Polarization-Independent LC Microdisplays
In collaboration with Imagineoptix, we have been successful in developing highly efficient Liquid Crystal On Silicon (LCOS) Microdisplays for projection applications. These revolutionary devices eliminate the need for polarizers through the innovative use of the unique diffractive properties of Liquid Crystal Polarization Gratings (LCPG). These gratings are capable of electrical contrasts approaching 1000:1 over the entire visible range. We recently demonstrated a prototype projection system using 256 x 256 LCOS microdisplay that proves this principle of operation. These microdisplays have a fast dynamic response with sub ms switching times that allows for field sequential operation. Figure 2 shows a schematic of the polarizer-free projector design using the LCPG microdisplay.
Motivation for Approach
Most commercial liquid crystal displays use polarizers which limit their maximum power efficiency to < 50%. Several alternatives for polarization independent modulation using liquid crystals have been suggested to overcome this particular limitation. Among these approaches, several types of polarization gratings have been experimented with. One particular type, shown in Fig. 3, is a continuous periodic structure that can be easily fabricated holographically on both transmissive and reflective surfaces. We have used this principle to engineer LCOS microdisplays that can modulate unpolarized light with high efficiency without the use for polarizers.
By eliminating the need for polarizers, LCPG modulators can almost double the power efficiency of any application based on conventional LC devices. In the case of portable applications such as compact projectors, this can mean almost twice as much battery time and also reduction in size by using a smaller power source (Imagine 2 power cells instead of 4!). Another manifestation of this efficiency advantage in the case of display applications is an increased brightness for the same amount of power. LC Modulation itself does not limit the efficiency of LCPG SLMs, but larger angular separation of the diffracted orders is necessary to fully utilize the efficiency advantage. We continue to strive toward small grating periods (< 2um --> diffraction > 20 degrees) by optimizing different material parameters that determine the grating properties. Another current limitation of these devices is the electrical contrast at low operating voltages which is determined by the residual retardation from the liquid crystal configuration. We have identified several compensation techniques that are currently being implemented, that should allow for much higher contrasts, even surpassing those reported for most LCDs.
1. RK Komanduri, C Oh, and MJ Escuti, "Polarization-Independent Liquid Crystal Microdisplays," SID Symposium Digest 2008 (Details:TBA),
2. RK Komanduri, WM Jones, C Oh, and MJ Escuti, "Polarization-Independent Modulation for Projection Displays Using Small-Period LC Polarization Gratings," Journal of the Society for Information Display, vol. 15, no. 8, pp. 589-594, 2007.
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