Abstract

Increased contrast ratios are essential to the continued success of DLP products in the marketplace, from rear projection televisions and home theatre systems to large venue displays and DLP Cinema. The contrast of DLP projection systems has increased steadily over the past 5 years due to improvements in illumination and projection optics, as well as changes to the DMD itself. The authors present an overview of contrast performance of DLP projectors over time as well as describe illumination and projection techniques that optimize the performance of the DMD. 1. Introduction

A key metric of image quality for a projected or directly-viewed image is the contrast ratio. Contrast ratio itself consists of two measurements, “on/off contrast”, or full-screen contrast, and ANSI contrast which uses a field of 16 black and white rectangles. ANSI contrast can be used to describe the influence of light scattering on the display, and ANSI contrast cannot exceed the value of full-screen contrast. For computer graphics displays, where images have large areas of white or other bright colors displayed, the ANSI contrast value is a very useful metric. An ANSI contrast value of 300:1 is usually considered sufficient due to the dynamic range limitations of the human eye [1].

For video applications where the average luminance level of a scene can vary substantially, many times approaching values of 5% of full white, the full-screen contrast ratio becomes the important metric. As the average luminance drops, the black level perceived by the viewer increases due to adaptation of the eye to the lower scene brightness. For this reason, full-screen contrast ratios of more than 1000:1 are needed to reproduce movie content in a pleasing manner. The author’s experience with DLP Cinema has shown that a contrast ratio exceeding 1300:1 is required to give the viewer an experience comparable to film projection. With regard to television, measurements have revealed that the perceived contrast ratio for images that have an average luminance level of 5% and above are often considered to be nearly equivalent between a rear projection DLP.... television (that is capable of 1500:1) and a direct view CRT television (that is capable of >9000:1)[2]

For this reason, Texas Instruments’ DLP Products division has engaged in a program to enhance contrast of both DLP Cinema and home entertainment products, specifically front and rear projection home theater products. In each case the incumbent technology, film and the CRT, respectively, can achieve contrast ratios in excess of 5000:1.

2. Contrast Improvements 1996-2001

Since DLP technology does not use polarized light, the mechanisms limiting contrast are completely different from those of liquid crystal technologies, e.g. birefringence, skew rays, compensation films, etc. Instead, DLP contrast is limited by the geometrical aspects of the projection lens pupil, the pupil light distribution, scattering, and diffraction effects. Earlier efforts to improve contrast, up until 2001, were concentrated on the device itself rather than the optical system in which it was used [3].

Contrast of displays using the digital micromirror device (DMD) have improved steadily since the first products were brought to market in 1996 as seen in Figure 1. The first commercial product used the “hidden hinge” (HH) design. Full screen contrast for these products was in the 200:1 to 250:1 range.

Following closely were two improvements to the pixel mirror itself, small rotated via (SRV) and “small mirror gap” (SMG). The via is a hollow post which joins the mirror to the structure below. This presents as a small hole at the mirror surface. Small rotated via was simply the rotation of the via geometry 45 degrees from the original design, along with reducing its dimensions from 4x3 microns to approximately 2x3 microns. Analysis had shown that diffraction and scattering from the via was a significant contributor to lowered contrast, and SRV allowed for a 50% increase in full-screen contrast. Similarly, small mirror gap reduced the gap between pixels, thereby increasing the fill factor of the pixel itself. By reducing the amount of light that can pass between pixels and impinge on the structure under the mirror, scattered light, and therefore the black level, is reduced. SMG allowed for another 30% improvement in contrast ratio.