The effect of an elastic deformation on optical properties of an aeolotropic medium (piezo-optic effect) is explicitly established from an energy function. Then is deduced the number of distinct coefficients ruling this effect in the special case of quartz. The author gives a new method to measure the absolute dephasage ; method based on photometric exploration of the three beams diffracted image, around the focus. By this method a phase variation of 2π/2000 has been disclosed. This method has been used for the direct measure of plaza-optic effect in quartz. With a single type of cubic sample, 5 out of the 8 coefficients of quartz have been accurately settled, and an upper limit has been given for the one of others. Other applications of this measurement method are considered.