Pulse LED luminous intensity time characteristics - spectral characteristics test device

1. Introduction
Pulse LED can emit strong light in a very short time and has a wide range of applications in landscape lighting. Its luminous intensity, time characteristics and spectral characteristics are important characteristic indicators of pulse LED. This work aims to develop a simple and applicable test system for pulse LED manufacturers, which has a relatively high performance-price ratio and can simultaneously test the time characteristics and spectral characteristics of pulse LED luminous intensity, for product development and inspection.
There are national standards for the measurement method of the color of lighting sources. In addition, some national standards and industry standards have been established for the measurement methods of photoelectric parameters of specific types such as high-pressure sodium bulbs, fluorescent high-pressure mercury bulbs and tubular dysprosium lamps. This project work is carried out under the guidance of the above standards. The
entire device consists of an integrating sphere, a light source and its dedicated power supply, a time characteristic test system, a spectral characteristic test system and a general control system.
The light source to be tested is a white light LED and a monochrome LED produced by multiple domestic manufacturers. We have made a matching pulse power supply by ourselves, and adopted a digital control voltage regulation method to make the working voltage continuously adjustable by 0.1V, with an adjustment range of 0.1-5V, and the trigger ignition voltage adjustable in steps to adapt to flash tubes of different specifications. In terms of the way the lamp is lit, this system uses two methods: external circuit triggering and manual triggering. It also uses photoelectric isolation to isolate the external triggering circuit from the light source circuit to avoid interference. The spectrometer generates a trigger pulse for synchronization, and the spectrometer completes the pulse light measurement at the same time.
2. 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. The whole is welded into two hemispheres in blocks. The movable hemisphere is connected to the fixed hemisphere with a hinge. The fixed hemisphere is fixed to the bracket base and is used to install the bracket (or hanging wire) and measurement hole of the lamp and screen.
A layer of white barium sulfate paint is coated on the inner surface of the sphere and the surface of the accessories (such as the screen, etc.). During the coating process, a barium sulfate aqueous solution mixed with a small amount of polyvinyl alcohol is evenly sprayed on the inner surface of the integrating sphere several times.
3. Standard light source and its power supply
The standard light source consists of a calibrated halogen tungsten filament bulb and its matching power supply. It is used to calibrate the spectrometer system. The halogen tungsten filament bulb is calibrated by the Optics Department of the China Institute of Metrology, and its working voltage (or current) when the color temperature is around 2855.6K is given. The matching power supply adopts a voltage stabilization (or constant current) circuit to provide a stable working voltage (or current) for the bulb, and the power supply stability reaches 0.1%. The standard light source used in this system adopts digital soft start and soft shutdown technology, so that the bulb is gradually ignited under program control to reach the working voltage (or current), and when it is extinguished, the lamp voltage (or current) is gradually reduced until it is completely extinguished.
4. Time characteristic test system
The light intensity-time characteristic test system is mainly used to measure the time characteristic curve of the luminous intensity of the LED. This is an important parameter of the pulse light source and its corresponding power supply. The whole system measures the light signal of the LED after passing through the integrating sphere. This system mainly consists of the following parts:
Photodetector and its amplifier: PIN photodiode using silicon material, the main indicators: the photosensitive area is 0.5 square millimeters, the pulse response time is less than 1ns, and it is mainly used for ultrafast light measurement. The amplifier uses a broadband amplifier with FET input and a current amplifier circuit to amplify the photocurrent of the photodiode to reach the voltage required by the data acquisition card.
Filter: Use QB21 color glass filter to filter out the infrared light of LED, and only keep the visible light required for measurement.
Data acquisition card: The voltage signal measured by the photodetector is digitized and stored in the computer and displayed. This system uses the data acquisition card of NI Company of the United States, with a minimum data acquisition interval of 50ns, and has the characteristics of programmable adjustment of trigger voltage.
Software system: LABVIEW of NI Company of the United States is used to complete the acquisition and calculation of pulse light signals.
The main parameters calculated by this software are: the time interval from the light signal rising to about 1/3 of the maximum light signal to the light signal falling to about 1/3 of the maximum light signal as the flash time, and the integral value of the flash time characteristic curve as the total luminous flux.
