The Optical Properties of Some Azo Pigments

Crystals of a number of azo pigments have been prepared by suitable recrystallisation methods, of sufficient size (about 200 μ) for light transmission and refractive index measurements to be made on them. The respective measurements were made with a specially designed microspectrophotometer and by a combination of immersion and retardation methods. X‐ray diffraction photographs showed that the crystal structure had not been changed by recrystallisation. In all cases the pigment crystals were birefringent and had very different absorption curves for polarised light travelling in the slow and in the fast directions in the crystal, resulting in considerable colour differences. This explains why the particle‐size distribution of different samples of such pigments must be very closely similar if dispersions of them are to display the same colour to the eye. It also follows from the high refractive indices found in some instances that these pigments must contribute considerably to the opacity of films in which they are dispersed.