In the past few years, high-dynamic-range (HDR) imaging has been studied extensively to visualize scenes containing ten orders of magnitude or more in intensity between the extremes of dark and light in many real-world settings. Because we cannot show such images on current cathode-ray tubes (CRTs) or LCDs, which are characterized by a dynamic range of several thousand to one, ‘tone mapping’ is used to compress their dynamic range. While this is an excellent technique, current displays are still inadequate for HDR image display.
New direct-view HDR displays have recently been developed with a much higher dynamic range than current CRTs and LCDs. One of these consists of an LCD panel and LED backlights that are partially modulated.1 We are pursuing development of an ultra-high-definition television2 that has 16 times higher resolution than high-definition television (HDTV) and needs a much larger screen. We developed a projector for a large-screen display using dual modulation of light to simultaneously achieve high dynamic range and high resolution.3
The basic projector design (see Figure 1) consists of a serial combination of two modulation blocks, one for chrominance and the second for luminance. While a conventional projector contains one block with three devices to modulate red, green, and blue (RGB) light, our projector also includes another block containing a luminance device to further modulate the combined RGB light. These modulation blocks have two main advantages. One is a dramatically improved dynamic range. The second is the ability to project high-quality color images by combining one high-resolution device for luminance modulation and three relatively low-resolution devices for chrominance modulation. The capability of the human eye to see fine detail depends largely on luminance-pattern differences. The projector needs only one high-resolution panel, thus also reducing cost significantly.
Figure 1. Basic design of our high-dynamic-range, high-resolution projector. RGB denotes the modulation of red, green, and blue light, while the Y signal is used for luminance modulation.
A photograph of our projector4 is shown in Figure 2 and its basic specifications are listed in Table 1. We used a newly developed 1.75-inch liquid-crystal-on-silicon (LCOS) device with 8192×4320 pixels5 for luminance modulation, and three 4096×2160-pixel devices for chrominance modulation. We confirmed subjectively that the image was not degraded by halving the chrominance resolution. The dynamic range of the projector from illuminances 10cm away from the projection lens is 1.1 million to one, which is extremely high. Next, we measured the tone reproduction for low-input video levels. In conventional digital projectors with 8-bit signal processing and γ=2.2 (where γ is the power-law exponent of the projector's input-voltage/luminance relationship), tone reproduction for dark areas is sparse, even for high dynamic range. However, our projector can display smooth tone reproduction for dark areas because it attenuates the output level in two stages. We confirmed that the projector can display an equivalent 10-bit tone reproduction with a low input level. A sample photograph of an image from the projector (see Figure 3) comprises 16 images, each with HDTV resolution of 1920×1080 pixels, indicating that the projector has 16 times higher resolution than HDTV.
Table 1. Projector specifications.
|Light output||1200 [lumen]|
|Devices||Liquid crystal on silicon (LCOS)|
1.7-inch diagonal, 4096×2160 pixels ×3
1.75-inch diagonal, 8192×4320 pixels ×1
Figure 2. High-dynamic-range, high-resolution projector.
Figure 3. Sample of a projected photograph comprising 16 images, each of which has high-definition-television resolution.
In conclusion, we have developed a projector with dual modulation of light to display high-quality images. It produces high-dynamic-range (1.1 million to one) and high-resolution (8192×4320 pixels) color images by combining one high-resolution LCOS device for luminance modulation and three relatively low-resolution LCOS devices for chrominance modulation. In addition, the projector produces a fine 10-bit tone reproduction. These results support the dual-modulation scheme as an effective way of improving dynamic range and displaying high-resolution color images. The projector currently only displays still images, because signal processing is not executed in real time. We will next design new hardware to process signals in real time and display moving images.
The authors wish to express their gratitude to the Victor Corporation of Japan Ltd. for developing the projector.
Yuichi Kusakabe, Masaru Kanazawa, Yuji Nojiri
NHK Science & Technical Research Laboratories
NHK (Japan Broadcasting Corporation)
Yuichi Kusakabe received his MS in applied physics from the University of Tokyo in 1999, and has since been working for NHK. He is a research engineer engaged in developing extremely high-resolution imagery systems and signal processing.
1. H. Seetzen, W. Heidrich, W. Stuerzlinger, G. Ward, L. Whitehead, M. Trentacoste, A. Ghosh, A. Vorozcov, High dynamic range display systems, ACM Trans. Graphics 23, pp. 760-768, 2004.
2. M. Kanazawa, Y. Kusakabe, Y. Nojiri, Y. Haino, M. Sato, K. Doi, Super Hi-Vision projection display for next-generation TV, JSID 15, no. 10, pp. 837-843, 2007.