8 Kinds of General LED Lighting Detection Technology
The distribution of LED light sources and traditional light sources in terms of physical size, luminous flux, spectrum, and light intensity are very different. LED detection can not duplicate the traditional light source detection standards and methods. Here are 8 common LED lamp detection techniques:
1. Luminous intensity detection
Light intensity is the intensity of light and is the amount of light emitted within a particular angle. Due to the concentration of light in the LED, the inverse square law does not apply to close distances. The CIE 127 standard specifies the measurement method for measuring light intensity. Measurement conditions A (far field conditions) and measurement conditions B (near field conditions) are two averaging methods. For light intensity conditions, the detector area under both conditions is 1 square centimeter. Normally, the luminescence intensity was measured using standard condition B.
2. Luminous flux and efficacy check
The luminous flux is the sum of the light emitted by the light source, ie the amount of light emitted. There are two main methods of detection:
1) Integral method. The standard lamp and the lamp to be tested are lighted one by one in the integrating sphere, and their readings on the photoelectric converter are recorded as Es and ED, respectively. The standard luminous flux is called Φs, then the measured luminous flux ΦD=ED×Φs/ Es. The integration method uses the “point light source” principle and is simple to operate. However, due to the color temperature deviation between the standard lamp and the lamp under test, the measurement error is large.
2) Spectrophotometry. Luminous flux is calculated from the spectral energy P(λ) distribution. Using a monochromator, the 380 nm to 780 nm spectrum of a standard lamp was measured in an integrating sphere, and then the spectrum of the lamp under test was measured under the same conditions, and the lamp flux was measured and compared with the calculation of the lamp under test.
Light efficiency refers to the ratio of the luminous flux emitted by a light source to the power it consumes. Constant current method is usually used to measure the light efficiency of the LED.
3. Spectral feature detection
The spectral characteristics of LED detection include spectral power distribution, color coordinates, color temperature, and color rendering index.
The spectral energy distribution means that the light of the light source is composed of many different wavelengths of color radiation, and the radiation power of each wavelength also has a different size, and this difference is arranged in the order of the wavelength of the spectral power distribution of the light source. Use a spectrophotometer (monochromator) and a standard lamp to measure the alignment of the light source.
The color coordinate is the light color of the light source in the figure. Color coordinates have a variety of coordinate systems, often using the X and Y coordinate systems.
The color temperature is the amount of light source color appearance (appearance color appearance) seen by human eyes. When the light emitted by the light source is the same color as the light emitted by the absolute black body at a specific temperature, the temperature is the color temperature. In the field of lighting, color temperature is an important parameter that describes the optical characteristics of a light source. Color temperature theory originates from black body radiation and can be obtained from the color coordinates of the black body locus by the color coordinates of the light source.
The color rendering index indicates that the light emitted from the light source correctly reflects the color of the object to be photographed, and is generally expressed by the general color rendering index Ra. Ra is the arithmetic mean of the color rendering index of the light sources of the eight color samples. The color rendering index is an important parameter for the quality of the light source. It determines the application scope of the light source, and improving the color rendering index of the white LED is one of the important tasks for the development of the LED.
4. Light intensity distribution test
The relationship between light intensity and spatial angle (direction) is called pseudo light intensity distribution. 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 photometer is usually used for measurement.
5. The influence of temperature on the optical properties of the LED
Temperature affects the optical properties of the LED. A large number of experiments can explain the effect of temperature on 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 within a unit projected area in that direction. Surface brightness is usually measured using a surface brightness meter and an aiming light meter, and there is an aiming light path and a measuring light path.
Measure other LED lamp performance parameters
1. Measure the electrical parameters of the LED lamp
Electrical parameters mainly include forward and reverse voltages and reverse currents. This is related to the ability of the LED lamp to work properly. This is one of the foundations for determining the basic performance of LED lights. There are two kinds of electrical parameter measurement of LED lamp: When the electric current is constant, test the voltage parameter; If the voltage is fixed, test the electric current parameter. The specific method is as follows:
1) Forward voltage. The forward current is applied to the LED lamp to be detected and a voltage drop occurs on the LED lamp. Adjust the current value to determine the power and record the relevant reading on the DC voltmeter. This is the forward voltage of the LED lamp. According to the related common sense, when the LED is turned on, the resistance is small and the external method using the ammeter is more accurate.
2) Reverse current. Apply reverse voltage to the LED fixture under test and adjust the regulated power supply. The ammeter reading is the reverse current of the LED under test. This is the same as measuring the forward voltage because of the large resistance of the LED during the reverse conduction and using the ammeter connection method.
2. LED lamp thermal characteristics test
The thermal characteristics of the LED have an important influence on the optical and electrical properties of the LED. Thermal resistance and junction temperature are the two main thermal characteristics of LEDs. The thermal resistance refers to the thermal resistance between the PN junction and the surface of the housing, ie, the ratio of the temperature difference on the heat flow channel to the power consumed on the channel. The junction temperature refers to the PN junction temperature of the LED.
Methods for measuring LED junction temperature and thermal resistance generally include: infrared microscopy, spectroscopy, electrical parameter method, thermal resistance scanning method. The surface temperature of the LED chip is measured using an infrared temperature microscope or a miniature thermocouple as the junction temperature of the LED, and the accuracy is not sufficient.
The commonly used electrical parameter method utilizes the forward voltage drop of the LEDPN junction and the PN junction temperature characteristics. The junction temperature of the LED can be obtained by measuring the forward voltage difference at different temperatures.