1. Luminous intensity detection
Light intensity is the intensity of light, which refers to the amount of light emitted at a certain angle. Because the light of the LED is concentrated, the inverse square law does not apply at close distances. The CIE127 standard stipulates two measurement averaging methods for the measurement of light intensity: measurement condition A (far field condition) and measurement condition B (near field condition). For the condition of light intensity, the detector area for both conditions is 1cm2. Typically, standard condition B is used to measure luminous intensity.
2. Luminous flux and light effect detection
Luminous flux is the sum of the amount of light emitted by the light source, that is, the amount of light emitted. The detection methods mainly include the following two:
(1) Integral method. Light the standard lamp and the lamp under test in turn in the integrating sphere, and record their readings in the photoelectric converter as Es and ED, respectively. The standard light flux is known Φs, then the luminous flux of the tested lamp is ΦD=ED×Φs/Es. The integration method utilizes the principle of "point light source" and is easy to operate, but is affected by the color temperature deviation between the standard lamp and the lamp under test, and the measurement error is large.
(2) Spectroscopy. The luminous flux is calculated from the spectral energy P(λ) distribution. Using a monochromator, measure the 380nm-780nm spectrum of the standard lamp in the integrating sphere, then measure the spectrum of the lamp under test under the same conditions, and compare and calculate the luminous flux of the lamp under test.
The luminous efficiency is the ratio of the luminous flux emitted by the light source to the power consumed, and the luminous efficiency of the LED is usually measured by a constant current method.
3. Spectral characteristic detection
The spectral characteristics detection of LED clean lamps includes spectral power distribution, color coordinates, color temperature, color rendering index, etc.
The spectral power distribution indicates that the light of the light source is composed of color radiation of many different wavelengths, and the radiation power of each wavelength is also different. The light source was measured by comparison with a spectrophotometer (monochromator) and a standard lamp.
Color coordinates are the quantities that numerically represent the color of light emitted by a light source on a coordinate graph. There are various coordinate systems for coordinate graphs representing colors, usually X and Y coordinate systems are used.
Color temperature is the amount that expresses the color table (appearance color representation) of the light source seen by the human eye. When the light emitted by the light source is the same color as the light emitted by an absolute black body at a certain temperature, that temperature is the color temperature. In the field of lighting, color temperature is an important parameter to describe the optical properties of light sources. The related theory of color temperature is derived from black body radiation, which can be obtained from the color coordinates of the black body locus, which is contained in the color coordinates of the light source.
The color rendering index indicates the amount of light emitted by the light source that correctly reflects the color of the illuminated object. It is usually expressed by the general color rendering index Ra, which is the arithmetic mean of the color rendering index of the light source to 8 color samples. The color rendering index is an important parameter of the quality of the light source, which determines the application range of the light source. Improving the color rendering index of white LEDs is one of the important tasks of LED research and development.
4. Light intensity distribution test
The relationship between light intensity and spatial angle (direction) is called false light intensity distribution, and the closed curve formed by this distribution is called light intensity distribution curve. Because there are many measuring points, and each point is processed by data, an automatic goniophotometer is usually used for measurement.
5. The influence of temperature effect on the optical characteristics of LED purification light
Temperature affects the optical properties of LEDs. 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 unit projection area of the direction. Generally, the surface brightness meter and the aiming brightness meter are used to measure the surface brightness. There are two parts: the aiming optical path and the measuring optical path.
Measurement of other performance parameters of LED lamps
1. Measurement of electrical parameters of LED clean lamps
Electrical parameters mainly include forward, reverse voltage and reverse current, which are related to whether LED lamps can work normally and are one of the basis for judging the basic performance of LED lamps. There are two types of electrical parameters measurement for LED lamps: that is, when the current is constant, the voltage parameters are tested; when the voltage is constant, the current parameters are 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 occurs across it. Adjust the power supply determined by the current value, and record the relevant reading on the DC voltmeter, which is the forward voltage of the LED lamp. According to relevant common sense, when the LED is conducting in the forward direction, the resistance is small, and it is more accurate to use the external method of the ammeter.
(2) Reverse current. Apply a reverse voltage to the LED lamp to be tested, adjust the regulated power supply, and the reading of the ammeter is the reverse current of the LED lamp to be tested. The same is true for measuring the forward voltage, because the resistance of the LED is relatively large when the LED is reversed, so the internal connection method of the ammeter is used.
2. Thermal characteristics test of LED lamps
The thermal properties of LEDs have an important impact 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 case, that is, the ratio of the temperature difference along the heat flow channel to the power dissipated on the channel, and the junction temperature refers to the temperature of the PN junction of the LED.
The methods of measuring LED junction temperature and thermal resistance generally include: infrared micrograph method, spectroscopy method, electrical parameter method, photothermal resistance scanning method, etc. Using an infrared temperature measuring microscope or a micro thermocouple to measure the surface temperature of the LED chip as the junction temperature of the LED is not accurate enough.
At present, the commonly used electrical parameter method is to use the characteristic that the forward voltage drop of the LED PN junction has a linear relationship with the PN junction temperature, and obtain the junction temperature of the LED by measuring the forward voltage drop difference at different temperatures.
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