chAPter | 31 Non-Destructive Testing 579 the use of defect echo heights for the purpose of size assess- ment is not advisable if the spectral "signatures" of the defect echoes show significant differences. 31.4.6 other Ways of Presenting Information from ultrasonics Many other techniques of processing ultrasonic information are available, and in the following the general principles of two main groups: (1) ultrasonic visualization or (2) ultra- sonic imaging will be outlined. More detailed information may be found in the references given at the end of this chapter. Ultrasonic visualization makes use of two main tech- niques: (1) Schlieren methods and (2) photoelastic methods. A third method combining both of these has been described by d. M. Marsh in Research Techniques in Non-destructive Testing (1973). Schlieren methods depend on detecting the deviation of light caused by refractive index gradients accompanying ultrasonic waves. in most cases this tech- nique has been used in liquids. Photoelastic visualization reveals the stresses in an ultrasonic wave using crossed Polaroids to detect stress birefringence of the medium. Main uses have been for particularly intense fields, such as those FIgure 31.26 Schlieren visualization.