Abstract
Purpose: :
Instruments that include an algorithm for automatically determining refraction (auto-refractors, wavefront aberrometers), generally include a fogged internal fixation target so that the far-point refraction can be determined. The efectiveness of this fixation target directly impacts the accuracy of the instrument refraction (relative to manifest refraction) and the amount of instrument myopia.
Methods: :
Using a research version of the iDesign combined Aberrometer/CT instrument, the fixation target presentation could be varied in a programmed sequence. A series of comparison clinical studies was conducted with different sequences and end points. The target was presented with varying defocus, but was not compensated for astigmatism.
Results: :
Several factors were found to be important for effective fogging: fogging rate, total fogging amount, rest eye vergence position, and total fogging range. It was found that in subjects with large astigmatism (>2D), the fogging range needed to minimize instrument accommodation was greater, by an amount of ~C/2. In addition, single button sequences were as effective at minimizing accommodation as were two button sequences with some delay between autorefract and final acquisition. The target type, pattern or arrangement had a secondary effect on instrument accommodation.
Conclusions: :
The target presentation sequence is critical for developing instruments to minimize accommodation. Target fixation must be changed at a speed that the human eye can track, and must have sufficient range so that all meridians are sufficiently fogged at the time of measurement.
Keywords: accommodation • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • aberrations