Optical low-coherence reflectometry (OLCR) was used as a noncontact method to measure the central corneal thickness of three patients intraoperatively during photorefractive keratectomy. Continuous on-line measurements were performed on the intact cornea immediately before the beginning of surgery, after the removal of the corneal epithelium, during laser tissue photoablation, and for 3 min after the ablation process. Corneal thinning due to evaporation was studied on a separate patient with the OLCR instrument, and it was found to be -0.14 microm/s during the first 5 min after epithelium removal. This baseline corneal thinning rate was used as a fit parameter to calculate actual from measured ablation depths. The measurements showed a maximum difference of +/-10 microm between planned ablations (34-92 microm) and measured ablation depths.
An optical low-coherence reflectometer is presented that uses a fiber Michelson interferometer with a rotating cube to generate rapid depth scans at a high repetition rate. A folded optical path geometry allows the reference arm to scale up the scan range, scan speed, and scan repetitiveness. Thickness measurements with a repetitiveness of 384 Hz and a longitudinal scanning speed of 21 m/s in air over a range of ~3 mm are demonstrated.
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