WO2008035245A2

WO2008035245A2 - Multicolor illumination source having reduced cri variability and method

Info

Publication number: WO2008035245A2
Application number: PCT/IB2007/053355
Authority: WO
Inventors: Gaines James M.
Original assignee: Koninklijke Philips Electronics, N.V.
Priority date: 2006-09-22
Filing date: 2007-08-22
Publication date: 2008-03-27
Also published as: WO2008035245A3

Abstract

The variability of the CRI among white light lamps obtained by mixing the light from multicolor LEDs (e.g., R, A, G, B or R, G, B LEDs) is reduced and the absolute value of CRI is increased by selecting the LEDs for each color to have a minimum difference and/or distribution of peak wavelengths, such as by choosing the LEDs from selected pre-sorted bins of LEDs having a designated range of peak wavelengths for each color.

Description

Multicolor illumination source having reduced CRI variability and method

This invention was made with Government support under Contract No. DE- FC26-05NT42342; W(A)-05-038, CH-1320 awarded by DOE. The Government has certain rights in this invention.

This invention relates to multicolor illumination sources employing LEDs, and more particularly relates to such sources employing LEDs of multiple peak wavelengths to generate mixed, e.g., white light.

Illumination sources are known in which LEDs of different colors are used to generate white light. See, for example, WO 2005/022030 A2, in which blue and green LEDs are used in combination with a red phosphor-converted LED to produce white light.

Red, green and blue (RGB) or red, amber, green and blue (RAGB) LEDs have the advantage that high colour rendering indexes (CRI) can be obtained, if the correct wavelengths of LEDs are chosen. See, for example, US 2006/0012987 A9, in which LEDs of different peak wavelengths are used to achieve a prescribed luminous flux spectrum with a high CRI.

The CRI of white light obtained by mixing the light from RAGB (or RGB) LEDs is strongly dependent on the wavelengths of the component LEDs. Unfortunately, it is difficult to control the manufacturing process to obtain LEDs with the same peak wavelength, so that often a considerable variation from the intended peak wavelength occurs.

Thus, manufacturers are known to resort to the expedient of sorting the LEDs into bins having specific designated ranges of peak wavelengths, which surround a dominant peak wavelength. (Lumileds, for example, has 3, 5, 6, and 6 wavelength bins for R, A, G and B LEDs, respectively.)

An RGB or RAGB lamp may contain one or more LEDs of each color, depending on the amount and character of the white light desired to be produced. Since the LEDs are randomly selected from one or more color bins, the peak wavelengths can vary randomly from the desired peak wavelength, leading to an unacceptable CRI variation from light source to light source.

According to one aspect of the invention, the variability of CRI is considerably reduced in a multicolor LED white light source by restricting or otherwise controlling the variation of peak wavelength of the LEDs of each component color. According to this aspect of the invention, a multicolor illumination source comprises at least two component color sources, preferably selected from the group consisting of red, amber, green and blue color sources, in which at least one of the component color sources comprises at least two LEDs having a variation in peak wavelengths within the range of about 2.5 nm to about 32 nm, whereby the spectral width of the component color source is broadened, leading to a reduction in the variation in CRI among multiple multicolor illumination sources.

According to a preferred embodiment of this aspect of the invention, the at least one component color source is a red source comprising at least two red LEDs having a variation in peak wavelengths within the range of about 4 nm to about 32.5 nm.

According to another preferred embodiment of this aspect of the invention, the at least one component color source is an amber source comprising at least two amber LEDs having a variation in peak wavelengths within the range of about 2.5 nm to about 12.5 nm.

According to another preferred embodiment of this aspect of the invention, the at least one component color source is a green source comprising at least two green LEDs having a variation in peak wavelengths within the range of about 5 nm to about 30 nm. According to another preferred embodiment of this aspect of the invention, the at least one component color source is a blue source comprising at least two blue LEDs having a variation in peak wavelengths within the range of about 4 nm to about 30 nm.

According to another aspect of the invention, the CRI of a multicolor LED white light source is increased by combining two or more LEDs having prescribed differences in peak wavelengths for each color component.

According to a preferred embodiment of this aspect of the invention, the red source comprises at least one group of two red LEDs whose peak wavelengths are spaced apart by about 12 nm between 620.5 nm and 654 nm.

