From Conoscopy to Sphenoscopy: New Experimental Methods in Crystals Inspections

Naturally birefringent crystals are fundamental components of devices with many fields of application, from high energy physics and biomedical devices, to lasers. Crystals functional properties are related to their quality. Non-destructive tests are mandatory to check and improve the quality of these expensive materials as well as to control the growth process. In this paper, a set of methods are presented, based on photoelasticity. The classical diffuse light Photoelasticity, laser Conoscopy and laser Sphenoscopy are discussed and compared. Each technique has shown reliability and sensitivity to the crystal stress condition. The work discusses the main characteristics of these techniques; from the simple optical set-up of the diffuse light technique, limited to volume measurements, to the extremely high resolution of the laser Conoscopy achieved through a more complex optical lay-out and, finally, to the fast and easy data analysis of Sphenoscopy.