It should be noted that the design idea of ​​this test system is to realize the simultaneous rapid testing of the two main characteristic indicators of the time characteristic and spectral characteristic of the pulse LED luminous intensity. In order to facilitate switching during testing, the test systems of the two characteristics are unified together. The integrating sphere is used to eliminate the spatial anisotropy of the LED spectral characteristics. The measurement accuracy of the LED time characteristics is mainly determined by the time response characteristics of the photodetector, its amplifier and the data acquisition card, and the integrating sphere has no substantial effect on this.
5. Spectral characteristics test system
The light color characteristics of the lighting source and its characterization quantities 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 spectral photometry. Specific equipment includes: optical fiber, flat field grating spectrometer, linear array CCD and its driving circuit, electrical signal processing system and spectral software.
1. Measuring the relative spectral energy distribution of the light source
When measuring the spectral energy distribution with an integrating sphere, it is necessary to compare it with the spectral energy standard light source, and the power supply of the standard light source is 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 light source is measured by comparing it with the standard lamp. The spectral radiation intensity of the 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 replace 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 photoelectric detector reading of the light source to be measured, and Rs(λ) is the photoelectric detector reading of the standard light source.
In order to adapt to the spectral distribution measurement of LEDs, a time-integrated multi-channel photoelectric measurement method is used. 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 steady light source.
2. Calculation of the chromaticity coordinates of the light source
This work uses the measurement results of the spectral photometric colorimetry method to calculate the chromaticity coordinates of the light source. Because we only need to find 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 colorimetry system or the relative values ​​of the three stimulus values ​​X, Y, and Z called the equal energy spectrum, and finally find the chromaticity coordinates x, y in the CIE1931XYZ colorimetry system or the color coordinates u, v in the CIE1960 "UCS" uniform color system.
3. Calculation of the correlated color temperature TCP
of the light source After calculating the u and v color coordinate values ​​of the light source, the black body locus and isotherms on the CIE1960 "UCS" diagram (u, v color coordinate diagram) can be used to find the point on the black body locus that is closest to the color coordinates of the light source to be measured. A circle is made with the u and v color coordinate points of the light source to be measured as the center and the isotherms that intersect with them as the radius. It must be tangent to the black body locus curve at one point, that is, the point where the isotherms intersect the black body locus curve vertically. The coordinates of the tangent point are calculated through a certain calculation procedure, and the correlated color temperature of the light source to be measured is obtained.
4. Measurement and calculation of the color rendering index of the light source
Color rendering is one of the important characteristic parameters of light sources. The evaluation of the color rendering of a light source is generally based on the total chromaticity shift of the test color sample under the reference light source and the light source to be tested. The color perception of the illuminated object is not only related to the spectral energy distribution of the light source and the reflectivity of the object, but also to the adaptation state of the human eye, which is called the color adaptation effect of the human eye (color constancy).
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) with a color temperature close to or even equal to that of the light source to be tested is selected. The reference light source does not use a specific light source, but a reference illuminant. For light sources with a color temperature below 5000K, a black body is used as the reference light source; for light sources with a color temperature above 5000K, CIE synthetic daylight is used 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 light source to be tested (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
VI. Master control system
The master control system of the instrument is performed by a computer. The computer uses the spectrum measurement software to control the spectrometer to collect spectrum signals through the serial port, and sets the flash trigger time at the same time. At the same time, the computer can set the flash signal trigger level of the data acquisition card through the time characteristic measurement software. The spectrometer triggers the LED power supply, collects the spectrum signal at the same time as the flash, and the data acquisition card collects the luminescence time characteristic curve. The spectrum characteristic and time characteristic curve are measured at the same time. 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 completed by the cooperation of various subsystems.
VII. 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 optical fiber)
Spectral intensity dynamic range: 300 (Pk-Pk)
Spectral range: 380-780nm
Measurement time: 5ms-2s
Test parameters: spectral distribution, CIE1931 color coordinates, CIE1960 color coordinates, correlated color temperature, color rendering index