Method for rapid calculation of tear film lipid and aqueous layer thickness and ocular surface refractive index from interferometry spectra

1. A method for determining optical properties of a corneal region, comprising the steps of:

obtaining a combined tear film aqueous layer plus lipid layer thickness;

obtaining a tear film lipid layer thickness;

subtracting the tear film lipid layer thickness from the combined tear film aqueous layer plus lipid layer thickness to obtain a tear film aqueous layer thickness; and

determining a corneal layer refractive index based on the tear film lipid layer thickness and the tear film aqueous layer thickness,

wherein at least one of obtaining a combined tear film aqueous layer plus lipid layer thickness and obtaining a tear film lipid layer thickness comprises measuring a tear film aqueous layer plus lipid layer relative reflectance spectrum using a wavelength-dependent optical interferometer.

2. The method of claim 1 , wherein obtaining a tear film lipid layer thickness comprises

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

converting the measured tear film aqueous plus lipid layer relative reflectance spectrum to a calculated absolute reflectance spectrum.

3. The method of claim 2 , wherein obtaining a tear film lipid layer thickness further comprises determining a tear film lipid layer thickness using an iterative curve fitting procedure.

4. The method of claim 2 , wherein determining a corneal layer refractive index based on the tear film lipid layer thickness and the tear film aqueous layer thickness comprises a matrix fitting calculation based on the tear film lipid layer thickness and the tear film aqueous layer thickness.

5. The method of claim 4 , wherein each of the lipid layer thickness and the aqueous layer thickness comprises an initial estimate and wherein the matrix fitting calculation determines a final lipid layer thickness and a final aqueous layer thickness.

6. The method of claim 5 , wherein the matrix fitting calculation comprises finding a best fit curve for an observed a tear film reflectance spectrum obtained using a wavelength-dependent optical interferometer.

7. The method of claim 4 , wherein the matrix fitting calculation comprises fitting the calculated absolute reflectance spectrum to a mathematical construct based upon a characteristic mathematical matrix of a plurality of thin film layers comprising in sequence, from top to bottom: air as a boundary, a tear film lipid layer, a tear film aqueous layer, and a corneal tissue as a semi-infinite substrate.

8. The method of claim 7 , wherein the corneal tissue comprises an epithelium.

9. A system for determining optical properties of a corneal region, comprising:

a wavelength-dependent optical interferometer; and

a controller in communication with the interferometer, the controller configured to

obtain a combined tear film aqueous layer plus lipid layer thickness,

obtain a tear film lipid layer thickness,

subtract the tear film lipid layer thickness from the combined tear film aqueous layer plus lipid layer thickness to obtain a tear film aqueous layer thickness, and

determine a corneal layer refractive index based on the tear film lipid layer thickness and the tear film aqueous layer thickness,

wherein the controller, in order to at least one of obtain a combined tear film aqueous layer plus lipid layer thickness and obtain a tear film lipid layer thickness, is further configured to measure a tear film aqueous layer plus lipid layer relative reflectance spectrum using the wavelength-dependent optical interferometer.

10. The system of claim 9 , wherein the controller, in order to obtain a tear film lipid layer thickness, is further configured to

measure a tear film aqueous layer plus lipid layer relative reflectance spectrum using the wavelength-dependent optical interferometer, and

convert the measured tear film aqueous plus lipid layer relative reflectance spectrum to a calculated absolute reflectance spectrum.

11. The system of claim 10 , wherein the controller, in order to obtain a tear film lipid layer thickness, is further configured to determine a tear film lipid layer thickness using an iterative curve fitting procedure.

12. The system of claim 10 , wherein the controller, in order to determine a corneal layer refractive index based on the tear film lipid layer thickness and the tear film aqueous layer thickness, is further configured to perform a matrix fitting calculation based on the tear film lipid layer thickness and the tear film aqueous layer thickness.

13. The system of claim 12 , wherein each of the lipid layer thickness and the aqueous layer thickness comprises an initial estimate and wherein the matrix fitting calculation determines a final lipid layer thickness and a final aqueous layer thickness.

14. The system of claim 12 , wherein the controller, to perform the matrix fitting calculation, is further configured to fit the calculated absolute reflectance spectrum to a mathematical construct based upon a characteristic mathematical matrix of a plurality of thin film layers comprising in sequence, from top to bottom: air as a boundary, a tear film lipid layer, a tear film aqueous layer, and a corneal tissue as a semi-infinite substrate.

15. The system of claim 14 , wherein the corneal tissue comprises an epithelium.

16. The system of claim 15 , wherein the controller, to perform the matrix fitting calculation, is further configured to find a best fit curve for an observed a tear film reflectance spectrum obtained using the wavelength-dependent optical interferometer.