The reciprocal of the photon fluence rate, E p 0, at wavelength λ, causing identical photoresponse, Δ y, per unit time (Δ y Δ t). The effectiveness spectrum is directly proportional to the conversion spectrum of the sensory pigment, if spectral attenuance is negligible.
Source:
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2275 (https://doi.org/10.1351/pac199668122223)