BACKGROUND: Excimer laser photoablation for refractive and therapeutic
keratectomies has been demonstrated to be feasible and practicable. H
owever, corneal laser ablations are not without problems, including th
e delivery and maintenance of a homogeneous beam. We have developed an
excimer laser calibration system capable of characterizing a laser ab
lation profile. METHODS: Beam homogeneity is determined by the analysi
s of a polymethylmethacrylate (PMMA)based thin-film using video captur
e and image processing. The ablation profile is presented as a color-c
oded map. Interpolation of excimer calibration system analysis provide
s a three-dimensional representation of elevation profiles that correl
ates with two-dimensional scanning profilometry. Excimer calibration a
nalysis was performed before treating a monkey undergoing phototherape
utic keratectomy and two human subjects undergoing myopic spherocylind
rical photorefractive keratectomy. Excimer calibration analysis was pe
rformed before and after laser refurbishing. RESULTS: Laser ablation p
rofiles in PMMA are resolved by the excimer calibration system to .006
mu m/pulse. Correlations with ablative patterns in a monkey cornea we
re demonstrated with preoperative and postoperative keratometry using
corneal topography, and two human subjects using videokeratography. Ex
cimer calibration analysis predicted a central-steep-island ablative p
attern with the VISX Twenty/Twenty laser, which was confirmed by corne
al topography immediately postoperatively and at 1 week after reepithe
lialization in the monkey. Predicted central steep islands in the two
human subjects were confirmed by videokeratography at 1 week and at 1
month, Subsequent technical refurbishing of the laser resulted in a be
am with an overall increased ablation rate measured as microns/pulse w
ith a donut ablation profile. A patient treated after repair of the la
ser electrodes demonstrated no central island. CONCLUSIONS: This excim
er laser calibration system can precisely detect laser-beam ablation p
rofiles. The calibration system correctly predicted central islands af
ter excimer photoablation in a treated monkey cornea and in two treate
d human subjects. Detection of excimer-laser-beam ablation profiles ma
y be useful for precise calibration of excimer lasers before human pho
torefractive and therapeutic surgery.