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In this paper, to our best knowledge, it is the first time to present a precise simulation and detailed design of angular correlated color temperature (CCT) distribution of white LEDs covering a range of CCT from 2800K to 6500K. An optimized design of packaging structure with a silicone lens covering a phosphor dome performed an extreme small angular CCT deviation of 105K in the simulation and 182K in a corresponding real sample for a white LED with the CCT near 6500K.
©2012 Optical Society of America
1. Introduction
Solid-state light sources such as white LEDs (light emitting diodes) have been extensively studied for the applications on general lighting owing to the advantages of low cost, compact size, high efficiency, long life, fast response and acceptable color performance [1–3]. In a simpler and cost effective approach, a GaN die covered with yellow (or green/red) phosphor in a package can perform white light with CRI (Color Rendering Index) around 70, or with CRI larger than 90 through a specific phosphor recipe [4–9]. Even the CRI can reach 90 or above, a serious problem of chromatic performance by a white LED may occur, in which the correlated color temperature (CCT) varies one by one and/or angle by angle. This problem is associated with phosphor recipe and processing in blue LED packaging. The shortage of such a phenomenon is that a white LED may emit a light beam at a certain direction with extreme high CCT, and low CCT at the other directions. Figure 1 shows the measurement of angular CCT distribution of three commercial high-power white LEDs, where the angular CCT deviations (so-called ACCTD) of the three kinds of white LED with the CCT around 6500 K are all larger than 1500 K, and some of them even reaches 3000 K. Such a property may cause the LEDs disqualified in general lighting. The worse condition is that the blue leakage could be harmful to human body.
There are several ways proposed to reduce ACCTD. Liu et al. proposed a design for conformal coating of phosphor within a lens, and the simulation showed that an ACCTD around 900 K was achieved as the CCT of the white LED was around 6000 K [10]. Simmer et al. proposed to use a planar color conversion element to reach an ACCTD as small as 300 K in simulation when the CCT was around 5600 K [11]. Wang et al. proposed a design of a silicone lens to make a uniform CCT distribution, and the simulation showed that an ACCTD could achieved as small as 40 K when the CCT was 5000 K, which can be regarded as the smallest value [12]. Shuai et al. proposed to have a phosphor structure in a convex shape over a metal cup, and the simulation showed that the ACCTD was 200 K when the CCT was 5000 K [13]. Generally, the ACCTD depends on the CCT of a white LED. When the CCT is small, the ACCTD could be relatively small since the lights inside the heavy phosphor volume scatter more to well mix the lights in different color. In contrast, the ACCTD in a cool white LED as shown in Fig. 1 is always relatively large. Some reports even addressed a white LED with the CCT of around 9870 K performing an ACCTD as large as 5600 K and more [14,15]. Besides, a white LED could perform different CCT with or without an encapsulation lens [16] and so does the ACCTD. Therefore, in order to have a white LED with an extreme small ACCTD, a specific design of the packaging geometry, precise simulation and well-control fabrication process are demanded. In this paper, to our best knowledge, it is the first time to present a precise simulation of ACCTD for white LEDs with a CCT covering a range from 2800 K to 6500 K. Besides, a special design to perform extreme small ACCTD will be presented, and corresponding experiment and measurement will be demonstrated.
2. The first insight for performing small ACCTD
Practically, the phosphor in white-LED packaging can be divided two different structures. The first is the conformal structure, where a phosphor film is coated on the top surface of the LED die. Such as approach makes the light path at different angles accumulate different optical path lengths so that the CCT at different viewing angle will be different from the others. The second is the dispensing structure, where the LED die is attached on the bottom side of a metal cup, which is also used to contain phosphor. In such a structure, the first problem is the same as the conformal coating, and the second problem is the sidewall of the cup may act an important role in photon recycling [17] so that the ACCTD could be a function of the reflectivity of the sidewall as well. Therefore, to reduce ACCTD in such two structures seems difficult.
A basic concept to have smaller ACCTD is to enable all lights emitted from the blue die to go through an equal path in the phosphor volume. Thus choosing a light source with a spatial coherence as small as possible could be working for the goal. The first thinking is that the LED die can emit collimating lights so that the phosphor can be made a planar plate or film to make each light to have equal path length in the phosphor, as shown in Fig. 2(a) . However, even we have a collimating light source, owing to scattering property of the phosphor, the ACCTD is not truly small since the angular distribution for blue and yellow lights could be different. The second idea is that the LED die is selected as small as possible so that it can be regarded as a point source. Then the phosphor can be in a form of a hemi-sphere to reach equal path length, as shown in Fig. 2(b) [18]. Unfortunately, such cases are not possible for a white LED operated at high power since the LED is neither a plane wave emitter nor a point light source. Another design for obtaining a small ACCTD is to make an additional cavity to mix the blue and yellow lights [12,19]. One of the simple structures is to build up a cavity filled with volume scattering diffusing particle. However, such a structure may cause large backward scattering so that the packaging efficiency is lower than the other packaging structures. Actually, a white LED with low CCT perform the similar light mixing owing to larger scattering by the heavy-doped phosphor. Therefore, a low ACCTD can be easily observed in a white LED with low CCT. In contrast, when the CCT is above 5000 K, to achieve an extreme small ACCTD, the geometry of the phosphor in the package should be designed carefully.
