[Experimental studies on influence of alignment errors on measurement in a corneal topographic system--relationship...

To evaluate the influence of alignment errors in focusing on the results of measurements in corneal topography, we experimented using polymethyl-methacrylate balls with definite radii of curvature as test objects. We used 3 different corneal topographic systems with different working distances to clarify the relationship between the degree of errors and the working distance. We determined the refractive power of the test balls by the topographic systems and studied changes in the values as the balls were slightly displaced from the alignment position axially, or horizontally. The test balls were set on the stage with a micrometer so that the distance of displacement could be determined precisely. When the test balls were placed precisely in alignment, there was no significant relationship in the magnitude of measurement errors with working distance of the topographical systems. When the test balls were displaced axially, the measurement values decreased as the balls come near from the alignment position and increased as they moved away from the alignment position. The magnitude of the measurement errors caused by back and forth displacement of the object became larger as the working distance of the topographic system became shorter. There was no specific relationship between the degree of measurement errors and the working distance when the balls were displaced horizontally.