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Stereoscopic laser display system using six primary colors

Klaus Ch. Rohwer, Helmut Jorke and Arnold Simon INFITEC GmbH, Lise-Meitner-Strasse 9, 89081 Ulm, Germany

Abstract The requirements on laser wavelengths necessary to display 3D content using the wavelength multiplex technology and encompassing the DCI color space are presented. As an example of the technique a system consisting of a rebuilt six primaries projector and corresponding passive 3D glasses is shown.

1. Introduction The application of lasers as projection light sources has regained attraction in the past few years. The display industry has shown high interest in introducing lasers because benefits such a low étendue, pure polarization and spectrally narrow band emission are desired characteristics for a projection light source [1,2].

The idea of the wavelength multiplexing technology (WMT) initially aimed at six laser like light sources. Due to the absence of affordable laser light sources the idea was adapted to projectors using broadband light sources [3]. Narrow spectral emission bands of the projector are being generated by spectral filtering using narrow band interference filters. These filters are built into the projector as fixed filters or as a spinning filter wheel. For stereoscopic applications left and right images are coded in complementary spectral ranges; stereo images are again decoded at the viewer side by the corresponding glasses filters. This technology is multiply applied to show stereoscopic 3D content, especially by Dolby 3D™ in the cinema area.

Figure 1. Wavelength multiplexing technology (WMT) for a

conventional 3D projection system; top: typical Xenon projector spectrum (black line – white light, other line colors

correspond to the light color); middle: typical Infitec 4+3 band WMT projector filter spectrum (red – left eye, green – right eye); bottom: typical WMT spectrum presented to the

observer (red – left eye, green – right eye).

WMT is a passive glasses based stereoscopic 3D technology dividing the visual spectrum in at least six color bands, of which three are shown on the left and other three to the right eye of the observer. The proper adjustment of the filter edges produces a well-balanced colorful image for left and right channel [4] (figure 1).

As the WMT is independent of polarization it can be applied to any screen. The passive glasses do not show any flickering effects and they don’t need any synchronization or battery replacement.

Regarding the color gamut in digital cinema the DCI specifications [5,6] have to be met. With the upcoming technology of laser projection, this is easily achieved for 2D content using only three laser wavelengths. In order to make WMT work with lasers, at least six different wavelengths are required. These correspond to six primary colors (6P), two different ones each in the red, green, and blue range, respectively (figure 2). In contrast to the current WMT technology, for laser light sources no projector filters are required. Therefore the light flux of 3D mode is the same as for the 2D mode.

Figure 2. Wavelength multiplexing technology (WMT) for a 6P laser 3D projection system (schematic illustration); top: assumed laser projector spectrum (solid line – left image,

dashed line – right image); middle: assumed glasses spectra (red – left eye, green – right eye); bottom: transmitted laser

light (solid line – left image, dashed line – right image).

In addition to DCI specifications, stereoscopic image separation requires a certain spectral distance of the adjacent wavelengths in order to separate the images for the left and right eye by corresponding glasses filters. On the projector side, no filters will be needed even if the laser light sources emission is artificially broadened for the sake of speckle reduction. Furthermore, only certain laser wavelengths are commercially available and

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economically feasible. From these, appropriate ones have to be selected to make a six-primaries stereoscopic laser display system work with wavelength multiplex technology.

2. Color gamut In order to meet the DCI specifications for both eyes one has to select laser wavelengths carefully. The starting point is the red corner of the DCI color gamut, since it is closest to the CIE chromaticity curve (figure 3). This determines the shortest wavelength to be used for one of the red lasers (616 nm). From this point the shortest green wavelength can be determined (525 nm). This one in turn determines the shortest blue wavelength (452 nm).

The corresponding longest wavelengths are determined starting as well with the red one. Theoretically the longest visible wavelength of 780 nm could have been chosen. We recommend a maximum wavelength of 660 nm, otherwise the perceived brightness will be significantly reduced. From this the longest possible blue wavelength (468 nm) can be determined, and this in turn determines the longest possible green wavelength (545 nm). The results are displayed in table 1.

Figure 3. DCI color gamut and compliant laser wavelengths for six primary color wavelength multiplex stereoscopic 3D

Minimum wavelength

Maximum wavelength

Relative separation

red 616 660 6,9%

green 525 545 3,7%

blue 452 468 3,5%

Table 1. Digital cinema compliant wavelengths of laser projection light sources for use with the wavelength

multiplexing technology

Since interference filters shift in the transmission spectrum to shorter wavelengths with increasing viewing angle, the spectral design of the glasses has to consider these characteristics. Using a broader transmission range than required ensures that changes of the spectral characteristics over the viewing angle always lie within the transmission band of the filter. Therefore a separation of 3% of the laser wavelengths (red > 20 nm, green > 16 nm, blue > 14 nm) is recommended. For extremely large viewing angles the application of curved glasses reduces the angular shift in such a way that this condition is still ensured.

