LED light source and optical characteristics detection

1 Overview

As a new type of optoelectronic light-emitting device, LED is getting more and more popular because of its low carbon, environmentally friendly, small size, light weight, low cost and long life. The development momentum is extremely fast, which is unexpected for photoelectric experts. LED is widely used not only in various illumination sources, indicator lamps, but also in large quantities for LCD backlights. It has replaced traditional fluorescent tubes and become the new favorite of LCD TVs. In addition, LEDs can be used as a light source for many measuring instruments.

The development of LED TV in the full sense is also very fast. Although it is mostly used outdoors, due to his high brightness and vivid color and dynamic effects, it is only a matter of time before entering thousands of households. Therefore, let us fully understand the new small partner of LED, it is very urgent.

Since the first red LED was introduced in 1976, after 30 years of development, LED has formed a variety of spectrum products, and the power of a single LED has grown from the initial zero to several watts to several tens of watts. In 2001, white LED was successfully developed. It is expected that LEDs will eventually enter the field of lighting and even enter home lighting. The latest research results of white LED are even more exciting.

The luminous efficiency of low power LEDs has reached 100 lm/W. In particular, RGB-LED research results show that LEDs, like conventional three-primary fluorescent lamps, can achieve a variety of different color temperatures and uniform illumination environments.

The progress of LED light sources and people's expectations for its application in the field of lighting have also placed new demands on the corresponding optical inspection technology. Since the optical characteristics of LEDs are quite different from those of conventional light sources, research and development of measurement methods suitable for this new type of light source are needed.

2.LED light source

LED (Light-Emitting-Diode) is a semiconductor that converts electrical energy into visible light. It changes the principle of incandescent tungsten filament illumination and energy-saving lamp trichromatic toner illumination, and uses electric field illumination. According to analysis, the characteristics of LED are very obvious, long life, high luminous efficiency, no radiation and low power consumption. The spectrum of the LED is almost entirely concentrated in the visible light band, and its luminous efficiency can reach 80 to 90%. Comparing LED with ordinary incandescent lamp, spiral energy-saving lamp and T5 trichromatic fluorescent lamp, the results show that the luminous efficiency of ordinary incandescent lamp is 12lm/W, the life is less than 2000 hours, the luminous efficiency of spiral energy-saving lamp is 60lm/W, and the service life is less than 8000 hours, T5 fluorescent lamp is 96lm / W, life expectancy is about 10000 hours, and white LED with diameter of 5 mm is 20 ~ 28lm / W, life can be more than 100000 hours. Some people also predict that the future LED life limit will be infinite.

High power means that the luminous rate is large, generally 0.5W, 1W 3W 5W or higher. Light intensity and lumens are larger than small power, but the same heat dissipation is also great. Now high power is a single application, plus a large heat sink. The low power is generally around 0.06W. Plugins and piranhas, etc. Now LED flashlight is generally used with low power, the light does not scatter, depending on the angle of illumination of the LED, there are large angles and small angles, small angles are not scattered, large angles are scattered.

The brightness of the LED is necessarily related to the angle of illumination of the LED. The smaller the angle of the LED, the higher its brightness. If it is 5mm LED 180 degree angle of white light, the brightness is only a few hundred MCD, if it is 15 degree angle brightness, it will go to more than 10,000 20,000 MCD brightness, the brightness difference is several tens of times, if it is used for lighting Use, in the outdoor is best to use high-power LED, the brightness is even higher, a single power has 1W, 3W, 5W, and some use a combination of multiple high power into a high-power LED, power to hundreds There are.

LED lighting principle

LED (Light Emitting Diode), a solid-state semiconductor device that converts electricity directly into light. The heart of the LED is a semiconductor wafer. One end of the wafer is attached to a holder, one end is the negative pole, and the other end is connected to the positive pole of the power supply, so that the entire wafer is encapsulated by epoxy resin. The semiconductor wafer consists of two parts, one part is a P-type semiconductor, in which the hole dominates, and the other end is an N-type semiconductor, which is mainly electrons here. But when the two semiconductors are connected, they form a "PN junction" between them. When a current is applied to the wafer through the wire, the electrons are pushed toward the P region. In the P region, electrons recombine with the holes, and then the energy is emitted in the form of photons. This is the principle of LED illumination.

