Sequential-Color LED Illumination for Reflective Microdisplays

Integrated silicon microdisplays, such as liquid-crystal-on-silicon (LCOS) devices, are becoming the most effective image source for high-resolution viewfinders, head- mounted displays, and helmet-mounted displays (HMDs). Since these microdisplays are reflective in nature, they require new arrangements of illumination and viewing optics compared to previous transmissive displays. A complete display might comprise a reflective microdisplay panel lit from the front, through a beam splitter, by a light-emitting- diode (LED) illuminator, and viewed through an eyepiece optic. Full-color display, without resolution-degrading color filter triads, can be achieved with one simple microdisplay panel by utilizing field sequential color — easily attained by making the illuminator from a few red, green, and blue LEDs. The best image quality is provided, though, when the illuminator appears as an extended diffuse white spot, rather than red, green, and blue points. This can be achieved by spacing a diffuser between the LEDs and the display, with the light from the differently colored LEDs overlapping on the diffuser to make a white spot. An especially efficient arrangement is achieved with a specularly reflecting microdisplay when the illuminator and eyepiece lens are positioned so that the lens simultaneously makes a virtual image of the display panel, and a real image of the illuminator. By arranging the image of the illuminator to fall in the same position as the pupil of the viewer’s eye, the largest possible amount of illumination light is made usable by the viewer, while the amount wasted is minimized. For microdisplays that operate in polarized light, high optical efficiencies are obtained if the needed beam splitter is a polarizing beam splitter (PBS). The overall system can be simplified if the PBS is curved, which saves space and combines the function of an illuminator condenser or collimating lens. PBS function can also be obtained from suitable edge-illuminated holographic illuminators.