ANSI Z80.23-1999 pdf download Corneal Topography Systems – Standard Terminology, Requirements
4 Requirements Corneal topographers complying with this standard shall meet the requirements of 4.1 and 4.2.
4.1 Area measured When measuring an 8 mm spherical surface, a corneal topographer shall directly measure locations on the surface whose radial perpendicular distance from the corneal vertex is at least 3.75 mm. If the maximum area covered by a corneal topographer is reported, it shall be reported as the maximum radial perpendicular distance from the corneal vertex sampled on this 8 mm spherical surface.
4.2 Measurement sample density Within the area bounded by the requirement of 4.1, the surface shall be directly sampled in sufficient locations so that any surface location within the area has a sample taken within 0.5 mm of it. 4.3 Measurement and report of performance When the performance of a corneal topographer for the measurement of either curvature or ele- vation is assessed and reported, the testing shall be done in accordance with 5.1, 5.2 and 5.3 and the analysis and reporting of results shall be done in accordance with 5.4.
5 Test methods and test devices
Corneal measurements made with corneal topography systems collect data from a major portion of the corneal surface and consist of thousands of individual measurements. Each measurement is associated with a surface location and, when repeated measurements are made, some align- ment variability will always exist. This is especially true for human corneas as opposed to test surfaces. While this does not affect the overall validity of measurements, it does mean that the same identical points are not measured from one measurement to the next. Therefore it is best to use a method of analysis which will not overly penalize local, random error but will give an overall measure of system performance. It is possible and practical to align the system as identically as possible between repeated measurements and then treat the entire measurement (or specified parts of it) as an ensemble and find measures of mean error and standard deviation for the en- semble. Such measures will be used herein to assess corneal topography system performance.
5.1 Types of test
5.1.1 Accuracy An accuracy test shall be conducted by measuring a test surface specified in 5.2 using the method specified in 5.3.1 and analyzing the measured data using the method specified in 5.4. An accuracy test tests the ability of a corneal topography system to measure the absolute surface value of a known surface at known locations.
5.1.2 Repeatability A repeatability test shall be conducted by measuring human corneas as specified by 5.3.2 and analyzing the measured values using the method specified in 5.4. A repeatability test tests the ability of corneal topography system to report the same measured values at similar locations for a human cornea when these measurements are taken close together in time.