The wavelength of light determines the color of the light, which is determined by the formation of the PN junction material.

The LED is made of a III-IV compound such as GaAs (gallium arsenide), GaP (gallium phosphide), GaAsP (phosphorus gallium arsenide) and the like, and its core is a PN junction. Therefore, it has the IN characteristic of a general PN junction, that is, forward conduction, reverse cutoff, and breakdown characteristics. In addition, it has luminescent properties under certain conditions. At the forward voltage, electrons are injected into the P region from the N region, and holes are injected into the N region from the P region. A part of the minority carriers (small children) entering the other area is combined with the majority carriers (multiple sub-) to emit light.

Assuming that luminescence occurs in the P region, the injected electrons directly composite with the valence band holes to emit light, or are first captured by the luminescent center and then condensed with the holes. In addition to this luminescent composite, some electrons are trapped by the non-luminous center (this center is near the middle of the conduction band and the intermediate band), and then recombined with the holes, and the energy released each time is not large, and visible light cannot be formed. The greater the ratio of the composite amount of luminescence to the non-luminous composite amount, the higher the photon efficiency. Since the recombination emits light in the minority carrier diffusion region, light is generated only within a few μm of the PN junction surface.

Theoretically and practically, the peak wavelength λ of light is related to the forbidden band width Eg of the semiconductor material in the light-emitting region, that is, the unit of Eg in the formula λ≈1240/Eg(mm) is electron volt (eV). If visible light (wavelength between 380 nm and 780 nm red) is produced, the Eg of the semiconductor material should be between 3.26 and 1.63 eV. Light longer than the wavelength of red light is infrared light. There are now infrared, red, yellow, green and blue light-emitting diodes, but the blue light diodes are expensive and expensive, and are not commonly used.

3. Classification of LED

(1) According to the luminous color of the luminous tube

According to the color of the luminous tube, it can be divided into red, orange, green (also subdivided into yellow-green, standard green and pure green), blue light and so on. In addition, some LEDs contain chips of two or three colors.

The light-emitting diodes of the above various colors can be classified into four types of colored transparent, colorless transparent, colored scattering and colorless scattering according to whether the light-emitting diode is doped with or without a scattering agent, colored or colorless. Scatter-type LEDs are used for indicator lights.

(2) According to the characteristics of the light-emitting surface of the light-emitting tube

According to the characteristics of the light-emitting surface of the light-emitting tube, the round lamp, the square lamp, the rectangular, the surface light-emitting tube, the lateral tube, the surface-mounted micro tube, and the like. The circular lamps are divided into φ2mm, φ4.4mm, φ5mm, φ8mm, φ10mm, and φ20mm according to the diameter. In foreign countries, a φ3 mm light-emitting diode is generally referred to as T-1; φ5 mm is referred to as T-1 (3/4); and φ4.4 mm is referred to as T-1 (1/4).

The angular distribution of the circular luminous intensity can be estimated from the half value angle size.

There are three types from the angular distribution of luminous intensity:

A. High directivity. It is usually a pointed epoxy package or a metal reflective cavity package with no scattering agent. The half-value angle is 5°-20° or less, has high directivity, can be used as a local illumination source, or combined with a light detector to form an automatic detection system.

B. Standard type. Usually used as indicator light, its half value angle is 20 ° ~ 45 °.

C. Scattering type. This is an indicator light with a large viewing angle, the half value angle is 45° to 90° or more, and the amount of the scattering agent is large.

(3) According to the structure of the light-emitting diode

According to the structure of the light-emitting diode, there are structures such as a full epoxy encapsulation, a metal base epoxy package, a ceramic base epoxy package, and a glass package.

(4) According to luminous intensity and working current

LEDs with ordinary brightness according to luminous intensity and working current (lighting intensity 100mcd); high-intensity light-emitting diodes with luminous intensity between 10 and 100mcd. Generally, the operating current of LEDs is between ten mA and tens of mA, while the operating current of low-current LEDs is below 2 mA (the brightness is the same as that of ordinary LEDs).

