It’s easy to forget that when we look at a display, we see an image because rays of light travel from the screen to our eye. All other rays are wasted and we want to save power by making displays which concentrate their emission into the viewers’ eyes but how do you do that?
It would have been hard with old-fashioned TV’s, but liquid crystal displays have the great advantage that they work in two stages. First there is a backlight which is just a slim lamp, then there is the liquid crystal panel which is an electrically switchable stained glass window.
Until now, it hasn’t been possible to know where the viewer is so conventional backlights just glow – they are made deliberately diffuse. They comprise a transparent sheet surrounded by LEDs pumping light into its edges: the LEDs emit light at all possible directions into the sheet, then the sheet is made rough so as to make everything as random as possible.
If instead, we line one edge of the sheet with parallel lasers and the sheet is smooth except for a single grating on its surface, then light from the lasers all travels in the same direction within the sheet as if through an optical fiber, and the grating diffracts light out from all parts of the sheet in the same direction. Instead of emitting diffuse light, our sheet emits collimated light so that when we put a Fresnel lens in front, all the emitted rays concentrate into an eye at the focal point of the lens.
Lasers are expensive, perhaps 100 times more so than LEDs, but we don’t need many because, remember, we are no longer wasting any light, so we expect our components to cost no more than is normal.