According to another preferred embodiment of this aspect of the invention, the amber source comprises at least one group of from two to five amber LEDs whose peak wavelengths are spaced apart by about 2.5 nm between the 584.5 nm and 597 nm.

According to another preferred embodiment of this aspect of the invention, the green source comprises at least one group of three green LEDs whose peak wavelengths are spaced apart by about 5 nm between the 530 nm and 545 nm. According to another preferred embodiment of this aspect of the invention, the blue source comprises at least one group of three blue LEDs whose peak wavelengths are spaced apart by about 5 nm between 460 nm and 475 nm.

According to another aspect of the invention, there is provided a method of manufacturing a multicolor illumination source comprising at least two component color sources, preferably selected from the group consisting of red, amber, green and blue color sources, in which at least one of the component color sources comprises at least two LEDs, the method including the step of selecting the at least two LEDs of the at least one component color sources to have a variation in peak wavelengths within the range of about 2.5 nm to about 32 nm, whereby the spectral width of the component color source is broadened, leading to a reduction in the variation in CRI among multiple multicolor illumination sources.

According to a preferred embodiment of this aspect of the invention, the component color source is a red source comprising at least two red LEDs, and the LEDs are selected to have a variation in peak wavelengths within the range of about 4 nm to about 32.5 nm.

According to another preferred embodiment of this aspect of the invention, the at least one component color source is an amber source comprising at least two amber LEDs, and the LEDs are selected to have a variation in peak wavelengths within the range of about 2.5 nm to about 12.5 nm.

According to another preferred embodiment of this aspect of the invention, the at least one component color source is a green source comprising at least two green LEDs, and the LEDs are selected to have a variation in peak wavelengths within the range of about 5 nm to about 30 nm. According to another preferred embodiment of this aspect of the invention, the at least one component color source is a blue source comprising at least two blue LEDs, and the LEDs are selected to have a variation in peak wavelengths within the range of about 4 nm to about 30 nm.

According to another preferred embodiment of this aspect of the invention, the LEDs are selected from pre-sorted bins of LEDs having known ranges of peak wavelengths. Therefore, a procedure in accordance with the invention has been developed to deliberately select LEDs from several different bins, and combine the correct proportion of LEDs from each bin to make a lamp that has the correct color overall.

This invention can be applied in the production of LED light sources used to generate light by mixing the light from LEDs of different colors. It is of particular interest in cases where a high CRI, and/or a small variation in the CRI among different light sources is desired. These and other aspects of the invention will be further elucidated with reference to the Figures, in which: Fig. 1 is a graph of light intensity in arbitrary units (a.u.) versus wavelength in nanometers (nm) showing representative spectra of red (R), green (G) and blue (B) LEDs;

Fig. 2 is a graph of calculated CRI values of an RAGB white light source of the prior art showing variation of CRI over a range of peak wavelengths: R: 613.5-645nm; A: 584.5-597nm; G: 520-550; and B: 460-490; Fig. 3 is a graph similar to that of Fig. 2 for an RAGB white light source of the prior art showing variation of CRI with variation in peak wavelengths within a 5nm band centered on a desired peak wavelength;

Figs. 4A and 4B are diagrammatic representations of white light sources composed of R, G, A and B LEDs of different wavelengths; Figs. 5 A and 5B are graphs similar to that of Fig. 1 each showing the individual and combined spectra of two R LEDs having peak wavelengths separated by 20 nm and 10 nm, respectively;

Fig. 6 is a graph similar to the graph of Fig. 2 for an RAGB white light source of the invention showing variation of CRI with variations in peak wavelengths of R, A, G and B LEDs with a single LED taken from each of the 3 R bins, each of the 5 A bins, each of the 6 G bins, and each of the 6 B bins; and

Fig. 7 is a graph similar to the graph of Fig. 6 for an RAGB white light source of the invention showing variation of CRI with variation in peak wavelengths of R, A, G and B LEDs, except that the shortest-wavelength R bin, the longest and two shortest -wavelength G bins and the three longest-wavelength B bins were excluded.

The Figures are diagrammatic and not necessarily drawn to scale.

