Non-invasive glucose meter

1. A method of determining the concentration of a substance in an eye, the method comprising:

providing a nearly parallel measurement light beam having a first wavelength at which the substance has a non-zero first absorption coefficient;

providing a nearly parallel reference light beam having a second wavelength at which the substance has a second absorption coefficient which is substantially equal to zero;

irradiating the retina of the eye with the measurement light beam;

irradiating the retina of the eye with the reference light beam;

reflecting at least a portion of the measurement light beam from the retina, thereby producing a retro-reflected measurement light beam having the first wavelength;

reflecting at least a portion of the reference light beam from the retina, thereby producing a retro-reflected reference light beam having the second wavelength;

generating a measurement signal in response to the retro-reflected measurement light beam;

generating a reference signal in response to the retro-reflected reference light beam; and

determining the concentration of the substance in the eye in response to the measurement signal and the reference signal.

2. The method of claim 1 , wherein the measurement signal is generated without contributions from other reflections from the eye.

3. The method of claim 1 , wherein the reference signal is generated without contributions from other reflections from the eye.

4. The method of claim 1 , wherein the measurement light beam and the reference light beam are focused at a point on the retina.

5. The method of claim 1 , wherein the substance comprises glucose.

6. The method of claim 1 , wherein the measurement light beam is provided by a first infrared light source and the reference light beam is provided by a second infrared light source.

7. The method of claim 1 , wherein the measurement light beam is focused at a point on the retina.

8. The method of claim 1 , wherein the reference light beam is focused at a point on the retina.

9. An apparatus for measuring a concentration of a substance in an eye having a retina, the apparatus comprising:

a measurement light source producing a nearly parallel measurement light beam having a first wavelength, the substance having a non-zero first absorption coefficient for light at the first wavelength, wherein the measurement light source is positionable such that at least a portion of the measurement light beam retro-reflects from the retina;

a reference light source producing a nearly parallel reference light beam having a second wavelength, the substance having a second absorption coefficient which is substantially equal to zero for light at the second wavelength wherein the reference light source is positionable such that at least a portion of the reference light beam retro-reflects from the retina;

a detector 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; and

an electrical circuit operatively coupled to the detector, the electrical circuit responsive to the measurement signal and the reference signal to measure the concentration of the substance in the eye.

10. The apparatus of claim 9 , wherein the detector is positionable to primarily receive the retro-reflected measurement light beam and the retro-reflected reference light beam without receiving other reflections from the eye.

11. The apparatus of claim 9 , wherein the measurement light source and the reference light source are positionable such that the measurement light beam and the reference light beam are focused at a point on the retina.

12. The apparatus of claim 9 , wherein the substance comprises glucose.

13. The apparatus of claim 9 , wherein the measurement light source comprises a first infrared light source and the reference light beam comprises a second infrared light source.

14. A substance-measuring meter comprising:

an optical system adapted to irradiate the retina of an eye with a nearly parallel measurement light beam having a first wavelength at which a substance has a non-zero first absorption coefficient and adapted to irradiate the retina of the eye with a nearly parallel reference light beam having a second wavelength at which the substance has a second absorption coefficient which is substantially equal to zero, such that at least a portion of the measurement light beam and at least a portion of the reference light beam are retro-reflected from the retina, thereby producing a retro-reflected measurement light beam having the first wavelength and a retro-reflected reference light beam having the second wavelength;

a detection system adapted to generate a measurement signal in response to the retro-reflected measurement light beam and to generate a reference signal in response to the retro-reflected reference light beam; and

an electronic circuit responsive to at least the measurement signal and the reference signal by determining the concentration of the substance in the eye.

15. The meter of claim 14 , wherein the detection system is positionable to primarily receive the retro-reflected measurement light beam and the retro-reflected reference light beam without receiving other reflections from the eye.

16. The meter of claim 14 , wherein the detection system is positionable such that the retro-reflected measurement light beam and the retro-reflected reference light beam propagate along an optical path of at least 100 mm between the eye and the detector.

17. A method of using a meter adapted to measure a concentration of a substance in an eye, the method comprising:

irradiating the retina of the eye with a nearly parallel first light beam having a first wavelength;

irradiating the retina of the eye with a nearly parallel second light beam having a second wavelength;

receiving at least a portion of the first light beam retro-reflected from the retina;

receiving at least a portion of the second light beam retro-reflected from the retina;

generating a first signal in response to the received portion of the retro-reflected first light beam;

generating a second signal in response to the received portion of the retro-reflected second light beam; and

determining the concentration of the substance in the eye in response to at least the first signal and the second signal.

18. The method of claim 17 , wherein the substance comprises glucose.