Possible Errors in Determining Axial Length Changes During Accommodation with the IOLMaster

Purpose: The Zeiss IOLMaster uses partial coherence interferometry to measure the optical path length within the eye. This is converted into the axial length. There are errors in measuring possible changes in axial length as a result of accommodation, because the instrument uses an average refractive index in the eye. We determined the likely extent of these errors.

Method: Errors were estimated using Gullstrand's no. 1 schematic eyes. One set of these had the shell structure of the eyes, and another set had the gradient index structure specified by Gullstrand.

Results: Errors were 18 to 26 microm for an accommodation of 10.9 D.

Conclusions: The worth of the IOLMaster would be increased if it used an equation for conversion from optical length to geometrical length that took into account variations in lens thickness and if it were able to be modified to provide component distances and the axial length using partial coherence tomography.