| As the development of light-emitting diodes(LED) technology,great progress of the white LED in luminous efficiency, service life andcost-control have been achieved, and it has been applied in various lightingareas. As the best choice to replace incandescent lamp, LED bulbs can beused in family lighting, school lighting, bar lighting, office lighting, corridorlighting and so on, its lighting effect is obvious.For the existing LED bulbs, white LED consist of a blue chip and ayellow-emitting yttrium aluminum garnet (YAG) phosphor is used widely.This kind of LED is maturity in production process, with low consumptionand high efficiency. However, its color rendering index (CRI) is low due tothe deficiency in red spectral components. Thus it could not satisfy therequirements of quality lighting. In order to improve CRI of LED bulbs,influences of different light-mixing schemes on the spectrum of LED arestudied and an optimize scheme is proposed and analyzed.1、The mathematical model for LED spectra is studied. And the accuracy ofthis model when used for phosphor-converted LED is proved by contrastingthe data of experiment and simulation. Then the numerical model of LEDbulbs is obtained. Combined with evaluation theory of light source, thevalues of CRI(Ra), luminous efficiency(LER), correlated colortemperature(Tcp) and CIE1931(x,y) chromaticity coordinates can becalculated by using Matlab software.2、Influences of different combinations and proportions of different colorphosphors on the value of Raand LER of LED are studied. It is confirmed bysimulations that the Ravalue and LER value of light source are determinedsolely by its spectrum. But there is a fundamental trade-off relationshipbetween Raand LER, that is, to get higher Ravalue, more comprehensivespectrum is need. While higher LER need the spectral close to555nmband.(one of the two quantities can be maximized only at the expense of theother). It also can be concluded that the spectra of LED can be compensatedby using and mixing different phosphors, but some spectral composition may be reduced or missed. High CRI on some color temperature stage can berealized by adjusting proportions of different colors. But due to the difficultencapsulated process, the scheme of the phosphor mixing is not suitable forLED bulbs.3、Methods of improving CRI of LED both at home and abroad are studied. Akind of LED unit consists of white LEDs of blue chip pumps YAGphosphor(marked as A), white LEDs of blue chip pumps YAG phosphor withsmall amount nitride red phosphor(marked as B) and red LEDs(marked as C)is proposed based on practical application, which is easy to implement.Influences of changing the number of each kind of LED on results areanalyzed. When the Tcpvalue of A and B are6174K and4326K respectively,results with Ravalue above88and LER value more than272lm/W areobtained. Tcpvalue of these results are located in3400K~5600K. Comparedwith A alone, CRI of LED unit improved obviously, from74~80to90.Compared with the combination of A and C, CRI of LED unit also showsbetter. Combinations of A and B with different Tcpvalue(such as3500K,5000K,6500K) respectively are analyzed. Results with high Ravalue87~94and LER value275~290lm/W whose Tcpvalue are located in lowerstage(such as2700K~3000K) and higher stage(such as6000K~6300K) areobtained. The result shows that red spectrum is compensated by a few redLEDs and a small amount of nitride red phosphors. The spectra of LED unit ismore comprehensive due to the differences of the peak wavelength and halfspectral width of red phosphor and red LED.So that the value of CRI becomebetter. Different numbers of those LEDs can be set according to the demandin order to get optimized results with different correlated color temperature. |
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