An optical system for non-invasive monitoring includes a light source to direct one or more pulses of light into an eye, a subject and a light receiver to receive light returning from the eye. The light receiver, or a processor coupled to the light receiver, records a frequency or intensity of the returning light at a selected timing relative to the one or more pulses of light from the light source. This timing may be selected to reduce a proportion of light reflected from a cornea, iris, or lens of the eye.