Considering the relation of OD with total transmittance (Tt) and reflectance (Rt), the differential coloration efficiency K(λ, x) can be written as K(λ, x) = − 1 Tt(λ, x) dTt(λ, x) dq − 1 [1 − Rt(λ, x) ] dRt(λ, x) dq (8)