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why triple beam system is better than double beam system?

Jul 13, 2018

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Among the 3D  devices currently used in theaters, the polarized 3D devices occupy a  considerable proportion, and the process is mature and stable, and the  cost is relatively cheap, which has been widely recognized by the  theater. In the polarized 3D device, the luminous  efficacy of the double beam system and the triple beam system can  reach 30% compared with the 16% light effect of the single optical path  3D device, which is collectively referred to as high luminous efficiency  3D, which doubles the brightness of the screen 3D picture. The most important type of technology in cinema 3D technology.


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The double beam system uses a conventional polarization beam splitting polarization beam  splitter (PBS) to polarize the entire incident light, transmit the  P-polarized beam and reflect the S-polarized beam, and then use a  mirror, a polarization rotating device, and a liquid crystal phase  modulation. The device adjusts the reflected beam,  and uses a liquid crystal phase modulation device to adjust the  transmitted beam, and cooperates with a lens device to make the two  beams overlap on the screen as an image, thereby improving light  utilization.

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The 3D technology of  triple beam system is based on the principle of 3D technology of double beam system. The structure of the polarization beam splitting prism is  redesigned into a K-shape. The reflected beam can be divided into two,  which are divided into two upper and lower beams, respectively. Lower reflection. The transmitted beam is a beam of light that is a complete image on the screen. The  reflected beam is only a half image, and two reflective S-polarized  beams are modulated by two mirrors, two polarization rotating devices,  and two liquid crystal phase modulation devices, respectively. The transmitted P-state polarized beam is modulated by the liquid crystal phase modulation device and the lens group. The  upper and lower images are stitched together on the screen into a  complete image that overlaps with the entire transmitted image on the  screen, improving light utilization.


First, the size and weight comparison
Figure  1 and Figure 2 show the main optics and optical path diagrams of the double beam system and the triple beam system, respectively, based  on the same divergence angle and spot size. Since  the three light paths divide the light into two upper and lower half  images, the optical path is significantly shortened in the entire  optical path, so the size of the prism is significantly reduced, and the  thickness of the device is thinned. Under  the same projection ratio, the three-light 3D equipment is  significantly smaller than the double beam system, and the  thickness is thinner and the installation environment is lower. At the same time, due to the reduction in volume, the weight will also be significantly reduced.

Second, light efficiency
In  principle, double beam system and triple beam system use  polarization splitting devices to convert unpolarized light from the  lens into linearly polarized light. The polarization splitting devices  used in the two technologies have the same spectral efficiency. At the same time, both use mirrors as the light direction adjustment device, and their reflectivity is basically the same. After  the light is converted, it is finally passed through a liquid crystal  modulator and adjusted to left-handed circularly polarized light and  right-handed circularly polarized light according to the frame order. The performance of the devices used is essentially the same throughout the optical conversion and modulation process.
Therefore,  in theory, the light efficiency of the double beam system and the triple beam system is the same, and there is no problem that the triple beam system 3D light effect is higher than the double beam system 3D.


Third, the crosstalk rate
In  the polarization 3D technology, the crosstalk rate is a parameter  indicator describing the crosstalk of the left and right eye images. In  terms of percentage, the industry standard is usually a crosstalk rate  of no more than 2%. When the crosstalk rate is increased, ghosting is aggravated and image quality is deteriorated. The  crosstalk rate is not only an important parameter for measuring  polarized 3D equipment, but also an important parameter for measuring  the entire 3D movie theater. It involves various related equipments  including digital projectors, metal screens, and polarized 3D glasses. In the case of a polarized 3D device itself, the crosstalk rate is  primarily determined by the optical parameters and drive circuitry of  the liquid crystal modulation device.
The  double beam system and the triple beam system use the same  manufacturing process to fabricate the liquid crystal modulation device  and drive using the same driving circuit. However,  since the area of the liquid crystal modulation device of the optical  path on the dual optical path 3D device is too large to be completely  driven, the contrast of the polarization modulator of the upper optical  path is seriously degraded. Generally, the double beam system designed according to a  projection ratio of 1.2:1 has a contrast ratio of 60 to 80:1, which is  converted to a crosstalk ratio of 1.25% to 1.7%.
The  internal optical path of the 3D equipment of triple beam system is obviously  smaller than that of the double beam system. The spot area is  reduced and divided into two parts. Two liquid crystal modulation  devices with smaller area are used respectively. There is no problem of  insufficient driving capability, so the contrast of the triple beam system is obvious. Higher than double beam system. Generally, the triple beam system designed according to the  projection ratio of 1.2:1 has a contrast ratio of 150 to 180:1, which is  converted into a crosstalk ratio of 0.56% to 0.67%.
Therefore, in terms of crosstalk rate and ghosting, the triple beam system is much better than the double beam system. The use of three-light path can effectively improve the contrast of  the polarized 3D device and reduce the crosstalk rate, which can  effectively improve the ghosting problem of the 3D picture on the  screen.

Fourth, light source compatibility
Currently,  the digital light source used in cinema digital projectors includes:  mercury lamps, xenon lamps, phosphor lasers, and RGB lasers (including  6-color and 9-color lasers). Except for RGB laser, the spectrum of other light sources is  continuous spectrum or partial continuous spectrum. It belongs to  incoherent light source and has no problem when paired with double beam system and triple beam system 3D equipment.
The RGB laser source uses three kinds of semiconductor lasers of a fixed wavelength as a light source. The  half-wave width of each wavelength of RGB laser is very narrow. It  belongs to the coherent light source and retains almost all the  characteristics of the laser, including: monochromaticity, coherence,  linear polarization and so on. The  K-shaped polarizing beam splitting prism used in the 3D equipment of  triple beam system is glued and spliced by a plurality of prisms, and the  splicing seam is located in the middle of the optical path. Usually, the  seam is optically considered to be discontinuous. When  the RGB laser is used in a three-light 3D device, due to the coherence  of the laser, horizontal color streaks often appear in the middle of the  screen. The  prism used in the dual-optical 3D device has no stitching in the  optical path, so it is optically continuous and there is no problem of  color streaks.


