Rainbow glare (glare in Rainbow) is a rare undesirable optical effect of the LASIK all laser (femtosecond laser). It has caused the diffraction light induced by the pattern of the network created at the back of the flap by the impacts of the femtosecond laser. It has been described by Krueger and coll, who then used the laser femtosecond Intralase 15 KHz (Intralase Corp., USA) in 2008. We also reported the first observation of the Rainbow glare with laser Wavelight FS200 as well as the first surgical correction of a rainbow glare case.
Rainbow glare matches the perception of bands or coloured light trailsdistributed according to the order of the visible light spectrum, and located around but away from the sources of white lights (where the use of the term "Rainbow", although the phenomenon responsible for the perception of the Rainbows is different, see below).
The distribution of these bands around the source is usually very symmetrical, and has a geometric character. The orientation of the light streaks is variable, but usually predominates in a horizontal direction of sprawl: Blue is the closest color to the Central light source, and the more distant red. These multicolored streaks can be seen in a more visible way that the observed light source is bright, and located on a dark background. The dimensions of the observed source can also affect the appearance of the rainbow glare.
Characteristically, there is always a free interval between a point source and the first multicolored light strips. If the light source is monochromatic (one colour), the strips are narrower, and identical to the source color.
The following image was obtained thanks to the insertion of a network of diffraction similar to that created by a femtosecond laser in the cornea. The observed source is strong: it is the flash of a smartphone (white light).
This more 'realistic' image is taken through the same device "etched" by the laser impacts, but the observed source is a neon light tube, which the luminance is weaker, and the most important spatial extension. Rainbow glare is reduced, and the colored bands are less defined than in the case of a vivid and more punctual source.
We have made the first effective treatment for the rainbow glare: the issue of impacts to the excimer laser on the deep surface of the flap helped two patients to heal immediately the effects of this complication. The intensity of this phenomenon generally decreases with time, and disappears in a few months in the vast majority of the case. When it persists, of surgical treatment to deliver a photoablation intended to destroy the pattern of diffraction to the deep surface of the flap seems so effective.
Mechanism of the rainbow glare
The intimate mechanism of the rainbow glare is linked to the diffraction light waves; This is also the only known side effect mainly caused by the diffraction of light within a biological structure (the Diffractive implants used in cataract surgery use the properties of the diffraction by design - creating additional homes - but they are in synthetic material).
The origin of the rainbow glare in summer attributed initially to the pattern created by the impacts of femtosecond laser within the stroma corneal, to allow the cleavage of the corneal flap in LASIK procedure any laser. The femtosecond laser delivers very brief light, focused courses at a depth chosen (around 110 microns) in the corneal tissue, and whose delivery is a mode 'raster' (line by line) with most of the lasers femtosecond currently used. It takes about 1 million spots to create a standard flap interface.
This interface is created by issuing the contiguously laser, "line by line" spots. The Exxx_xxx_5197ment between the spots for each line is customizable, as well as the Exxx_xxx_5197ment between each of the rows (the distance between the spots and lines being generally close to 8 microns).
Because of the extreme brevity of the impacts laser (on the order of about 10-14 seconds), their peak power is high, and causes an ionization of matter within a sphere around the point of impact: the radius of this sphere depends among other delivered energy. It can reach 5 to 10 microns. The issue of the impacts thus creates a regular pattern within the corneal tissue, is observable in confocal microscopy in vivo, in post operative; It's lines of "hyper-reflective" points, spots located at the interface level, probably located on the flater ACE of the flap, and not on the deeper stroma (which was carved by the excimer laser). We recently published a clinical observation where the Exxx_xxx_5197ment between the spots visualized within the stroma (7.5 cm) corresponded to the direction and layout of the colored bands (the first red band appearing at 16 cm from the white source located at a distance of 1 meter).
Indeed, once the flap raised (this is the first time of the realization of the LASIK), the underlying stroma is carved by the excimer laser to correct the optical defect second time: photoablation for myopia or hyperopia, and/or astigmatism), which causes the disappearance of the print by the femtosecond laser. However, it seems this still at the back of the flap, which is not exposed to the excimer laser.
