1. A method for evaluating oxygen saturation of the optic nerve and retina, said method comprising the steps of: registering a set of multi-spectral images of a retina, locating blood vessels in each of the multi-spectral images, evaluating the width of the blood vessels, selecting sample points for calculating optical density, calculating the optical density ratio, where the optical density is the logarithm of the light reflected from background divided by light reflected from vessel and said optical density ratio is the ratio of said optical density at two different wavelengths, evaluating oxygen saturation level, and presenting results indicating oxygen saturation of the optic nerve and retina.