A simulation method to determine adaptation luminance is proposed for implementation of the CIE mesopic photometry system. The simulation takes four factors into account: luminance distribution, eye movement of observers, surrounding luminance effect and area of measurement. Each factor is modelled as a two-dimensional geometrical function. The method determines an adaptation luminance for the area of measurement through four calculation steps. The simulation method was applied to examples of luminance distributions of outdoor lit scenes and the results were compared with possible simple predictors of adaptation luminance. The comparisons suggest that the average luminance of the area of measurement can be considered as a good approximation in most of the cases. Exceptions are scenes for pedestrians in which there are many bright sources surrounding the area of measurement.
CIE 191:2010 recommends a mesopic photometry system that defines the luminous efficiency function for peripheral visual tasks, which vary depending on the adaptation state of observers. For implementation of the system, an adaptation field to determine the adaptation state needs to be defined. To address this issue, vision experiments have been conducted to measure surrounding luminance effects on the adaptation state at a peripheral task point. The results reveal that the adaptation state depends mainly on the local luminance at the task point but there is also a small effect of the surrounding luminance. The results suggest that the surrounding luminance effect is larger than the veiling luminance predicted with existing foveal models; nevertheless, it is not significant for the mesopic luminance on uniform luminance distributions.
CIE 191:2010 recommends a mesopic photometry system in which the luminous efficiency function is determined from the adaptation luminance. However, it cannot be practically implemented because the adaptation field used to determine the adaptation luminance is not defined. Surrounding luminance effect is a key factor in defining the adaptation field. Vision experiments have been conducted to measure the surrounding luminance effect on peripheral adaptation state with a point source at various intensities placed at different positions. The results reveal that the effect can be described as veiling luminance and that the magnitude of the effect is larger than that of the foveal veiling luminance. A model for the peripheral veiling luminance proposed by Stiles and Crawford in 1937 shows the best prediction for the surrounding luminance effect.
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