Diffraction by the femtosecond laser impact areas
Each impact is a change in the local physical properties of the corneal tissue, although these changes remain to be explored. It is possible that the refractive index is different from that of the adjacent corneal stroma. The regular pattern, different alternating areas of refractive index is able to produce a phenomenon of constructive and destructive interference by diffraction of the light beyond these areas. These have indeed the close dimensions of the micron, of the same order as the wavelength of visible light. The mathematical expression for calculating the dispersion of each colored radiation angle is relatively simple. On the other hand, the calculation of the distribution of energy carried away in each order of diffraction is more complex.
The following diagram represents a constructive interference between two waves (monochromatic light) diffracted by an impact area (these areas are contiguous: it is of order 1 of diffraction (the optical path differs from a wavelength between two parallel paths).) The zero-order of diffraction (light "continues its path without deviation" is not shown for clarity). The wavelength, the more the angle that is constructive interference is important.
The diffraction pattern is represented here for a wavelength shorter than that of yellow:
Shorter wavelengths are diffractées with an angle less than the longer wavelengths, which explains the appearance of multicolored bands (Rainbow) when the source is polychromatic (ex: white light).
White light is partly divided into different levels of diffractions by regular impact areas, which act as a network of diffraction. The direction of these orders depends on the considered wavelength. This light is then refracted by the posterior face of the cornea, then the lens to be focused on the retina.
The directions where the "bands of colour" are observed are related to the orientation of the lines of spots. In general, the consistency being largest in horizontal (this is related to the constant spacing of the spots 'on-line', while there is a vertical offset due to the juxtaposition of lines, including the Starter wife the circular edge of the flap), the bands adopt a rather horizontal direction.
Rainbows observed in nature are produced by a phenomenon of refraction and not of diffraction (incident, emitted sunlight behind the observer is refracted several times inside the droplets of rain in suspension and returned to the observer; each refraction "separates" the different wavelengths). The 'rainbow glare' should rather have been baptized 'diffraction glare. " the phenomenon of dispersion of the colors of the rainbow glare is close to that of the surface of CD or DVD discs: the micro-gravures are at the origin of the diffraction of incident light after reflection at the surface of the disc.
Refractive surgery techniques that do not require the use of the laser femtosecond (ReLex, Smile) are initially exposed has the occurrence of rainbow glare. The optical interface Associates here l coalescence of two surfaces created by the issuance of femtosecond impacts. The concentric circular distribution of impacts expected to lead radiaires skies arches, giving an aspect of kaleidoscope has these colorful lights. of A date, experience this effect after Relex or Smile n has not yet been reported.
Rainbow glare is a rare phenomenon that can be observed after femto-LASIK. It fades with time and not occaseionne important visual discomfort only in a very small number of case. If the induced gene persists in time and intensity, a photoablation excimer to the deep surface of the flap is possible and is effective (see:) Rainbow Glare Gatinel JRS 2015 correction)
(1) Krueger RR, Thornton he, Xu M, Bor Z, van den Berg TJ. Rainbowglare have year optical side effect of IntraLASIK. Ophthalmology, 2008; 115:1187 - 1195.
(2) Bamba S, Rocha KM, Ramos-Esteban JC, Krueger RR. Impact of rainbow glare after laser in-situ keratomileusis flap creation with a 60 kHz femtosecond laser. J Cataract Refract Surg. 2009; 35:1082 - 1086.
(3) Sarayba MA, Ignacio T, PS, Tran DB Binder. Comparative study of stromal bed quality using mechanical microkeratomes has IntraLase femtosecond laser 15 - and 30 - kHz microkeratomes. Cornea. 2007; 26:446 - 451.
(4) Binder PS, Sarayba MA, Ignacio T, Juhasz T, Kurtz RM. Characterization of submicrojoule femtosecond laser corneal tissue dissection. J Cataract Refract Surg. 2008; 34:146 - 152.
(5) Binder PS, Brownell M Martiz J. Rainbow glare mechanism not confirmed by SEM. Presented as a poster at the American Academy of Ophthalmology