I. Introduction
Pulsed LED can emit very strong light in a very short time, and it has a wide range of applications in landscape lighting and other aspects. Its luminous intensity, time characteristics, and date spectrum characteristics are important characteristics of pulsed LEDs. The purpose of this work is to develop a simple and applicable test system for pulse LED manufacturers, which has a relatively high cost performance and can simultaneously test the time and spectral characteristics of the luminous intensity of pulse LEDs, and is used for product development and inspection.
Regarding the measurement method of the color of the illumination light source, there are already national standards. In addition, some national standards and industry standards have been established that involve specific types of photoelectric parameter measurement methods such as high-pressure sodium bulbs, fluorescent high-pressure mercury bulbs, and tube-shaped dysprosium lamps. The project work was carried out under the guidance of the above standards.
The whole device is composed of integrating sphere, light source and its special power supply, time characteristic test system, spectrum characteristic test system and overall control system.
The light source to be tested is white light LED and monochromatic LED produced by many domestic manufacturers. We have made a self-made pulse power supply and adopted the numerical control voltage regulation method, so that the working voltage is continuously adjustable every 0.1V, and the adjustment range is 0.1- 5V, the ignition ignition voltage is adjustable in order to adapt to different specifications of flash tube. In the way of lighting the lamp, this system adopts two methods of external circuit trigger and manual trigger, and uses photoelectric isolation to isolate the external trigger circuit from the light source circuit to avoid interference. The spectrometer generates trigger pulses to synchronize and The spectrometer completes the pulsed light measurement.
Second, the integrating sphere
The integrating sphere used in this device is manufactured by Dandong Wanguan Testing Equipment Factory according to relevant standards. The inner diameter of the integrating sphere is 20 cm, and the whole is welded into two hemispheres in pieces. The movable hemisphere is connected to the fixed hemisphere with a hinge edge. The fixed hemisphere is fixed on the base of the bracket and is used to install the lamp and screen bracket ( Or suspension wire) and measuring hole.
The inner surface of the sphere and the surface of the accessories (such as screens) are coated with a white coating of barium sulfate. During the coating process, an aqueous solution of barium sulfate mixed with a small amount of polyvinyl alcohol was sprayed uniformly on the inner surface of the integrating sphere several times.
3. Standard light source and its power supply
The standard light source is composed of a calibrated halogen bulb and its supporting power supply. Used to calibrate the spectrometer system. Tungsten halogen bulbs are calibrated by the Optical Division of the Chinese Academy of Metrology, and give the operating voltage (or current) when their color temperature is around 2855.6K. The matching power supply adopts a voltage stabilizing (or constant current) circuit to provide a stable working voltage (or current) of the bulb, and the power supply stability reaches 0.1%. The standard light source used in this system adopts numerical control soft start and soft turn off technology, so that the light bulb is gradually ignited to reach the operating voltage (or current) under the program control, and when it is extinguished, the lamp voltage (or current) is gradually reduced until it is completely Go out.
4. Time characteristic test system
The time characteristic test system of the light intensity jujube is mainly used to measure the time characteristic curve of the luminous intensity of the LED. This is an important parameter for pulsed light sources and their corresponding power supplies. The entire system measures the optical signal of the LED after passing through the integrating sphere. This system is mainly composed of the following parts:
Photodetector and its amplifier: PIN photodiode using silicon material, the main indicators: the area of ​​the photosensitive part is 0.5 square millimeters, the pulse response time is less than 1ns, mainly used for ultrafast light measurement. The amplifier uses a broadband amplifier with FET input, and uses a current amplifying circuit to amplify the photocurrent of the photodiode so that it reaches the voltage required by the data acquisition card.
Filter: Use QB21 color glass filter to filter the infrared part of the LED, leaving only the visible part needed for measurement.
Data acquisition card: digitally store the voltage signal measured by the photoelectric detector in the computer and display it. This system uses the NI NI company data acquisition card, the minimum data collection interval is 50ns, and has the characteristics of programmable trigger voltage adjustment.
Software system: LABVIEW of American NI company is used to complete the collection and calculation of pulse optical signal.
The main parameters calculated by this software are: the time interval from when the optical signal rises to about 1/3 of the maximum optical signal to when the optical signal falls to about 1/3 of the maximum optical signal is taken as the flash time, and the integrated value of the flash time characteristic curve is taken as the total luminous flux .
It should be noted that the design idea of ​​this test system is to realize the two main characteristic indexes of the time characteristic and the spectral characteristic of the rapid test of the luminous intensity of the pulsed LED at the same time. It is easy to switch when the test is not completed, and the two characteristic test systems are unified. The integrating sphere is used to eliminate the spatial anisotropy of the LED spectral characteristics. The measurement accuracy of the LED's time characteristics is mainly determined by the time response characteristics of the photodetector and its amplifier and data acquisition card, but the integrating sphere has no substance. Sexual influence.
V. Spectral characteristic test system
The light color characteristics and characterization quantities of the lighting source, such as color coordinates, color temperature and color rendering index, are determined by the spectral energy distribution of the lighting source. The color measurement in this work is based on spectrophotometry. Specific equipment includes: optical fiber, flat-field grating spectrometer, linear CCD and its driving circuit, electrical signal processing system, and spectral software.
