Building a high-dynamic-range projection system
In the past few years, the limited dynamic range of both imaging devices and displays has been extensively studied in computer graphics. In addition to methods for producing high-dynamic-range (HDR) imagery, such as physically-based rendering 1 algorithms for capturing both still images2 and videos3 with extended dynamic range, have been developed.
Several ways to display HDR images have been tried. One alternative is to transform the original range of intensities into the (smaller) range of intensities a normal monitor can reproduce: a process called tone mapping. Various tone-mapping operators4 have been developed. While these allow HDR images to be displayed, the dynamic range of conventional displays is still inadequate for creating the appropriate visual sensation of an HDR scene.
Recently developed HDR displays provide an alternative.5 These displays have a contrast ratio of more than 50000:1 and peak intensities up to 8500cd/m2. In comparison, traditional displays usually have a contrast ratio of about 300:1 and peak intensities of about 300cd/m2. The principle behind an HDR display is to filter the light twice: once in the spatially-varying backlighting system, and then again in the front liquid-crystal-display (LCD) panel. If the maximum contrast of the backlight image is c1:1, and the maximum transmissive contrast of the front LCD panel is c2:1, then the theoretical contrast ratio of the overall system is (c1.c2):1.
Our new HDR projection system6 (Figure 1) is designed as an external attachment for an unmodified data projector. The system works by first adapting the projected image using a set of lenses to form a smaller image. This image is then modulated by an LCD panel and the result is projected through another lens system onto any screen, just as in traditional projection scenarios.
We built our prototype using an Optoma EP755™ projector, with a 600:1 contrast ratio and a 2000lm peak output. Its color wheel was removed to increase the maximum light output. As the projector's optics are designed to create an image more than one meter away, we added a 100mm focal-length achromatic lens directly in front of the projector to create a small-yet-bright image at a distance of about 100mm. This small image is then modulated a second time by the LCD, in our case a Sharp QA-75 computer VGA projection panel. We use only a small central portion of the display, about 40×30mm, or 120×90 pixels. Subsequently, two 120mm plano-convex lenses, acting as a field lens, gather and direct the light towards an objective lens. This single achromatic lens, with a focal length of 125mm, projects the doubly-modulated image onto a screen—in our case a white wall—about 90cm away.
The algorithm for controlling the system during the measurements was the same as used in previous work.5 We used the ANSI display-testing procedure to measure the system's contrast and brightness. At 90cm, the image produced by the system was about 150×200mm. Under these conditions, the minimum and maximum luminance of the system were 0.6 and 425 cd/m2, respectively, which translates to a dynamic range of 708:1. The measured contrast of the projector was 136:1, the LCD panel 9.56:1, and the maximum transmissivity of the LCD panel 17.2%.
Another implementation of the technology would integrate the LCD panel into the projector, or use another spatial-light-modulator technology for the second light-modulation step.
This research has shown that adding an LCD panel to the optical path of a DLP™-based projection system can noticeably improve its contrast ratio, bringing it closer to the dynamic range of luminance levels found in nature. In our prototype, we achieved an ANSI contrast of 708:1, with a maximum luminance level about four times that of typical cathode ray tubes. These specifications are not very impressive compared to HDR displays,5 mainly because of the low contrast ratio of the LCD panel used. However, there are various obvious areas for improvement: for example, the optical path would benefit from better optical elements and a proper enclosure.
