Projection-based topography systems include those using slit photography, rasterstereography, laser interferometry or moiré interference. An image is projected onto the surface of the cornea in the same way as a slide is projected onto a screen and viewed directly. Some of these systems have the advantage of obtaining data from irregular and non-reflective surfaces and potentially from a wider area of cornea. They directly measure corneal height (elevation), from which slope, curvature and power can be calculated. Corneal thickness (pachymetry) measurements can also be made by slit systems.
The projection-based technique most commonly used in commercial systems is slit photography, which gives an accuracy similar to other commercially available devices. However, the other techniques have potentially a higher accuracy and other advantages but need further development before being introduced to regular clinical practice.
KeywordsCorneal topography Projection-based topography Corneal shape Corneal elevation Slit photography Rasterstereography Laser interferometry Moiré interference Topography accuracy Three-dimensional topography Corneal pachymetry
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