Abstract
Four methods of heterochromatic photometry were employed, using the same four observers in each case. These were (i) two types of direct heterochromatic photometry (direct comparison with white, and step by step), (ii) flicker photometry, and (iii) the minimally distinct-border method (MDB). The MDB method is shown to yield results that are linear and obey Abney’s law. Flicker and MDB methods generate relative luminous-efficiency functions that agree well with each other and also with the CIE standard observer as modified by Judd; the methods of direct heterochromatic photometry yield data that agree fairly well with each other, whereas they differ greatly from the data obtained by flicker or MDB. Luminous efficiency as measured by the direct methods seems to receive a contribution from two sources, (a) achromatic signals of the photopic visual system, which exclusively determine the MDB setting, and (b) chromatic signals of the visual system, which produce extra brightness, the amount of which is related to the saturation of the stimulus used.
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