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)

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    Let us consider the measurement of spectral colours against one taken as the standard of illumination, or against a white. On the critical frequency-log. I diagram, the standard and the colours to be measured are each represented by straight lines. At a given illumination a quantity of any coloured radiation may be chosen such that flicker will disappear with it at the same speed as with a chosen standard. If the rise and fall or the contour of the flicker sensation is independent of the speed or changes in the same manner for all colours with change of speed, then this coloured light may be alternated with the standard, and the speed may be reduced to the point where colour flicker is just about to appear—forming a flicker photometer. The speed not altering the equality of the two contours, the relative quantities of the standard light and the coloured will be the same as was required to make flicker disappear with each separately at the critical frequency for the standard. If this is the state of affairs the same relationship between critical frequency and slit widths exists for both standard and colour in the flicker method as with the method of critical frequency. That is F = KslogI+ps, F = K\ log Sx+^\. We may then substitute for the critical frequency in question its value in terms of the standard illumination, and obtain as a result KslogI + C = Kx logSx, that is, the logarithms of the slit-widths will plot as a straight line against the logarithms of the illuminations. On the second assumption the relative quantities of the standard and the coloured radiation would be read off the diagram (fig. 4) for the speed at which the flicker photo- meter is run. The possibility is, however, practically ruled out by the fact that, as shown by reference to the data of the first paper, the flicker photometer speeds for an illumi- nation showing a strong reversed Purkinje effect are 8 to 10 cycles per second ; therefore, from the data in the present paper they are well down in the Purkinje region. Speed cannot therefore be the dominant factor. Considerable time and care have been spent in testing the first possibility. The data in the previous paper were plotted lo S\ in the form of ]Qg g ^ against log I. The resultant lines