4. LED optical characteristics research

The International Commission on Illumination (CIE) Technical Committee's research on the technical characteristics of LEDs is divided into two divisions. Namely: visual and color division (D1) and light and radiation measurement division (D2). The color rendering of white LEDs and related metering issues are under investigation and have been forwarded to D1: TC1-65, TC1-62, the two drafts of the color chart for the color measurement and the color rendering of the LED.

The TC1-62 document "Colour Rendering of White LED LightSources" may partially replace the CIE 13.3-1995 publication. These two documents have entered the voting phase.

The CRI calculation method is specified in the CIE 13.3-1995 publication. If the result of the calculation of the CRI by the white LED is inconsistent with the visual result, the document determines that this contradiction exists. The technical report concludes that CIE CRI does not apply when applying color rendering calculations including white LEDs. The Technical Committee recommended that D1 establish a new set of color rendering indices that do not immediately replace the current CIE color rendering index calculation method. As a supplement to CIE CRI, the new CRI can only be used to determine the alternative CRI calculation method after successfully applying a new color rendering index. D2 set up a special technical committee TC 2-45 to study the measurement method of LED: TC 2-45 document "Mea-surement of LEDS" is voting, it will replace CIE 127 publication.

5. LED luminous efficiency limit value

For a long time, semiconductor research experts have explored various new technologies to improve the internal and external quantum efficiency of LEDs. In 2006, there have been reports of low-power white LEDs with luminous efficiencies of 100 lm/W. In order to determine the reasonable expectation of LED luminous efficiency, it is necessary to calculate the LED luminous efficiency limit value based on photometry and colorimetry.

In October 1979, the 10th International Metrology Conference (CGPM) defined New Candela (cd). Candela (cd) is the luminous intensity of a light source emitting a single-color radiation frequency of 540.0154×1012 Hz (wavelength 555 nm) in a given direction, and the radiation intensity in this direction is:

1 cd = (1/683) W / sr (wavelength 555 nm); 1 cd = 1 lm / sr; 1 W = 683 lm (wavelength 555 nm).

If the power loss, internal quantum efficiency, and external quantum efficiency values ​​are ignored, the luminous efficiency limit values ​​of various light sources and LEDs can be calculated.

The spectral efficiency of the human eye spectrum is not the same as the spectral power distribution of the ideal energy white light. Due to the spectral response characteristics of the human eye, the ideal iso-energy white light can be weight-calculated to obtain an ideal iso-energy white light limit luminous efficiency of 182.45 lm/W in the visible spectrum.

In the field of illumination, the birth of a new type of light source, its life and light efficiency are important quality indicators, but its color rendering properties for various colors is another important quality indicator of the lighting environment. The theoretical luminous efficiency of the two yellow spectral lines of the low-pressure sodium lamp is up to 450 lm/W, and the actual luminous efficacy exceeds 200 lm/W. However, due to its poor color rendering properties, it was eventually replaced by high pressure sodium lamps and metal halide lamps.

Investigating the new light source of LED, compared with the ideal white light at the expense of some color rendering index Ra, the limit luminous efficiency of white LED will be higher, about 200lm. For a white LED actually used in the field of illumination, it is reasonable to set the target value of the luminous efficiency at 150 to 160 lm/W.

In addition to white LEDs for lighting applications, the luminous efficiency of LEDs of various spectra can also be estimated from data. Red (643 nm), green (535 nm), and blue (460 nm) LEDs are used as the three primary colors, and the limit luminous efficiency values ​​can also be estimated based on the data.

6. Comparison of LED and traditional light source

The comparison of LEDs with conventional light sources has the following characteristics:

(1) LEDs are small in size and available in a variety of different form factors for different applications.

(2) LED has a variety of colors, ultraviolet, purple, green, yellow, red to infrared, white LED spectrum.

(3) LED optical parameters are related to temperature; (4) LED optical parameters are related to observation angle; (5) LEDs have various light distribution curves, and there is no determined optical axis.

The above characteristics of the LED bring many problems to the measurement of the optical characteristics of the LED.

