LED light source and optical characteristics detection

As a new type of photoelectric light-emitting device, LED is becoming more and more popular due to its advantages of low carbon, environmental protection, small size, light weight, low cost and long life. Its development momentum is extremely rapid, which is beyond the expectation of photoelectric experts. LED is not only widely used as a variety of lighting sources and indicator lights, but also used in large quantities as LCD backlight sources, and is likely to replace traditional fluorescent tubes and become the new favorite of LCD TVs. In addition, LED can also be used as a light source for many measuring instruments.

LED TVs in the full sense are also developing very fast. Although they are mostly used outdoors now, it is only a matter of time before they enter thousands of households due to their high brightness, realistic colors and excellent dynamic effects. Therefore, it is very urgent for us to fully understand this new partner, LED.

Since the first red LED was introduced in 1976, after 30 years of development, LED has formed a series of products with various spectrums, and the power of a single LED has also grown from the initial 0.01 watt to several watts or even tens of watts. In 2001, white LED was successfully developed, and people expect LED to eventually enter the lighting field, even into 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, the research results of RGB-LEDs show that LEDs, like conventional three-primary-color fluorescent lamps, can achieve a variety of color temperatures and a uniform lighting environment.

The progress of LED light sources and people's expectations for its application in the field of lighting have also put forward new requirements for corresponding optical detection technology. Since the optical characteristics of LED are quite different from those of traditional light sources, it is necessary to research and develop measurement methods that are suitable for this new light source.

2.LED light source

LED (Light-Emitting-Diode) is a semiconductor that can convert electrical energy into visible light. It changes the principle of tungsten filament luminescence of incandescent lamps and trichromatic powder luminescence of energy-saving lamps, and adopts electric field luminescence. According to analysis, the characteristics of LED are very obvious, with long life, high light efficiency, no radiation and low power consumption. The spectrum of LED is almost entirely concentrated in the visible light band, and its luminous efficiency can reach 80-90%. Comparing LED with ordinary incandescent lamps, spiral energy-saving lamps and T5 trichromatic fluorescent lamps, the results show that the luminous efficiency of ordinary incandescent lamps is 12lm/W, and the life is less than 2000 hours, the luminous efficiency of spiral energy-saving lamps is 60lm/W, and the life is less than 8000 hours, and the luminous efficiency of T5 fluorescent lamps is 96lm/W, and the life is about 10,000 hours, while the luminous efficiency of white light LED with a diameter of 5 mm is 20-28lm/W, and the life can be more than 100,000 hours. Some people also predict that the upper limit of LED life will be infinite in the future.

High power refers to high luminous efficiency, generally 0.5W, 1W 3W 5W or higher. The light intensity and lumen are greater than low power, but the heat dissipation is also greater. Now high power is used in single application, with a large heat sink. Low power is generally around 0.06W. Plug-in and piranha, etc. Now LED flashlights are generally used with low power. Whether the light is scattered or not depends on the light-emitting angle of the LED. There are large angles and small angles. Small angles do not scatter, but large angles scatter.

The brightness of LED is inevitably related to the light-emitting angle of LED. The smaller the angle of LED, the higher its brightness. If it is a 5MM LED, the brightness of white light at an angle of 180 degrees is only a few hundred MCD, and if it is at an angle of 15 degrees, the brightness will reach more than 10,000 or 20,000 MCD, which is dozens of times different. If it is used for lighting, it is best to use high-power LED outdoors, the brightness will be higher, the single power is 1W, 3W, 5W, and some use multiple high-power LEDs to combine into a high-power LED, the power can reach hundreds.

The light-emitting principle of LED

LED (Light Emitting Diode) is a solid-state semiconductor device that can directly convert electricity into light. The heart of LED is a semiconductor chip. One end of the chip is attached to a bracket, one end is the negative pole, and the other end is connected to the positive pole of the power supply, so that the entire chip is encapsulated by epoxy resin. The semiconductor chip consists of two parts, one part is a P-type semiconductor, in which holes dominate, and the other end is an N-type semiconductor, which is mainly electrons. But when these two semiconductors are connected, a "PN junction" is formed between them. When current acts on the chip through a wire, electrons will be pushed to the P region, where electrons and holes recombine, and then they will emit energy in the form of photons. This is the principle of LED light emission.

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

LED is made of semiconductors such as III-IV compounds, such as GaAs (gallium arsenide), GaP (gallium phosphide), and GaAsP (gallium arsenide phosphide), and its core is a PN junction. Therefore, it has the IN characteristics of a general PN junction, that is, forward conduction, reverse cutoff, and breakdown characteristics. In addition, under certain conditions, it also has luminescence characteristics. Under forward voltage, electrons are injected from the N region into the P region, and holes are injected from the P region into the N region. A part of the minority carriers (minority carriers) that enter the other region recombine with the majority carriers (majority carriers) to emit light.

