Halos, or halo effect, are seen by LASIK patients with large pupils. Halos and starbursts are forms of "spherical aberration" which is caused by pupils dilating larger than the effective optical zone of the laser treatment. As with other aberrations induced by laser eye surgery, halos vary in several ways. Some are bigger or smaller, and some are denser or more transparent. Some patients report halos nested several layers deep. On close examination, some halos may appear to consist of fine starburst rays, so much that there is no clear distinction between the two.
Halos can appear around any relatively brighter area or reflective object. In the image below, the yellow caution sign and the white line on the edge of the road have a halo, whereas a bright light source such as a streetlight has a starburst.
Some images courtesy VisionSimulations.com
LASIK patients may see various forms of visual aberrations at night including starbursts, halos, and multiple images, as well as loss of contrast sensitivity (inability to see fine detail). The LASIK industry does not consider night vision impairment at night as a "complication", even though patients may no longer be able to drive safely at night or participate in activities in dim light situations.
These visual aberrations may be psychologically disturbing and emotionally devastating for patients.