7. Measurement of LED optical characteristics

The optical characteristics of LEDs should be considered from the following characteristics:

(1) Luminous intensity (cd)

Luminous flux is the total light energy that a certain light source emits into the surrounding space. The distribution of the luminous flux emitted by different light sources is different in space. The unit of luminous intensity is Candela, the symbol is cd, which represents the luminous flux emitted by the light source in a unit spheroidal solid angle (the angle formed by the surface of the object to the point source). 1cd = 1lm/1sr (sr: steradian unit of solid angle).

The luminous intensity of an LED generally refers to the intensity of illumination in the direction of the normal (for the axis of the cylindrical light-emitting tube). If the radiation intensity in this direction is (1/683) W/sr, then the light can be 1 candela (symbol cd). Since the luminous intensity of a general LED is small, the luminous intensity is usually a candle (candela, mcd) as a unit.

Due to the structural characteristics of the LED, in order to improve its luminous efficiency, a reflector is installed at the bottom, which is actually a luminaire itself. The light emitted by each area has different focus points, it is not a point source. Therefore, in evaluating the luminous intensity of LEDs, the inverse square law of distance in photometry is not applicable. Two current internationally accepted measurement conditions are specified in the CIE127 publication.

The measurement results using the above two measurement conditions can be compared internationally. The measurement conditions of A and B are not strictly in accordance with the definition of luminous intensity, and are therefore referred to as "average luminous intensity" (ALI).

Correction of the measuring probe: Since the measuring error of the measuring probe will cause the measurement error of the "average luminous intensity" (ALI), the matching error has a more serious influence on the measurement results of the red and blue LEDs, and the spectral correction method can be used. Improve measurement accuracy.

Requirements for LED luminous intensity measuring instruments:

A. The measured solid angle should be correct dΩ = 0.001 sr (A condition) d Ω = 0.01 sr (B condition).

B. Measuring the mechanical axis is correct; C. Effective stray-proof light design; D. Precision V(λ) photodetector; E. Providing V(λ) photodetector spectral data for easy correction of measured values; F. Stable power supply.

(2) Total luminous flux (lm)

Since the human eye has different sensitivities to electromagnetic waves of different wavelengths, we cannot directly measure the light energy by the radiant power or radiant flux of the light source. It is necessary to use the unit of luminous flux based on the amount of light perceived by the human eye. measure. Luminous flux is represented by the symbol Φ in lumens (lm).

LED luminous flux measurement, using a distributed photometer to accurately measure the total luminous flux of the LED (the detector spectral response curve has been corrected). This is an absolute measure of the total luminous flux of the LED, but the test instrument is expensive, and the integrating sphere is commonly used in the industry for measurement.

A. The size of the integrating sphere is as large as possible, which can reduce the absorption of the screen and the foreign matter error; B. The larger the reflectance of the surface of the coating, the less the difference in the response rate of the inner surface of the ball. Currently in the LED test, the surface reflectance of the coating is even greater than 98%.

C. Pay attention to the installation position of the LED to be tested, and align the emitted light with the area where the inner surface of the integrating sphere responds uniformly; D. Apply the auxiliary light source to reduce the absorption of the screen and the foreign object error.

(3) Spectral characteristics, chromaticity coordinates, dominant wavelength measurement

Spectral characteristics, chromaticity coordinates, measurement of dominant wavelengths, according to the technical exchanges of the International LED Commission (CIE) three-LED international expert meeting and relevant international comparison results, the following recommendations are as follows:

A. The national metrology department should adopt a dual monochromator measurement system; B. The monochromator measurement system can meet the industrial sector application; the C.1nm and 5nm spectral measurement bandwidths have relatively close chromaticity test results, and can be measured with a 5 nm bandwidth; The contrast measurement of the dominant wavelength is very small; the relative error of the E.CCD measuring instrument is large.

(4) Spatial distribution of luminous intensity and total luminous flux

(5) Brightness (cd/m2)

Brightness is the intensity of light emitted by an object seen by an eye from a certain direction. The unit is candela/square [cd/m2], and the symbol is L, indicating the luminous flux of the illuminant in a unit of solid angle unit area in a specific direction, which is equal to the luminous intensity of 1 candela on the surface of 1 square meter.

(6) Color temperature (Co1or Temperature)

When the color of the light emitted by the light source is the same as the color of the black body radiated at a certain temperature, the temperature of the black body is referred to as the color temperature of the light source, expressed by the absolute temperature K. When the color of the light emitted by the light source is close to the color of the black body radiated at a certain temperature, the temperature of the black body is referred to as the correlated color temperature of the light source.

