The LED market in horticultural lighting is growing rapidly, and there is an increasing demand for horticultural LED calibration. The evaluation of LED lighting for plant growth needs to be based on photon systems, such as photon flux (PF) rather than a photometry system which is used to evaluate how bright a LED is for human eyes. The integrating sphere and spectroradiometer are selected as the key instrument of the test system for LED photon flux calibration. Standard and improved calibration methods are presented. The standard method includes calibration of the test system, verification, spectral absorption correction, and measurement of the test LED lamp. The improved method simplifies the absorption correction in the calculation. Three LED lamps for plant growth are calibrated by the two methods, and the photon flux scale is traced to total spectral radiant flux (TSRF) standard lamps with the relative uncertainty Urel = 2.5% (k = 2).
NIM has developed LED filament standard lamps for total luminous flux with excellent long-term stability, uniform distribution of luminous intensity, and the E27 Edison screw base which is compatible with present incandescent standard lamps. The standard lamp consists of six high-power LED filaments which are sealed in a G150 glass bulb and filled with He gas. All the 12 lamps were aged for 500 hours before the 315 days’ long-term stability test. 9 lamps are lighted for 1 hour every day. One lamp is continuously operated for about 6000 hours, and 2 lamps are well stored on a shelf. The results show that the long-term stabilities of the 3 cases are better than 0.07%. This LED filament lamp could be promising as a transfer standard of total luminous flux.
KEYWORDS: Light emitting diodes, Photometry, Monte Carlo methods, LED lighting, Light sources and illumination, LED displays, Luminous efficiency, Metrology, Numerical simulations, Mathematical modeling
Nowadays, Light Emitting Diode (LED) and related products are popular in everyday lighting. Especially colour LEDs are widely used in colour application such as outdoor display, traffic signal light and cars. Precise photometric measurement results for those LED products are necessary due to needs of the international or domestic commercial trade. In the testing lab, people usually use photometer and related photometry equipment to measure LEDs. The photometer mimics the human eyes however it is not exactly the same. Thus, colour correction factor (CCF) is applied when the relative spectral response of the photometer S*(λ) is different from the photopic luminous efficiency function V(λ). Since the CCF is the most significant uncertainty component in LED total luminous flux and averaged LED luminous intensity measurement, the evaluation of uncertainty of CCF become more important in National Institute of Metrology China (NIM). Recently the Monte-Carlo method, as known as numerical simulation technique, is utilized to evaluate uncertainty by generating millions of random variable with related distribution function. The input mathematical model of the photometer S*(λ) and spectra of measurand PLED(λ) are described. The propagation of the uncertainty is also introduced. Examples of the evaluation of CCF uncertainty are shown.
LED light sources are replacing incandescent lamps gradually and inevitable, but using traditional incandescent lamps as standard lamps to measure LED light sources introduces greater uncertainty due to the great difference of their spectrum. In order to reduce the measurement uncertainty, a standard lamp based on LED filament is developed. The lamp is free of cumbersome cooling devices, and its luminous intensity distribution is uniform in 4π geometry. However, its luminous flux is sensitive to the ambient temperature. Therefore, a linear model of luminous flux as a function of ambient temperature and the lamp voltage is established. Then driven by a constant current, the lamp voltage, the relative luminous flux and the ambient temperature of the LED standard lamp are monitored. The model parameters can be obtained from the test data. Finally, by measuring the lamp voltage, the luminous flux drift caused by the ambient temperature can be compensated, so that the luminous flux of the LED standard lamp maintains a stable magnitude.
The SI derived unit lumen is a unit of total luminous flux, describing a total quantity of visible light emitted from a light source. The realization of lumen unit is implemented by the facility called goniophotometer in National Institute of Metrology China (NIM). The goniophotometer is a facility measuring the luminous intensity of a light source at various angles, and the total luminous flux is calculated by integrating the spatial distribution of luminous intensity. Because of sampling characteristic of the goniophotometer, the spatial distribution of luminous intensity is a discrete curve. In our previous method, the goniophotometer measure the luminous intensity value at polar angle step size of 1°, 2° or 5°. For measurement of LED-light sources, which are likely oriented light sources, the spatial distribution of light is not homogeneous. Carefulness must be taken because the choice of angle sampling interval is going to be a considerable uncertainty component. Recently, a timeline-based sampling method is developed and applied in the primary standard goniophotometer in NIM, to realize the lumen unit, as well as measuring the LED light sources. The control computer is not to record the luminous intensity at exact integer angle, but to record the luminous intensity and its related time, and to record the angle and its related time. These two record threads are running respectively and simultaneously during the whole testing process. An interpolation is used to calculate out the luminous intensity and the angle at the same timeline. Thus, the spatial distribution of luminous intensity curve has much more effective data than that from the previous method. It is an improvement to help to get a more accurate result. The repetitive fluctuation is evaluated to be 0.025%, a much lower level than that of the previous method.
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