Calibration error on the measurement of back vertex power for contact lenses with method using focimeter with manual focusing.
OBJECTIVE
The International Standard ISO 9337-1, which sets forth the method for measuring back vertex power of contact lenses with manually focusing focimeters, specifies that test lenses conforming to ISO 9342 may be used to calibrate focimeters on its spectacle lens support, and correction values obtained in this way can be used directly in measuring contact lenses on its contact lens support. This study was conducted because of concern that the method mentioned in ISO 9337-1 is not good enough for the calibration of focimeters used to measure contact lenses.


METHODS
To test the validity of this method, a research group from China National Institute of Metrology (NIM) studied it theoretically and carried out a series of comparison experiments, respectively, with the conventional test lenses conforming to ISO 9342 and test lenses made at NIM (with an expanded uncertainty of 0.025 D).


RESULTS
The results show that the measurement error between the two calibration methods will exceed 0.50 D if the specification described in ISO 9337-1 is adopted. This error also exceeds allowable tolerances for focimeters and for contact lenses themselves. Experiments and theoretical calculations done by the NIM group show that these errors mainly come from spherical aberration. Vertex error induced by the lens support is not negligible.


CONCLUSIONS
When focimeters are calibrated to find correction values with the method specified in ISO 9337-1, measurement error will not be eliminated if these correction values are used in measuring contact lenses, and the resulting deviation is too large to be ignored. Therefore, special test lenses should be used to calibrate focimeters to find correction values when these focimeters are used to measure back vertex power of contact lenses because contact lenses are a special product being used to correct human vision.
Wang
Li Ru
LR
Optical Engineering Division, China National Institute of Metrology, Beijing. star@public.bta.net.cn
Zhang
Ji Yan
JY
Ya
Zhen
Z
D002138:Calibration / Q000592:standards*; D003261:Contact Lenses; D006801:Humans; D055095:Optics and Photonics