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

Theoretical and practical proofs show that the peak wavelength λ of light is related to the bandgap width Eg of the semiconductor material in the light-emitting region, that is, λ≈1240/Eg (mm), where the unit of Eg is electron volt (eV). If visible light (wavelength between 380nm purple light and 780nm red light) can be generated, the Eg of the semiconductor material should be between 3.26 and 1.63 eV. Light with a longer wavelength than red light is infrared light. There are now infrared, red, yellow, green and blue light-emitting diodes, but the cost and price of blue light diodes are very high and they are not widely used.

3. Classification of LEDs

(1) According to the color of the light emitting tube

According to the color of the light emitting tube, it can be divided into red, orange, green (further subdivided into yellow-green, standard green and pure green), blue light, etc. In addition, some light emitting diodes contain two or three colors of chips.

According to whether the light emitting diode is doped with scattering agent or not, and whether it is colored or colorless, the above-mentioned light emitting diodes of various colors can also be divided into four types: colored transparent, colorless transparent, colored scattering and colorless scattering. Scattering type light emitting diodes are used as indicator lights.

(2) Classification by 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, it can be divided into round lamps, square lamps, rectangular lamps, surface light-emitting tubes, lateral tubes, and micro tubes for surface mounting. According to the diameter, round lamps can be divided into φ2mm, φ4.4mm, φ5mm, φ8mm, φ10mm, and φ20mm. In foreign countries, φ3mm light-emitting diodes are usually recorded as T-1; φ5mm as T-1 (3/4); and φ4.4mm as T-1 (1/4).

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

There are three categories based on the luminous intensity angular distribution diagram:

A. High directivity. It is usually a pointed epoxy package or a package with a metal reflective cavity without adding a scattering agent. The half-value angle is 5° to 20° or less, with high directivity. It can be used as a local lighting source, or used in conjunction with a light detector to form an automatic detection system.

B. Standard type. Usually used as indicator light, with a half-value angle of 20° to 45°.

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

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

According to the structure of the light emitting diode, there are full epoxy encapsulation, metal base epoxy packaging, ceramic base epoxy packaging and glass packaging.

(4) According to luminous intensity and working current

According to the luminous intensity and working current, there are ordinary brightness LEDs (luminous intensity 100mcd); those with luminous intensity between 10 and 100mcd are called high brightness LEDs. The working current of general LEDs is between tens of mA and tens of mA, while the working current of low current LEDs is below 2mA (the brightness is the same as that of ordinary light-emitting tubes).

4. Research on LED optical properties

The technical committee of the International Commission on Illumination (CIE) has two divisions for the study of the technical characteristics of LEDs, namely, the Vision and Color Division (D1) and the Light and Radiation Measurement Division (D2). The color rendering of white light LEDs and related metrology issues are being studied, and the draft documents D1: TC1-65 and TC1-62, which study the visual measurement of color tables and the color rendering of LEDs, have been forwarded.

TC1-62 document "Colour Rendering of White LED Light Sources" may partially replace CIE 13.3-1995 publication. Both documents have entered the voting stage.

The TC1-62 document "Colour Rendering of White LED LightSources" introduces the visual test results of the white light LED color rendering index CRI.

The CIE 13.3-1995 publication specifies the calculation method of CRI. If the result of CRI calculation for white light LEDs is inconsistent with the visual result, the document confirms the existence of this contradiction. The conclusion of the technical report is that CIE CRI is not applicable when color rendering calculations are applied including white light LEDs. The technical committee recommends that D1 establish a new set of color rendering indices that do not immediately replace the current CIE color rendering index calculation method. The new color rendering index is a supplement to CIE CRI. Only after the successful application of the new color rendering index combination can it be determined to replace the current CRI calculation method. D2 established a special technical committee TC 2-45 to study the measurement method of LEDs: TC 2-45 document "Measurement of LEDS" is under voting, and it will replace CIE 127 publications.

5. LED luminous efficiency limit

Semiconductor research experts have long been exploring various new technologies to improve the internal and external quantum efficiency of LEDs. In 2006, there were reports that the luminous efficiency of low-power white LEDs reached 100lm/W. In order to determine a reasonable expected value of LED luminous efficiency, it is necessary to calculate the limit value of LED luminous efficiency based on photometry and colorimetry.

