Definition:
CIE S 026 is the international standard published by the International Commission on Illumination (CIE) that defines a system for measuring and quantifying light’s impact on the human circadian system. It introduces metrics such as melanopic equivalent daylight illuminance (melanopic EDI) and other α-opic EDIs, which describe the biological effectiveness of light based on its spectral composition.
Why it matters:
Before CIE S 026, lighting standards primarily focused on visual performance, ignoring non-visual effects such as circadian regulation, alertness, and hormone production. By adopting CIE S 026 metrics, researchers, designers, and regulators can assess how light influences human health and wellbeing — supporting compliance with frameworks like the WELL Building Standard.
How it works:
CIE S 026 defines five α-opic sensitivities: melanopic, cyanopic, chloropic, erythropic, and rhodopic. These correspond to the five photoreceptor types in the human eye. Light measurements are weighted according to each sensitivity to calculate biologically relevant illuminances. These can then be compared to reference daylight conditions.
Applications:
- Circadian lighting design in workplaces, schools, and healthcare environments
- Research into sleep, alertness, and light therapy
- Compliance with health-based building standards
- Field measurement using spectral light dosimeters such as those developed by PhotoSpec Labs
Related terms: