Application of LED light source in micro-projection system

1 Introduction

With the advent of blue LED, LED has three primary colors of light, and has been gradually applied to projection display in recent years. This article briefly summarizes the characteristics of LED light source , analyzes its application status and application advantages in micro projectors, summarizes the main problems currently existing, and finally looks forward to the application prospects of LED light source in micro projection system.

2 Introduction to LED

LED (Lighting emitting diode) is a light emitting diode that uses a solid semiconductor chip as the light-emitting material. The excess energy released by carrier recombination in the semiconductor causes photon emission, and directly emits red, yellow, blue, green, cyan, orange, and purple light.

As the fourth generation of green and energy-saving lighting source in the 21st century, LED has broad development prospects.

2.1 Development history and research status of LED light sources

In 1964, the world's first red light-emitting diode was successfully developed, followed by yellow LED and green LED. In 1994, the successful development of blue LED made the development of white LED possible.

In 1996, Nichia Corporation of Japan successfully developed a two-wavelength white light LED (based on a blue light single-chip substrate with YAG yellow phosphor mixed to produce white light) and successfully commercialized it. In the same year, it obtained a patent for this technology worldwide. The current development direction is the three-wavelength white light LED that produces white light by mixing inorganic ultraviolet light chips with red, blue and green three-color phosphors. After commercialization, it will replace fluorescent lamps, compact energy-saving fluorescent lamps and two-wavelength LEDs and become a new generation of light sources. Table 1 shows the technical indicators of white light LEDs in various countries at the end of 2007.

Table 1 Comparison of technical indicators of power-type white light LEDs at home and abroad (December 2007)

2.2 Characteristics of LED light sources

2.2.1 High luminous efficiency

LED light source is a primary light source. In the process of emitting light, electrical energy is directly converted into light energy. Incandescent lamps and halogen lamps have a light efficiency of 12~24 lm/W, fluorescent lamps have a light efficiency of 30~70 lm/W, and sodium lamps have a light efficiency of 90~140 lm/W. Most of the power consumption is converted into heat loss. After improvement, the light efficiency of LED can reach 50~200 lm/W. Moreover, its light has good monochromaticity, narrow spectrum, and can directly emit colored visible light without filtering. With the in-depth research of various countries, the light efficiency of LED will be further improved, and it will definitely become a new type of lighting source.

2.2.2 Low power consumption

The power of a single LED tube is 0.03~0.06 W. It is driven by DC or pulsed DC. The driving voltage of a single LED tube is 1.5~3.5 V, the current is 15~18 mA, the response time is in microseconds, and it can be operated at high frequency. Under the same lighting effect, the power consumption is 1/8 of that of an incandescent bulb and 1/2 of that of a fluorescent tube.

Taking bridge guardrail lights as an example, a fluorescent lamp with the same effect consumes more than 40 watts, while each LED consumes only 6 to 8 watts (calculated based on 100 LED tubes with red, green and blue evenly distributed), and can change colors.

2.2.3 Long service life

LED light sources use electron light field radiation to emit light, and do not have the disadvantages of filament light, such as easy burning, heat deposition, and light attenuation. Generally speaking, the life of ordinary incandescent lamps is about 1,000 hours, and the life of fluorescent lamps and high-intensity gas discharge lamps is no more than 10,000 hours. The service life of LEDs can be as long as tens of thousands of hours. Moreover, they are small in size and light in weight. The epoxy resin packaging structure can withstand high-intensity mechanical shock and vibration.

The service life of LED lamps can reach 5 to 10 years, which can greatly reduce the maintenance costs of lamps.

2.2.4 High safety and reliability, flexible control mode

LED is a cold light source with low heat generation and no heat radiation. It can accurately control the light type and emitting angle. The light color is soft and glare-free. It does not contain mercury, sodium and other substances that may harm the environment. The microprocessor (CPU) system can control the luminous intensity and adjust the luminous mode. The combination of red, green and blue can achieve 8 kinds of changing effects. If grayscale control is used, 256 × 256 × 256 (16,777,216) color effects can be produced in theory, which truly realizes the perfect combination of light and art.

3 Application Status of LED Light Sources in Projectors

The projector using LED light source, on the one hand, changes the complex optical path structure required by the original light source; on the other hand, since LED is a cold light source, it does not generate a lot of heat during operation, which can reduce the requirements for the projector's cooling system and reduce the configuration of cooling. Therefore, the volume of the projector can be much smaller than before. In terms of weight, LED light source projectors are generally kept below 1 kg, and some are even only about 0.5 kg. Such a weight is easy to carry, and LED projectors also consume much less power than projectors using traditional light sources. The design of the heat dissipation device completely eliminates the annoying noise of traditional projectors.

3.1 Application Status

The first projector product using LED light source was unveiled at the SID annual meeting in May 2003. Its compact body caused a great response; however, it was only a concept product at that time. The real practical significance was at the CES exhibition in January 2005, when five companies simultaneously exhibited pocket projectors using LED light sources. People began to really consider the advantages and disadvantages of LED light source projection products in terms of application. After another year of technical and product development, LED light source became a prominent highlight at the CES exhibition in January 2006. A large number of LED light source papers were published at the SID discussion meeting in June 2006, marking the entry into a new stage of LED development. In 2008, the bottleneck of LED light source brightness was broken. Osram, a German projector lamp manufacturer, announced at the end of 2008 that it had developed the latest LED light source system. The brightness of this light source using 5 LED bulbs has been increased to more than 500 lm. Several LED 1080p projectors exhibited at the CES 2009 Consumer Electronics Show also claimed to have a brightness of more than 700 lm. Among them, Qiling Technology from Taiwan announced that the brightness of its LED projector has exceeded 1,000 lm. These products all show that the brightness of LED light sources is very close to that of ordinary projection lamps.

