In all these cases, the dependence on the wavelength of light has been ignored. In reality, the BRDF is wavelength dependent, and to account for effects such as iridescence or luminescence the dependence on wavelength must be made explicit: f r ( λ λ i , ω ω i , λ λ r , ω ω r ) . Note that in the typical case where all optical elements are linear, the function will obey f r ( λ λ i , ω ω i , λ λ r , ω ω r ) = 0 except when λ λ i = λ λ r : that is, it will only emit light at wavelength equal to the incoming light. In this case it can be parameterized as f r ( λ λ , ω ω i , ω ω r ) , with only one wavelength parameter.