New Application of White Light LED--Structure and Principle of Card Light

LED is the abbreviation of light emitting diode, which is an electric light source made of semiconductor technology. The core part of LED is a chip composed of P-type semiconductor and N-type semiconductor. There is a transition layer between P-type semiconductor and N-type semiconductor, which is called P2N junction. When forward conduction occurs, the majority carriers and minority carriers in the semiconductor recombine, and the released energy is emitted in the form of photons or part of it. A large number of photons form a photon stream, that is, light emission. When reverse voltage is applied to the P2N junction, it is difficult for minority carriers to be injected, so no light is emitted. Due to the different band gap energies of semiconductor materials, LEDs made of different semiconductor materials can emit light of different colors.

LED light source It has the characteristics of small size, long life, low power consumption, fast response speed, rich colors, and flat packaging. It is an environmentally friendly and energy-saving cold light source . Due to the above characteristics of LED, it is widely used in display and lighting fields.

In 1965, the first commercial LED (emitting red light) was introduced with an efficiency of only 0.1 lm/W. In 1968, the efficiency of LEDs made of GaAsP material reached 1 lm/W, and they could emit red, orange, and yellow light. In the early 1990s, the development of two new materials, GaAlInP emitting red and yellow light and GaInN emitting green and blue light , greatly improved the light efficiency of LEDs . In 1998, the development of white light LEDs marked the arrival of a revolution in lighting technology.

1 Latest progress of white light LED

In December 2006, Nichia Chemical Industry of Japan released a white LED with a luminous efficiency of 150 lm/W at a forward current of 20 mA. Its luminous efficiency is about 1.7 times that of fluorescent lamps with improved color rendering , 11.5 times that of incandescent lamps, and even exceeds the high-pressure sodium lamps that are generally considered to have the highest luminous efficiency. Showa Denko Group (SDK) has developed a new process for manufacturing high-quality compound semiconductors based on gallium nitride (GaN) and other nitrides, mainly for blue and white LEDs. In February 2007, Philips Lumileds announced LED epitaxial technology (, which basically solves the problem that the light efficiency of white light high-power LED decreases with the increase of current, and can increase the light efficiency with the increase of driving current. Cyberlux claims that the use of plastic materials can make the price of white light LEDs lower. This technology can not only significantly reduce the manufacturing cost, but also has better lighting brightness than traditional white light LEDs. In March 2007, Seoul Semiconductor (SSC) launched an octagonal 2 W single-chip Acr ic he with a life of 35,000 hours and a luminous flux of 96 lm , which can be directly plugged into a 110 V or 220 V AC power supply without the need for an AC/DC converter. SSC plans to increase the light efficiency of the Acr ic he to 120 lm/W by 2008.

At present, the thickness of LED chips is only 0.4 mm, and the light extraction technology of LED has also made great progress. Therefore, backlight sources with LEDs are gradually used in small displays such as third-generation mobile phones, PDAs, portable DVDs, digital cameras, etc. In addition, various lights on cars, such as front and rear lights, side lights, headlights, interior lights and instrument panel displays, will be fully developed and applied. According to statistics from Strategies Un2limited, Kohido, a large Japanese car lighting manufacturer, estimates that by 2008, all new cars will use LEDs as headlights.

2 White LED Card Lights

A domestic Optoelectronics Device Co., Ltd. has brought a new white light LED product to Japan, named "Card Light", and the actual product is shown in Figure 1. This is a new application of white light LED and a patent has been applied for.

2.1 Structure and principle of LED card light

Most LED card lights currently manufactured are small in size, with the light-emitting area ranging from 3.81 cm (1.5 inches) to 6.0 cm (2.4 inches). The structure is divided into five parts: frame, backlight module, flashing circuit, power supply, and switch.

(1) Framework

The frame is used to install and integrate other parts to form a whole. It can be processed into different geometric shapes according to different needs. The card light in Figure 1 is a rectangle.

(2) Backlight module

The backlight module is the most important part of the card light, which determines the light-emitting area and light quality of the card light. The backlight module is composed of LED lights , reflectors, light guides , diffusers, shading plates, anti-reflection prisms and backlight frames. The LED lights are the light source, the light guides spread the light emitted by several LED lights to the entire light-emitting surface, the diffusers make the emitted light more uniform through the principle of diffuse reflection, the anti-reflection prism can change the angle of the emitted light and gather more light in the direction of the front viewer, and the shading plates are used to control the light-emitting area and the shape of the light-emitting surface.

(3) Flashing circuit

A flashing circuit is usually installed inside the card light, and the light-emitting part of the card light is adjusted to emit several different modes of light by controlling the switch.

Ordinary flashing circuits can provide three modes: constant light, long interval flashing, and fast flashing.

(4) Power supply

The working voltage of LED is about 3 V. Since LED has the characteristic of saving power, the power supply for the card light can be a button battery. The card light powered by a button battery has a compact structure and can be made into a product with a small thickness. A detachable rechargeable lithium battery can also be used. The advantage of using a rechargeable lithium battery is that it saves resources. If the card light is used frequently, it is more reasonable to use a rechargeable lithium battery structure.

(5) Switch

Used to connect the circuit and power supply, and adjust the card light on, off and lighting mode.

2.2 Brightness test

The brightness of the test card lamp uses a multi-point luminance meter (BM-7). BM-7 has 4 measurement angles that can be switched: 2°/1°/0.2°/0.1°, and the minimum measurement area can reach φ0.1 mm. It can measure brightness L, chromaticity X, Y, tristimulus values ​​X, Y, Z, color temperature , response time, contrast , etc. It is widely used in the measurement of LCD , BLU, LCM and other fields, and is the standard of the LCD industry.

The 3.8 cm (1.5 inch) card light has two white light LEDs. The brightness of the card light with a light emitting area of ​​3.8 cm (1.5 inch) was tested using the BM-7. The brightness test adopted the nine-point test method. The test point positions are shown in Figure 2.

Select 5 LED backlights, use the test instrument TOPC ONBM -7, the test distance is 50 cm, and the test condition is current I = 20mA. The brightness values ​​of the 9 points are measured as shown in Table 1 (unit is cd/㎡):

2.3 Data Analysis

The calculation formula of uniformity R is:

Calculated by the above formula, the uniformity of the five groups of backlight effective luminous parts is 92%, 86%, 93%, 92%, and 88% respectively, of which the maximum uniformity has reached 93% and the minimum is 86%. All uniformities are above 85%, which shows that the card light and other light sources have a high uniformity. The last item in the table is the average brightness of each group of 9 points. The fourth group has a lower value, which may be due to the installation process.

Except for the fourth group, all of them are over 3400 cd/m2. Usually, brightness over 200 cd/m2 can meet the application requirements.

In order to visually compare the brightness of each test point, five average brightness values ​​are taken for each point to make a three-dimensional surface diagram as shown in Figure 3, where the horizontal axis and vertical axis represent the positions of the measurement points, marked with digital numbers of the points, and the vertical numerical axis represents the average brightness of the 9 points in cd/m2.

The above data show that the brightness and uniformity of the card light are very good.

However, as can be seen from Figure 3, the brightness of the edge of the luminous surface is high and the brightness of the center is low, which needs to be improved in the future.

2.4 Application of card lights

LED card lights have many applications in daily life. White LED card lights are thin and light, environmentally friendly, and are usually integrated into other product structures. Translucent cards (such as business cards, labels, holiday cards, etc.) can be attached to the light-emitting part of the card light. When the card light is lit, especially in the dark, the content of the card will be highlighted. In addition to white card lights, red, green, and yellow LEDs have been used to make colored card lights. Gift packaging boxes equipped with such colored card lights can play a decorative role. In addition, since the card light is equipped with a flashing circuit, the flashing light can attract people's attention, so the card light is also used in security.

3 Conclusion

With the successful development of 100 lm/W white LED, the popularization of white LED in display and lighting has taken another step forward, and more and more new devices made of white LED are being produced. The European Union has stated that it will completely replace incandescent lamps with LEDs within two years, and the Chinese government has also launched a green lighting project. It is certain that with the increasing application of white LED in automobiles, display backlights, and lighting appliances, the LED era will soon arrive.