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.

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