This technical report presents a comprehensive review of intrinsically photosensitive retinal ganglion cell (ipRGC) biology, from the initial discovery by Berson et al. (2002) through the characterisation of five subtypes (M1-M5), their distinct central projections, and the functional circuits they subserve. We examine the melanopsin phototransduction cascade, the spectral sensitivity profile (peak ~480 nm, extended range to ~560 nm), and the properties of sustained, non-adapting responses that distinguish ipRGCs from rods and cones. Particular attention is given to two ipRGC-mediated pathways with direct clinical relevance: the retinohypothalamic tract (RHT) to the suprachiasmatic nucleus (SCN) governing circadian melatonin regulation, and the projection to posterior thalamus where convergence with trigeminal nociceptive afferents drives migraine photophobia (Noseda et al., 2010). The melanopsin action spectrum is presented quantitatively as the foundation for wavelength-selective lens design, and the implications for FL-41 migraine lenses, amber circadian lenses, and daytime performance lenses are systematically analysed. Twenty-two peer-reviewed references are cited. Patent disclosures included for transparency.
Dubey et al. (2026) studied this question.