In October 1979, the 10th General Conference on Weights and Measures (CGPM) defined the new candela (cd). Candela (cd) is the luminous intensity of a light source emitting monochromatic radiation with a frequency of 540.0154×1012Hz (wavelength 555nm) in a given direction. The radiation intensity in this direction is:

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

If we ignore the power supply loss, internal quantum efficiency and external quantum efficiency, we can calculate the luminous efficiency limits of various light sources and LEDs.

The spectral light efficiency of the human eye is not exactly the same as the spectral power distribution of the ideal equal-energy white light. Due to the spectral response characteristics of the human eye, after weighted calculation, the ideal equal-energy white light's limit luminous efficiency in the visible spectrum range can be obtained as 182.45lm/W.

In the field of lighting, a new type of light source is born. Its lifespan and light efficiency are important quality indicators, but its color rendering characteristics for various colors are another important quality indicator of the lighting environment. The theoretical luminous efficiency of the two yellow spectrum lines of low-pressure sodium lamps can reach 450lm/W, and the actual light efficiency exceeds 200lm/W. However, due to its poor color rendering characteristics, it was eventually replaced by high-pressure sodium lamps and metal halide lamps.

In terms of LED, a new type of light source, the ultimate luminous efficiency of white light LED is higher than that of ideal white light with equal energy, at the expense of some color rendering index Ra, which is about 200lm. For a white light LED actually used in the field of lighting, it is reasonable to set the target value of luminous efficiency at 150-160lm/W.

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

6. Comparison between LED and traditional light sources

Compared with traditional light sources, LED has the following characteristics:

(1) LED is small in size and comes in a variety of sizes, suitable for different applications.

(2) LEDs have a variety of colors, from ultraviolet, purple, green, yellow, red to infrared and white light LED spectrum.

(3) LED optical parameters are related to temperature; (4) LED optical parameters are related to observation angle; (5) LED has various light distribution curves and no definite optical axis.

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

7. Measurement of LED optical properties

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

(1) Luminous intensity (cd)

Luminous flux is the total light energy emitted by a light source to the surrounding space. The distribution of luminous flux in space is different for different light sources. The unit of luminous intensity is candela, symbolized as cd, which indicates the luminous flux emitted by a light source within a certain unit steradian solid angle (the angle formed by the surface of the object to the point light source). 1cd = 1lm/1sr (sr: steradian unit of solid angle).

The luminous intensity of LED usually refers to the luminous intensity in the normal direction (for cylindrical light-emitting tubes, it refers to the axial direction). If the radiation intensity in this direction is (1/683) W/sr, then the light is 1 candela (symbol: cd). Since the luminous intensity of general LEDs is small, the luminous intensity is usually measured in candela (mcd).

Due to the structural characteristics of LED, in order to improve its luminous efficiency, a reflector is installed at its bottom. In fact, it is a lamp itself. The light emitted from different areas has different focal points, and it is not a point light source. Therefore, when evaluating the luminous intensity of LED, the inverse square law of distance in photometry is not applicable. CIE127 publication stipulates two currently internationally recognized measurement conditions.

The measurement results of the two measurement conditions mentioned above can be compared internationally. The A and B measurement conditions are not strictly based on the definition of luminous intensity and are therefore called "average luminous intensity" (ALI).

Regarding the correction of the measuring detector: Since the matching error of the measuring detector will cause the measurement error of the "average luminous intensity" (ALI), the matching error has a more serious impact on the measurement results of red and blue LEDs. The use of spectral correction method can improve the measurement accuracy.

Requirements for LED luminous intensity measuring instruments:

A. To measure the solid angle correctly, dΩ=0.001sr (condition A) dΩ=0.01sr (condition B).

B. Correct measurement of mechanical axis; C. Effective anti-stray light design; D. Precision V (λ) light detector; E. Provide V (λ) light detector spectrum data to facilitate correction of measurement values; F. Equipped with high-stability power supply.

(2) Total luminous flux (lm)

Since the human eye has different sensitivities to electromagnetic waves of different wavelengths, we cannot directly use the radiation power or radiation flux of the light source to measure the light energy. We must use the unit based on the human eye's perception of light - luminous flux. Luminous flux is represented by the symbol Φ and the unit is lumen (lm).

The measurement of LED luminous flux can be accurately measured by using a distributed photometer (under the condition that the detector spectral response curve has been corrected). This is an absolute measurement method for the total luminous flux of LEDs, but the test instrument is expensive. Integrating spheres are often used for measurement in industry.