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However, the triple beam system is not completely unable to  solve the problem of horizontal color stripe. By special treatment of  the prism edge and optimization of the prism glue and splicing process,  the problem of horizontal color stripe can be basically solved.

Five, image clarity
When the double beam system is working, its final screen image is made up of two overlapping images. The  picture of the triple beam system is spliced into two images by  two half images, and then coincides with another whole image. Therefore, the accuracy of image overlap will directly determine the sharpness of the picture. Even  under ideal optics conditions, there are still large differences  between double beam system and triple beam system , which are  determined by the structure of the two, that is, the optical path  difference is different. In  the first two optical path diagrams, the red part is used to indicate  the optical path difference between the transmitted beam and the  reflected beam. For a projection distance of 18 meters, the screen width  is 15 meters, the projection ratio is 1.2:1, and the optical path  difference in the double beam system is about 150 mm. That is, the center light of the upper and lower beams is about 150 mm. In  the triple beam system designed according to the 1.2:1  projection ratio, the optical path difference between the transmitted  beam and the reflected beam is only about 60 mm. The  center beam of the upper beam in the double optical path 3D needs to be  deflected downward by 0.48 degrees, while the triple beam system need  only be deflected by 0.21 degrees. Therefore, the focal plane of the upper beam is also deflected by 0.48 degrees. In  the case where the screen is placed vertically, the upper edge of the  screen is closer to the lens than the focal plane of the lower beam by  about 34 mm, and the lower edge of the screen is about 34 mm away from  the focal plane of the lower beam. Referring to the angle of the light  of the 1.2:1 projection ratio, the relative angle can be calculated. In  the lower beam, the upper beam is about 8.4 mm smaller on the left edge  of the screen, and the upper beam is about 8.4 mm below the edge of the  screen. Calculated at 2K resolution, the 18-meter wide screen has a size of 8.79mm per pixel. Therefore,  for the double beam system, in the most ideal case, the pixel deviation  around the screen is also about 1 pixel, but often due to the excessive  size of the double beam system optics, the accuracy cannot be achieved  ideally, so the dual optical path 3D device The screen overlap is generally 1 to 3 pixels.

The  beam and the down beam need to be deflected by 0.21 degrees on the  triple beam system, and the focal plane of the upper and lower beams is  also deflected by 0.21 degrees. Also  in the case where the screen is placed vertically, the upper edge of  the screen is about 7.4 mm closer to the lens than the focal plane of  the middle beam, and the horizontal line of the screen is about 7.4 mm  away from the lens than the focal plane of the middle beam, with  reference to the angle of light of the projection ratio of 1.2:1. It  can also be inferred that the upper beam is about 1.8 mm smaller on the  left edge of the screen than the intermediate beam, and the upper beam  is about 1.8 mm in the middle of the screen, and the lower beam is  consistent with the upper beam. According  to the 2K resolution calculation, in the most ideal case, the pixel  deviation around the screen and the middle horizontal line is about 0.2  pixels, and the volume of the triple beam system is much  smaller than that of the double beam system, and the accuracy  is higher than that of the double beam system. Therefore, the excellent screen alignment of the triple beam system can be controlled at 0.5 pixels or even higher precision.

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Double beam system shows severe trapezoidal deformation when images coincide
The  image of the double-beam reflected beam is smaller than the  transmission image on the upper edge of the screen, and the lower edge  of the screen is larger than the transmission image, so that the picture  cannot completely overlap in the four corners of the screen. The lower  the projection ratio, the larger the volume of the double beam system. The larger the optical path difference  between the upper and lower beams, the more the trapezoidal deformation  will be intensified. Therefore, at present, the  two-beam path with a projection ratio of 1.2:1 or less has a problem  that the images cannot overlap, and the pixel deviation is 2 to 4 pixels  or more.


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                 Double beam system 3D images cannot be completely coincident

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The trapezoidal deformation of the image of triple beam 3D is not obvious
In  the 3D device of triple beam system, since the optical path difference  between the transmitted beam and the reflected beam is small, the angle  at which the reflected beam needs to be deflected is also small, so the  reflected image has substantially no trapezoidal deformation, and can be  done in most areas of the screen. To the pixel level alignment.

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                             Triple beam system3D image perfectly coincides


Therefore,  the triple beam 3D modulator presents far more clarity than the  double beam system, especially for 4K digital projectors.


Through the  above horizontal comprehensive comparison, although the cost of triple beam system is slightly higher than that of the double beam system, it is small  in size, light in weight, low in installation environment, accurate in  image alignment, clearer in image, high in light efficiency, and  slightly ghosting. The most mature cinema 3D solution.


At present, more  and more cinemas are beginning to transform RGB lasers, but since most  of the triple beam system are not compatible with RGB laser projectors,  the theater can only choose double beam 3D system and RGB laser  projectors, but the RGB laser-modified theaters Most  of them are giant screen theaters, and their projection ratio is  generally between 1 and 1.3:1. In the range of double beam system,  there is a problem that images cannot overlap, which causes the theater  to spend huge sums of money to upgrade RGB laser and double beam system. However, the picture obtained was ambiguous, and the ghost of the screen was serious, and huge amounts of money were vainly.