Color Wheel and Color Processing Technology of DLP Back Projector
As is well known, due to the use of DMD micro lens reflection technology in DLP, both single and two-piece DMD methods use color wheels to separate and process colors in color processing.
Generally speaking, a color wheel is a combination of red, green, blue, white and other color separation filters that can separate the transmitted white light into different colors. It is then rotated by a high-speed motor and sequentially divided into different monochromatic lights on a designated light path. Finally, it is synthesized and projected into a full-color image through other optical components.
From a physical structure perspective, the surface of the color wheel is a very thin metal layer, which is coated using vacuum film technology. The thickness of the coating corresponds to the spectral wavelengths of red, green, and blue. When white light passes through the metal coating layer, the color of the corresponding spectral wavelength will pass through the color wheel, while other colors will be blocked and absorbed, thus completing the separation and filtering of white light.
In a single-chip DMD projection system, the input signal is converted into RGB data, which is sequentially written into the DMD's SRAM. The white light source is focused on the color wheel through a focusing lens, and the light from the color wheel is then imaged on the surface of the DMD. When the color wheel rotates, red, green, and blue light are sequentially projected onto the DMD. The color wheel and video image are performed sequentially, so when the red light shines on the DMD, the lens tilts to the position and intensity that should be displayed according to the red information; Open” Green and blue light, as well as video signals, work in the same way. The human visual system concentrates red, green, and blue information and sees a full-color image. The image formed on the surface of the DMD can be projected onto a large screen through a projection lens.
In two DMD projection systems, in order to improve brightness and compensate for the red deficiency of metal halides, the color wheel uses two auxiliary colors -; Magenta and yellow. The magenta segment allows red and blue light to pass through, while the yellow segment allows red and green light to pass through. And the three DMDs use color separation prisms, without the need for color separation wheels.
The following mainly discusses several color wheel technologies of single-chip DMD currently used in DLP rear projection units.
1、 Several commonly used color wheel processing techniques and their characteristics
Due to the fact that the color wheel of a single DMD projector can only process one color at a time, it may result in some loss of brightness. Additionally, due to the inherent differences in the spectral wavelengths of different colors of light, there may be variations in color reproduction, resulting in less vivid red colors in the image. Therefore, how to make the projector have sufficient display brightness while ensuring the true reproduction of colors is a key issue for every projector manufacturer in product design, and one of the most important factors is the design solution of color wheel technology.
The following are several commonly used DLP color wheel technologies:
1. Three stage color wheel RGB
Composed of three colors: red R, green G, and blue B, products from different manufacturers have different designs for the opening angles of red, green, and blue. Generally speaking, red has a larger opening angle, which can compensate for the shortcomings of red in the image. The prerequisite for using this color wheel technology is that the projector's optical part has sufficient brightness, otherwise it may cause brightness problems in the image. At the same time, the use of three-stage color wheel technology has relatively good color reproduction.
2. Four stage color wheel RGBW
It consists of four colors: red R, green G, blue B, and white W. The purpose of adding white color is mainly to further improve the brightness of the projector, which can generally be increased by about 20% compared to a three-stage color wheel. However, at the same time, this color wheel technology also brings about the problem of insufficient color reproduction of the projector, causing color distortion in the image and reducing the image quality. In addition, in the design, the pulse signal can be synchronously locked in the W segment, and the pulse width corresponds to the width of the W segment, which can reduce the flickering phenomenon of the image to a certain extent. This technology is mainly applied in conference rooms and teaching projectors.
3. Six stage color wheel RGBRGB
Due to the increasingly widespread application of DLP technology in home theater projection and rear projection televisions with huge market potential, people have put forward higher requirements for the color representation and continuous dynamic video playback effect of DLP.
The six segment color wheel is composed of six colors in RGBRGB. As the speed of the color wheel increases (180Hz) and more images are processed per unit time, this design effectively reduces the rainbow effect of motion images and edges, resulting in better video dynamic effects and richer and more vibrant colors in the image. However, due to the large number of segmented partitions in the six color system, there is also a significant amount of time loss when the light collecting column passes between each color segment. As a result, the brightness of the projector is often relatively low. Therefore, a few projector manufacturers have started to design and use 7-segment color wheel RGBRBW technology to improve the brightness of the projector and reduce flickering on the screen. This technology is mainly used for household consumption and applications with high video requirements.