"Down inside the computer are three lasers - a red one, a green one, and a blue one. They are powerful enough to make a bright light, but not powerful enough to burn through the back of eyeball and broil your brain, fry your frontals, lase your lobes. As everyone learned in elementary school, these three colours of light can be combined, with different intensities, to produce any colour that Hiro's eye is capable of seeing."
Snowcrash, page 22

VRD or Virtual Retinal Display is an offshoot of HMD display technology, which, instead of placing a pair of display screens in front of the eyes, actually projects an image directly onto the human retina with low-energy lasers or LCDs.

A Virtual Retinal Display thus bypasses the eye almost completely, itself doing all the work to refocus, and just delivering the result. This bypasses the eyestrain problems of looking at a 3D image on a 2D display, and trying to focus on an image that is perceived to be further away. . In principle the technology can provide full-colour, high-resolution dynamic displays, but in practice the technology is simply not advanced enough yet.

The original technology was was invented by the University of Washington in the Human Interface Technology Lab (HIT) in 1991, however the technology at the time was not sufficient for practical use.

VRD displays are actually a lot less complex than flatscreens to create, requiring a fraction of the hardware components. This has the effect of making the display systems much lighter and more reliable - even if the components they use are a lot more intricate than normal. Once the technology is fully realised, it is very likely they will replace mobile phone, ipod, and other such handheld display systems very quickly, moving to other systems from there.

Power Usage

In the modern, power-centred world, it is worth noting that a VRD is much more efficient than CRT or LCD flatscreens, and requires much less power than even a mobile phone display. Using about a microwatt of power to run and fill an entire eye with vision, they provide a sharp, clear image regardless of external lighting conditions.

Composition

A VRD system consists of four stages:

• Information Stream
• Laser / LED light source
• Scanner matrix: Rows of horizontal scanners interlaced with rows of vertical scanners
• Lens to focus the image
  

The scanner bank oscillates like shutter glasses, allowing colour frequencies to cycle past it, in sequence to build up a pixel display which is blasted through the lens by the laser beams, and directly onto the retina of the eye, blanketing it.