1. A system for determining a relative thickness of optic disc tissue, comprising: a light source configured to project broad spectrum light towards the optic disc tissue; a receiver configured to receive light reflected from the optic disc tissue at at least two spectral wavelengths; a digitizer in communication with the receiver configured to form a first image from the reflected light at each wavelength at a first time, and configured to form a second image from the reflected light at each wavelength at a second time; and a processor configured to determine a normalization point in each of the first and second images, compute a difference in the absorption of light by the optic disc tissue at selected image points in each image at each of the two spectral wavelengths to determine a thickness at each selected image point, determine relative thicknesses of the optic disc tissue of each image by compassing the thickness at each selected image point to the normalization point, compute an average of the relative thickness for each image to obtain an average thickness for each image, select one average thickness from the images to be a reference average thickness, and compute the difference between the average thickness and the reference average thickness for the first and second images to determine a relative thickness of the optic disc tissue over time.