1. A method implemented by a computer-based imaging system for evaluating oxygen saturation of optic nerve and retina, said method comprising: registering a set of spectral images of a retina within a multi-spectral image, which comprises using border regions to estimate orientation of each spectral image, equalizing the orientation of each spectral image by rotating the spectral image, edge-detecting each spectral image, estimating a translation between the spectral images based on the edges of adjacent spectral images, and transforming the spectral images to a stack of registered spectral images, locating blood vessels in each of the spectral images, evaluating a width of the blood vessels, selecting sample points for calculating optical density, calculating an optical density ratio, wherein the optical density is a logarithm of light reflected from background divided by light reflected from the blood vessel and said optical density ratio is ratio of said optical density at two different wavelengths, evaluating oxygen saturation level, and presenting results to indicate the oxygen saturation level of the optic nerve and retina.