Abstract: A method and apparatus for determining the transmission capability of a substrate and any damage which may affect said transmission capability comprising directing a focused infrared illumination beam through the substrate and observing the reflected or scattered light to detecting any scratch, fracture, blemish or the like in the substrate surface, or within the substrate itself, by detecting any change in the energy level of the sensed beam or change in reflected scattered light. The illumination beam provides a spot on the substrate surface and, by mapping an entire substrate of said spots adjacent one another and comparing the energy or light level of adjacent spots, any internal or external damage can be determined. A portable battery operable tester with an illumination beam and associated optics and a transmitted beam and associated optics enables such detection of transmission loss.