Outstanding problems in photometry

ABSTRACT DR. J. W. T. WALSH gave a thoughtful address on “Outstanding Problems in Photometry” to the Illuminating Engineering Society on February 4. He pointed out that the advent of the

gaseous discharge lamp has made the problem of heterochromatic photometry a very urgent one. It is well known that if two surfaces are judged to be equally bright by a number of observers

with normal colour vision when the brightness is one candle per square foot, they certainly do not necessarily appear equally bright when the energy they emit is halved (Purkinje effect).

Another class of photometric measurements which presents new and difficult problems is the photometry of projection apparatus, such as motorcar headlights, signal lights and similar devices.

For such apparatus the conception of candle-power often becomes meaningless. The different optical elements produce beams for which the effective light centres are at such widely separated

positions that there is no point that can be approximately regarded as the effective light centre of the whole. Dr. Walsh suggests two methods of attack. The first is to abandon candle-power

measurements altogether and give a figure or figures for the illumination produced at a specified distance from the face of the projector. The other is to make measurements of candle-power

at very great distances from the projector. In the case of a railway signal, this distance may be half a mile. This calls for the use of a telephotometer. A sensitive instrument of this type

will probably in the future become part of the regular equipment of any photometric laboratory which has to deal with projection apparatus. In addition, problems arise in deciding what type

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Photometry. _Nature_ 137, 610 (1936). https://doi.org/10.1038/137610a0 Download citation * Issue Date: 11 April 1936 * DOI: https://doi.org/10.1038/137610a0 SHARE THIS ARTICLE Anyone you

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