LED light source and traditional light source have great differences in physical size and spatial distribution of light flux, spectrum and light intensity. LED detection cannot copy the detection standards and methods of traditional light sources. Xiaobian below introduces the detection technology of common LED lamps.
Detection of optical parameters of LED lamps
1, luminous intensity detection
Light intensity, the intensity of light, refers to the amount of light emitted at a particular angle. Due to the concentrated light of the LED, the inverse square law is not applicable in the close range. The CIE127 standard specifies two measurement averaging methods: measurement condition A (far field condition) and measurement condition B (near field condition) for measurement of light intensity. In the case of light intensity, the detector area of â€‹â€‹both conditions is 1 cm 2 . Normally, the luminescence intensity is measured using standard condition B.
2, luminous flux and light efficiency detection
The luminous flux is the sum of the amount of light emitted by the light source, that is, the amount of luminescence. The detection methods mainly include the following two types:
(1) Integration method. The standard lamp and the lamp under test are sequentially ignited in the integrating sphere, and their readings in the photoelectric converter are recorded as Es and ED, respectively. The standard light flux is known as Î¦s, then the luminous flux of the lamp under test is Î¦D=ED&TImes; Î¦s/Es. The integral method utilizes the principle of "point source", and the operation is simple, but it is affected by the deviation of the color temperature of the standard lamp and the lamp under test, and the measurement error is large.
(2) Spectroscopic method. The luminous flux is calculated from the spectral energy P(Î») distribution. Using a monochromator, the spectrum of the standard lamp from 380 nm to 780 nm is measured in the integrating sphere, and then the spectrum of the lamp under test is measured under the same conditions, and the luminous flux of the lamp to be measured is calculated and compared.
The luminous efficiency is the ratio of the luminous flux emitted by the light source to the power consumed by it, and the luminous efficacy of the LED is usually measured by a constant current method.
3. Spectral characteristics detection
The spectral characteristic detection of the LED includes spectral power distribution, color coordinates, color temperature, color rendering index and the like.
The spectral power distribution indicates that the light of the light source is composed of many different wavelengths of color radiation, and the radiation power of each wavelength is also different. This difference is sequentially arranged with the wavelength, which is called the spectral power distribution of the light source. The light source is obtained by comparison measurement using a spectrophotometer (monochromator) and a standard lamp.
The color coordinate is a digital representation of the amount of illuminating color of the light source on the graph. The coordinate graph representing the color has multiple coordinate systems, usually in the X and Y coordinate systems.
The color temperature is the amount of the light source color table (appearance color appearance) that the human eye sees. When the light emitted by the light source is the same as the color of the light emitted by the absolute black body at a certain temperature, the temperature is the color temperature. In the field of illumination, color temperature is an important parameter describing the optical properties of a light source. The theory of color temperature is derived from blackbody radiation, which can be obtained from the color coordinates of the blackbody locus by the color coordinates of the source.
The color rendering index indicates the amount by which the light emitted by the light source correctly reflects the color of the object, which is usually expressed by the general color rendering index Ra, which is the arithmetic mean of the color rendering index of the eight color samples. Color rendering index is an important parameter of light source quality, which determines the application range of light source. Improving the color rendering index of white LED is one of the important tasks of LED research and development.
4, light intensity distribution test
The relationship between the light intensity and the spatial angle (direction) is called the pseudo-light intensity distribution, and the closed curve formed by such a distribution is called the light intensity distribution curve. Since there are many measuring points and each point is processed by data, it is usually measured by an automatic distribution photometer.
5. Effect of temperature effect on optical characteristics of LED
Temperature affects the optical properties of the LED. A large number of experiments can show that the temperature affects the LED emission spectrum and color coordinates.
6, surface brightness measurement
The brightness of the light source in a certain direction is the luminous intensity of the light source in the projected area of â€‹â€‹the light source. Generally, the surface brightness meter and the aiming brightness meter are used to measure the surface brightness, and there are two parts of the aiming light path and the measuring light path.
Measurement of other performance parameters of LED lamps
1. Measurement of electrical parameters of LED lamps
The electrical parameters mainly include forward and reverse voltages and reverse currents. It is related to whether LED lamps can work normally. It is one of the basis for judging the basic performance of LED lamps. There are two kinds of electrical parameter measurement of LED lamps: that is, when the current is constant, the test voltage parameter; when the voltage is constant, the current parameter is tested. The specific method is as follows:
(1) Forward voltage. A forward current is applied to the LED lamp to be detected, and a voltage drop is generated across the two ends. Adjust the current value to determine the power supply, record the relevant reading on the DC voltmeter, which is the forward voltage of the LED luminaire. According to the common sense, when the LED is conducting in a forward direction, the resistance is small, and the external connection method using the ammeter is relatively accurate.
(2) Reverse current. Apply a reverse voltage to the LED luminaire being tested, adjust the regulated power supply, and the current meter reading is the reverse current of the LED illuminator under test. The same as measuring the forward voltage, because the resistance of the LED is reversed when the reverse conduction is large, the current meter is connected internally.
2, LED lamp thermal characteristics test
The thermal characteristics of LEDs have an important influence on the optical and electrical properties of LEDs. Thermal resistance and junction temperature are the main thermal characteristics of LED2. Thermal resistance refers to the thermal resistance between the PN junction and the surface of the housing, that is, the ratio of the temperature difference along the heat flow path to the power dissipated on the channel. The junction temperature refers to the temperature of the PN junction of the LED.
Methods for measuring LED junction temperature and thermal resistance generally include: infrared micro-imager method, spectroscopy method, electrical parameter method, photothermal resistance scanning method, and the like. The surface temperature of the LED chip is measured by an infrared temperature measuring microscope or a miniature thermocouple as the junction temperature of the LED, and the accuracy is insufficient.
The commonly used electrical parameter method is to use the characteristic that the forward voltage drop of the LEDPN junction is linear with the PN junction temperature, and the junction temperature of the LED is obtained by measuring the difference of the forward voltage drop at different temperatures.
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