Since the result is obtained by a single photographic operation, it is very convenient to measure the time dependent phase distribution or analyze the stress and strain of a specimen in motion. 3) When the phase difference of a specimen is very small as compared with the wavelength, the phase distribution can be presumed approximately from Hilbert transformation of the image intensity which is yielded in a system with half stop. 4) When a polychromatic light source is adopted instead of a monochromatic one and a prism is placed in front of specimen, a coloured image pattern can be obtained. The colour distribution in the image is equivalent to the distribution of phase gradient in the specimen. Of course the analysis of stress and strain is possible by these methods as well as by the mask scanning method described in Part I. Especially the following consideration is given in Appendix. When the phase distribution has a singly periodic sinusoidal construction, the distribution can be determined quantitatively by the photometric observation of the average intensity in the image plane.
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