Kruithof's rule revisited using LED illumination

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Summary

This study revisits Kruithof's rule using LED lighting, finding that visual performance depends primarily on luminance rather than CCT, while perceived brightness is influenced by CCT (higher CCT appears brighter). For lighting designers, this suggests that CCT choices may be better guided by subjective comfort and ipRGC activation considerations than by performance outcomes alone.

Key Findings

Visual performance tasks were largely dependent on luminance level and only slightly influenced by correlated colour temperature (CCT).
Observers consistently rated high CCT illumination as appearing brighter than low CCT illumination at equivalent illuminance levels.
High colour rendering index LEDs (Ra > 90) were used across multiple CCT and illuminance combinations, and spectral power distributions were found to differentially activate intrinsically photosensitive retinal ganglion cells (ipRGCs) and affect pupil response.
Results challenge the practical basis of Kruithof's rule, suggesting the 'pleasing' effect is driven by subjective appraisal rather than facilitated visual performance.

Categories

The Science of Light: Investigates spectral power distributions, CCT effects on ipRGCs, pupillary light reflex, and melanopic responses under LED illumination.

Workplace Performance: Tests whether Kruithof's rule reflects real visual performance benefits or subjective preference under varying CCT and illuminance conditions.