A. The size of the integrating sphere should be as large as possible to reduce screen absorption and foreign matter errors; B. The greater the reflectance of the coating surface, the less the difference in the response rate of the inner surface of the sphere. Currently, in LED testing, the reflectance of the coating surface is even greater than 98%.

C. Pay attention to the installation position of the LED to be tested, and aim the emitted light at the area with uniform response on the inner surface of the integrating sphere; D. Use auxiliary light sources to reduce screen absorption and foreign matter errors.

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

The measurement of spectral characteristics, chromaticity coordinates, and dominant wavelength is recommended as follows based on the technical exchanges of the three international LED expert meetings of the International Commission on Illumination (CIE) and related international comparison results:

A. The national metrology department should adopt a dual monochromator measurement system; B. The monochromator measurement system can meet the application needs of the industrial sector; C. The chromaticity test results of 1nm and 5nm spectral measurement bandwidths are relatively close, and 5nm bandwidth measurement can be used; D. The comparative measurement difference of the main wavelength is very small; E. The relative error of the CCD measurement 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 the eye from a certain direction. The unit is candela/square meter [cd/m2], symbolized by L, indicating the luminous flux per unit solid angle per unit area of ​​a luminous body in a specific direction, which is equal to the luminous intensity of 1 candela emitted on a surface of 1 square meter.

(6) Color temperature

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

8. Standard LED

(1) Theoretical and technical basis for measuring LED optical properties

A. Based on the above analysis of LED optical characteristics, national metrology departments and the industry can use conventional photometric, chromatic and radiometric instruments to measure LED total luminous flux, spectral characteristics, chromaticity coordinates, dominant wavelength, color temperature and other parameters.

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

C. In order to improve the measurement uncertainty and measurement efficiency in the process of transferring values ​​such as average luminous intensity and luminous flux, CIE has established technical committees such as Tc2-45, Tc2-46, and Tc2-50 to carry out related research and evaluation work, as well as research on standard LEDs.

D. The basic theories of photometry, colorimetry, and radiometry are the basis for measuring LEDs. Standard A light source is an important benchmark for measuring the spectral power distribution characteristics of standard LEDs.

E. The accurate standard LED luminous flux value can be determined by measuring with a distributed photometer. As a supplementary test method, the United States (NIST), Hungary, the United Kingdom (NPL), Germany (PTB) and other countries as well as my country are actively carrying out standard LED research.

(2) Requirements for standard LED characteristics

A. The working temperature of standard LED is generally higher than the ambient temperature, and there are also cooling technology solutions; B. The samples of standard LED need to be aged for hundreds of hours, and the stable ones are selected for later calibration; C. The standard LED must have the same spectral power distribution as the test sample, and it is necessary to establish a variety of standard LEDs with different colors. Especially for white light LED, since it can be composed of different spectra, it is almost impossible to develop a universal white light LED standard; D. The standard LED must have the same luminous intensity distribution curve (light distribution curve) as the test sample. If the color (spectrum) of the LED to be tested is different from the standard LED (spectrum), the photometer detector needs to be spectrally calibrated.

(3) Advantages of using standard LED measurements

A. No spectral calibration of the photometer probe is required; B. No strict positioning of the photometer reference plane is required.

9. Application of LED

(1) Application of display screens, traffic signals, multimedia in advertising, and urban lighting display sources LED lights have the characteristics of earthquake resistance, impact resistance, fast light response, power saving, and long life. They are widely used in various indoor and outdoor display screens. They are divided into full-color, three-color, and monochrome display screens. There are more than 100 units in the country developing and producing them. 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 updated nationwide, and the promotion speed is fast, and the market demand is huge.

(2) Application in the automotive industry Automotive lighting includes the dashboard, audio indicator lights, switch backlights, reading lights, and external brake lights, taillights, side lights, and headlights. Incandescent lamps for automobiles are not resistant to vibration and impact, are easily damaged, have a short lifespan, and need to be replaced frequently. In 1987, my country began to install high-mounted brake lights on cars. Due to the fast response speed of LEDs, they can remind drivers to brake early and reduce rear-end collisions. In developed countries, central rear high-mounted brake lights made of LEDs have become standard parts for cars. In 1996, the American HP company launched an LED car taillight module with three colors of LED lights, which can be combined into various car taillights at will. In addition, the light source of the dashboard and other lighting parts of the car can be served by ultra-high brightness lamps, so they are gradually adopting LED displays. my country's automobile industry is in a period of great development, which is an excellent opportunity to promote ultra-high brightness LEDs. In the next few years, it will generate an annual output value of 1 billion yuan, and in 5 years, it will generate an annual output value of 3 billion yuan.

