LED light sources and traditional light sources are very different in terms of physical size, luminous flux, spectrum, and spatial distribution of light intensity. LED flashlight detection cannot copy the detection standards and methods of traditional light sources. Here are eight detection techniques for common LED flashlights.
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 relatively concentrated, the inverse square law is not applicable at close distances. The CIE127 standard stipulates that the measurement of light intensity proposes two measurement averaging methods: 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 2 types:
(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", which is simple to operate but is affected by the deviation of the color temperature between the standard lamp and the lamp under test, and the measurement error is relatively large.
The luminous flux is calculated by 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.
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 color 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 order of 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 coordinate graphs that represent colors, 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 is 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 is derived 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 illuminated 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 the white light LED 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 flashlight
Electrical parameters mainly include forward, reverse voltage and reverse current, which are related to whether the LED flashlight can work normally, and are one of the bases for judging the basic performance of the LED flashlight. There are two types of electrical parameter measurements for LED flashlights: 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 determined by the current value and record the relevant readings on the DC voltmeter, which is the forward voltage of the LED flashlight. According to the relevant 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 reverse voltage to the tested LED flashlight, adjust the regulated power supply, and the reading of the ammeter is the reverse current of the tested LED flashlight. It is the same as measuring the forward voltage because the resistance of the LED is large when the reverse conduction is conducted, the internal connection method of the ammeter is adopted.
LED flashlight thermal characteristics test
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 two main thermal characteristics of LEDs. 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 an infrared micro-imaging method, spectroscopy, electrical parameter method, and photothermal resistance scanning method. The surface temperature of the LED chip measured by an infrared temperature microscope or a miniature thermocouple is used as the junction temperature of the LED, which is not accurate enough.
At present, the commonly used electrical parameter method uses the characteristic that the forward voltage drop of the LEDPN junction has a linear relationship with the PN junction temperature, and the junction temperature of the LED is obtained by measuring the forward voltage drop difference at different temperatures.
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