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Common LED Lighting Detection Technology

Nov 27, 2019

Common LED Lighting Detection Technology


There are great differences between LED light sources and traditional light sources in terms of physical size and luminous flux, spectrum, and spatial distribution of light intensity. LED detection cannot copy the detection standards and methods of traditional light sources. The editor 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 in a specific angle. Due to the concentrated light of the LED, the inverse square law is not applicable at short distances. The CIE127 standard provides two measurement averaging methods for measurement of light intensity: measurement condition A (far field condition) and measurement condition B (near field condition). In the direction of the light intensity, the area of the detector in both conditions is 1 cm2. Normally, the luminous intensity is measured using standard condition B.

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 2 types:

(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 measured light flux ΦD = ED × Φs / Es. The integration method uses the "point light source" principle, which is simple to operate, but affected by the color temperature deviation of the standard lamp and the lamp under test, 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 calculate the luminous flux of the lamp under comparison.

The light effect is the ratio of the luminous flux emitted by the light source to the power it consumes. Usually, the light effect of the LED is measured by a constant current method.

3.Spectral characteristic detection

The detection of LED's spectral characteristics includes spectral power distribution, color coordinates, color temperature, and color rendering index.

Spectral power distribution indicates that the light of the light source is composed of many color wavelengths of different wavelengths, and the radiation power of each wavelength is also different. This difference is called the spectral power distribution of the light source according to the order of the wavelength. Spectrophotometer (monochromator) and standard lamp are used to compare and measure the light source.

The black coordinate is an amount that represents the light emitting color of a light source on a coordinate chart in a digital manner. There are many coordinate systems for the color coordinate graphs. X and Y coordinate systems are usually used.

The color temperature is an amount indicating the color table (appearance color expression) of the light source as seen by the human eye. When the light emitted by the light source is the same color as the light emitted by the absolute 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 is derived from black body radiation, which can be obtained from the color coordinates containing the black body locus through the color coordinates of the light source.

The color rendering index indicates the amount of light reflected by the light source that correctly reflects the color of the object. It is usually expressed by the general color rendering index Ra, where Ra is the arithmetic average of the color rendering index of the eight color samples. The color rendering index is an important parameter of light source quality, it determines the application range of the light source, and 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 false light intensity distribution, and the closed curve formed by this distribution is called the light intensity distribution curve. Because there are many measuring points, and each point is processed by data, it is usually measured by an automatic distribution photometer.

5.The effect of temperature effect on the optical characteristics of LED

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

6. Surface brightness measurement

The brightness of a light source in a certain direction is the luminous intensity of the light source in a unit projected area in that direction. Generally, surface brightness meters and aiming brightness meters are used to measure the surface brightness.

Measurement of other performance parameters of LED lamps

1.Measurement of electrical parameters of LED lamps

Electrical parameters mainly include forward, reverse voltage and reverse current, which are related to whether the LED lamp can work normally. There are two types of electrical parameter measurement of LED lamps: the voltage parameter is tested under a certain current; and the current parameter is tested under a constant voltage. The specific method is as follows:

(1) Forward voltage. Applying a forward current to the LED lamp to be detected will cause a voltage drop across its ends. Adjust the power source with 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 forward, the resistance is small, and the external method of the ammeter is more accurate.

(2) Reverse current. Apply reverse voltage to the tested LED lamps and adjust the regulated power supply. The reading of the ammeter is the reverse current of the tested LED lamps. It is the same as measuring the forward voltage, because the LED has a large resistance when it conducts in the reverse direction.

2, Thermal characteristics test of LED lamps

The thermal characteristics of LEDs have an important impact on the optical and electrical characteristics 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, which is the ratio of the temperature difference along the heat flow channel to the power dissipated on the channel. The junction temperature refers to the temperature of the PN junction of the LED.

The methods of measuring LED junction temperature and thermal resistance are generally: infrared micro-imager method, spectrometry method, electrical parameter method, photothermal resistance scanning method and so on. The temperature of the LED chip was measured as the junction temperature of the LED with an infrared temperature microscope or a miniature thermocouple, and the accuracy was insufficient.

Currently, the electrical parameter method is commonly used to make use of the linear relationship between the forward voltage drop of the LEDPN junction and the temperature of the PN junction, and obtain the junction temperature of the LED by measuring the difference in forward voltage drop at different temperatures.