Background stimulus for invariantspectral sensitivity

Increment‐threshold spectral sensitivity was determined with backgrounds of four different chromaticities and two different troland (td) levels. One background was equivalent to a field delivering to the receptors an equal quantal flux at all visible wavelengths. Another background approximated CIE Illuminant A. Light from a chromatically unfiltered xenon‐arc lamp provided a third background. The fourth background was monochromatic at 470 nm. Backgrounds were presented at 10 and 1000 td. Foveal spectral sensitivity determined on each of these backgrounds was compared to that measured for the dark‐adapted state. The 10‐ and 1000‐td equal‐quantal fields yielded the functions that most closely approximated the curve resulting from the dark‐adapted condition, suggesting that such an equal‐quantal field, which appeared as a saturated blue, is neutral for measurements of spectral sensitivity. Because of the putative contribution of the opponent chromatic channels to spectral sensitivity determined using increment thresholds, the results further suggest that at the detection stage of these channels an equal‐quantal field provides a balanced state of adaptation