The raw image obtained by a topography device can be viewed to provide qualitative information about the corneal shape. However it is the analysis of the data and its subsequent display as a colour-coded contour map which adds considerable value to the investigation. Additional displays include cross sections and three-dimensional wire nets.
When viewing the output, before looking at the map, it is important to check the patient details and date, the eye (right or left). The scale lists the step interval of the coloured contours and their units of measurement. When comparing two maps, these should be the same for both maps. The colour scale uses warm colours (red and orange) for the steep areas where the videokeratoscopy mires are close together and the cooler colours (green and blue) for the flatter areas.
Overlays may be applied to the map to show the location of the videokeratoscopy rings, pupil, steep and flat meridian and a measurement grid. Multiple map displays can include various measures at one time point (e.g. including posterior corneal shape or pachymetry), serial maps over time or difference maps (subtraction of an earlier map from a later one).
Numerical and statistical displays can give the actual values at one or more points or calculate indices which summarise a particular feature of the corneal surface (e.g. astigmatism, regularity or asymmetry).
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