Do CIE-recommendations for proper light at the proper time conflict with glare?

The International Commission on Illumination recently issued a position statement advocating for proper light at proper time, specifically recommending a melanopic equivalent daylight illuminance of 250 lx at eye level during daytime. However, the statement also cautions that avoiding glare from windows requires ‘advanced lighting design guidance’, implying a conflict between achieving healthy lighting and managing daylight-induced glare. This study examines this potential conflict. Through theoretical analysis and a user study across six locations, we found that between the recommended illuminance levels for healthy lighting and the vertical illuminance levels at which glare becomes an issue lies a difference of a factor 10 roughly. This provides façade designers with considerable flexibility in selecting appropriate shading solutions. Furthermore, vertical illuminance levels between 250 lx and 1000 lx from daylight typically reduce perceived glare from façades. A supporting simulation study positioned a sample office in three European climates and employed typical shading devices and controls, confirming these findings for positions up to 2 m from the façade. For deeper room positions in multi-user spaces, solutions become more complex and require advanced lighting design, with higher diffuse transmittance shading appearing beneficial. Overall, the results demonstrate that healthy daytime lighting through daylight does not inherently increase glare risk.