New Horizons of Science, Technology and Culture Vol. 10

Radiometry measures optical radiation in terms of physical energy. Photometry, in contrast, weights that radiation by the spectral sensitivity of human vision. This study addresses a fundamental limitation in lighting science: the absence of a standardised, physically rigorous method for converting photometric quantities (lumens, lux) into photon-based metrics required for biological, environmental, and astronomical applications. Existing estimates of photon flux per lumen vary widely due to inconsistent wavelength assumptions and incomplete radiometric treatment. A first-principles framework is developed by linking the SI definition of photometry (683 lm·W⁻¹ at 555 nm) with Planck’s photon energy relation, yielding a reproducible monochromatic reference conversion of 4.09 × 10¹⁵ photons·s⁻¹·lm⁻¹. This reference is then generalised to broadband light sources through spectral integration of spectral power distributions (SPDs), demonstrating that photon flux per lumen is inherently source dependent. To bridge photometric measurement and biological exposure assessment, a unified metric—Ocular Photon Exposure (OPE)—is introduced, integrating corneal illuminance, pupil geometry, and photon flux conversion. A worked example using a digitised white LED SPD shows that photon exposure can differ by a factor of approximately five compared with monochromatic assumptions for identical illuminance levels. The proposed framework resolves longstanding inconsistencies in lumen–photon conversion and establishes a robust foundation for interdisciplinary applications, including circadian photobiology, outdoor trespass lighting (OTL), environmental light pollution, and human-centric lighting design.