Studies in the photometry of lights of different colours / by Herbert E. Ives. : spectral luminosity curves obtained by the equality of brightness photometer and the flicker photometer under similar conditions.

  • Ives, Herbert Eugene, 1882-1953.
Date:
[1912]
Catalogue details

Licence: In copyright

Credit: Studies in the photometry of lights of different colours / by Herbert E. Ives. : spectral luminosity curves obtained by the equality of brightness photometer and the flicker photometer under similar conditions. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

    7/106 (page 151)
    of lights of different hue or saturation. This difficulty has long been recognized, and various opinions as to the solva- bility of the problem have been expressed. Various plans to accomplish its solution have been proposed. The belief has been expressed by some that comparison of the luminosities of two differently coloured lights is no more possible of exact accomplishment than the exact comparison of a sound with an odour. Helmholtz declared his lack of confidence in his own ability to make such comparisons. On the other hand, some observers have by practice acquired skill and apparent certainty in heterochromatic photometry. Again, methods of photometry have been discovered and developed which, in the process of measurement, evade and perhaps eliminate the differences in hue and saturation. By these different methods different criteria for equality of luminosity are obtained. What are the relations between the criteria ? Are two coloured lights which are equally bright as measured by one method, of equal brightness as measured by another ? The relative brightness of differently coloured lights is not constant under all conditions. Physiological factors enter to a large degree. The u Purkinje effect is the name given to the increase of blue sensitiveness at low illumination. The yellow spot effect is the name given to the change in the relative brightnesses of different colours occurring when the size of the field of view is changed. To what degree do these enter in the various methods of colour photometry ? The advent of high efficiency incandescent lamps and of new illuminants with wide range of colour, the prospect of an even greater variety of illuminants in the future, diverging still further in colour from the present flame and carbon lamp standards, all render the problem of measuring the luminosity of lights of various hues one of the most important in photometry. A complete solution of the problem can come only through knowledge of the relative behaviour of each method of comparison under the various conditions known to affect the relative brightness of different colours. That knowledge acquired, we may proceed to a process of test and selection, and so hope to determine that method which meets to the fullest degree the requirements of a method of measurement. The chief obstacle in the way of giving preference to one method of colour photometry over another is at present our lack of data. Some forms of flicker photometers have been compared with some forms of direct comparison photometers. Spectral luminosity curves for the eye have been derived by tests of visual acuity, by the flicker photometer, by equality