Applanation tonometer using light reflection to determine applanation area size

DETAILED DESCRIPTION Our tonometer objectively determines the size of an applanated area of the cornea.
It does this by exploiting Snell's law of refraction which describes the trigonometric relationship between incidence and refraction angles that occur as light encounters an interface between two media having different refractive indexes.
By choosing a suitable refractive index for a tonometer prism relative to the 1.
0 index of refraction of air and the 1.
33 index of refraction of water at the wetted surface of the cornea, and by using an appropriate wavelength of light at an appropriate incidence angle, our tonometer can transmit a portion of the incident light through the applanated area and can reflect another portion of the light around the applanated area.
Either of these light portions is a function of the size of the applanated area, which can then be determined by the amount of light reaching a detector.
To accomplish this, we prefer a prism 10 made so that light directed toward the eye is internally incident on applanating surface 15.
There, the internally incident light is divided into two portions--one that transmits through the applanated area into the eye, and one that reflects internally from surface 15 around the applanated area, where surface 15 is in contact with air.
In other words, the internally incident light reflects completely from the portion of surface 15 that contacts air, with its 1.
0 index of refraction, while the incident light passes through the wetted region where the cornea is applanated and presents an index of refraction of 1.
33.
Transmitting light through the applanated area and reflecting light around the applanated area effectively divides the incident light into two portions, both of which are functions of the size of the applanated area.
In the preferred embodiment illustrated in FIGS.
1 and 2, the reflected light from around the applanated area is directed to a detector 11 that receives an amount of reflected light that is inversely related to the size of the applanated area