What are the characteristics of various projection display technologies?

Before the advent of projectors, we mainly used slide projectors. In fact, today's projectors are also special forms of slide projectors. Some scholars have shown that the development of projectors, animation, and movies are inseparable.

The earliest use of light and shadow technology began in the 17th century when a Jesuit priest named Athanasius Kircher invented a "magic lantern". Later, it became a toy, and its modern name is a projector. Initially, this technology was mainly used for entertainment, using the principles of light and shadow to tell stories on a screen, which is somewhat similar to the shadow play of the Tang Dynasty. However, later, with the development of light and shadow technology, projectors were separated from movies and animations.

The ancestor of projectors——CRT projection technology

The history of CRT projectors can be traced back to the 1950s, when they were mainly used on business aircraft to play videotapes. In the 1980s, the rapid development of personal computers led to an increasing market demand for text and data presentation, which prompted the rapid development of CRT projection technology. The application areas of projection technology began to penetrate into conference rooms, classrooms, and theaters. In the mid-to-late 1980s, with the widespread use of computer workstations and graphics processing software, graphic projectors that could project high-resolution graphics and animations were produced accordingly. In 1989, the first LCD projector was launched, ending the situation where only CRT technology was available in the projector market. In 1994, the home projection market sprouted, and CRT projectors were more mature than LCD projectors at the time, so they began to enter high-end home theaters. However, in 1996, when 3LCD technology was launched and the first DLP projector came out, CRT projection technology began to decline and quickly faded out of people's sight.

LCD projection technology

When it comes to LCD projection technology, we must mention Epson. The LCD chip, as the core of the LCD projector, is the heart of the entire projection system. One of Epson's most important contributions to the development of LCD projectors is the successful development of the world's first LCD projection panel chip, and in 1989, it manufactured the world's first LCD projector VJP-2000, thus creating a new era of LCD projectors.

Through the continuous advancement of technology and products, LCD projection has maintained its leading position in the global digital projector technology to this day. Compared with the up-and-coming DLP and LCOS technologies, LCD's market share is as high as over 55%.

Classification of LCD projection technology

In the process of LCD projector technology evolution, its technology types have also been differentiated. In addition to the three-chip high-temperature polysilicon liquid crystal panel (HTPS) technology that currently occupies the absolute mainstream, there have also been single-chip technology, liquid crystal light valve projectors, etc.

The working principle of a single-chip LCD projector is shown in the figure:

The single-chip LCD projector uses an ordinary small-screen LCD panel as the display core chip, and is equipped with other necessary equipment to form a complete working optical path. It provides a large-size real picture through the lens system to show users a shocking image effect. In the optical path, this product is difficult to reduce in size due to the large chip area, and the total light efficiency utilization rate of the optical path is also very low. The process structure of the internal optical and mechanical components is accurate to 0.1mm. When choosing the main optical components, such as projection bulbs, reflective bowls, reflectors, projection lenses, focusing lenses, Fresnel lenses, etc., it has a mature industrial chain and can provide a good product experience.

Liquid crystal light valve projector is a historical technology. It is essentially a product that uses the optical switching function of liquid crystal to improve the imaging effect of CRT projector.

Liquid crystal light valve projectors use CRT tubes and liquid crystal light valves as imaging devices. They are products designed to solve the contradiction between image resolution and brightness of CRT projectors. A general light valve is mainly composed of three parts: a photoelectric converter, a mirror, and a light modulator, which is a controllable switch. The light signal output by the CRT is irradiated onto the photoelectric converter, which converts the light signal into a continuously changing electrical signal. The external light source generates a beam of strong light, which is projected onto the liquid crystal light valve and reflected by the internal mirror. Through the light modulator and the processing of the polarization filter, the light valve projects the light and the CRT signal, and is projected onto the screen to form a picture.

3LCD projection technology

3LCD projection technology is the mainstream technology of LCD projectors. Its market share accounts for the vast majority of the LCD projector market and has become the de facto standard in the industry.

The optical path principle of the 3-chip LCD display projector is that the light emitted by the high-brightness, high-color-temperature metal halide lamp (UHE) is reflected by the parabolic lamp bowl to form parallel light, which is then shot into the first lens array (unit focusing lens group), and the utilization rate of polarized light is improved by the second lens array (polarized light conversion lens group). Then, it passes through the split light path to form three beams of three primary colors, which are respectively shot into the three liquid crystal panels of R, G, and B. Then, the three colors of light are converged by the integrated prism, and the projection lens projects the three-color composite image onto the screen.

In this projection process, various optical devices, LCD panel chips, and light bulbs play their respective roles. Among them, metal halide lamps, UHP (Ultra-High Performance) lamps, and UHE lamps are commonly used as light sources. Metal halide lamps are low-cost and cheap, but their defects are high heat generation and short half-life. Half-life refers to the time it takes for the brightness of the lamp to drop to half of the designed brightness. In order to overcome the defects of high heat generation and short half-life of metal halide lamps, cold light sources came into being, and UHP and UHE metal halide lamps appeared. UHP and UHE lamps generate much less heat than metal halide lamps. In addition, new light sources including laser light sources and LED inorganic light-emitting diodes have also become the focus of various companies in developing new technologies.

In addition to the light source, the role of a series of optical devices cannot be underestimated. The projection process requires a planar, uniform, parallel, three-primary-color linearly polarized light source. The light emitted by an ordinary light bulb is white, spherical, circularly polarized light. Through the adjustment of a series of lenses, prisms, and filters, the light emitted by the bulb can become a light source that meets the projection needs of the projector.

In the imaging process of the LCD projector, the LCD panel chip is at the core. In actual work, the LCD panel chip plays the role of an optical switch. Each pixel under the control of the independent active matrix on the chip can change independently, thereby controlling the amount of light projected through the half of the liquid crystal. Through the three LCD panels to control the three elements separately, and after the final picture synthesis, it can become a colorful, light and dark color projection picture. After being magnified by the lens and projected on the screen, consumers can enjoy the excellent large-size image like a movie.

Chips for LCD projection technology

The LCD chip is the core component of the LCD projector. It is not only an important cost component of the LCD projector, but also an important component that determines the quality, performance and life of the entire product.

LCD is the abbreviation of Liquid Crystal Display. Liquid crystal has both the fluidity of liquid and the optical anisotropy of crystal, and is often called "liquid crystal". The working temperature of liquid crystal material is -55-77℃. The interaction between liquid crystal molecules is small. Under the action of electric field, the molecular arrangement changes, resulting in changes in the transmittance and reflectivity of liquid crystal to light, which is called the electro-optical effect of liquid crystal. In liquid crystal display technology, this special material property is formally used to make liquid crystal materials into switches that can control the entry and exit of light, thereby achieving imaging effects.

The liquid crystal display panel is composed of liquid crystal material filled between two glass substrates, and linear polarizers, scanning circuits, thin film transistors (TFTs) and the like attached to the surface of the glass substrates. When a scanning voltage is applied to the row electrode of a certain pixel, the thin film transistor (TFT) is turned on, and the liquid crystal pixel is transparent. The liquid crystal units between the glass substrates constitute optical switches, which control the amount of light passing through the liquid crystal and display the changes in color brightness.

In actual product development, the LCD panel chips used in projectors must withstand the huge energy (mainly heat) emitted by the light bulb behind them. This determines that the LCD used in LCD projectors is inevitably different from the LCD used in ordinary LCD monitors and LCD TVs. At present, LCD projectors mainly use a technology called "high-temperature polycrystalline silicon LCD panel (HTPS)".