The impact of room surface reflectance on corneal illuminance and rule-of-thumb equations for circadian lighting design

Recently, corneal illuminance attracts much attention because it is closely related to important functions of indoor lighting. Especially, applying circadian light in the built environment places a challenging requirement on indirect corneal illuminance. In this work, rule-of-thumb equations are proposed to guide circadian lighting design: (i) for artificial lighting, E _{cor,avg (i)} = (Φ/C ₁) · ρ/(1−ρ′), where E _{cor,avg (i)} is the average indirect corneal illuminance at standing or sitting positions, Φ is the initial flux from luminaires, C ₁ is a constant comparable to the total room surface area, ρ is the reflectance of the surface where the first reflection occurs, and ρ′ is the area-weighted average of surface reflectance; and (ii) for daylighting, E _{cor,avg (i)} = C ₂ · WWR · ρ/(1−ρ′), where C ₂ is a constant, and WWR represents the window-to-wall ratio. The equations above are validated by comparing against numerical simulation data obtained with the Radiance software. For artificial lighting simulation, various combinations of room surface reflectance, initial light distribution, and WWR are investigated; and for daylighting simulation, different combinations of surface reflectance, WWR, and geographic location are analyzed. The good fits to simulation data indicate that the proposed simple equations can provide reasonably accurate results for quick feedback at the field. It is also demonstrated that room surface reflectance has a dominant impact on indirect corneal illuminance. The approach of improving surface reflectance is more favorable than increasing luminaire flux or expanding window area, and therefore should be the recommended approach to achieve quality and efficient circadian lighting.