The color of the object is revealed by the light. The color of an object under illumination from a light source will vary depending on the spectral distribution of the source. For example, the red object observed in the daylight fluorescent lamp is observed as a sauce red under the low pressure sodium lamp. Due to the different spectral distribution of the light source, the color difference caused by the illumination of the object reflects the difference in the color rendering performance of the light source. The color effect caused by the light source after illuminating the object is called the color rendering of the light source.
At present, the calculation method of the color rendering index of the light source is mainly the 'dam 0 color method†formulated by CIE. The "color measurement method" was formulated by CIE in 1965, and was officially recommended after revision in 1974. The color rendering index was evaluated by test color. It is the most effective method, which is consistent with the visual effect and is the standard method for calculating the color rendering index. CIE specifies that the color rendering index is divided into general color rendering index Ra and special color rendering index Ri. A set of 14 test standards is used in the evaluation. Color sample: R, light gray red; Rz, dark grayish yellow; R: saturated yellowish green; R4, medium yellowish green; R, light blue green; R6, light blue; R, pale purple blue; Rs, light red Purple; R9, saturated red; Ro, saturated yellow; R saturated green; R:: saturated blue; Rl3' Caucasian skin color; R, leaf green. Among them, 1 to 8 test colors are used for general color rendering index Ra. Calculated, these 8 color samples are selected from the Munsell color scale, which contains various representative tones, all of which have medium chroma and lightness; the latter 6 test colors are used to calculate the special color rendering index for testing the light source. A special color rendering performance is selected, respectively, red and yellow with higher chroma Green, blue and leaf green and European and American skin color; In addition, China's method of calculating the color rendering index of light source has also increased the color sample of Chinese women's skin color. The 8 color samples specified in the 1210 CIE standard for calculating the general color rendering index have Medium chroma and lightness, which are used to measure the color rendering of light sources with continuous spectrum and wide frequency band, and the LED light source has steep waveform and narrow frequency band. Currently, LED mainstream lighting products are yellow-green phosphor + red fluorescent The powder combination matches the blue light chip to emit white light with a general color rendering index Ra=80. Only the special color rendering index saturated red R may have a negative value. Therefore, in order to correctly reflect the color rendering of the light source, the Rq requirement is proposed on the basis of Ra. Based on the combination of phosphors, this paper studies the effect of phosphor combination on R9.
LED different index finger spectrum and R9 analysis
The white LED implementation is mainly:
(1) Combination of Blu-ray film and single phosphor, with about 70 indicators:
Blue chip + yellow phosphor;
(2) The combination of the blue chip and the two phosphors has an index of about 80:
Blue chip + yellow-green phosphor + red phosphor;
(3) The combination of the blue chip and the two phosphors has a display index of about 90%:
Blue chip + green phosphor + red phosphor;
The yellow phosphor matches the blue chip to obtain a pure white light spectrum, however the average Ra is low, only 72.7. Positive white light spectrum
Figure 1 shows the white light spectrum obtained by matching the yellow phosphor with the blue light chip. As shown in Fig. 1, the white light spectrum distribution is narrow, which is mainly due to the lack of red and green components in the spectrum.
Figure 1 shows the white light spectrum obtained by matching the yellow phosphor with the blue light chip and the yellow light phosphor and the nitride series red phosphor and then matching with the blue light chip. As shown in Fig. 2, it can be seen that the red light region is significantly increased, and the test results show that the average Ra reaches 80.
Figure 2: White light spectrum obtained by matching yellow-green phosphor, red phosphor and blue light chip
In order to further enhance the white light Ra to compensate the green component in the spectrum, the white light spectrum obtained by matching the green phosphor and the red phosphor with the blue chip is used. As shown in Fig. 3, it can be seen that the green light region is significantly increased, the spectral distribution is relatively uniform, and the test results show that the average Ra reaches 91.
Figure 3: White light spectrum obtained by matching green and red phosphors with blue light chips
Table 1 shows the optical parameters of the above three phosphors matching the blue chip to obtain white light. Adding a red phosphor to the yellow-green phosphor, Ra reaches 80, and the special color rendering index increases significantly. The special color rendering index Rq representing saturated red increases from a 4O to 0; after the green phosphor is added to the system, The average Ra reached 91, the special color rendering index was further enhanced, and the R increased to 78. However, with the addition of red and green phosphors, the relative luminescence brightness gradually decreases. This phenomenon is caused by the contradiction between Ra and light effect. High Ra requires the white light spectrum to be evenly distributed in the visible light region, and the high light effect required spectrum is distributed as much as possible in the region surrounded by the visual function.
