According to digital light processing and imaging technology, projectors are divided into 3 types: DLP, LCD and LCoS. Each projection technology has its own fundamental differences, advantages and disadvantages. In this article, we will talk about DLP technology in detail and compare it with others.
The principle of obtaining a projection
To understand how a digital projector works, you need to understand the very principle of obtaining a projection. There are 2 ways – diaprojection and epiprojection. In the first case, light passes through a transparent image-forming element, such as a film. In the second, light is reflected from the image and projected onto the screen using optics. In digital projectors, the same principles are preserved, only matrices are used instead of a film or paper picture.
Technologies 3 LCD and LCoS
LCD technology uses three LCD matrices. The light from the lamp is divided into 3 streams, each of which first passes through its own light filter (red, blue and green), and then through its own matrix.
The matrix adjusts the light transmission at each point (pixel) and gives the desired picture. After passing through the matrices, the light is summed by the prism. As a result, the red, blue and green images are superimposed on each other, giving a full-fledged color image, which is projected through the lens onto the screen.
Along with such advantages as high brightness and color saturation, LCD projector technology has 2 significant disadvantages:
The liquid crystal matrix cannot completely darken the points where light should not pass through 100%. As a result, the screen does not achieve the depth of black, which leads to a loss of contrast.
The second minus is the burnout of the matrix. The projector requires a comfortable environment and regular maintenance. For example, if you constantly smoke in the cinema, the life of the matrices will be drastically reduced.
Trying to eliminate these shortcomings, the developers have created a new LCoS technology based on epiprojection. The technology also used three LCD matrices, only located on a reflective substrate. This made it possible to better remove heat through the substrate and, thereby, to increase the resource of the matrices. In addition, the light passes through the matrix twice – to the substrate and back, which gives a stronger darkening of the black dots and increases the image contrast.
DLP projectors use the principle of epiprojection – the reflection of light from controlled micromirrors of the DMD matrix. The matrix itself is a semiconductor chip, on which there are many movable microscopic mirrors fixed on the substrate.
The mirrors are controlled by addressable electrodes connected to the cells and can be deflected instantly. In the upright position, the mirror reflects light from the lamp through the lens onto the screen, forming a bright point – a pixel. In the deflected position, the light is reflected onto the light collector and is displayed as a dark dot on the screen.
Thus, each mirror is responsible for its own pixel. Deviating to the desired position, together the mirrors form a projection of the image on the screen. The size of the mirrors depends on their number – the more there are, the smaller they are, and the distance between the mirrors is fractions of a micron. In modern DLP projectors with 4K resolution, the number of mirrors on the matrix can exceed 8 million.
Getting a color image
A color wheel is used to obtain color in a design. This is a rapidly rotating disk, divided into 3 sectors of the light filter – red, blue and green. Before reaching the matrix, the light from the source passes through the windows of the disk and, thus, all 3 colors alternate.
Due to the synchronization of the disk rotation and the operation of the matrix, 3 pictures of different colors are alternately projected onto the screen. The overall picture is achieved by inertial overlay of colors, resulting in the desired shade. For example, the alternation of red and green.
the human eye perceives as brown, etc. In fact, a single-matrix DLP projector does not give a multi-colored image to the screen, and at any moment of the projection, an image of one of three colors is displayed on the screen. A full-fledged color image appears already in the viewer’s head due to the peculiarities of human vision.
Advantages and disadvantages of DLP projection technology
The main advantage of DLP technology was the highest contrast, since the projector’s light does not reach the areas of the screen that should be displayed in black at all. Also, the resource of the DMD matrix is approximately 10 times greater than the resource of the LCD matrix in LCD technology. Mirrors on a DMD matrix are made of heat-resistant aluminum alloy and do not burn out, unlike liquid crystals.
It is impossible to speak unambiguously about the shortcomings of DLP technology, since modern projector models have many modifications. The features of the modern design are designed to reduce undesirable effects and compensate for shortcomings to one degree or another. Therefore, to begin with, we will talk about the disadvantages of the basic technology and design features that solve the problem.
The fact is that the light from the lamp always passes through one light filter of the color wheel, which at this point in time is on its way and loses up to 50% of its brightness.
We managed to solve this problem with the help of the fourth transparent sector in the color wheel. In the algorithm of this design, with a full turn of the disk, the first 3 pictures form a color image, and the fourth one is white, as if highlighting the image, except for those areas that should be black.
This solution can be considered a half-measure, since increasing the brightness in this way leads to a decrease in color saturation. The imbalance in the brightness of white and color occurs because white light is formed by four flashes – each color and additionally white. The color is formed only by three, and sometimes two flashes.
To even out this imbalance, another dark green, and sometimes also a yellow sector is added to the color wheel, thus correcting the shift of the spectrum to color standards.