LED lighting fixtures eight tests you must know


Eight tests on LED lighting fixtures you must know

Eight tests on LED lighting fixtures you must know

Luminous intensity detection

Light intensity is the intensity of LED lighting fixtures, which refers to the amount of light emitted at a certain angle. Because the light of the LED is relatively concentrated, the inverse square law is not applicable in the short-distance situation. The CIE127 standard provides two measurement averaging methods for the measurement of light intensity: measurement condition A (far-field condition) and measurement condition B (near-field condition) Under the condition of light intensity, the detector area of ​​the two conditions is 1cm2. Normally, standard condition B is used to measure the luminous intensity.

Luminous flux and light effect detection

Luminous flux is the total amount of light emitted by the light source, that is, the amount of light emitted. The detection methods mainly include the following

(1) Integral method. Light the standard lamp and the lamp under test in the integrating sphere, and record their readings in the photoelectric converter as Es and ED respectively. The standard luminous flux is known Φs, then the luminous flux of the lamp under test ΦD=ED×Φs/Es. The integral method uses the principle of “point light source” and is simple to operate, but is affected by the deviation of the color temperature of the standard lamp and the lamp under test, and the measurement error is relatively large.

(2) Spectroscopy. The luminous flux is calculated by the spectral energy P(λ) distribution. Use a monochromator to measure the 380nm~780nm spectrum of the standard lamp in the integrating sphere, and 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.

Spectral characteristics detection

The detection of the spectral characteristics of LEDs includes content such as spectral power distribution, color coordinates, color temperature, and color rendering index.

The spectral power distribution means that the light of the light source is composed of many different wavelengths of colored radiation, and the radiant power of each wavelength is also different. This kind of difference is called the spectral power distribution of the light source when arranged in the order of the wavelength. Use a spectrophotometer (monochromator) and standard lamp to compare and measure the light source.

The color coordinate is the amount of the luminous color of the light source that is represented on the coordinate graph in a digital way. There are multiple coordinate systems for the color coordinate graph, and X and Y coordinate systems are usually used.

The color temperature represents the amount of the light source color table (appearance color expression) seen by the human eye. When the light emitted by the light source has the same color as the light emitted by an absolutely black body at a certain temperature, the temperature is the color temperature. In the field of lighting, color temperature is an important parameter describing the optical characteristics of a light source. The related theory of color temperature originates from black body radiation, which can be obtained from the color coordinates containing the black body locus by 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 object. It is usually expressed by the general color rendering index Ra, which is the arithmetic average of the color rendering index of the light source for 8 color samples. The color rendering index is an important parameter of the quality of the light source. It determines the application range of the light source. Improving the color rendering index of white light LEDs is one of the important tasks of LED research and development.

Light intensity distribution test

The relationship between the light intensity changing with the spatial angle (direction) is called the false light intensity distribution, and the closed curve formed by this distribution is called the light intensity distribution curve. Since there are many measuring points and each point is processed by data, an automatic goniophotometer is usually used for measurement.

The influence of temperature effect on the optical characteristics of LED

The temperature will affect the optical characteristics of the LED. A large number of experiments can show that temperature affects the LED emission spectrum and color coordinates.

Surface brightness measurement

The brightness of a light source in a certain direction is the luminous intensity of the light source per unit projected area in that direction. Generally, a surface luminance meter and an aiming luminance meter are used to measure the surface luminance. There are two parts: aiming light path and measuring the light path.

Measurement of electrical parameters of LED lighting fixtures

Electrical parameters mainly include forward and reverse voltages and reverse currents, which are related to whether the LED lamps can work normally, and are one of the bases for judging the basic performance of LED lamps. There are two types of electrical parameter measurements for LED lamps: test voltage parameters when the current is constant; test current parameters when the voltage is constant. The specific method is as follows:

(1) Forward voltage.

When a forward current is applied to the LED lamp to be tested, a voltage drop will occur at both ends. Adjust the power supply with the determined current value and record the relevant reading on the DC voltmeter, which is the forward voltage of the LED lamp. According to related common sense, when the LED is forward-conducting, the resistance is small, and it is more accurate to use the ammeter external method.

(2) Reverse current.

Apply a reverse voltage to the tested LED lamp, adjust the stabilized power supply, and the reading of the ammeter is the reverse current of the tested LED lamp. It is the same as measuring the forward voltage, because the resistance of the LED is large when the LED is reversed, so the internal connection method of the ammeter is adopted.

Thermal characteristics test of LED lamps

The thermal characteristics of the LED have an important influence on the optical and electrical characteristics of the LED. Thermal resistance and junction temperature are the main thermal characteristics of LED. Thermal resistance refers to the thermal resistance between the PN junction and the shell surface, 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 for measuring the junction temperature and thermal resistance of LEDs generally include the infrared micro-imaging method, spectroscopy, electrical parameter method, and photothermal resistance scanning method. 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, the accuracy is not enough.

The current commonly used electrical parameter method uses the characteristic that the forward voltage drop of the LED PN junction has a linear relationship with the temperature of the PN junction, and the junction temperature of the LED is obtained by measuring the forward voltage drop difference at different temperatures.

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