Comparison between CIE 2° and 10° field photopic luminosity functions V(λ) for calculating daylight discomfort glare metrics

The spectral sensitivity of the average human eye in photopic conditions is represented by the photopic luminosity function V (λ). The CIE has established the photopic luminosity functions for the 2o and 10o visual fields for a standard observer applicable for foveal and para-foveal light sources, respectively. These functions differ in short wavelength region where V10° (λ) has higher sensitivity. However, V10°-(λ) function is not implemented in any of the discomfort glare metrics even though, for most glare scenarios, the glare source is located further than 2o from the fovea. This can result in an underestimation of the short wavelength contribution of the glare sources’ spectra, and, a fortiori, in the blue-colored light sources. In this paper, we aim to determine the impact of replacing V2° (λ) with V10°-(λ) in the daylight discomfort glare metrics for scenarios where the visible sun disk lies very much outside the 2° zone and acts as a glare source through blue-tinted and color-neutral tinted low transmittance glazing. We compare three types of colored glazed façade: color-neutral glazing, blue-tinted electrochromic (EC) glazing and an extreme case of saturated blue-tinted glazing. We found that the difference in derived glare source luminance and discomfort glare metrics is statistically significant only for the saturated blue glazing with an average 70% increase in luminance and 20% increase in DGP (i.e. one category higher discomfort) and 9% increase in CGI, when using V10° (λ). We conclude that the impact of replacing V2° (λ) with V10°-(λ) is negligible for standard EC or color-neutral glazing types in commercial buildings. However, specific cases of saturated blue light sources that peaks at 450nm are more accurately quantified by V10° (λ), that produces higher values of glare metrics.