Fig.l shows examples of R, G and B spectra of LEDs that could be used to make a multicolor RAGB white light lamp according to the invention. The arrows at each peak indicate the variability (within limits) in the exact position of the peak wavelength which occurs during the manufacture of each color.

Fig. 2 (prior art) shows how much the CRI of a lamp can vary with uncontrolled variations of peak wavelengths of R, A, G and B LEDs over the ranges: R: 613.5-645nm; A: 584.5-597nm; G: 520-550; and B: 460-490. CRI was calculated for LEDs modeled with second-order Lorentzians. The peak wavelength was stepped between calculations by 1.5 nm for R, G and B and by 1 nm for A. As may be seen from Fig. 2, the CRI can vary widely, from less than 30 to as high as 97, which is likely to be unacceptable for many applications.

One approach is to controlling this variability is to restrict the peak wavelength variation for each color to a narrow band by selecting the LEDs for each color from one of the available bins. Representative color bins and their wavelength ranges are shown in Table I.

TABLE I color peak wavelength range (nm) Red (R) Bin l 613.5 - 620.5 Bin 2 620.5 - 631 Bin 3 631 - 645

Amber (A) Bin l 584.5 - 587 Bin 2 587 - 589.5 Bin 3 589.5 - 592 Bin 4 592 - 594.5 Bin 5 594.5 - 597 Green (G) Bin l 520 - 525 Bin 2 525-530 Bin 3 530-535 Bin 4 535-540 Bin 5 540 - 545 Bin 6 545-550 Blue (B) Bin l 460 - 465 Bin 2 465 - 470 Bin 3 470 - 475 Bin 4 475 - 480 Bin 5 480-485 Bin 6 485 - 490

Fig. 3 is a graph illustrating this approach by showing variation of CRI with R, A, G and B peak wavelengths within a 5 nm band centered on a desired or nominal peak wavelength. The calculation is the same type which was used to generate the graph of Fig. 2, except that the peak wavelengths were stepped by 1 nm between calculations for all four colors. As can be seen from Fig. 3, the variability of CRI is significantly reduced, from a range of about 30-97 in Fig. 1, to a range of about 75 - 90 in Fig. 3. However, this approach of selecting LEDs from a fixed bin per color will result in higher costs, which would prove to be unacceptable for most applications.

An essential feature of the invention is to use LEDs from several different bins

(of each color) in each multicolor LED white light source to overcome the strong dependence of CRI on wavelength, to produce a higher and more consistent CRI. For example, if such a white light source consists of 20 LEDs (5R, 5 A, 5G, 5B) then the 5 R LEDs are chosen from different bins (e.g., 1 from bin 1, 2 from bin 2 and 2 from bin 3), the 5 A LEDs from 5 different bins (e.g., 1 each from bins 1, 2, 3, 4 and 5), the 5 G LEDs from different bins (e.g.,

1 each from bins 1, 2, 3, 4 and 5) and the 5 B LEDs from different bins (e.g., 1 each from bins 1, 2, 3, 4 and 5). This is in contrast to the approach of selecting all 5 R LEDs from the optimum R bin, all 5 A LEDs from the optimum A bin, etc.

Figs. 4 A and 4B are diagrammatic representations of multicolor LED white light sources having a 7-LED configuration composed of 2 R, 2 A, 2 G and 1 B LED. In Fig.

4A, all LEDs of a given color have identical spectra. In Fig. 4B, the R, A and G LEDs are deliberately chosen from different color bins that are separated from each other by several nm. For example, LED Ai is selected from bin 1 (584.5-587nm) and LED A₂ is selected from bin 6 (592-594.5nm), guaranteeing at least 5nm difference between the peaks. Such selections of the LEDs from multiple bins results in broader spectra than the spectra of the individual LEDs. Of course, a multicolor white light source could include one or more groups or clusters of LEDs, such as the 7-LED configuration shown in Figs. 4A and 4B, depending on the amount and character of light desired.

Figs. 5A and 5B show examples of combined LED spectra for two R LEDs

(peak 1 and peak 2) selected from different R color bins. The separation between peaks is 20nm in Fig. 5A and IOnm in Fig. 5B. As may be seen, the combined spectra are wider in both cases, although the wider separation in Fig. 5A results in two distinct peaks, while the narrower separation in Fig. 5B results in a single but broader peak.

