The reciprocal of the @[email protected], \(E_{\text{p}}^{0}\), at @[email protected] \(\lambda \), causing identical photoresponse, \(\Delta \text{y}\), per unit time (\(\frac{\Delta \text{y}}{\Delta t}\)). The @[email protected] spectrum is directly proportional to the @[email protected] of the sensory pigment, if spectral @[email protected] is negligible.