In the Star Trek fictional universe, a VISOR is a device used by the blind to artificially provide them with a sense of sight. The device scans the electromagnetic spectrum, creating visual input, and transmits it into the brain of the wearer via the optic nerves. It is a thin, curved device, with the sensors on the convex side, that covers the eyes and attaches at small input jacks implanted in the temples. The only VISOR seen on screen was used by Geordi La Forge, who was blind from birth. VISOR stands for "Visual Instrument and Sensory Organ Replacement", though the complete term never appeared in the series, only in novelizations and other written spin-offs.
The VISOR also caused him persistent pain, which could not be treated without interfering with the device. It can be presumed that use of a VISOR is very rare, since no other characters in the franchise have appeared with a similar enhancement, and Beverly Crusher and Katherine Pulaski, the two high-ranking doctors who served on the ship, were unfamiliar with the device.
The device does not reproduce normal human vision, but does allow the character to "see" energy phenomena invisible to the naked human eye, as well as allowing him to view things at infrared and at microscopic levels. This also allowed the character to see human vital signs such as heart rate and temperature, giving him the ability to monitor moods and even detect lies. In the TNG episode "Heart of Glory", Captain Picard keys the main viewer to Geordi's visor allowing him to see the way he does. Seeing all the overlapping and different wavelengths was highly confusing to the Captain, prompting him to ask Geordi how he was able to differentiate between them all. Geordi's response made the comparison of child hearing many different sounds at once and eventually being able to pick out what they needed; it is a learned talent. The character's special visual abilities were responsible, at least in part, for his unusually rapid advancement in rank seeing as how his enhanced eyesight was a kind of built-in diagnostic tool allowing him to sidestep certain mechanical sensory tools.
Twice in the series, Geordi refused to be granted natural vision, first by Commander Riker who had been given extraordinary amounts of power by Q and later by Doctor Katherine Pulaski. Geordi refused the gift of natural eyesight in the first instance because it would have come at the cost of Riker's humanity.
Sometime between 2371 and 2373, before the time of the film Star Trek: First Contact, the VISOR was replaced by prosthetic implants, performing not only the same functions, but according to Memory Alpha, the Star Trek Wiki, the new implants have even more enhanced functions. On film, they are depicted using a combination of cosmetic contact lenses and CGI.
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