(3) LED backlight is the most eye-catching with its high-efficiency side-emitting backlight. As an LCD backlight, LED has the characteristics of long life, high luminous efficiency, no interference and high cost performance. It has been widely used in electronic watches, mobile phones, BP machines, electronic calculators and card readers. As portable electronic products become increasingly miniaturized, LED backlight has more advantages. Therefore, the backlight manufacturing technology will develop towards thinner, low power consumption and uniformity. LED is a key component of mobile phones. An ordinary mobile phone or PHS needs about 10 LED components, while a color screen and camera mobile phone needs about 20 LED components. At present, the consumption of mobile phone backlight is very large, with 3.5 billion LED chips used per year. At present, my country's mobile phone production is very large, and most LED backlight sources are imported. This is an excellent market opportunity for domestic LED products.

(4) Early LED lighting sources had low luminous efficiency, and the light intensity could only reach a few to dozens of mcd. They were suitable for indoor occasions and were used in home appliances, instruments, communication equipment, microcomputers and toys. The current direct goal is to replace incandescent lamps and fluorescent lamps with LED light sources. This replacement trend has begun to develop from local application areas. In order to save energy, Japan is planning a light-emitting diode project to replace incandescent lamps (called "Lighting Japan"), with a budget of 5 billion yen for the first five years. If LED replaces half of the incandescent lamps and fluorescent lamps, it can save energy equivalent to 6 billion liters of crude oil each year, which is equivalent to the power generation of five 1.35×106kW nuclear power plants, and can reduce the production of carbon dioxide and other greenhouse gases, improving people's living environment. In 2004, my country invested 5 billion to vigorously develop energy-saving and environmentally friendly semiconductor lighting plans.

(5) Other applications include a pair of flashing shoes popular among children, whose built-in LEDs flash when children walk. In Wenzhou alone, 500 million LEDs are used each year. LEDs are used as power indicator lights for electric toothbrushes. According to a domestic manufacturer that is currently producing electric toothbrushes, the company has a small number of health toothbrushes on the market and estimates that 300 million LEDs will be needed each year for mass production. LED Christmas lights are popular. Due to their novel shapes, rich colors, unbreakable properties, and low-voltage safety, they have recently been selling strongly in Hong Kong and other Southeast Asian regions and are widely welcomed by people. They are threatening and replacing the existing Christmas market for electric light bulbs.

10. Some problems faced by LED

Under the guidance of the strategic task of promoting the optimization and upgrading of industrial structure 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 plans for industrial development, and a wave of LED investment has been set off in China. As Guangdong accounts for nearly half of the total output value of domestic LEDs, the "Several Opinions on Accelerating the Transformation of Economic Development Mode" issued in May this year listed the semiconductor lighting (LED) industry as one of the three major strategic emerging industries that Guangdong will focus on in the near future. The LED industry in Guangdong has formed a situation of hundreds of boats competing for the current. Shenzhen, Dongguan, and Foshan have all listed the LED industry as a pillar industry for development. The LED industry has shown a trend of full blossoming. Domestic LED companies have risen rapidly and rapidly expanded their production capacity, launching strategies to compete for the LED market. From Sanan Optoelectronics, the leading enterprise in the domestic LED industry, to Silan Microelectronics, which is expanding its production capacity, to Elec-Tech, which has just started, the LED industry boom has almost swept the entire industry. Due to the low overall level of domestic LED product technology, domestic enterprises are generally small in scale, weak in technical strength, and low in product quality, the LED market is highly competitive, and related manufacturers are fighting price wars. At the same time, disorderly investment and vicious competition have also appeared 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 duplicate investment.

In addition, some local governments have intentionally opened up local markets to local enterprises in the process of economic development, or locked up local markets by exchanging investment for markets, and local protectionism has been on the rise. If such local administrative barriers are not broken, it will inevitably lead to the convergence of industrial projects between regions, directly increase industrial bubbles, and hinder the process of regional integration and development. Different regions should highlight their different characteristics and avoid simple duplication of construction for short-term interests, which will lead to vicious competition in the future.

In addition, there are a series of other problems in China's LED lighting. For example, China currently has no national standards for LED lamps, only some local standards; my country's local LED lamps' innovative design capabilities are obviously insufficient, etc. Only by solving these problems can the LED industry develop more healthily and LED products can better meet the needs of the public. Market dominance is the foundation for the long-term healthy development of the LED lighting industry. The government must guide and support policies, regulations, R&D investment, application demonstrations and standards. Only by planning the whole country as a whole can the LED industry be guided to continue to develop healthily.