Fig. 2 Two possible ways to perform low spatial CCT deviation. (a) A phosphor plate with an LED die emitting collimating lights. (b) A phosphor dome with an LED die as small as a point source.
3. Design and simulation
Since a planar structure of phosphor could not perform a small ACCTD, a special shape for phosphor structure should be found. The most reasonable approach is to deign a dome such as a hemi-sphere of phosphor to cover the blue die attached on a board. The problem causing large ACCTD comes from the size of the LED die, which is not a point. An alternative way is to figure out the effect of die size on ACCTD. In principle, if the phosphor dome is large enough, the effect by the die size may be reduced. To check the detailed effect on the size, a precise simulation should be made.
The simulation is based on a phosphor model that we developed [16]. From the phosphor model, we need to figure out the scattering property of the phosphor. Then obtain the absorption coefficient of the blue light through Monte Carlo ray tracing [20–22] with Mie scattering from the blue lights from the white LED detected with an integrating sphere. Finally, we can obtain the conversion coefficient by measuring the yellow lights from the white LED. Through the experimental measurement incorporated with Monte Carlo ray tracing, we built up a precise simulation program with important parameters such as phosphor particle size, absorption coefficient and conversion coefficient. Besides, phosphor concentration needs to be carefully adjusted for each phosphor dome to reach requested CCT.
In the simulation, the phosphor in the packaging was YAG-based. The dimensions of the LED die were fixed at 0.66 mm × 0.66 mm × 0.1 mm, which is one of the typical sizes of high-power LED. The phosphor dome was a hemi-sphere covering the die attached on a board, where the reflectivity of the top surface of the board was set 85%. Figure 3 shows a simulation of the ACCTD as a function of the diameter of the phosphor dome. As shown in the figure, the ACCTD reduces as the dome size increases. This makes sense for the initial design, but is not applicable in practical products if the phosphor dome is too large. An acceptable diameter of white LEDs is suggested to a limited size, e.g., 6 mm or below. A more reasonable way is to use an epoxy or silicone lens to cover the phosphor dome to protect the phosphor and also to enhance the light extraction [16]. Thus we made a simulation with adding a silicone lens with a diameter of 6 mm to cover the phosphor, where the structure is shown in Fig. 4 . The simulation result is shown in Fig. 5 . We found three important characteristics with introducing the silicone lens as illustrated below. First, the packaging efficiency increases from around 65% to 67%. Second, the CCT varies from near 6500 K to around 7800 K with the silicone lens. It is caused by the different light extraction efficiency for blue and yellow lights. The lens is more effective in extracting blue light, so the CCT increases with adding a lens [16]. Third, the ACCTD also increases according to the silicone lens. The reason is similar to that for variation of CCT. Thus the rise of the CCT causes the rise of the ACCTD. Therefore, the optimization for ACCTD should be made with adding a silicone lens. Besides, since the effect of unequal enhancement of light extraction in blue and yellow lights becomes obvious as the CCT is large, the optimization for cool white LEDs will be more complicated. So we focus on the simulation of the white LEDs with the CCT near 6500 K. The simulation was made by adjusting the phosphor concentration with the appointed geometry so the phosphor concentration is different for each phosphor dome size. Figure 6 shows the simulation result, where the ACCTD can be reduced to around 468K with the CCT around 6500 K.
Fig. 3 Simulation of both ACCTD and packaging efficiency of the white LEDs without lens, where the number in the figure is the CCT of the LED.
Fig. 5 Simulation of both ACCTD and packaging efficiency of the white LEDs with silicone lens, where the number in the figure is the CCT of the LED.
Fig. 6 Simulation of the optimized ACCTD based on the white LEDs with silicone lens, where the number in the figure is the CCT of the LED.
4. Experimental verification
The design for performing extreme small ACCTD is so critical that the dome size, phosphor concentration and die position should be controlled to a certain precise level. In the experiment, we have to adjust the phosphor concentration to match the designed CCT. The process for making the white LED with a phosphor dome is described as follows. First, the LED dies we chose was entitled EZ700 by CREE, and the peak wavelength was selected 449 ± 1 nm to fit the parameters in the simulation of phosphor. Through die bonding on a designed substrate of Al2O3, the LED with blue die was wire-bounded and was ready to attach a YAG-based phosphor dome. In the manufacture process, the phosphor dome was contained by a silicone lens. Thus we made a molding lens of silicone, where the central bottom side is a hollow dome for containing the phosphor. After the phosphor concentration was adjusted to the designed value, we dispensed the phosphor into the molding lens, where the hollow dome was faced up. The blue die on the board was attached on the up-side-down molding lens. Then the LED was put into an oven with the temperature reaching 150°C. In the experiment, we made white LEDs with CCT covering from 2800 K to 6500 K. However, it was not possible to use the yellow YAG-based phosphor to reach the CCT below 4000 K when the color coordinate of the white LED was restricted at the black body radiation curve. Therefore, we mixed a nitride-based red phosphor to reach warm white with a CCT around 3800 K and below. Figure 7 shows one of completed white LEDs.
