LED light sources and traditional light sources are very different in size and spatial distribution of luminous flux, spectrum, and light intensity. LED test cannot use the traditional light source detection standards and methods.
Below we will introduce you to the testing technology of common LED lamps.
Testing of optical parameters of LED lamps
1、Luminous intensity test
The intensity of light is the amount of light emitted at a particular angle. Because of the LED light is more concentrated, in the case of close proximity is not applicable to the square inverse ratio law, Standard CIE127 for the measurement of light intensity proposed by the measurement conditions A (far-field conditions), measurement conditions B (near-field conditions) two measurement of the average normal light intensity conditions, two conditions of the detector area is 1cm2.
Typically, the luminous intensity is measured using standard condition B.
2.Luminous flux and light effect testing
Luminous flux is the sum of the amount of light emitted by the light source, i.e. luminous flux. There are two main test methods.
The standard light flux is Φs, then the luminous flux of the measured lamp ΦD = ED×Φs/Es. The integration method uses the principle of “point light source”, the operation is simple, but due to the color temperature deviation of the standard lamp and the measured lamp, the measurement error is larger.
The luminous flux is calculated from the spectral energy P(λ) distribution. Using a monochromator the spectra of the standard lamp are measured in the integral sphere from 380nm to 780nm, and then the spectra of the lamp are measured under the same conditions, and the luminous flux of the lamp under test is compared and calculated.
The luminous efficacy is the luminous flux emitted by the light source and the ratio of its power consumption, usually using a constant current method to measure the luminous efficacy of LEDs.
3、Spectral characteristic testing
The spectral characteristics of LEDs include spectral power distribution, color coordinates, color temperature, color rendering index and other contents.
The spectral power distribution indicates that the light source is composed of many different wavelengths of color radiation, the radiation power of each wavelength is also different, such differences in the order of arrangement with the wavelength is called the spectral power distribution of the source. Comparative measurements of the light source were obtained using a spectrophotometer and a standard lamp.
A color coordinate is a numerical representation of the amount of light emitting color of a light source on a coordinate map. There are various coordinate systems for color, usually X and Y coordinate systems.
Color temperature is the amount of the light source’s color chart (the appearance of color) that the human eye sees. When the light emitted by a light source is the same color as the light emitted by an absolute blackbody 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 a light source. The theory of color temperature correlation derives from blackbody radiation and can be obtained from the color coordinates of a light source containing a blackbody track.
The color rendering index(CRI) indicates the amount of light emitted by the light source that correctly reflects the color of the subject, and is usually expressed in terms of the general color rendering index Ra, which is the arithmetic average of the light source for eight color samples. The CRI is an important parameter of the quality of the light source, which determines the scope of application of the 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 of light intensity with spatial angle (direction) is called light intensity distribution, and the closed curve formed by this distribution is called light intensity distribution curve. Due to the large number of measurement points and the fact that each point is processed with data, measurements are usually made using an automatic distribution photometer.
5、The effect of temperature effect on the optical properties of LED
Temperature affects the optical properties of the LED. A large number of experiments can illustrate 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 per unit projected area in that direction. Generally, a surface brightness meter and an aiming brightness meter are used to measure the surface brightness. There are two parts: an aiming optical path and a measuring optical path.
Measurement of other performance parameters of LED luminaires
1、Measurement of electrical parameters of LED lamps
Electrical parameters mainly include forward, reverse voltage and reverse current, which is about whether the LED lamps can work normally, and is one of the basis for judging the basic performance of LED lamps. There are two types of electrical parameter measurement for LED lamps:
voltage parameters are tested under a certain current;
and current parameters are tested under a certain voltage.
The specific method is as follows:
(1)Forward voltage.When a forward current is applied to the LED lamp to be detected, a voltage drop will occur at both ends. 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 related common sense, when the LED is conducting forward, the resistance is small, and it is more accurate to use the ammeter external connection method.
(2)Reverse current.Apply reverse voltage to the tested LED luminaire, adjust the regulator power supply, the ammeter reading is the measured LED luminaire reverse current. The same as the measurement of forward voltage, because of the large resistance when the LED reverse guide pass, using the ammeter internal connection method.
2、LED lamp thermal properties test
The thermal properties of LEDs have an important influence on the optical and electrical properties of LEDs. Thermal resistance and junction temperature are the two main thermal properties of LED. Thermal resistance refers to the thermal resistance between the PN junction to the surface of the shell, that is, the temperature difference along the heat flow channel and the ratio of power dissipated on the channel, junction temperature refers to the temperature of the PN junction of LED.
The methods of measuring LED junction temperature and thermal resistance are incl.: infrared micro-imager method, spectroscopy method, electrical parameter method, photothermal resistance scanning method, etc.
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, and the accuracy is not enough.
At present, the commonly used electrical parameter method is to use 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 difference in forward voltage drop at different temperatures.