1. A method for analyzing material moving in an essentially stationary and unchanging spectral background, comprising the steps of: (i) producing at predetermined intervals of time, at least two images at a first wavelength of said moving material in said background; (ii) comparing at least among each other, images obtained from at least one set of at least two of said at least two images for determining regions of said images having a changed intensity level at said first wavelength over at least one of said predetermined intervals of time; (iii) performing steps (i) and (ii) at at least a second wavelength; (iv) performing spectral analysis on said regions of said images having a changed intensity level determined at said first and at said at least a second wavelength; and (v) determining from said spectral analysis the quantitative level of chromophores in said moving material.