In the measurement, the LED was located at the rotational center of a rotational stage. A spectrometer, which is at a distance of 150 cm from the rotational stage, was used to measure the CCT of the white LED, which was operated at 350 mA with thermal equilibrium. The measurement of angular CCT distribution is shown in Fig. 8 and the average ACCTD for each case is shown in Fig. 9 , while the lowest ACCTD in measurement is illustrated in Fig. 10 .
Fig. 8 The measured result of ACCTD for the LEDs with different CCTs. The lowest three CCT were performed by the white LEDs with two phosphors.
5. Further optimization
From the simulation as well as the corresponding experiment, we found that the smallest measurement in the ACCTD was lower than the simulated value. The reasons for the deviation could come from the simulation error in the phosphor model and the error in making the real samples. However, we found that the ACCTD can be further reduced since the measurement showed that the CCT along the normal direction was higher than others in the best case. It means that the more blue lights can be extracted in the normal direction. Therefore, a simple way to remove the phenomenon is to extend the vertical thickness of the phosphor along the normal direction so that the blue light in the phosphor dome can be depressed along the normal direction. Thus the phosphor dome will be no more a hemi-sphere. A further simulation for a white LED with the CCT of 6500 K was made by extending the phosphor thickness. The result is shown in Fig. 11 , where the extended thickness of the phosphor dome around 0.3 mm is dramatically useful in reducing the ACCTD. The ACCTD was found as small as 105 K when the extended thickness was 0.3 mm, as shown in Fig. 11. Besides, the packaging efficiency in the simulation was kept around 65%.
The corresponding experiment was made with a similar process as before. Figure 12 shows the measured angular CCT distribution of the samples. The LEDs with a CCT of 6101 K, 6301 K and 6463 K performed an ACCTD of 182 K, 200 K and 194 K, respectively. The extended thickness in the three samples were 4 mm, 3mm, and 3 mm, respectively. To our best knowledge, the ACCTD of 194 K is the smallest value in a real white LED with a CCT near 6500 K. In comparison to the white LED without the extended thickness of the phosphor dome, the ACCTD was reduced to around 200 K from around 500 K. The extended thickness of the phosphor dome indeed played an important role in depressing the blue light and reducing the ACCTD.
Fig. 12 The angular CCT distribution of the simulated and a real white LED with extended thickness of the phosphor dome.
6. Summary
In this paper, we have presented a study of white LEDs with extreme small ACCTD. The study started from a simple physics insight in angular CCT distribution and then designed the packaging structure of white LEDs with a phosphor dome at a shape of hemi-sphere. The optimization in simulation followed the developed phosphor model with real packaging parameters. Since a white LED with high CCT could perform larger ACCTD, all the optimization was done at a CCT around 6500 K. With adding a silicone lens on the phosphor dome, the ACCTD was achieved as small as 500 K. As shown in Fig. 9, the real packaged white LEDs performed an average ACCTD of 48 K (CCT / 2824 K), 50 K (CCT / 3144 K), 60 K (CCT / 3708 K), 186 K (CCT / 4568 K), and 232 K (CCT / 5879 K) for two phosphors, and 32 K (CCT / 4243 K), 48 K (CCT / 4587 K), 110 K (CCT / 4928 K), 152K (CCT / 5616 K), 332 K (CCT / 6252 K), and 488 K (CCT / 6541 K) for single phosphor.
The further optimization was made with adding an extended thickness of the phosphor dome to depress the blue light in the normal direction. The simulation for a white LED with the CCT around 6500 K showed that the ACCTD as small as 105 K could be achieved when the extended thickness was around 0.3 mm. The corresponding experiment echoed the simulation though the measured ACCTD was a little bit larger than the simulation, where an ACCTD of 182 K was found at a real white LED with the CCT of 6463 K. Through the designed packaging structure, to our best knowledge, these samples as well as the corresponding simulation perform the smallest ACCTD of white LEDs at a range of CCT covering from 2800 K to 6500 K.
Acknowledgments
This study was supported in part by the National Central University’s Plan to Develop First-class Universities and Top-level Research Centers Grants 995939 and 100G-903-2, and also was sponsored by the National Science Council of the Republic of China with the contracts of no. 97-2221-E-008-025-MY3, 99-2623-E-008-002-ET and NSC100-3113-E-008-001. The authors would like to thank Breault Research Organization and Howard Huang for the support of simulation with ASAP.
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