8. Standard LED

(1) Theoretical and technical basis for measuring optical properties of LEDs

A. According to the above analysis of the optical characteristics of LEDs, national metrology departments and industry can apply conventional luminosity, chromaticity and radiance instruments to the total luminous flux, spectral characteristics, chromaticity coordinates, dominant wavelength, color temperature and other parameters of the LED. measuring.

B. For the measurement of LED luminous intensity, since the LED illumination characteristics do not follow the photometric inverse square law, the CIE127 document recommends using the A and B conditions to measure the LED average luminous intensity (ALI).

C. In order to improve the measurement uncertainty and increase the measurement efficiency during the measurement of average luminous intensity, luminous flux, etc., CIE has established technical committees such as Tc2-45, Tc2-46 and Tc2-50 to carry out related research and evaluation work. And research on standard LEDs.

D. The basic theory of photometry, colorimetry, and radiosity is the basis of LED measurement. The standard A source is an important benchmark for determining the spectral power distribution characteristics of standard LEDs.

E. Accurate standard LED luminous flux values ​​can be determined using distributed photometric measurements. As a supplementary test method, the United States (NIST), Hungary, the United Kingdom (NPL), Germany (PTB) and other countries and China are actively carrying out standard LED research work.

(2) Requirements for standard LED characteristics

A. The operating temperature of the standard LED is generally greater than the ambient temperature, and there is also a technical solution for cooling; B. The sample of the standard LED needs to be aged for several hundred hours, and the stabilizer is selected for the post-calibration work; C. The standard LED must have the test sample The same spectral power distribution requires the creation of standard LEDs in a variety of different colors. Especially for white LEDs, since it can be composed of different spectra, it is almost impossible to develop a universal white LED standard; D. Standard LEDs must have the same luminous intensity distribution curve (light distribution curve) as the test sample, if the color of the LED to be tested If the (spectrum) is different from the standard LED (spectrum), the photometric detector needs to be spectrally corrected.

(3) Advantages of applying standard LED measurements

A. There is no need to perform spectral correction on the photometric probe; B. There is no need to strictly position the photometric reference plane.

9. LED application

(1) Display, communication number, advertising industry multimedia, urban lighting display light source LED light has the characteristics of shock resistance, light response, power saving and long life, widely used in various indoor and outdoor displays. It is divided into full-color, three-color and monochrome displays, and more than 100 units in the country are developing and producing. Traffic lights mainly use ultra-high brightness red, green and yellow LEDs. Because LED signal lights are energy-saving and highly reliable, traffic lights are gradually being upgraded nationwide, and the promotion speed is fast, and the market demand is large.

(2) Applications in the automotive industry Automotive lights include dashboards, audio indicators, backlights for switches, reading lights and external brake lights, tail lights, side lights, and headlights. Automotive incandescent lamps are not resistant to shock, damage, and short life, and need to be replaced frequently. In 1987, China began to install high-position brake lights on cars. Because the LED response speed is fast, it can remind the driver to brake early and reduce the rear-end collision accident. In developed countries, the central rear high-position brake light made by LED has become the standard part of the automobile. The LED car taillight modules can be combined into a variety of car taillights. In addition, the light source of the car dashboard and other various lighting parts can be used with ultra-high brightness light, so the LED display is gradually adopted. China's automobile industry is in a period of great development and is an excellent opportunity to promote ultra-high brightness LEDs. In the next few years, an annual output value of 1 billion yuan will be formed, and an annual output value of 3 billion yuan will be formed within 5 years.

(3) LED backlights are most attractive for high-efficiency side-emitting backlights. LEDs are used as LCD backlights with long life, high luminous efficiency, no interference, and high cost performance. They have been widely used in electronic watches, mobile phones, and BP. On the machine, electronic calculator and credit card machine, with the increasingly miniaturized portable electronic products, LED backlights have more advantages, so the backlight production technology will develop toward thinner, lower power consumption and uniformity. LED is a key component of mobile phones. A regular mobile phone or PHS requires about 10 LED devices, while a color screen and a camera-equipped mobile phone require about 20 LED devices. At this stage, the amount of backlight used in mobile phones is very large, and 3.5 billion LED chips are used every year. At present, China's mobile phone production is very large, and most of the LED backlights are still imported. For domestic LED products, this is an excellent market opportunity.

