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.
Desing of a portable Spectrophotometer
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.
Basic ECE R43 / ISO 3538 measuring arrangement for light transmission
Implementation of ECE-R-43
The implementation of an inverted measuring geometry of ECE-R-43 can be made considerably more compact. This follows the Helmholtz reciprocity principle for the reversibility of a beam path. The measuring geometry also corresponds to the luminance measuring arrangement according to CIE 130  and DIN 5036 .
Schematic representation of the inverted ECE R43 measuring arrangement for light transmission
When implementing the inverted ECE R43 measuring arrangement in a mobile spectrophotometer for measuring the light transmission, special attention must be paid to compliance with:
- Size ratio of the illuminated field of the integrating sphere light source and the measuring beam of the luminance meter.
- Integrating sphere light source with spectral monitor detector for simulation of the CIE standard illuminants A, D50 and D65 as well as for the correction of back reflections of the specimen into the integrating sphere.
- Robust semiconductor source.
- Pulsed operation of the light source and matched detection to enable the rejection of ambient light.
- Luminance measuring head with quasi-parallel measuring beam.
- Luminance probe with spectral detector to simulate the CIE photometric function.
- Luminance probe with camera-assisted positioning of the luminance measurement beam relative to the illuminated field of the integrating sphere light source
The light source and luminance meter are two independent units which must first be placed together in order to establish a 100% level. Subsequently, the light source and luminance meter are positioned on each side of the glass sample and aligned. Thereafter, the measurement of light transmission in % is measured with CIE standard illuminants A, D50 or D65 as well as the spectral transmission.
Measurement Solution for Light Transmission and ECE R43
Gigahertz Optik GmbH offers the LCRT-2005H-S portable spectrophotometer proven in many applications. Its compact size allows measurements on glass plates of any size. The device can be integrated into stationary measurement setups and is offered with accessories to support many varied applications.