Charles L. Bruzzone

10.4: High-Performance LCoS Optical Engine Using Cartesian Polarizer Technology

An efficient and economical LCOS optical engine has been designed for high definition rear projection television. The design uses a 3M™ VikuitiT Optical Core comprising three Cartesian PBSs and an X-Cube color combiner. The first prototype engine built with this core delivers 400 lm color corrected, near 9000K color temperature with contrast over 1000:1, using a 100W lamp without polarization conversion.

3M PBS for high-performance LCOS optical engine

A new 3M Polarizing Beamsplitter (PBS) enables high performance optical engines for Liquid Crystal on Silicon (LCOS) projectors. It overcomes the limitations of previous LCOS optical engines that have insufficient light efficiency, contrast, and dark state uniformity. These limitations are the direct result of the performance of existing MacNeille PBSs: poor transmission of p-polarized light (Tp) and reduced contrast at modest beam angles and wavelength sensitivity. 3M has addressed these problems by creating a plastic polarizing film made of alternating layers of different plastics with the refractive indices tailored so that they match in one direction but not in the other. In the unmatched direction, a quarter-wave stack is formed that is highly reflective, while the matched direction appears as a transparent slab of plastic. This film is laminated between glass prisms to form a PBS with none of the problems associated with MacNeille PBS’s. For an F/2 beam, Tp exceeds 92% across the visible and photopic contrast exceeds 2000:1. High contrast is achievable in an optical engine without the use of a post-polarizer, avoiding this 15% loss that is necessary with a MacNeille PBS. Finally, the input light need not be highly polarized, allowing the engineer additional design freedom.

Compact, high-brightness LED illumination for projection systems

— An arrangement consisting of a polarizing beam splitter, retarders, and dichroic mirrors provides a compact and efficient combiner for red, green, and blue light from separate LED packages. The resulting illuminating beam is unpolarized. In initial tests, a 0.55-in. DMD engine based on this illuminator produced an on-screen flux of 188 lm from 58 W of LED input power, using an f/2.4 pupil. Subsequent work with the same imager and LED set has resulted in a compact and efficient 300-lm projector.

Invited Paper: Detailed optical characteristics of multi‐layer optical film polarization beam splitter

— 3M polarization beam splitter (PBS) technology has been shown to be the most light-efficient solution currently available for use in LCOS projection. It also provides short back focal length, very high contrast, and extremely uniform dark states without the use of lead-glass prisms. Recent improvements in contrast performance, increased understanding of the effects of pupil shape and size on contrast, effects of temperature on optical performance, and improved photostability are reported. New light-engine architectures employing the 3M PBSs with associated light budget analyses are suggested.

Compensation of birefringence in lead-free polarizing beam splitters for LCOS projectors

Most optical designs for delivering light to LCOS imagers and then from the imagers to the projection lens use polarizing-beam-splitter (PBS) technology. Most of the PBSs used in commercial LCOS projectors contain glass with a significant amount of lead (Pb). Such glasses have inherently low stress birefringence, and therefore maintain the polarization state of light passing through them. However, Pb-bearing glass is an expensive, difficult to process, and hazardous material with special disposal requirements and is therefore not desirable in consumer-electronic products. On the other hand, Pb-free wire-grid plate PBSs require a longer back focal length than would be optimal. Data and modeling results show that uniform high-contrast dark states may be obtained from lead-free-glass Cartesian PBS prisms when a quarter-wave compensator is used between the imager and the PBS.

6.1: Photostability of Polymeric Cartesian Polarizing Beam Splitters

Polymeric Cartesian Polarizing Beam Splitters provide significant cost and performance advantages for LCOS RPTV engines. This paper describes work to demonstrate photo-stability in these components. Film resins and adhesives both affect photostability, and must be selected carefully. Care must also be taken in constructing the tests and interpreting results.

Advances in polymeric Cartesian polarizing beamsplitters and light engines employing them

3M Polarization Beam Splitter (PBS) technology has been shown to be the most light efficient solution to the needs of LCOS projection. It also provides very high contrast and extremely uniform dark states without the use of lead in the glass prisms. We report on recent improvements in contrast performance, increased understanding of the effects of pupil shape and size on contrast, effects of temperature on optical performance, and improved photostability. We also suggest new light-engine architectures employing the 3M PBSs with associated light budget analyses.

LCoS microdisplay throughput measurements

We propose a straightforward method to measure the absolute throughput of an LCoS microdisplay. The method requires two measurements. The first measurement is of the lumens delivered from a testbed, which includes the microdisplay under test, with a folded light path. The second measurement is of the lumens delivered by the unfolded testbed with the microdisplay removed. This second measurement is used to normalize the first to obtain a fractional transmission that a designer can use to make a meaningful prediction of the system brightness of a LCoS-based projection system.