LED is the abbreviation of light emitting diode. It is an electric light source made of semiconductor technology. The core 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, called p2n junction. When conducting in the forward direction, most carriers and a few carriers in the semiconductor compound, and the released energy is emitted in the form of photons or part of photons. A large number of photons form a photon flow, that is, luminescence. When p2n junction is applied with reverse voltage, it is difficult to inject a few carriers, so it does not emit light. Due to the different band gap energy of semiconductor materials, LEDs of different semiconductor materials can emit different colors of light.

LED light source has the characteristics of small volume, long service life, low power consumption, fast reflection speed, rich color and flat packaging. It is an environmental protection and energy-saving cold light source. Because of the above characteristics, LED is widely used in the field of display and lighting.

In 1965, the first commercial LED (red light) came out with an efficiency of only 0.1 LM / W. In 1968, the LED made of GaAsP material had an efficiency of 1 LM / W and could emit red, orange and yellow light. In the early 1990s, two new materials, GaAlInP with red and yellow light and GaInN with green and blue light, were successfully developed, which greatly improved the light efficiency of LED. In 1998, the successful development of white LED marked the arrival of lighting technology revolution.

1 the latest development of white LED

In December 2006, Japan’s Nichia chemical industry released a white LED with a forward current of 20 mA and a luminous efficiency of 150 LM / W. its luminous efficiency reached about 1.7 times that of the fluorescent lamp with improved color rendering, 11.5 times that of the incandescent lamp, and even exceeded the high-pressure sodium lamp with the highest luminous efficiency. Showa Electric Group (SDK) has developed a new process for manufacturing gallium nitride (GAN) based and other nitride based high-quality composite semiconductors, mainly for blue and white LEDs. In February 2007, Philip s Lumileds announced LED epitaxial technology (, it basically solves the problem that the light efficiency of white high-power LED decreases with the increase of current, which can make the light efficiency increase with the increase of driving current. Cyberlux claims that the use of plastic materials can make the price of white LED cheaper. This technology can not only significantly reduce the manufacturing cost, but also better than the traditional white LED. In March 2007, Seoul semiconductor (SSC) Launched an octagonal 2W single chip acriche with 35000 hours and 96 LM luminous flux, which can be directly plugged into 110 V or 220 V AC power supply without AC / DC converter. SSC plans to improve the luminous efficiency of ariche to 120 LM / W by 2008.

At present, the thickness of LED chip is only 0.4 mm, and the light extraction technology of LED has also made great progress. Therefore, the backlight with LED is gradually used in the third generation mobile phones, PDAs, portable DVDs, digital cameras and other small displays. In addition, all kinds of lights on the car, such as front and rear lights, side lights, headlights, interior lights and instrument panel display, will be fully developed and applied. According to the statistics of strategies un2limited, kohido, a large Japanese lamp factory, estimates that all new cars will use LED as lamps by 2008.

2 white LED card lamp

A domestic optoelectronic device Co., Ltd. is a new white LED product for Japanese tape workers, named “card lamp”, and the physical product is shown in Figure 1. This is a new application of white LED, which has been patented.

Structure and working principle analysis of white LED card lamp

2.1 structure and principle of LED card lamp

At present, most led card lamps are small in size, and the area of the luminous part ranges 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 part and switch:

(1) Frame

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

(2) Backlight module

Backlight module is the most important part of card lamp, which determines the luminous area and luminous quality of card lamp. The backlight module is also composed of LED lamp, reflector, light guide plate, diffusion plate, shading plate, antireflection prism and backlight frame. The LED lamp is the light source. The light guide plate extends the light emitted by several LED lamps to the whole light-emitting surface. The diffusion plate makes the light emitted more uniform through the diffuse reflection principle. The antireflection prism can change the angle of the emitted light and gather more light in the direction of the front viewer. The shading plate is used to control the light-emitting area and the shape of the light-emitting surface.

(3) Scintillation circuit

A flashing circuit is usually installed inside the card lamp, and the luminous part of the card lamp is adjusted to emit several different modes of light through the control switch.

Ordinary flicker circuit can provide three modes: constant light, long interval flicker and fast flicker.

(4) Power supply part

The working voltage of LED is about 3 V. due to the power saving characteristics of LED, the power supply for card lamp can be button battery. The card lamp powered by button battery has compact structure and can be made into products with small thickness. Detachable rechargeable lithium battery can also be used. The advantage of using rechargeable lithium battery is to save resources. If the card lamp is often used, the structure of rechargeable lithium battery is more reasonable.

(5) Switch

It is used to connect circuit and power supply, and adjust card lamp on, off and luminous mode.

2.2 brightness test

The brightness of the test card lamp adopts a multipoint luminance meter (BM-7). BM-7 has 4 measurement angles such as 2 ° / 1 ° / 0.2 ° / 0.1 °, which can be switched, and the minimum measurement area can reach φ 0. 1 mm。 It can measure brightness L, chroma x, y, trichromatic values x, y, Z, color temperature, response time, contrast, etc. It is widely used in the measurement of LCD, Blu, LCM and other fields. It is the standard of LCD industry.

3. The 8 cm (1.5 inch) card lamp has two white LEDs. Use BM-7 to test the brightness of the card lamp with a luminous area of 3.8 cm (1.5 inch). The nine point test method is used for the brightness test. The location of the test point is shown in Figure 2.

Structure and working principle analysis of white LED card lamp

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

Structure and working principle analysis of white LED card lamp

2.3 data analysis

The calculation formula of uniformity R is:

According to the above formula, the uniformity of the effective light-emitting part of the five groups of backlight is 92%, 86%, 93%, 92% and 88% respectively, of which the maximum uniformity has reached 93% and the minimum value is 86%. All uniformity is above 85%, which shows that the uniformity of light source of card lamp is very high. The last item in the table is the average value of the brightness of 9 points in each group. The fourth group is low, which may be the reason for the installation process.

Except for the fourth group, they all exceeded 3400 CD / m2. Generally, the brightness of more than 200 CD / m2 can well meet the application.

In order to visually compare the brightness of each test point, take five average brightness values for each point and make a three-dimensional surface diagram, as shown in Figure 3. The horizontal axis and vertical axis of the horizontal plane represent the position of the measurement point, marked with the digital label of the point, and the vertical value axis represents the average brightness value of 9 points, with the unit of CD / m2.

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

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

Structure and working principle analysis of white LED card lamp

2.4 application of card lamp

Led card lamp has many applications in daily life. White LED card lamp is light, thin and environment-friendly. It is usually integrated into other commodity structures. Translucent cards (such as business cards, labels, festival cards, etc.) can be attached to the luminous part of the card lamp. When the card light is on, especially in the dark, the card content will be highlighted. In addition to the white card lamp, the red, green and yellow LEDs have been made into color card lamps. This color card lamp can play a decorative role in the gift box. In addition, because the card lamp is equipped with a flashing circuit, the flashing light can attract people’s attention, so the card lamp is also used in safety.

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 are made by white LED. The European Union has said 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 automobile, display backlight and lighting appliances, the LED era will come soon.

Source; Electronic Engineering Network

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