Wet cell measurement of soft lens power

The view has been expressed by Chaston 1 that "the dimension of power is the most critical of all soft lens dimensions because such small differences can give what the patient may think of as a large reduction in visual acuity". Sarver, Harris, Mandell and Weissman 2 described how the power of a soft lens can be measured by blotting the surfaces with a lint-free tissue to remove excess moisture, then air drying for 15 to 30 seconds before positioning on a focimeter. This method is probably the one in most common use at the present time both by laboratories and practitioners. Kaplan 3 adapted a vertical contact lens modification unit to function as a centrifuge so that a soft lens, supported by an afocal hard lens, could be spun for a few seconds in order to "dry" it prior to power measurement. Sarver and others 2 established that in the case of polyHEMA lenses, power readings should be completed within about five minutes after removal of the lens from a vial or storage case to prevent degradation of the focimeter image as a result of dehydration. Whilst spherical, single vision lenses can be measured within this period, speed and additional dexterity may be necessary in the case of ultra-thin, toric, or bifocal lenses. Special care is required when handling thin high water content lenses since their fragility may result in breakage during blotting. The essential advantage of measurement of power in air is that it can be applied to lenses of unknown refractive index. Chaston 4 mentioned that the principal disadvantage of this method is "that the indistinct image from the blotted lens gives a spread of readings". A further disadvantage is that the ambient air temperature and humidity may be uncontrolled. Poster 5 argued that the use of a saline-filled cell is "more convenient and reliable as well as determining the power in the condition that the lens will be worn (hydrated state)". When a soft lens is immersed in saline, the measured power is significantly less than that measured in air. It is essential, therefore, that measurements are made using a focimeter of very high accuracy. Mechanical modification of a conventional focimeter to increase its accuracy was described by Dormon-Brailsford 6 7 and Hampson 8. A dial guage was attached to the instrument to determine back vertex focal length in hundredths of a millimetre and this value was converted to dioptres. Slides shown to illustrate the manufacture of the Softens R have revealed that Bausch and Lomb replaced the standard, graduated power drum of a focimeter with one of much larger diameter in order to increase precision of power readings when using a wet cell. It is surprising that Nakajima, Shibata, Magatani, Hirano and Terao 9 used a projection focimeter with a scale marked only in intervals of 0.25 D. for wet cell power measurement. The major advantage of measuring the power of a soft lens immersed in a saline-filled cell is that the environment can be controlled. According to Holden lo, "the only way of checking soft lenses is to evaluate the finished product in the hydrated state, i.e. in a wet cell. Ideally, all parameters should be measured in this way because of the susceptibility of lenses to the environmental conditions". Another advantage of wet cell power measurement which seems to have been neglected is that it permits verification of very high powered lenses which would otherwise be beyond the range of the focimeter. This advantage is worthy of consideration in the case of both soft and hard lenses. A disadvantage of wet cell power measurement is that the refractive index of the lens must be known. Hampson 8 has pointed out that surface aberrations of the lens may be masked as a consequence of its