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Ron R. Rammage, Daniel Neal, James Copland, Thomas D. Neal, Wei Xiong, Steve Farrer, Phillip Riera; Comparison Of Automatic Fogging Methods To Minimize Instrument Accommodation. Invest. Ophthalmol. Vis. Sci. 2012;53(14):1359.
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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.
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.
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.
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.
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