In a method and apparatus, a property of an optically diffuse medium including a first optical absorber having a first concentration and a second optical absorber having a second concentration is determined. A surface area of the medium is imaged at multiple wavelengths around an isosbestic wavelength of the first absorber and the second absorber. A reflectance spectrum of the medium at the surface area at the multiple wavelengths is determined. A derivative of the determined reflectance spectrum around the isosbestic wavelength is determined. From the derivative, a concentration ratio of the first concentration and the second concentration is estimated.