The Luminosity Curve as Affected by the Relation between Rod and Cone Adaptation*

Abstract

Previous experimental determination of the luminosity curve of the eye at intermediate intensities has shown a series of transitional curves intermediate between rod and cone, the appearance of which has been attributed to rod-cone interaction. However, examination of mesopic curves predicted on the basis of no interaction indicates that their major characteristics are dependent instead on the relation between rod and cone functional levels. Residual discrepancies between predicted and empirical curves require the assumption of summation between the two mechanisms at wavelengths for which sensitivity is nearly equal for both. In addition to contributing to an understanding of relations between rod and cone function, the analysis emphasizes the lack of validity of mesopic curves for photometric calculations.