The image-forming response of the eye to light is normally mediated by three classes of cone photoreceptors, whose overall light sensitivity is described by the photopic luminous efficiency function. The eye also has a non-image-forming response mediated by intrinsically photosensitive retinal ganglion cells, which contain melanopsin and are important in entraining circadian rhythms. The response of these cells depends on scene luminance. But the standard luminous efficiency function has a maximum at approximately 555 nm whereas melanopsin has a maximum spectral sensitivity at approximately 480 nm. Is the difference in wavelengths important in natural scenes? This question was addressed by calculating luminance and melanopsin signals in over 30 hyperspectral radiance images of outdoor scenes under natural lighting. The scenes contained mixtures of herbaceous vegetation, woodland, barren land, rock, and artificial objects, such as rural and urban buildings. In a regression of melanopsin signals on luminance signals for each scene the proportion R2 of the variance explained varied between 68% and 99% across scenes and phases of daylight. Within scenes the variation in R2 may have been greater. These results suggest that luminance measures alone are an uncertain predictor of the response of intrinsically photosensitive retinal ganglion cells and that colour or spectral data are also needed.
|Title of host publication||Seeing Color, Regensburg, Germany|
|Publication status||Published - 2016|