At present, LED light sources are mainly used in two types of projection products: integrated small projectors and pocket-sized miniature projectors [5]. Integrated small projectors are not made into independent models. They are mainly integrated with micro display devices such as mobile phones, digital cameras, and laptops. They are battery-powered and can project images about the size of A4 paper or standard letter paper, making it easy to share the screen with others. Pocket-sized handheld projectors are complete independent models. They are small and light, and can be easily picked up with one hand. They are easy to carry and can be placed in a briefcase. They can be used for both AC and DC.

3.2 Application Advantages

3.2.1 Small size, long life

LED light sources are common light-emitting diodes, which are much smaller than the bulb modules and cooling systems used in projectors. This is also the key reason why mini projectors can be made into palm-sized mini projectors. The life of the bulbs used in ordinary projectors is only 2,000 to 4,000 hours, and its light effect will decrease with the increase of usage time, as shown in Figure 1. The life of LED light sources is tens of thousands of hours, and there is no need to consider the cost of replacing bulbs for several years, which greatly reduces the user's usage cost.

Figure 1 The relationship between the light efficiency of a common projector bulb and its usage time

3.2.2 Fast response speed

LED does not require the preheating time of ordinary projector bulbs, so it can light up immediately when turned on and there is no need to wait for shutdown to dissipate heat. The cooling airflow cannot be removed immediately after the high-pressure mercury bulb is turned off, otherwise it will easily cause the lamp to explode. LED light sources do not generate much heat, so there is no need to worry about damage, so they can be turned off and stopped immediately.

3.2. Wide color range

The three primary colors of RGB LED are very pure and can achieve a very wide color gamut (as shown in Figure 2). However, ordinary projection bulbs have a limited spectrum range and require color wheel separation. Color purity is often limited by brightness requirements. The color gamut of ordinary projectors is only 60~70% of NTSC, while LED light source projectors can easily reach more than 100%, with better color reproduction.

Figure 2 LED light source wide color range

3.2.4 Color wheel separation required

The color of a single-chip DLP comes from the decomposition of the light from the bulb by the color wheel. The rotation speed of the color wheel directly affects the color refresh speed of the picture. The common rainbow phenomenon is caused by the existence of the color wheel. The three color luminous bodies of RGBLED emit light in turn in time sequence, which is equivalent to the color separation of the color wheel, but the speed can reach the range equivalent to the 48-times speed color wheel. The rainbow phenomenon is greatly reduced, and there is no noise caused by the high-speed rotation of the color wheel. In addition, compared with UHP lamps, LED light sources do not emit light in the ultraviolet and infrared bands, and UV and IR filters are not required in the system, so the projection system can be made smaller and the cost is lower. Figures 3 and 4 are system structure diagrams when ordinary bulbs are used as light sources and three-color LED lamps are used as light sources.

Figure 3 System structure diagram of ordinary light bulb as light source

Figure 4 System structure diagram of three-color LED lamp as light source

4 Problems with LED as a projector light source

4.1 Luminous flux needs to be improved

There is still a big gap between the luminous efficiency of LED and the UHP and UHE lamps used in traditional projectors. If LED light sources want to become a new alternative to traditional projector metal halide bulbs and popular ultra-high pressure mercury bulbs, they must emit more light and have higher energy conversion efficiency. Currently, countries around the world are stepping up research on LED luminous efficiency.

4.2 High production cost

High production cost is the main reason affecting the popularity of LED light sources.

However, in recent years, with the improvement of chip production technology and the lifting of the ban on various patented technologies (the patent protection period of Japan's Nichia Corporation is until 2006), manufacturing costs are dropping sharply, and LED light sources are developing in the direction of high efficiency and low cost, which provides favorable conditions for the widespread application of LED light sources.

4.3 Product consistency and reliability need to be improved

Since the projection needs to be composed of multiple LED monomers, the discreteness of its parameters is an important technical issue. In addition to ensuring consistency as much as possible through pre-selection and classification, it is also necessary to design a reasonable LED arrangement structure and study a suitable driving circuit to prevent accidental energy concentration from burning some LEDs.

4.4 Poor beam quality

At present, the outgoing beam diffusion angle of LED devices or chips is large, and the optical extension is large. Its geometric shape is not conducive to collection and transmission, nor is it conducive to improving the relevant performance and maturity of the projection engine. The projection light source constructed with LED devices or chips is an extended surface light source with multiple discrete light-emitting elements, which increases the difficulty of increasing the engine light efficiency and improving image quality indicators such as brightness and uniformity. How to efficiently use the light energy of LED has become a very valuable research issue.

5 Conclusion

The bottleneck of LED light source brightness has been broken, which means that the time is ripe for LED light sources to be used in projectors. The advantage of low heat generation of LED light sources can make projectors more compact, and its 20,000-hour life basically eliminates the need to replace the bulb after a period of use. The user's cost of use will be greatly reduced, which makes it possible for projectors to enter the home market. With the in-depth research and technological breakthroughs of LED light source technology in various countries around the world, LED light sources will surely replace existing metal halide bulbs and popular ultra-high pressure mercury bulbs in the near future and become a new light source for projectors.