Table 1 Using different phosphors to match the blue chip to obtain the luminescence properties of white light (a) yellow phosphor, (b) yellow-green + red phosphor, (C) green + red phosphor
It can be seen from Table 1 that when Ra is close to 70, its saturated red value is much smaller than 0. When Ra reaches 9O, its saturated red R is much larger than 0, so the combination of the above two cases does not need to consider its saturated red R9 value. Only when Ra is close to 80, its saturated red Rq is close to 0, and a negative value may occur. It is necessary to analyze the phosphor combinations of different 80 indicators, study the change of R9, and give a phosphor scheme to increase the R9 value.
R9 analysis of Ra=80 phosphor combination
Figure 4 (A): Spectral brightness coefficient of color samples No. 1-8; (B): Spectral brightness coefficient of No. 15 color sample. The mainstream fluorescent powder in the market is yellow-green phosphor with 530-535 nm band and 623-626 nm. Band of red phosphors. Ra is the arithmetic mean of R ~ Rs, so the spectral brightness coefficient of the color sample of Figure 1 (A) 1 ~ 8 can be obtained, the spectral brightness coefficient in the yellow-green part of the composite maximum is about 532nm, long-band spectral brightness The coefficient has reached a large value around 625 nm, so the yellow-green phosphor in the 530-535 nm band and the red phosphor in the 623-626 nm band can be selected to achieve both Ra and brightness. However, from the spectral brightness coefficient of the color sample No. 9-15 of Figure 4(B), Ro~R has a large value between 40 and 600 rim, and only the spectral brightness coefficient of the saturated red No. 9 color sample is very special. It is almost 0 before 600rim, which is why the Rq value is -40 when the blue chip is combined with the single yellow powder. However, the red phosphor near 625 nm can have a larger spectral brightness coefficient of the 9th color sample, but because the red phosphor has a lower brightness and a smaller proportion of the powder, the adjustment value can be selected in the following ways. :
a) selecting a red phosphor of 626 nm wavelength in the wavelength range of 623 to 626 nm;
b) the red phosphor has the same peak wavelength and selects a red phosphor with higher brightness;
c) Select a greener yellow-green powder and increase the proportion of red phosphor. Based on these three methods, two green powders and three red phosphors shown in Table 2 were cross-tested.
It can be seen from Table 3 that when the chip wavelength is 452.5 nm: when the green powder is GB, the combination of GB+RA is Ra=80, R9=0, and the red phosphor RB with increasing wavelength is selected to increase Ra. And, the combination of GB+RB has Ra=81 and R9=5.
When the green powder is GA, the combination of GA+RB has Ra=82 and R=7, and the wavelength coordinates can be selected similarly, but the red phosphor RC with enhanced brightness is matched with GA to improve the combination of Ra and R, GA+RC. The color index is Ra=83 and R9=10. Because the color coordinates and peak wavelength of RC are both equivalent to RB, but the brightness is higher, so the combination of Ra and R of GA+RB. Both are high. When the red phosphor is RA with a wavelength of 624 nm, the combination of GB+RA has Ra=80 and R9=0. By selecting the shorter wavelength green powder GA and RA, the GA+RA combination has Ra=81 and R9=2. Because the peak wavelength of GA is small, when the same red phosphor is used, the proportion of red phosphor increases, Ra and R. The value increases.
in conclusion
(1) The eight color samples used in the CIE standard for calculating the general color rendering index have limitations in evaluating the light source. It is a good result for measuring the color rendering of a light source with a continuous spectrum and a wide frequency band, but not Suitable for LED light sources with steep waveforms and narrow frequency bands. At present, the mainstream LED lighting products in the market are yellow-green phosphor + red phosphor combined with white light of Ra=80. Under this color rendering index, the special color rendering index may only have a negative value when saturated red, in order to correctly reflect the light source. Color rendering, based on the evaluation Ra, proposed to evaluate the Rq value.
(2) When Ra=70, R9 must be <0; when Ra=90, Rq must be >0, only when Ra=80, R9 values ​​appear as <0, =0, >0. Therefore, when Ra = 80, the detected value is more meaningful.
(3) The following three methods can be selected to increase the R9 value:
a) selecting a red phosphor of 626 nm wavelength in the wavelength range of 623 to 626 nm;
b) the red phosphor has the same peak wavelength and selects a red phosphor with higher brightness;
c) Select a greener yellow-green powder and increase the proportion of red phosphor.
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