LED lighting knowledge: 15 possible factors affecting the measurement error of the integrating sphere
The illumination optical measuring equipment includes an integrating sphere testing system, a light color distribution testing system, an illuminance meter, a luminance meter, an optical radiation power meter, and a photobiosafety measuring system.
The integrating sphere testing system is commonly used or used by testing organizations and enterprises to measure the light color and electrical parameters of the lamp. Regardless of the size of the impact, the main potential influencing factors in the integrating sphere test system are analyzed first.
The basic working principle of the integrating sphere measurement method
The principle of the integrating sphere measurement method is to measure the measured light in the integrating sphere first, then measure the measured light, and then compare the relative measurement by the computer.
When the lamp under test is similar to the standard reference lamp in all respects, the error is reduced to a minimum. But even then, there are many factors in the actual equipment and testing process that cause it to deviate, not to mention the large difference between most of the lamps tested and the standard reference lamps.
Although some factors deviate from the ideal situation, a single factor has little effect on the measurement results. However, there are too many factors that deviate from the ideal, and the greater the possibility of cross-correlation, the greater the error. Therefore, we should minimize the deviations in equipment and testing.
1. The working principle of the spectrometer in the integrating sphere measurement method
The integrating sphere measurement method is divided into photometry and spectrometry. In the early days, simply using the pointer photoelectric galvanometer to read the photocurrent in the photometric probe, and then using the manual calculation method, the error is very large, and it has long been used. It was later improved to read with a digital meter, but it was still out of date for a long time. At present, much more is used in integrating sphere spectroscopy. It can be divided into two types: photomultiplier tube and CCD array.
(1) Working principle inside the photomultiplier tube spectrometer
The complex color light emitted by the lamp under test is uniformly mixed in the integrating sphere and then received by the fiber input end, and transmitted by the optical fiber into the spectrometer, and then filtered into the input slit, and projected onto the grating to decompose the spectral optical power signal.
Since the photomultiplier tube as photoelectric conversion cannot distinguish the spectrum itself, the mechanical device rotates the grating to project a monochromatic optical power signal of a certain bandwidth to the output slit in accordance with the wavelength, and is received by the photomultiplier tube closely attached to the slit. The optical power signal is converted and multi-stage amplified into an electrical signal, and then further amplified by an external circuit and output to a computer for processing. In this series of processes, the technology is very complicated.
In addition, the measurement requires accurate wavelength scanning (this is not a problem for the measurement accuracy of the spectrum continuous lamp, but it is important for the light color parameters of the lamp of the separated spectrum, especially the color parameter measurement accuracy, such as the color temperature measurement accuracy of the incandescent lamp. The color temperature measurement accuracy of the three-color energy-saving lamps is very high. The resolution and accuracy of the measurement are closely related to the input and output slit width, wavelength positioning and scanning step size.
Therefore, the measurement speed of the photomultiplier tube spectrometer is slow, and the early spectrometer generally takes 2 to 3 minutes, and the recent one takes about 10 seconds.
The measurement accuracy of the photomultiplier tube spectrometer can be very high, but the photomultiplier tube also has disadvantages:
1 The sensitivity is lowered by strong light or because the irradiation time is too long, and will partially recover after stopping the irradiation. This phenomenon is called "tiredness". In a 90-minute continuous measurement of the same incandescent lamp using a photomultiplier tube spectrometer for many years, the luminous flux continuously decreased by 3% in one direction, but the color temperature change was very small within 5K (0.2%);
2 The sensitivity of each point on the surface of the photocathode is not uniform;
3 The voltage applied in the actual measurement is too high to generate noise.
(2) Working principle inside the array CCD spectrometer
Different from the photomultiplier tube type, the array CCD spectrometer decomposes the complex light of the lamp under test into a spectral light power signal arranged in order of wavelength, and simultaneously projects onto the CCD array capable of distinguishing the spectral wavelengths at one time. In this way, the image receiving and obtaining the optical power signals of the respective wavelengths is replaced by the need to scan and sequentially input the monochromatic light into the photomultiplier tube to receive the spectral optical power signals of the respective wavelengths in a "time-division" manner. And thus no mechanical scanning device for raster scanning is required.
Therefore, the measurement speed is very fast, up to the millisecond level. At present, the better CCD converter for illumination is 2048 bits, and the highest wavelength accuracy is calculated to be 0.2 nm. In general, the current accuracy is not comparable to the photomultiplier tube spectrometer.
The disadvantages of CCD are: 1) the noise of the substrate is large; 2 the dark current is closely related to the temperature, and needs to be cooled. For every 5 to 7 °C reduction, the undercurrent is reduced by half. The professional application of CCD is usually cooled by liquid nitrogen, so that the temperature is lower than - 110 ° C; semiconductor refrigeration is generally -10 ° C to -20 ° C, it is difficult to reach a high level; 3 CCD device pixel efficiency is inconsistent, will cause optical power measurement error of each wavelength, which is more than the above mentioned photomultiplier The effect of uneven sensitivity at each point on the surface of the photocathode is severe.
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