Overlay of melanopsin (Opn4) absorbance spectrum with normalized solar spectral radiance values during sunrise.

A hypothetical A₁-chromophore-based Opn4 spectrum is plotted with absorbance maximum of 480 nm (dashed trace) with solar radiance spectra recorded at five-minute intervals from the onset of civil twilight (06:35 EDT) until just after sunrise (07:00 EDT). Spectral radiance measurements (μW cm^-2 nm^-1 sr^-1) are normalized by the integral (i.e., area under the curve) and scaled from zero to one.