Further reductions in variability of CRI are possible by selecting LEDs of the same color with minimum differences in peak wavelengths. Where the LEDs are selected from color bins having LEDs with peak wavelength within a designated range, this minimum color difference can be achieved by deliberately selecting LEDs from only those bins having the desired minimum difference.

Fig. 6 shows the variation of calculated CRI with R, A, G and B peak wavelengths over the same range of available peak wavelengths as for Fig. 2: R: 613.5-645 nm; A: 584.5-597 nm; G: 520-550 nm; and B: 460-490 nm. However, in contrast to Fig. 2, these calculations were done for a single LED taken from each of the 3 R bins, each of the 5 A bins, each of the 6 G bins, and each of the 6 B bins having the wavelength ranges shown in Table 1. For each calculation, wavelengths from one of the R rows in Table II were combined with wavelengths from one of the A, G and B rows and CRI was calculated for all possible combinations of rows, and plotted as shown in Fig. 6.

Table II

Wavelength (nm) Red (R)

613.5 620.5 631

614.5 622 633

615.5 623.5 635

616.5 625 637

617.5 626.5 639

618.5 628 641

619.5 629.5 643

620.5 631 645

Amber (A)

584.5 587 589.5 592 594.5

585.3333 587.8333 590.3333 592.8333 595.3333

586.1667 588.6667 591.1667 593.6667 596.1667

587 589.5 592 594.5 597

Green(G)

520 525 530 535 540 545

521 526 531 536 541 546

522 527 532 537 542 547

523 528 533 538 543 548

524 529 534 539 544 549

525 530 535 540 545 550

Blue (B)

460 465 470 475 480 485

461 466 471 476 481 486

462 467 472 477 482 487

463 468 473 478 483 488

464 469 474 479 484 489

465 470 475 480 485 490

As may be seen from Fig. 6, CRI varies within the range of about 72 - 87. This range of variation has decreased greatly from that shown in Fig. 1. While this range is comparable to that shown in Fig. 3, which was obtained for the single bin approach, the cost of selecting LEDs from each of the available bins is lower than in the case of restricting the selection to a single bin per color. In actual practice, the strict relationship of wavelength combinations used to generate the calculations for Fig. 6 need not be adhered to, so long as the selection results in a degree of randomness in the actual distribution of CRIs that would be obtained if a large number of products were made in this way. Additional improvement can be made by excluding some bins. Fig. 7 shows the variation of calculated CRI with R, A, G and B peak wavelengths in which the calculations were similar to those of Fig. 6, but in this case the shortest -wavelength R bin, the longest and two shortest-wavelength G bins and the three longest -wavelength B bins were excluded. As may be seen from Fig. 7, the CRI now ranges from about 87.7 - 94.6, a dramatic improvement in both the absolute values of CRI and the variation in CRI values.

More LEDs at tighter wavelength spacing than the given number could also be selected, such as by selecting more than one LED from each bin, each LED having a different peak wavelength within that bin.

The invention has necessarily been described in terms of a limited number of embodiments. From this description, other embodiments and variations of embodiments will become apparent to those skilled in the art, and are intended to be fully encompassed within the scope of the invention and the appended claims. For example, while the description has been mainly in terms of white light sources in which the white light is produced by a mixture of RGB or RAGB LEDs, the invention also applies to any light source in which the light results from a mixture of two or more colors, where at least one of those colors is produced by one or more LEDs. Where not all colors are produced by LEDs, the other color or colors could be produced, for example, by a photoluminescent and/or electroluminescent material or device.

CLAIMS:

1. A multicolor illumination source comprising at least two component color sources, in which at least one of the component color sources comprises at least two LEDs having a variation in peak wavelengths within the range of about 2.5 nm to about 32 nm.

2. The multicolor illumination source of claim 1 in which the at least two component color sources are selected from the group consisting of red (R), amber (A), green (G) and blue (B) color sources.

3. The multicolor illumination source of claim 2 in which the at least one component color source is a red (R) source comprising at least two red (R) LEDs having a variation in peak wavelengths within the range of about 4 nm to about 32.5 nm.

