Measuring Index of Contact and Intraocular Lenses While Submerged in Hydrating Fluid

The conventional way to measure index of contact or intraocular lenses is using an Abbe refractometer where the lens is not immersed in liquid. Advantages of using the Metricon Model 2010/M for measuring such lenses include:

  • Maintains full hydration during measurement: A cell is available to allow the lens to be measured while it is completely immersed in hydrating fluid during measurement. Compared to techniques in which the lens is not immersed in fluid, this results in a much fuller hydration of the lens and a much more accurate measurement of the actual index of the lens when hydrated in the eye. In addition, complete immersion in a liquid minimizes or eliminates drying effects and squeezing of liquid out of the lens, resulting in index measurements which are much more stable with time and from measurement to measurement.
  • Less operator subjectivity: The 2010/M system shows very little variation from operator to operator since it is fully automatic and relies on computerized pattern recognition to find the critical angle knee. Abbe measurements often show much larger variation from operator to operator because the light-to-dark transition line used in the Abbe is often not sharp and requires judgment by the operator.- this is especially true for lenses with surfaces which are less than perfect (e.g. slightly rough or cloudy/hazy).
  • Speed: The 2010/M system can routinely make individual measurements (including the time to load and unload) in one minute or less.
  • Accurate determination of dispersion/Abbe number: The 2010/M system can easily be configured with three discrete laser sources to measure index at three wavelengths (for example, 405/473/633 or 445/532/650 nm). After measurement of index at the three wavelengths (measuring at three wavelengths requires a total of about two minutes including time to load and unload the lens) the Model 2010/M software will generate a complete index vs wavelength curve in five seconds (index at the Abbe number wavelengths and the Abbe number itself are also automatically calculated and displayed). The user can also specify up to eight additional pre-determined wavelengths for automatic index calculation with every fit, or additional wavelengths can be specified for index calculation after the fit.

Metricon’s proprietary fitting software has been tested extensively with a variety of materials and fitting error for materials with index less than 2.0 has been found to be less than .00005 (a factor of 10 better than conventional fitting techniques). This means that the fit adds no more than .00005 to the typical Model 2010/M measurement error of .0001-.0002 and, as a result, calculating index at any intermediate wavelength is only slightly less accurate than using a laser to measure the lens at that wavelength.

For further details or to discuss your application please contact Metricon at 609-737-1052 or info@metricon.com.