Detection of LED light source and its optical characteristics

1 Overview

As a new type of optoelectronic light-emitting device, LED is more and more popular because of its advantages of low-carbon, environmentally friendly, small size, light weight, low cost and long life. LEDs are not only widely used as a variety of lighting sources, indicator lights, but also in large quantities for LCD backlights. They have largely replaced 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.

The LED TV in the complete sense is also developing rapidly, although it is mostly used outdoors now, because of its high brightness and realistic colors and excellent dynamic effects, it is only a matter of time before entering thousands of households. Therefore, it is very urgent for us to fully understand the new LED partner.

Since the advent of the first red LED in 1976, after 30 years of development, LEDs have formed various spectrum series products, and the power of a single LED has also developed from a few zero watts to a few watts or even tens of watts. White LED was successfully developed in 2001, and people expect that LED will eventually enter the field of lighting, and even enter home lighting. The latest white LED research results are even more exciting.

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

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

2. LED light source

LED (Light-EmitTIng-Diode in Chinese means light-emitting diode) is a semiconductor that can convert electrical energy into visible light. It changes the principle of incandescent lamp tungsten filament light emission and energy-saving lamp three-color toner light emission, and uses electric field light emission. According to analysis, the characteristics of LED are very obvious, long life, high light efficiency, no radiation and low power consumption. Almost all of the LED spectrum is concentrated in the visible light band, and its luminous efficiency can reach 80 to 90%. Comparing LEDs with ordinary incandescent lamps, spiral energy-saving lamps and T5 three-color fluorescent lamps, the results show that: ordinary incandescent lamps have a light efficiency of 12lm / W and a life of less than 2000 hours, and spiral energy-saving lamps have a light efficiency of 60lm / W and a life of less than 8000 hours, T5 fluorescent lamp is 96lm / W, life is about 10000 hours, and white LED with a diameter of 5mm is 20 ~ 28lm / W, life can be more than 100,000 hours. Some people also predict that the upper limit of LED life in the future will be infinite.

High power refers to a large luminous power, generally refers to 0.5W, 1W 3W 5W or higher. The light intensity and lumen are higher than the small power, but the heat dissipation is also large. Now the high power is a single application, plus a large heat sink. Low power is generally around 0.06W. Plug-ins and piranhas. At present, LED flashlights are generally used with low power, and the light is not dispersed, depending on the angle of light emitted by the LED. There are large angles and small angles. Small angles are not scattered, and large angles are scattered.

The brightness of the LED is inevitable in relation to the light emitting angle of the LED. The smaller the angle of the LED, the higher the brightness. If it is a 5MM LED, the brightness of the white light at 180 degrees is only a few hundred MCD. If it is 15 degrees, the brightness will be more than 10,000 or 20,000 MCD. The brightness difference is dozens of times. If it is used for lighting Used, it is best to use high-power LEDs outdoors, the brightness is higher, a single power has 1W, 3W, 5W, and some are combined into a high-power LED with multiple high-power, the power goes to hundreds All.

The principle of LED light

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

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

LEDs are made of III-IV group compounds, such as GaAs (gallium arsenide), GaP (gallium phosphide), GaAsP (gallium arsenide) and other semiconductors, the core of which is the 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 luminescent properties. Under 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 (minor carriers) that enters the opponent's area recombine with the majority carrier (multiple carriers) and emit light.

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

Theory and practice have proved that 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, λ≈1240 / Eg (mm) where the unit of Eg is electron volts (eV). If it can produce visible light (wavelength from 380nm violet to 780nm red light), the Eg of the semiconductor material should be between 3.26 and 1.63eV. Light with a longer wavelength than red light is infrared light. Now there are infrared, red, yellow, green and blue light-emitting diodes, but the cost and price of blue diodes are very high, and they are not widely used.

3. Classification of LED

(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 (and subdivided into yellow green, standard green and pure green), blue light and so on. In addition, some light-emitting diodes contain chips of two or three colors.

The light-emitting diodes of the above colors can be divided into four types: colored transparent, colorless transparent, colored scattering, and colorless scattering according to whether the light emitting diode emits light with or without a scattering agent, colored or colorless. Scattering light-emitting diodes can be used as indicator lights.

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

According to the characteristics of the light emitting surface of the light emitting tube, it can be divided into round lights, square lights, rectangular, surface light emitting tubes, lateral tubes, and micro tubes for surface mounting. Round lamps are divided into φ2mm, φ4.4mm, φ5mm, φ8mm, φ10mm and φ20mm according to the diameter. Foreign countries usually record φ3mm LED as T-1; φ5mm as T-1 (3/4); φ4.4mm as T-1 (1/4).

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

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

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

B. Standard type. Usually used as indicator light, the 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 full epoxy encapsulation, metal base epoxy encapsulation, ceramic base epoxy encapsulation and glass encapsulation.

(4) According to luminous intensity and working current

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

4. LED optical characteristics

The study of the technical characteristics of LEDs by the International Lighting Commission (CIE) Technical Committee 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 measurement issues are being studied, and the draft documents for visual measurement of the two research color tables D1: TC1-65, TC1-62 and LED color rendering have been forwarded.

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 stage.

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

CIE 13.3－1995 publication stipulates the calculation method of CRI. If the result of the calculation of CRI by white LED is inconsistent with the visual result, the document determines that there is such a conflict. The technical report concluded that CIE CRI is not applicable when applying color rendering calculations including white LEDs. The Technical Committee recommends that D1 establish a new set of color rendering indexes. These color rendering indexes do not immediately replace the current CIE color rendering index calculation method. The new color rendering index complements the CIE CRI. Only after the successful combination of the new color rendering index can the calculation method of replace the current CRI be determined. 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 under voting and it will replace CIE 127 publication.