Reproducibility of contact lens power measurements using the phase shifting schlieren method

PURPOSE. To assess a new method of power measurement of soft and rigid contact lenses. The method is the phase shifting schlieren method, as embodied in the Nimo TR1504 instrument. MATERIALS and METHODS. Three Nimo TR1504 instruments were used to measure the power related dimensions of: a) a range of custom toric rigid lenses; b) a range of commercially available spherical hydrogel lenses; and c) a commercially available range of toric silicone hydrogel lenses. The measurements were carried out using a standard ISO ring test protocol where independent tests were carried out under conditions of reproducibility. The analysis of the measurements was carried out using ISO methods which enabled the reproducibility standard deviation, SR, of the method to be calculated. RESULTS. The results show that this new method has a reproducibility standard deviation SR of 0.048D for spherical soft (hydrogel) lenses. This means the back vertex power of spherical soft lenses having a power in the range ±20.0D can be determined to current ISO product tolerances with a single measurement. The method has SR of 0.059D for sphere power and 0.093D for cylinder power for toric soft lenses having powers in the range ±10.0D and cylinder powers in the range ±2.0D. A single measurement will determine sphere power to current ISO tolerance limits with 95% confidence while two measurements are required to determine the cylinder power to the same confidence level.