With continuous spectrum light sources the filter design of the glasses and projector filters accounts for this shift by introducing a spectral gap between the transmission bands. This is not the case with laser projection systems, since there are distinct wavelengths only. Furthermore, the unchanged transmission of the glasses over the entire viewing range enables flawless color uniformity on the screen.

The 3D transmission of a six-primary wavelength multiplexing laser projection system is only bounded by transmission of the glasses filters. Ideally, the laser 6P projection system is built as an assembly of 6 imagers. In the early stage of 6P, two projector setups will be used, where each of the projectors is equipped with a triple primary laser system. Thereby, both images for left and right eye are provided simultaneously. The constant stream of light to the eye contributes to a pleasing visual comfort. The low-loss 3D glasses with a transmission of 90 - 95% enable boundless 3D brightness [11]. Additionally, the high reflectivity of interference filters allow a perfect channel separation with values < 0.5% in crosstalk.

Figure 4. Measured transmission in the blocking ranges of one of the laser glasses used. Note the transmission scale that

ranges from 0% to 1%.

3. Setup In order to show the feasibility of a 6 chip laser projection system using the wavelength multiplex technology, we rebuilt a commercially available six chip LCOS projector (LG CF-3D) [7]. We replaced both projector lamps with an optical fiber providing laser light from external laser boxes. A vibrating “fiber shaker” was added to the system to reduce the speckle form the laser. For white balance the laser power (total 2 x 10 W) of each primary was tuned to meet the DCI white point. No additional color correction was applied. As glasses we applied sputtered metal oxide interference filters on flat glass substrates.

A general challenge with laser projection systems is speckle. In order to reduce this, the laser boxes applied were equipped with several lasers of slightly different wavelengths for each color (artificial broadening). The fact that the laser light is not strictly monochromatic reduces the visibility of speckles. For further speckle reduction we inserted an electromechanical fiber shaker between the lasers and the projector.

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Due to the fact that the light engine was not changed this projector still works in the polarizing 3D mode in combination with a silver screen. The setup is shown in figure 5.

Figure 5. Laser projection system using six primaries fed to a rebuilt projector via optical fibers.

4. Results and Discussion Images of this laser projector were compared with the conventional lamp based projection system in 2D and 3D mode. The colors of the laser projector system appear to be more saturated and vivid than the lamp based projector.

Regarding independently the DCI specifications for the left and right images, the DCI color gamut could not be met in the blue for the short laser wavelength (445nm) image and in the green (550nm) for the long wavelength image. The resulting triangles in the CIE chromaticity diagram are shown in figure 6.

However, in 6P mode the existing color difference of left and right primaries was not noticed by the viewer. According to related research studies [8,9] we assume binocular color fusion of the human perception averages the perceived color. Therefore the perceived color gamut of the viewer is somewhere in between the individual color gamut and the 2D color gamut.

We also compared the image homogeneity of the 6P system to the polarized 3D system. Most significant is the increase of speckle in the polarizing system. Comparing the white and silver screen in the 6P application indicates that silver screens worsen speckle of laser light.

5. Conclusions A 6-chip-LCoS system for laser six primary (6P) stereoscopic imaging was presented. Although primary colors were not color corrected the binocular color fusion was achieved.

Figure 6. Color space achieved by our rebuilt LCOS projector

using commercially available laser wavelengths.

6. References [1] B. D. Silverstein, Andrew F. Kurtz, Joseph R. Bietry, and

Gary E. Nothhard, “A Laser-Based Digital Cinema Projector”, SID Symposium Digest p.326 (2011)

[2] M. S. Brennesholtz ,” New-Technology Light Sources for Projection Displays”, SID Symposium Digest 858 (2008).

[3] H. Jorke, A. Simon, and M. Fritz, “Advanced stereo projection using interference filters”, J. SID 17 No. 5, p.407 (2009).

[4] A. Simon and H. Jorke, “Interference Filter System for High-Brightness and Natural-Color Stereoscopic Imaging”, SID Symposium Digest 317 (2011).

[5] Digital Cinema Initiatives, LLC, “Digital Cinema System Specification”, Version 1.2, March 07, 2008

[6] Digital Cinema Initiatives, LLC, “Stereoscopic Digital Cinema Addendum”, Version 1.0, July 11, 2007

[7] J. Seo, J. Kwon, Y. Kim, S. Lee, C. Yoon, J. Choi, S. Park, and S. Lee,”Color Separation 3D in a Laser Projection System Using a 2D MEMS Scanner”, SID Symposium Digest p. 637 (2012)

[8] Y. J. Jung, H. Sohn, S. Lee, Y. M. Ro and H. W. Park, “Quantitative measurement of binocular color fusion limit for non-spectral colors”, Optics Express, 19 No.8, p. 7325-38 (2011)

[9] D. Qin, M. Takamatsu, Y. Nakashima, and X. Qin, “Change of wavelength difference limit for binocular color fusion with wavelength and brightness of stimuli,” J. Light Vis. Environ. 30 No.1, p. 43–45 (2006).

[10] M. Schuck and G. Sharp, “3D digital cinema technologies”, Journal of the SID 20/12, p.669-79 (2012)

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