Images of absolute retardance L center dot Delta n, using the rotating polariser method

Modulation techniques for measuring changes in optical birefringence, such as the rotating-polariser method (Wood & Glazer, 1980, J. Appl.. Crystallog r. 13, 217), allow one to determine \sin delta\, delta = 2 pi L Delta n/lam bda, Delta n = double refraction, L = light path and lambda = wavelength. H owever, they generally suffer from not providing absolute values of the opt ical retardance or are limited to relatively low retardance values. In addi tion, knowledge of the absolute phase is required when establishing the cor rect values of optical orientation information. In this paper, it is shown how the phase delta, and thus optical retardance, can be extracted from com bining measurements of \sin delta\ at different wavelengths. The new approa ch works on each single point of a 2-D picture without the need to correlat e with neighbouring points. There is virtually no limit to the retardance, and the computational efforts are small compared with other methods (e.g. A jovalasit et al. 1998, J. Strain Analysis 33, 75). When used with imaging t echniques, such as the rotating polariser method of Glazer, Lewis & Kaminsk y 1996 (Proc. R. Sec. London Series A452, 2751) this process has the potent ial to identify automatically optically anisotropic substances under the mi croscope. The algorithm derived in this paper is valid not only for birefri ngence studies, but can be applied to all studies of interfering light wave s.