An apparatus and method for measuring a concentration of a substance in an eye using a retro-reflected measurement light beam having a first wavelength at which the substance has a non-zero first absorption coefficient and a retro-reflected reference light beam having a second wavelength at which the substance has a second absorption coefficient which is substantially equal to zero. The apparatus further includes a detector positionable to receive the retro-reflected measurement light beam and the retro-reflected reference light beam. The detector is responsive to light having the first wavelength by generating a measurement signal and responsive to light having the second wavelength by generating a reference signal. The apparatus further includes an electrical circuit coupled to the detector. The electrical circuit is responsive to the measurement signal and the reference signal to measure the concentration of the substance in the eye.