Modeling spectral sensitivity at low light levels based on mesopic visual performance

Meri Viikari, Aleksanteri Ekrias, Marjukka Eloholma, Liisa Halonen

Lighting Laboratory, Helsinki University of Technology, Finland

Abstract: The spectral sensitivity of the eye at low light levels, ie, mesopic conditions, is determined by the rod and cone photoreceptors of the retina operating together in varying degree as adaptation luminance shifts between the scotopic and photopic. Thus mesopic spectral sensitivity is different from photopic, where only cones contribute to vision. There are definite needs for a practical system of mesopic photometry to be used in assessing light at low light levels, especially in road and other outdoor lighting applications. However, neither of the recently proposed systems of mesopic photometry, the MOVE-model or the X-model, is found satisfactory by common consent of the lighting community. The most active debate has considered the upper luminance limit of the mesopic region, which is regarded to be too high for the MOVE-model and too low for the X-model. The present paper proposes a new modified MOVE-model whose upper luminance limit is adjusted to meet the actual road and street lighting luminance values measured in different weather conditions. The paper compares the MOVE-model, X-model, and the proposed modified MOVE-model with three independent visual performance data sets provided by different European universities. Based on the comparison, recommendations are given for future actions towards internationally accepted practice for mesopic photometry.

Keywords: spectral sensitivity, mesopic, visual performance, mesopic model, photometry, night-time driving