1. Measure the relative spectral energy distribution of the light source
When measuring the spectral energy distribution with an integrating sphere, it needs to be compared with the spectral energy standard light source, and the power supply of the standard light source is powered by a stable power supply. The relationship between the spectral radiation intensity of the light source and the wavelength is called the spectral energy distribution of the light source. The relationship between the relative value of the spectral radiation intensity and the wavelength is called the relative spectral energy distribution of the light source.
The relative spectral energy distribution of the measurement light source is compared with the standard lamp. The spectral radiation intensity of a standard lamp is known and can be expressed as Ss (λ). First put the standard light source into the integrating sphere to obtain the spectral response curve Rs (λ). Then change the light source to be measured into the integrating sphere to obtain the spectral response curve Rt (λ). The relative spectral energy distribution of the light source to be measured is:
Where: Ss (λ) is the relative spectral power distribution of the standard light source, Rt (λ) is the reading of the photodetector of the light source to be measured, and Rs (λ) is the reading of the standard light source photodetector.
In order to adapt to the spectral distribution measurement of LED, the time integration multi-channel photoelectric measurement method is adopted. The spectrometer is designed as a cross-C structure, the optical fiber couples the optical signal into the spectrometer, and the linear array CCD photoelectric receiver converts the spectral optical signal into an electrical signal. This system can also be used to measure the spectral characteristics of a stable light source.
2. Calculation of light source chromaticity coordinates
This work uses the measurement results of the spectrophotometric color measurement method to calculate the chromaticity coordinates of the light source. Because we only need the chromaticity coordinates, we only need to measure the relative spectral energy (power) distribution of the cursor. We only need to calculate the color matching function in the CIE1931XYZ chromaticity system or the tristimulus values ​​X, Y called the isoenergy spectrum. The relative value of Z can be obtained. Finally, the chromaticity coordinates x, y in the CIE1931XYZ chromaticity system or the color coordinates u, v in the CIE1960 "UCS" uniform color representation system are obtained.
3. Calculation of TCP related light source color temperature
When the u, v color coordinate values ​​of the light source are calculated, the black body locus and isotherm on the CIE1960 "UCS" diagram (u, v color coordinate diagram) can be used to find the point on the black body locus closest to the color coordinate of the light source to be measured, Take the u, v color coordinate points of the light source to be measured as the center of the circle, and the isotherm that intersects it as the radius to make a circle. By calculating the coordinates of the tangent point through a certain calculation program, the relative color temperature of the light source to be measured is known.
4. Measurement and calculation of the color rendering index of the light source
Color rendering is one of the important characteristic parameters of the light source. The evaluation of the color rendering of the light source is generally based on the quantitative evaluation of the color rendering of the light source based on the total chromatic displacement of the test color sample under the illumination of the reference light source and the light source to be tested. In addition to the spectral energy distribution of the light source and the reflectance of the object, the color perception of the illuminated object is also related to the adaptation state of the human eye, which is called the color adaptation effect of the human eye (permanent phenomenon of color vision).
In order to reduce the influence of the color adaptation effect, when evaluating the color rendering of any light source, a reference light source (reference light source) that is close to or even equal to the color temperature of the light source to be measured is selected. The reference light source does not use a specific light source, but uses a reference illuminant. For light sources with a color temperature below 5000K, use a black body as the reference light source; for light sources with a color temperature above 5000K, use CIE synthetic daylight as the reference light source. Even if the color temperature is the same, there is still a certain chromaticity difference between the reference light source and the side light source (should be less than 5.4 × 10-3).
Once the color temperature of the reference light source is determined, the spectral power distribution of the reference light source can be calculated.
5. Calculation of the average color rendering index Ra
Six, master control system
The overall control system of the instrument is served by a computer. The computer uses the spectrum measurement software to control the spectrometer to collect the spectrum signal through the serial port and set the flash trigger time at the same time. At the same time, the computer can set the trigger level of the flash signal of the data acquisition card through the time characteristic measurement software. The LED power supply is triggered by the spectrometer, and the spectrum signal is collected at the same time while the flash, the light emission time characteristic curve is collected by the data acquisition card, and the two measurements of the spectrum characteristic and the time characteristic curve are completed simultaneously. Then the corresponding software calculates the parameters to be measured. After setting the instrument test parameters using the corresponding software, no computer intervention is required during the test, and all tests are coordinated by various subsystems.
7. Performance and indicators of the instrument system
1. Light intensity-time characteristic system
Time resolution (sampling time interval): 50ns
Light intensity dynamic range: 46dB
Light measurement band: visible light (400-700nm)
Test parameters: flash time, luminous flux, flash rise time, flash fall time
2. Spectral system
Spectral resolution: better than 2nm (50um fiber)
Spectral intensity dynamic range: 300 (Pk-Pk)
Spectral range: 380-780nm
Measurement time: 5ms-2s
Test parameters: spectral distribution, CIE1931 color coordinate, CIE1960 color coordinate, correlated color temperature, color rendering index
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