Aspects of wet cell measurement of back vertex power of contact lenses

Abstract

Background:  The back vertex power of soft contact lenses can be measured when they are immersed in saline within a wet cell. Six equations have been published for the purpose of converting the power measured in saline to its corresponding power in air. Results provided by these equations have not been compared and the consequences of inaccuracies in the variables used in them have not been quantified.

Methods:  Sample data were used to solve these equations and inaccuracies were simulated in each of the variables required for their solution to determine the resultant magnitude of error in the calculated power in air.

Results:  Three equations gave identical results. Inaccuracies in the lens specification, distance of the back vertex of the lens from the cell wall, refractive index of the cell and its wall thickness were of very little consequence in the calculation of power in air. In the case of a high water content lens, immersed power must be measured to 0.02 D and its refractive index and that of saline must be known to ±0.001 to prevent errors in the calculated power in air exceeding tolerance limits.

Conclusion:  Measured power in saline can be converted to power in air using the original equation published by Wichterle in 1965. While the nominal specification of the lens can be used in this equation, it is essential to know its actual refractive index and that of the saline. Reproducibility of wet cell measurement of power using a suitable focimeter is unknown and should be established to allow this method to be used by practitioners and research laboratories.

Key words:

• back vertex power
• cuvette
• hydrogel lenses
• measurement
• soft lenses
• wet cell