Assessment of the performance of automated focimeters in the measurement of single vision spectacle lenses

Abstract

Background

The accuracy and precision of any instrument should not be taken for granted. While there is an international standard for checking focimeters, there is no report of any study on their performance.

Method

A sample set of 51 focimeters (11 brands), were used to measure the spherical power of a set of lenses and the prismatic power of two lenses complying with ISO 9342‐1:2005 and other calibrated prismatic lenses and the spherical power of some grey filters.

Results

The mean measured spherical power corresponded very closely with the calibrated values; however, the spread of results was substantial and 10 focimeters did not comply with ISO 8598:1996. The measurement of prism was much more accurate and precise and all the focimeters complied easily. With the grey filters, about one‐third of the focimeters either showed erratic reading or an error with the equivalent of category 4 sunglasses. On the other hand, nine focimeters had stable and accurate reading on a filter with a luminous transmittance of 0.5 per cent.

Conclusions

These results confirm that, in common with all other measurement instruments, there is a need to ensure that a focimeter is reading accurately and precisely over the range of refractive powers and luminous transmittances. The accurate and precise performance of an automated focimeter over its working life cannot be assumed. Checking before purchase with a set of calibrated lenses and some dark sunglass tints will indicate the suitability of a focimeter. Routine checking with the calibrated lenses will inform the users if a focimeter continues to indicate accurately.

Key words:

• focimeter
• optics
• refractive power
• spectacle lenses

Acknowledgements

The authors would like to acknowledge the assistance provided by the optical companies (Essilor Australia, HoyaVision Australia, Carl Zeiss Vision Australia and Raylane), the instrument importers (BOC Ophthalmic Instruments and Designs for Vision), the Optometry Clinic and the Centre for Eye Health at the University of New South Wales and the individual optometric practices in making their focimeters available for testing.

The cost of travel to Perth, Western Australia was provided by Raylane Pty Ltd.