(4) LED lighting source The early products have low luminous efficiency, and the light intensity can only reach several to several tens of mcd. It is suitable for indoor applications, and is applied in home appliances, instrumentation, communication equipment, microcomputers and toys. At present, the direct target is to replace incandescent lamps and fluorescent lamps with LED light sources. This alternative trend has been developed from the local application field. In order to save energy, Japan is planning to replace the incandescent lamp LED project (called "illuminate Japan"), the budget for the first five years is 5 billion yen, if the LED replaces half of the incandescent and fluorescent lamps, the annual savings can be equivalent The energy of 6 billion liters of crude oil is equivalent to the power generation of five 1.35 × 106 kW nuclear power plants, and can reduce the production of carbon dioxide and other greenhouse gases, and improve the environment in which people live and live. In 2004, China invested 5 billion yuan to develop energy-saving and environmentally-friendly semiconductor lighting plans.

(5) Other applications, such as a flashing shoe that is popular among children, the built-in LED will flash when walking, only 500 million LEDs will be used in Wenzhou in one year; the LEDs will be used as the indicator of the electric toothbrush. The manufacturer is putting into production, the company has a small number of health care toothbrushes, and it is expected that 300 million illuminating lamps will be needed every year. The popular LED Christmas lights are safe, colorful, not easy to break and safe for low pressure. Sexuality has recently been strong in Southeast Asia such as Hong Kong and has been widely welcomed by people. It is threatening and replacing the existing Christmas market for electric bubbles.

10. Some problems faced by LED

Under the guidance of the country's strategic task of promoting industrial structure optimization and upgrading and cultivating new industrial growth points, LED has become the development focus of local governments. Local governments such as Dongguan, Huizhou, Foshan, Shenzhen, Dalian, Nanchang and Xiamen have formulated Strategic planning for industrial development has set off a wave of LED investment in China. As a part of the country that accounts for nearly half of the total output of LEDs in China, in the "Opinions on Accelerating the Transformation of Economic Development Mode" issued in May this year, the semiconductor lighting (LED) industry has been listed as the three strategic emerging emerging focuses in Guangdong. One of the industries. The LED industry has formed a situation of competition in Guangdong. Shenzhen, Dongguan and Foshan have all listed the LED industry as a pillar industry. The LED industry has already shown a full bloom. Domestic LED companies have rapidly emerged and rapidly expanded their production capacity to launch a strategy for the LED market. From the domestic LED industry leading company Sanan Optoelectronics to the expansion of the production capacity of Shilanwei to the half-way Dehao Runda, the LED industry boom has almost swept the entire industry, due to the overall low level of domestic LED product technology, domestic enterprises are generally small, The technical strength is weak, the product grade is relatively low, the LED market is fiercely competitive, and related manufacturers compete for price wars. At the same time, there are also phenomena such as disorderly investment and vicious competition in the LED industry. Judging from the current domestic market dynamics and the distribution of the LED industry structure, there has been a serious problem of repeated investment.

In addition, some local governments intend to open local markets to local enterprises in the development of the economy, or to lock up local markets by means of investment for market, and local protectionism is on the rise. If we do not break such local administrative barriers, it will inevitably lead to the convergence of industrial projects between regions, directly increase the industrial bubble, and hinder the process of regional integration development. Different regions should highlight different characteristics, and avoid duplicating construction for short-term interests, leading to vicious competition in the future.

In addition, China's LED lighting still has a series of other problems. For example, China does not currently have a national standard for LED lamps, and only some local standards; China's local LED lighting design innovation design capabilities are obviously insufficient. In order to solve these problems, the LED industry can be developed healthily, and LED products can better meet the needs of the public. Market leadership is the basis for the long-term healthy development of the LED lighting industry. The government should guide and support policies, regulations, R&D investment, application demonstrations and standards. The national chess game and overall planning can guide the sustainable and healthy development of the LED industry.

Editor: Li Jie