Errors In Short Distance Photometry

J. G. Holmes; J. J. B. Moermann

doi:10.1117/12.959712

19 February 1982 Errors In Short Distance Photometry

J. G. Holmes, J. J. B. Moermann

Proceedings Volume 0262, Light Measurement '81; (1982) https://doi.org/10.1117/12.959712
Event: Light Measurement '81, 1981, Surrey, United Kingdom

Abstract

The errors involved in the short-distance photometry of projectors are evaluated and the same conclusions have been shown to apply to general purpose luminaires. The mathematical analysis from which the equations were derived has been published in Lighting Research and Technology (1981). The illuminance at a short distance from the projector does not follow the inverse square law; the errors depend on the angular subtense of the aperture of the projector relative to the divergence of the beam, and on the distribution of luminance across the aperture of the projector. At any particular distance, the errors are least in directions in which the curvature of the intensity distribution curve is least; the errors may therefore be greatest in the axial direction or in the direction of a shoulder on the curve, and they may change sign where the intensity distribution curve changes from convex to concave. In any particular direction, the error is greater if the outer zones of the projector have higher luminance or give a narrower relative spread; the worst case is a ring-shaped luminaire. If the relative error is less than 10 per cent, it is inversely proportional to the square of the distance of measurement. For general guidance, a nomogram relates the maximum likely percentage error to the beam divergence and to the relative distance of measurement; an empirical reference distance, to be known as the Beam Cross-over Distance, is suggested to replace the traditional 'cross-over distance' of a projector.

Citation Download Citation

J. G. Holmes and J. J. B. Moermann "Errors In Short Distance Photometry", Proc. SPIE 0262, Light Measurement '81, (19 February 1982); https://doi.org/10.1117/12.959712

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