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In Vivo Visualization of Rainbow Glare and Treatment With Undersurface Flap Phototherapeutic Keratectomy. Elahi S, Gatinel D. J Refract Surg. 2020;36(6):400-404

Link to the paper

 

Rainbow glare (RG) is a rare optical side effect of femtosecond laser-assisted LASIK (FS-LASIK) where patients complain of immediate postoperative rainbow-like glares when exposed to a polychromatic light source such as artificial lights. The presumed hypothesis is that they are caused by a diffractive grating pattern of geometrically-aligned, regularly-spaced femtosecond laser impacts. The crater left by the laser impact may be temporarily filled by various materials, from cavitation bubbles, which may explain the often transient nature of RG.

rainbow glare and diffractive slits

After the description of the first successful RG treatment by performing a 16-mm planar photoablation of the posterior surface of the flap -or undersurface photoablation, we analyze the case of a unilateral RG which required repeated undersurface photoablation using Wavelight EX-500 (Alcon Laboratories) excimer laser (flap created by Wavelight FS-200 femtosecond laser).

How many microns of back flap photoablation is necessary?

What is the origin of the invariably bright vertical lines with the typical spectral dispersion associated with rainbow glare – whereas the spots are not vertically aligned given that each horizontal line is shifted due to the line-wrap of the circular perimeter?

This paper proposes some answers to these questions.

  1. Undersurface photoablation is an efficient treatment of RG, and should at least include 15-20 mm thickness.
  2. The lateral spectral bands constitutive of the diffractive pattern in RG is easily comprehensible when considering the horizontal regular spacing of the femtosecond laser spots. However, there are invariably bright vertical lines with the typical spectral dispersion associated to RG  which cannot be explained by the even spot distribution. Since each horizontal line is shifted due to the line-wrap of the circular perimeter, the spots are not vertically aligned. However, the lines created by the spots simulate regularly spaced thin diffractive slits  which could explain the origin of the vertical spectral lines.

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