Non-destructive light transmission measurement of large glass panes
The amount of visible light transmitted by the glass used in buildings, automobiles, airplanes, trains, etc. is of utmost importance to the people occupying and using them. Therefore, the light transmission of window glazing, vehicle windscreens, etc. needs to be evaluated with respect to the photometric sensitivity of the human eye. Conventional measurements with laboratory based spectrophotometers, as with the statutory qualification of car windscreen light transmission in accordance with ECE R43  for example, permit only relatively small pieces of glass to be measured. Large panes of glass and windscreens must first be cut into suitably small samples. Portable spectrophotometers, on the other hand, help to avoid this time-consuming and costly effort and even make it possible to measure the light transmission of windows in-situ.
The big challenge in designing a portable spectrophotometer is the implementation of the required measurement geometry. The classical arrangement of conventional stationary measuring systems, as given in ECE R43 for example, is based on sample illumination by a parallel light beam and an integrating sphere as detector.