- High-Brightness Multilayer Optical Film Light Source
Anne L. Fischer
Liquid crystal on silicon projection systems are used in rear- and front-projection television, and in head-mounted displays, and they have the potential to be used in nanoprojectors. They are built on silicon wafers with a reflective plane and, instead of using the traditional transmissive LCD, the light enters the front, reflects off a mirror and exits.
The transmission of multilayer optical film (MOF), wire grid and MacNeille polarizing beamsplitters is the product of p-polarized light transmission (Tp) and s-polarized light reflectance (Rs). Image reprinted with permission of the Journal of the Society for Information Display.
Brightness efficiency is a key requirement in these systems. For a liquid crystal on silicon projection system to be bright enough, all of the optical components must handle cones of light with steep angles. Polarizing beamsplitters such as wire grid and MacNeille have difficulty dealing with the steep angles, but a group at 3M in St. Paul, Minn., and at the company’s Precision Optics division in Cincinnati, recently demonstrated multilayer optical film polarizing beamsplitters with very high optical efficiency and polarization contrast.
According to researchers Charles L. Bruzzone and Simon Magarill, when using a MacNeille beamsplitter, the performance decreases rapidly as the angle increases. And this type of beamsplitter has to be made in lead-bearing glass, which is harmful to the environment. Wire grid polarizers work well but are 30 percent to 40 percent less efficient than the multilayer optical film approach, according to the scientists. Bruzzone added that, because the wire grid is a plate polarizer with no prism around it, the projection lens has to look back through more air, which requires a lens with increased back focal distance.
The multilayer optical film approach had been introduced previously; however, the 3M researchers studied the reflection and transmission properties of the polarized beamsplitter, the contrast stability with temperature variations and changes in pupil configuration. They found that the temperature of the multilayer optical film polarized beamsplitter does not affect the contrast, but temperature gradients across the input prism do, as a result of birefringence in the glass. This finding led to work on an improved thermal design for the light engine.
Ongoing work is aimed at improving adhesives and films in an attempt to increase the lifetime of the films, which is problematic as projectors become brighter.
The multilayer optical film polarizing beamsplitter uniquely maintains high p-transmission and s-reflection over a large range of angles of incidence, enabling other types of optical devices that are under development. For example, small and effective devices to combine light from individual RGB light sources into a single low-étendue beam appear to be very promising.
Journal of the Society for Information Display, October 2007, pp. 811-816.
- liquid crystal
- A type of material that possesses less geometrical regularity or order than normal solid crystals, and whose order varies in response to alterations in temperature and other quantities. Liquid crystals are characterized by phase varieties, including cholesteric, nematic and smectic. The optical properties of liquid crystals are familiar from their use in displays, known as LCDs.
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