Modeling the conical corneal null-screen topographer with the Fermat principle

In this work we derive the system of equations that describe the passage of light through the conical corneal null-screen topographer from the Fermat principle; specifically, it computes the surface coordinates in which the reflection of the null-screen is produced, so that its normal field may be computed; since most of the surface information is obtained from it, this is an essential step on the evaluation process; the formulation here presented is an upgrade from previous works.