Alliance for Lighting Information

Mesopic Visual Sensitivity

by David M. Keith

The mesopic sensitivity functions describe the relationship between electro-magnetic radiation and human vision that is mediated by cone and rod photoreceptors, and is associated with luminances less than photopic but greater than scotopic. The definition has changed over the past years, according to IESNA documents, and now the mesopic range is defined as corresponding to luminances less than 3.4 cd/m2 and over 0.034 cd/m2 or 0.001 cd/m2 depending on the IESNA reference.

Regardless of the luminance value corresponding to the boundary between scotopic and mesopic, the mesopic range extends from luminances that allow for full foveal vision, with its corresponding color perception, down to luminances that are solely perceived by the rods and therefore have no foveal image or color perception. This transition range is a common adaptation state under outdoor lighting, and is significant in the understanding of nighttime visual performance.

Figures relating the mesopic range and outdoor lighting criteria are here. A discussion about the mesopic range and outdoor lighting criteria is here.

The performance of the visual system under mesopic adaptation has been the focus of recent research and development, but no single model has been adopted. One important aspect of mesopic vision is the sensitivity of peripheral vision, which may be significant for driving tasks. The differences in reaction time for peripheral visual tasks under mesopic conditions and different sources are discussed here.

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