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This study proposed a correlated color temperature (CCT) tunable phosphor-converted white light emitting diode (LED) with an omni-directional reflector (ODR). Applying current to each individual InGaN based ultraviolet, purple and blue source LED chip of the white LED package, we can achieve the CCT tunability. The optimum color properties of the resulting white light are (0.3347, 0.3384), 5398 K, 81, 3137~8746 K for color coordinates, CCT, color rendering index (CRI) and CCT tuning range, respectively. Roughening the ODR substrate, we solve the non-uniformity color distribution caused by the reflectance of the ODR and positioning of source LED chips.
©2009 Optical Society of America
Introduction
The basic white LED package is the phosphor-converting white LED. Excitation LEDs are coated with a phosphors resin blend for color mixing. Conventional white LEDs fabricated using color mixing of emission spectra from YAG:Ce yellow phosphor and blue LED excitation source. Although this method has the advantage of high phosphor conversion efficiency, a excessive CCT results from the high transmittance of the blue light [1]. To achieve high color rendering and better spectral characteristics, white LEDs can also be made by coating an ultraviolet (UV) LED with a mixture of multi-color phosphors. In a white LED with UV excitation, the disadvantages are low phosphors conversion efficiency caused by large Stokes shift and UV leak from the package [2]. However, real-time CCT tunability of phosphor-converting white LEDs cannot be obtained using phosphor-based white LEDs. To overcome this issue, a multi-chip package may be the choice for CCT tunability. To achieve a CCT tunable rage of 3000-6500K, the warm white (WW) and cool white (CW) LEDs are combined to tailor the CCT and color properties by individual drive circuitry [3].
Multi-chip packages offer the possibility of achieving CCT tunability like tri-color LED packages. Primary color red/green/blue (R/G/B) LED chips are deposited in the same package and tri-color mixing is controlled by each drive circuit to produce individual color properties. This method can produce monochromatic color with high color saturation and white light with a CCT of 3010~6762 K. However, low color rendering results from the narrow bandwidth of the emission spectrum of the color LED. Moreover, for different color LEDs, each epitaxial material used tends to degrade individually with heat and age, and the different degradation rates cause color changes in the mixing light. Therefore, complex issues of aging and drive circuitry are associated with tri-color LED packages [4]. To have high color rendering, Lumileds [5] use multi-chips of R/G/B/amber LEDs to mix the four colors for removing spectrum discontinuity between red and green light. However, the epitaxial material AlInGaP for the amber LED is more susceptible to thermal effects causing an increased rate of degradation. Specifically, as the AlInGaP LED junction temperature rises, the radiation power at 90°C is around 30% of that at 25°C. Therefore, to have enough amber LED power for color mixing, the drive circuitry must be more complex.
N. Narendran et al. [6] use a combination of WW/G/B LED chips to obtain a continuous emission spectrum and better color rendering. Using green and blue chips achieves a high CCT, but chromaticity coordinates shift and light intensity decays causing a low luminous efficacy for the WW LED. Furthermore, a combination of R/G/B/CW LEDs [7] can have the same color rendering and high luminous efficiency as a WW/G/B LED package. However, the same issues of aging and degradation for color LEDs are encountered.
To overcome the problems encountered in previous studies, this study proposed a new packaging structure by combining the advantages of a multi-chip package and phosphor-converted LEDs. The proposed package is comprised of multiple epitaxial growth InGaN chips including UV, purple and blue LEDs. Two individual spectra, emitted from red and green phosphors, excited by UV and purple wavelengths respectively, mixed with the light from the blue LED; compose the white light. The applied currents and power fractions of each source LED can be varied to achieve CCT tunability. Therefore, the emission spectrum of the white LED comprises the emission spectra of each phosphor and blue LED, tailored by individual applied current. Using the source LED chips from the same epitaxial material system, InGaN, we can mitigate the variation of light color and output power caused by the thermal degradation effect of different epitaxial growth materials. Moreover, the drive circuitry is simpler than other tunable multi-chip white LED’s.
