Correlated color temperature is not a suitable proxy for the biological potency of light

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Summary

This simulation study demonstrates that correlated color temperature (CCT) cannot reliably predict the biological potency of light as measured by mel-EDI, EML, or Circadian Stimulus (CS), meaning lighting designers must specify melanopic metrics directly rather than relying on CCT. The results show that even at a fixed CCT, large variations in circadian impact are possible depending solely on spectral power distribution choices.

Key Findings

At 300 lx and Rf ≥ 70, Circadian Stimulus (CS) varies from 17% to 41% across CCTs of 2500–6000 K, and varies by up to 23% at a single fixed CCT due to spectrum choice alone.
Mel-EDI varies from 123 to 354 lx across CCTs of 2500–6000 K at 300 lx; at a fixed CCT of 5000 K, mel-EDI still ranges from 196 to 319 lx (a 123 lx spread).
The range of achievable mel-EDI increases with higher CCT and decreases on average as color fidelity (IES TM-30 Rf) increases.
No reliable mathematical conversion exists between CS and mel-EDI when a spectrally diverse set of SPDs is considered.
The CS range is largest and notably discontinuous at 3500 K, corresponding to the inflection point of the CS model.

Categories

The Science of Light: Directly evaluates the relationship between CCT and melanopic metrics (mel-EDI, EML, CS), providing quantitative evidence that CCT is an unreliable proxy for biological potency.

Sleep & Circadian Health: Findings have direct implications for circadian lighting design, as specifying CCT alone is insufficient to achieve targeted melanopic stimulation for circadian health.