The spectral output quantity of a system such as a photomultiplier, diode array, photoimaging device, or biological unit divided by the spectral irradiance $s(\lambda )=\frac{\text{d}y\left(\lambda \right)}{\text{d}E\left(\lambda \right)}$, simplified expression: $s(\lambda )=\frac{Y_{\lambda }}{E_{\lambda }}$, where $Y_{\lambda }$ is the magnitude of the output signal for irradiation at wavelength $\lambda$ and $E_{\lambda }$ is the spectral irradiance of the parallel and perpendicular incident beam at the same wavelength.