On the optical measurement of corneal thickness. II. The measuring conditions and sources of error

Abstract

The optical measurement of corneal thickness based on oblique viewing of the optical section of the cornea is complicated by the finite width of the incident slit beam. In this report the theoretical and practical aspects of the effect of the slit width on the thickness reading are analysed. In practice, it was not possible to make slit-width independent thickness readings which were reproducible from one observer to another. In addition, the observed slit-width error was found to vary from one patient to another. The lack of reproducible estimate of the corneal thickness is attributed to difficulties associated with an exact definition of the edges of the visible bands of the optical section, which are determined by biological properties of the cornea as well as perceptive properties of the observer. Although inter-observer errors up to 0.02 mm were found, the intra-observer error amounted to only 0.005-0.006 mm (SD) between consecutive readings. Presumably this high intra-observer reproducibility is the result of the auxiliary pin-lights used. Changes in corneal thickness, measured by the same observer, can therefore be determined with great accuracy.