With q as the electronic charge, λ λ is the wavelength of interest, h is Planck's constant, c is the speed of light, k is Boltzmann's constant, T is the temperature of the detector, R 0 A is the zero-bias dynamic resistance area product (often measured experimentally, but also expressible in noise level assumptions), η η is the quantum efficiency of the device, and Φ Φ b is the total flux of the source (often a blackbody) in photons/sec/cm.