Assuming each photon absorbed in the depletion layer produces a viable electron-hole pair, and all other photons do not, N e t = Φ Φ ξ ξ λ λ h c where t is the measurement time (in seconds), Φ Φ o = incident optical power in watts, Φ Φ ξ ξ = optical power absorbed in depletion layer, also in watts.