Definition:
Alpha-opic quantities weight light according to the sensitivity of the eye’s five photoreceptors, as defined in CIE S 026. The five channels are melanopic (ipRGCs), cyanopic (S-cones), chloropic (M-cones), erythropic (L-cones), and rhodopic (rods). When applied to irradiance (at the eye) we get alpha-opic irradiance; when converted to a daylight-referenced, user-friendly unit we get alpha-opic equivalent daylight illuminance (alpha-opic EDI) reported in lux.
Why it matters:
Traditional photopic lux reflects visual brightness but not biological impact. Alpha-opic metrics describe how different spectra stimulate each photoreceptor class, enabling better assessment of non-visual responses (alertness, circadian timing, pupil control) alongside visual performance.
How it works (summary):
- Measure the spectral power distribution at the eye (corneal plane).
- Apply the five alpha-opic spectral weighting functions from CIE S 026.
- Report alpha-opic irradiances (biologically weighted) and, where helpful, convert to alpha-opic EDI in lux for daylight-equivalent comparison.
Measurement and standards:
Alpha-opic metrics are defined in CIE S 026 and are commonly used in research, design validation and building standards that consider circadian-effective light. Spectral light dosimeters from PhotoSpec Labs can report melanopic EDI and other alpha-opic channels to characterise light in real environments.
Applications:
- Comparing lamp or daylight spectra beyond photopic lux.
- Designing daytime scenes to support alertness while controlling evening stimulation.
- Post-occupancy and wearable field studies in offices, schools, healthcare and aged care.
Related terms: