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Models for spectral luminous efficiency in peripheral vision at mesopic and low photopic luminance levels
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Report / Helsinki University of Technology, Lighting Laboratory, Teknillinen korkeakoulu, valaistuslaboratorio, 37
Abstract
The aim of this work was to find out how the spectral sensitivity of the human eye varies in peripheral vision at mesopic and low photopic luminance levels. Two different measurement methods were employed for this purpose. Reaction time and contrast threshold measurements were conducted at various mesopic and low photopic luminance levels in foveal and peripheral vision. Luminance levels between 0.1 cd/m2 and 20 cd/m2 and visual target eccentricities of 0°, 10°, 30° and 60° were investigated in the measurements. Both measurement methods revealed the same observations: at low photopic luminance levels, the currently used V(λ) function is not the best descriptive spectral luminous efficiency function for peripheral vision. The short wavelengths were underestimated by the V(λ) function. A new spectral luminous efficiency function Vper(λ) was developed for peripheral vision at photopic light levels. This function is a linear combination of V(λ) and V10(λ) functions and takes into account the higher spectral sensitivity to short wavelengths as compared to the V(λ) function. Both measurement methods show that this function describes the spectral sensitivity of the eye better than V(λ), VM(λ) or V10(λ) functions at low photopic luminance levels (≤ 20 cd/m2). At mesopic luminance levels (≥ 0.1 cd/m2), it was investigated how a linear combination of photopic and scotopic spectral luminous efficiency functions describes the spectral sensitivity of the eye in peripheral vision. A model for the mesopic spectral luminous efficiency function Vmes(λ) was developed for peripheral vision. The results showed that this function describes the peripheral spectral sensitivity with best accuracy at mesopic luminance levels. The new function was compared with the proposed unified system of photometry by Rea, Bullough, Freyssinier-Nova and Bierman (2004). The function developed in this work described the measurement results with much higher accuracy. A sigmoid function was developed to describe the mesopic spectral luminous efficiency function as a combination of photopic and scotopic spectral luminous efficiency functions. This function can be used to calculate the weighting coefficient from 0.1 cd/m2 upwards.