Retinal implants are currently in their earliest stage of development: comparatively crude electrode arrays that provide an alternative to vision rather than restoring it. From here the path to ever more sophisticated systems seems assured, however:

A coming generation of retinal implants that fit entirely inside the eye will use nanoscale electronic components to dramatically improve vision quality for the wearer, according to two research teams developing such devices.

Current retinal prostheses, such as Second Sight's Argus II, restore only limited and fuzzy vision to individuals blinded by degenerative eye disease. Wearers can typically distinguish light from dark and make out shapes and outlines of objects, but not much more. The Argus II, the first "bionic eye" to reach commercial markets, contains an array of 60 electrodes, akin to 60 pixels, that are implanted behind the retina to stimulate the remaining healthy cells. The implant is connected to a camera, worn on the side of the head, that relays a video feed.

A similar implant, made by Bionic Vision Australia, incorporates just 24 electrodes. With so few electrodes, the amount of visual information transmitted to the brain is limited: text, for example, is difficult to read. Second Sight recently announced a method by which Argus II wearers are able to visualize Braille instead of traditional text.

Recognizing this limitation, both Second Sight and Bionic Vision Australia have announced that they are developing next-generation devices with 200-plus electrodes. But arrays of nanoscale electrodes, which are currently being incorporated into new retina devices, could someday give blind people 20/20 vision.

Link: http://www.technologyreview.com/news/508041/vision-restoring-implants-that-fit-inside-the-eye/

Source:
http://www.fightaging.org/archives/2012/12/a-brief-look-at-retinal-implants.php