Switchable Gratings

Replace the LEDs in a conventional backlight with lasers and replace its randomly rough surface by a grating and it will produce collimated illumination. The angle of collimated illumination depends on the angle of the lasers so we have a low power backlight which can target rays towards a user as they move around, provided they stay far away.

More likely is that a user will want to be close to the display and it might seem a good idea to add a Fresnel lens in front of the backlight so that we can concentrate rays into one eye of the viewer at a time. But the focal length of a lens is fixed, and viewers tend to move forward and back: how do we vary the angle at which rays converge?

Instead of embossing a single permanent grating on our backlight surface, suppose that we tile it with a line of gratings, each of which can be switched off. Now switch on our laser so that collimated light emerges from the backlight and there will be a ray from one grating which enters the viewer’s eye. That means we can switch off all the other gratings.

Alter the angle of collimation a bit, and a ray from a different grating will reach the eye, so we switch on this grating and keep the others off. Carry on like this and we can time-multiplex a fan of rays which concentrate into the eye, and we can simply alter the timing of things as the eye moves away or closer. That’s why we are interested in switchable gratings.