an improved grain quality analyzer , for analyzing the percentage concentration of various constituents , e . g . protein and water , in a grain sample , photo - optically measures the change in the optical density of the sample , δ od , in a range of characteristic wavelengths and , for protein , in a range of neutral wavelengths and uses these measured values to compute the percentage concentration of the constituents . a grain sample is irradiated with light , the wavelength of which sweeps across the infrared light - spectrum including a range of wavelengths termed characteristic wavelengths , and , for protein , a range of wavelengths termed neutral wavelengths . the characteristic wavelengths are those wavelengths at which the optical characteristics of the irradiated sample , that is , the optical density , reflectivity , transmissivity , and / or absorption , are known to vary as a function of the concentration of the measured constituent , and , for the protein measurement , the neutral wavelengths are those wavelengths at which the optical characteristics are substantially independent of the concentration of the protein . in the preferred embodiment , photo - optical sensors measure the light reflected from the sample and provide output signals indicative of the optical density of the sample as a function of the wavelength of the irradiating light . control and computing means sample the sensor output at spaced apart points in the range of characteristic wavelengths and in the range of neutral wavelengths to provide signals representative of the optical characteristics of the sample . computing circuitry computes the percentage concentration of water and , for protein , computes the percentage concentration using an algorithm in accordance with the present invention which eliminates the inaccuracies introduced by light scattering from the sample .