Abstract
Purpose :
To measure the refractive index (RI) of most commonly available soft contact lens materials and their packaging solutions (PS) and compare to the manufacturers’ nominal values.
Methods :
The RI of 18 commercially available single vision soft contact lens materials were measured using a digital refractometer CLR 12-70 (Index Instruments Ltd., Cambridge, UK) at wavelength of 546 nm. Three lenses of each material were measured once in their PS and then after soaking in standard phosphate buffered saline (PBS), both at 20 °C. The RIs of the PS were also measured for all types. Accuracy requirements for correct wet to dry power conversion based on the thick lens formula were projected.
Results :
The standard deviation between the three samples was generally less than 0.005 which is the ISO 18369-2 tolerance limit. The measured RI ranged from 1.3744 ± 0.001 to 1.4265 ± 0.0004 for the PBS soaked materials and from 1.3739 ± 0.0003 to 1.4264 ± 0.0024 for lenses soaked in PS. Comparing the nominal RI data with the mean PBS and PS measured values, 5 and 3 lens materials, respectively, fell outside the ISO tolerance. Dailies Aqua Comfort Plus PS had the largest difference, 0.0040, to PBS RI of 1.334. For converting lens power measured in PBS to the in-air power, the difference between measured and nominal material RI for the Proclear 1-Day lens of 0.0104 would result in wrongly calculated in-air power of 0.49 D for a -3.00 D lens and 0.99 D for a -6.00 D lens. RI of 4 silicone hydrogel materials could not be measured.
Conclusions :
The CLR 12-70 is a generally reliable and accurate refractometer for the measurement of soft contact lens materials, although it failed to measure four of the materials. Accurate RI measurements are of particular relevance with the increased use of wavefront sensors to measure lens power while immersed in solution. Even small (< 0.001) errors in solution or material RI can lead to significant errors in converted in-air power. In this respect, manufacturer provided RI data with only two decimal places precision may not be sufficient. To obtain valid in-air lens power results, measurement conditions, including wavelength, must match the material and solution RIs used for the conversion.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.