Fast absolute-reflectance method for the determination of tear film lipid layer thickness

1. A method of determining tear film lipid layer thickness, comprising the steps of:

measuring a tear film aqueous plus lipid layer relative reflectance spectrum using a wavelength-dependent optical interferometer;

converting the measured tear film aqueous plus lipid layer relative reflectance spectrum to a calculated absolute reflectance lipid spectrum, wherein the calculated absolute reflectance lipid spectrum is calculated based on wavelength-dependent refractive indices for air, lipid, and aqueous phases and wherein the wavelength-dependent refractive indices for lipid are based upon a Sellmeier equation form; and

comparing the calculated absolute reflectance lipid spectrum to a theoretical absolute reflectance lipid spectrum to determine a tear film lipid layer thickness.

2. The method of claim 1 , wherein the tear film aqueous plus lipid layer relative reflectance spectrum is measured relative to a reference material.

3. The method of claim 2 , wherein converting the measured tear film aqueous plus lipid layer relative reflectance spectrum to the calculated absolute reflectance lipid spectrum comprises multiplying the measured tear film aqueous plus lipid layer relative reflectance spectrum at a plurality of wavelengths times an absolute reference lipid spectrum for the reference material obtained at the plurality of wavelengths.

4. The method of claim 3 , wherein the reference material has a radius of curvature of 7.75 mm.

5. The method of claim 3 , wherein converting the measured tear film aqueous plus lipid layer relative reflectance spectrum to the calculated absolute reflectance lipid spectrum further comprises dividing by a correction factor to correct for differences between the theoretical absolute reflectance lipid spectrum and the calculated absolute reflectance lipid spectrum.

6. The method of claim 3 , wherein converting the measured tear film aqueous plus lipid layer relative reflectance spectrum to the calculated absolute reflectance lipid spectrum further comprises dividing by 100.

7. The method of claim 1 , wherein comparing the calculated absolute reflectance lipid spectrum to the theoretical absolute reflectance lipid spectrum comprises iteratively comparing the calculated absolute reflectance lipid spectrum to a plurality of theoretical absolute reflectance lipid spectra.

8. The method of claim 7 , wherein iteratively comparing the calculated absolute reflectance lipid spectrum to the plurality of theoretical absolute reflectance lipid spectra comprises minimizing a sum of least squares differences between the calculated absolute reflectance lipid spectrum and the plurality of theoretical absolute reflectance lipid spectra.

9. The method of claim 8 , wherein minimizing the sum of least squares differences comprises using a Levenburg-Marquardt algorithm.

10. The method of claim 7 , wherein iteratively comparing the calculated absolute reflectance lipid spectrum to the plurality of theoretical absolute reflectance lipid spectra comprises starting the iteration with the theoretical absolute reflectance lipid spectrum for a 65 nm thick lipid layer.

11. The method of claim 1 , wherein comparing the calculated absolute reflectance lipid spectrum to the theoretical absolute reflectance lipid spectrum does not include a comparison of the calculated absolute reflectance lipid spectrum to a theoretical absolute reflectance aqueous spectrum to determine a tear film aqueous layer thickness and a comparison of the calculated absolute reflectance lipid spectrum to a theoretical film stack to determine corneal surface refractive index.

12. A system for determining tear film lipid layer thickness, comprising:

a wavelength-dependent optical interferometer; and

a controller in communication with the wavelength-dependent optical interferometer, the controller configured to measure a tear film aqueous plus lipid layer relative reflectance spectrum using the wavelength-dependent optical interferometer,

convert the measured tear film aqueous plus lipid layer relative reflectance spectrum to a calculated absolute reflectance lipid spectrum, wherein the calculated absolute reflectance lipid spectrum is calculated based on wavelength-dependent refractive indices for air, lipid, and aqueous phases and wherein the wavelength-dependent refractive indices for lipid are based upon a Sellmeier equation form, and

compare the calculated absolute reflectance lipid spectrum to a theoretical absolute reflectance lipid spectrum to determine a tear film lipid layer thickness.

13. The system of claim 12 , wherein the tear film aqueous plus lipid layer relative reflectance spectrum is measured relative to a reference material.

14. The system of claim 13 , wherein the controller, in order to convert the measured tear film aqueous plus lipid layer relative reflectance spectrum to a calculated absolute reflectance lipid spectrum, is further configured to multiply the measured tear film aqueous plus lipid layer relative reflectance spectrum at a plurality of wavelengths times an absolute reference lipid spectrum for the reference material obtained at the plurality of wavelengths.

15. The system of claim 14 , wherein the reference material has a radius of curvature of 7.75 mm.

16. The system of claim 14 , wherein the controller, in order to convert the measured tear film aqueous plus lipid layer relative reflectance spectrum to the calculated absolute reflectance lipid spectrum, is further configured to divide by a correction factor to correct for differences between the theoretical absolute reflectance lipid spectrum and the calculated absolute reflectance lipid spectrum.

17. The system of claim 14 , wherein the controller, in order to convert the measured tear film aqueous plus lipid layer relative reflectance spectrum to the calculated absolute reflectance lipid spectrum, is further configured to divide by 100.

18. The system of claim 12 , wherein the controller, in order to compare the calculated absolute reflectance lipid spectrum to the theoretical absolute reflectance lipid spectrum, is further configured to iteratively compare the calculated absolute reflectance lipid spectrum to a plurality of theoretical absolute reflectance lipid spectra.

19. The system of claim 18 , wherein the controller, in order to iteratively compare the calculated absolute reflectance lipid spectrum to the plurality of theoretical absolute reflectance lipid spectra, is further configured to minimize a sum of least squares differences between the calculated absolute reflectance lipid spectrum and the plurality of theoretical absolute reflectance lipid spectra.

20. The system of claim 19 , wherein the controller, in order to minimize the sum of least squares differences, is further configured to use a Levenburg-Marquardt algorithm.

21. The system of claim 18 , wherein the controller, in order to iteratively compare the calculated absolute reflectance lipid spectrum to the plurality of theoretical absolute reflectance lipid spectra, is further configured to start the iteration with the theoretical absolute reflectance lipid spectrum for a 65 nm thick lipid layer.

22. The system of claim 12 , wherein the controller, in order to compare the calculated absolute reflectance lipid spectrum to the theoretical absolute reflectance lipid spectrum, is further configured not to include a comparison of the calculated absolute reflectance lipid spectrum to a theoretical absolute reflectance aqueous spectrum to determine a tear film aqueous layer thickness and a comparison of the calculated absolute reflectance lipid spectrum to a theoretical film stack to determine corneal surface refractive index.