On the optical measurement of a corneal thickness. I. Optical principle and sources of error

The optical principle for the estimation of corneal thickness with a commonly employed method is considered. The theoretical relationship between the apparent and true corneal thickness is described in a simple manner. Based on the theoretical results it could be confirmed that physiological variations in refractive index and radius of the cornea induce an insignificant error in the corneal thickness estimate. The theoretical relationship between apparent and true corneal thickness was compared to the performance of two commercially available pachometers (Haag-Streit and Zeiss). The actual reading of the pachometers was found to be lower than the theoretical reading by a non-linear error amounting to about 0.010 and 0.100 mm at a true corneal thickness of 0.50 and 1.00 mm, respectively.