4. The multicolor illumination source of claim 2 in which the at least one component color source is an amber (A) source comprising at least two amber (A) LEDs having a variation in peak wavelengths within the range of about 2.5 nm to about 12.5 nm.

5. The multicolor illumination source of claim 2 in which the at least one component color source is a green (G) source comprising at least two green (G) LEDs having a variation in peak wavelengths within the range of about 5 nm to about 30 nm.

6. The multicolor illumination source of claim2 in which the at least one component color source is a blue (B) source comprising at least two blue (B) LEDs having a variation in peak wavelengths within the range of about 4 nm to about 30 nm.

7. The multicolor illumination source of claim 3 in which the red (R) source comprises at least one group of two red (R) LEDs whose peak wavelengths are spaced apart by about 12 nm between the 620.5 nm and 654 nm.

8. The multicolor illumination source of claim 4 in which the amber (A) source comprises at least one group of from two to five amber (A) LEDs whose peak wavelengths are spaced apart by about 2.5 nm between the 584.5 nm and 597 nm.

Claims

9. The multicolor illumination source of claim 5 in which the green (G) source comprises at least one group of three green (G) LEDs whose peak wavelengths are spaced apart by about 5 nm between the 530 nm and 545 nm.

10. The multicolor illumination source of claim 6 in which the blue (B) source comprises at least one group of three blue (B) LEDs whose peak wavelengths are spaced apart by about 5 nm between 460 nm and 475 nm.

11. A method of manufacturing a multicolor illumination source comprising at least two component color sources, in which at least one of the component color sources comprises at least two LEDs, the method comprising the step of selecting the LEDs to have a variation in peak wavelengths within the range of about 2.5 nm to about 32 nm, whereby the spectral width of the component color source is broadened, and the variation in CRI among multiple multicolor illumination sources is reduced.

12. The method of claim 10 in which the at least two component color sources are selected from the group consisting of red (R), amber (A), green (G) and blue (B) color sources.

13. The method of claim 11 in which the at least one component color source is a red (R) source comprising at least two red (R) LEDs, and the LEDs are selected to have a variation in peak wavelengths within the range of about 4 nm to about 32.5 nm.

14. The method of claim 11 in which the at least one component color source is an amber (A) source comprising at least two amber (A) LEDs, and the LEDs are selected to have a variation in peak wavelengths within the range of about 2.5 nm to about 12.5 nm.

15. The method of claim 11 in which the at least one component color source is a green (G) source comprising at least two green (G) LEDs, and the LEDs are selected to have a variation in peak wavelengths within the range of about 5 nm to about 30 nm.

16. The method of claim 11 in which the at least one component color source is a blue (B) source comprising at least two blue (B) LEDs, and the LEDs are selected to have a variation in peak wavelengths within the range of about 4 nm to about 30 nm.

17. The method of claim 11 in which the at least two LEDs are selected from different bins of LEDs having known ranges of peak wavelengths.

18. The method of claim 17 in which there are four component color sources, a red (R) source, an amber (A) color source, a green (G) color source and a blue (B) color source, and the LEDs for each source are selected from a different bin.

19. The method of claim 18 in which the component color sources are grouped into one or more groups each consisting of 5 R, 5 A, 5 G and 5 B LEDs.

20. The method of claim 18 in which the component color sources are grouped into one or more groups each consisting of 2 R (R_h R₂), 2 A (Ai, A₂), 2 G (Gi, G₂) and 1 B LED.

21. The method of claim 18 in which the component color sources are grouped into one or more groups each consisting of 3 R, 5 A, 6 G and 6 B LEDs.

22. The method of claim 18 in which the component color sources are grouped into one or more groups each consisting of 2 R, 5 A, 3 G and 3 B LEDs.

23. The method of claim 22 in which each LED has a peak wavelength which falls within the range of one of the following bins:

Red (R) peak wavelength (nm)

Bin 2 620.5 - 631

Bin 3 631 - 645

Amber (A)

Bin l 584.5 - 587

Bin 2 587 - 589.5

Bin 3 589.5 - 592

Bin 4 592 - 594.5

Bin 5 594.5 - 597

Green (G)

Bin 3 530 - 535

Bin 4 535 - 540

Bin 5 540 - 545