One Hundred Years of V(λ): Predicting Spatial Brightness Using Vertical Illuminance Derived from Photopic Luminous Efficiency Functions

Many years later, as it faced the firing squad, the photopic luminous efficiency function was to remember that distant afternoon when its fathers developed it to quantify light. Drawing parallels from Gabriel García Márquez’s complex and multi-layered novel, this article details the cyclical nature of a recurring topic and the repetition of events across generations. About 100 years ago, the CIE 2° standard observer (V(λ)) was developed to enable a reliable, reproducible photometric system. V(λ)-derived photometric units (e.g., illuminance) were envisioned to predict a generic “visibility” response, but today they are widely used to quantify one of the most fundamental responses to light: brightness. Despite its limitations and several proposed alternatives, the lighting industry still uses V(λ) to calculate photometric measures. This study examines the predictive capacity of V(λ) and its three alternatives (CIE V₁₀(λ), V_F(λ), V_F,10(λ)) across three CCTs (2700 K, 4000 K, 6000 K) and three illuminance (50 lx, 100 lx, 300 lx) levels in predicting spatial brightness. Alternatives outperformed V(λ) under 2700 K and 50 lx, but overall photopic luminous efficiency functions (including V(λ)) could not predict spatial brightness consistently. Future studies should investigate the performance of specialized spatial brightness metrics.