Prescription compliance in ophthalmic lenses

Tolerances required for ophthalmic lenses are set down in national and international standards. It appears that the compliance of manufactured lenses has not been reported previously. Assembling a statistical quantity of lenses of a single prescription is usually an expensive process. It was, secondary to a lens impact study, possible to assemble a large number of plano lenses. In the assessment of the fracture velocity of lenses approximately 20 plano lenses of each material and thickness are required. Prior to using lenses for the impact study, they were checked for prescription. The results of the prescription measurements are reported here and the results of the impact study are reported in a separate paper.

Using an automated focimeter, 679 plano lenses in stock thickness, typical occupational eye protector thickness and up to 3.5 mm thick were measured. There were 21 combinations of material/thickness/coating from seven suppliers. The power was evaluated against Australian Standard 2228.1-1992, as the lenses were supplied in Australia. The permitted tolerances are ±0.09 D sphere and ±0.06 D cylinder.

When assessed for material/thickness/coating combination, failure rates varied from <0.0001 per cent to 77.5 per cent (with a further 17.3 per cent classified as borderline, because they were within the uncertainty of measurement of the required limit). Grouped by supplier, the failure rates ranged from <0.0001 per cent to 7.6 per cent (with a further 12.3 per cent borderline). To improve understanding of the result, it may be easier to quote the figures without considering uncertainties. When assessed by supplier, the failure rate varies from <0.0001 per cent to 12.6 per cent.

Compliance of plano lenses should be among the easiest of tasks for a laboratory. While we know of no defined or required acceptance rates for prescription lenses, a failure rate for a laboratory of 12.6 per cent, which includes a failure rate of 88 per cent in 2.8 mm thick refractive index = 1.53 hard coated lenses (n = 20) cannot be considered satisfactory and is a strong indication of a failure to check lenses before they leave the laboratory.