By Dick Whitney, Carl Zeiss Vision, ANSI Z80.1 Chair/Vision Council Lens Tech Committee Chair
ACCURACY VS. PRECISION OF READOUT
While many “electronic” or automated units have the ability to read out digitally to 0.01, this may not mean they are capable of that accuracy. In reality, both automated and manual focimeters have similar limitations with regard to accuracy, but each has merit when considering their use for lens inspection. The purpose of this section of the revision to ANSI Z80.1 for the year 2020 is to provide insight on pros and cons for both types when inspecting lenses.
There is confusion over the comparative accuracy of manual focus versus automated readout electronic devices. It is common for electronic devices to have readouts that display readings to 0.01. For some, this is interpreted as greater accuracy over the manual devices.
Digital readouts that display in 0.01s does help by eliminating the interpretation of the reading on the manual power drum. However, it does not mean the units are otherwise more accurate. Focimeters with the traditional about 6-mm diameter aperture and large measurement range (+/- 20 Diopters) have limitations which make affordably priced units incapable of meeting 0.01D accuracy.
STANDARD FOCIMETER ACCURACY
When powers are measured in the range of +/-5 D, it is generally accepted that errors of up to 0.06 D may be expected for standard low cost focimeters. This accuracy has not changed significantly over the decades and is not likely to be improved upon unless higher cost devices are used, which is not likely for typical laboratory operations.
Since this 0.06 error is half the standard power tolerance, it is also recognized that measuring lenses of marginal or low prescription power can be challenging. This situation leads to uncertainty when accepting/rejecting product to ANSI Z80.1 Standards. Visit Pro to Pro for a complete explanation.