A method of determining the ratio of the absolute absorption coefficients νa(μ2) and νa(μ1) of a light scattering medium at two different wavelengths comprising: a) passing light (as herein defined) through the medium; b) measuring a characteristic X of the light at two wavelengths μ1 and μ2 which is affected by the absorption coefficient during its passage through the medium; c) effecting changes Δνa(μ1) and Δνa(μ2) in the absorption coefficients at the two wavelengths; d) repeating step (b); e) determining the ratio of the absolute absorption coefficients νa(μ1) and νa(μ2) at said two wavelengths from the said changes therein Δνa(μ1) and Δνa(μ2) or from quantities representative thereof, from the characteristic X, and from the ratio of transport scattering coefficients of the medium νs'(μ1) and νs'(μ2) at the two wavelengths.