Experiments
For UV pumped white LED, Su et al. [8] have demonstrated that an ODR block UV leak and recycle the unabsorbed UV light to re-pump the phosphors until the UV light is exhausted. This study excited the phosphors of the proposed white LED by UV and purple LEDs with a wavelength of 372 nm and 410 nm, respectively; and mixed with a blue LED at 465 nm. Since the UV LED causes UV leak from the package, therefore, an omni-directional reflector (ODR) was implemented to enhance luminous efficacy and recycle unabsorbed UV light, as shown in Fig. 1 .
To have good matching between phosphor excitation and source LED emission spectrum, the chosen phosphor should only be excited by the LED chip with specified wavelength. Since white light is composed of three RGB colors, the red and green proportions of the composite spectrum are the emission spectra of the individual phosphors. For white light in which the blue LED provides the blue light, the red and green light are emitted from the Mg8.5(AsO4)2(AsO6)O2 and Ca8Mg(SiO4)4Cl2O:Eu, respectively. Although having better color rendering, the yellow YAG phosphor was not chosen to widen the color gamut for its blue light excitation. Table 1 shows the expected peak emission wavelength and peak excitation wavelength for red and green phosphors, respectively. Figure 2 shows integrating sphere measurement results for the emission spectra of red, green phosphors and blue LED individually. Mixing the emission spectra of the phosphors and the blue LED, the sum of total spectrum demonstrates a continuous curve of white light in Fig. 2. Therefore, we can study the feasibility of color temperature tuning for white LED with better color rendering.
Table 1. Emission peak wavelength of phosphors by source LED
Results and discussion
In Fig. 3 , comparing the emission spectra of white LEDs with and without ODR shows that the white LED with ODR reduced UV leak from the emission spectra. However, to have continuous emission spectrum and better color rendering, choice of appropriate phosphors is required.
Figure 4 shows the international Commission on Illumination (CIE) 1931 color space, and the point D65 represents the CIE standard illuminat D65 as the reference light source for this study. With the chromaticity coordinates of test points, Fig. 4 shows the range of tunable color gamut by applying different current to each source LED chip. The variation of individually applied current affects the chromaticity coordinates and CCT of the test points. To demonstrate the CCT tunability of the proposed white LED, a total of seven test points or conditions were labeled (see Fig. 4) and the color characteristic properties listed (see Table 2 ). From Table 2, the tunable color temperature is 3137 K~8746 K, including the full range of color temperature in daily life applications. Moreover, the range of CRI is from 57 to 84. Specifically, the color properties of the fourth test point are closest to the D65. The color properties for the point are (0.3347, 0.3384), 5398 K and 81 for color coordinates, CCT and CRI, respectively.
Table 2. Color properties of each test point
The non-uniform radiant power distribution can result from the reflectance of ODR, which slightly reduced blue light transmittance at normal incidence; and the positioning of the source LED chips. To reduce the non-uniform radiant power pattern, the opposite side of the ODR substrate was randomly roughened using sandpaper with average particle diameter of 100 μm to diffuse the emitted light pattern. The peak-to-peak roughness of the diffuser made on the ODR substrate surface is about 3.4 μm. Figure 5 shows the measured angular dependent CCT, and Fig. 6 shows the chromaticity coordinates (x, y) and output radiant power (Pout) distribution. The angular luminescence distribution was around ± 80 degrees and the range of the CCT is 6284~7499 K within an emitting angle of ± 70 degree. Obvious enhancement of uniformity of color distribution occurs.
Fig. 6 Chromaticity coordinates and output power distributions of white LED with a diffuser and ODR.
Figure 7 shows the electrical measurement of the white LED. Under a constant 20 mA drive current, the applied voltages of the source LED with an emission wavelength of 372 nm, 410 nm and 465 nm were 3.353 V, 3.190 V and 3.146 V, respectively. Therefore, the source LED chips with the same epitaxial growth material can get approximately the same applied voltage. For different test points in color space, the different drive currents of different source LED chips are required. However, the voltages for the source LED chips are in the same range of 3.1~3.8V for the applied current of 20~80mA. This reduces the complexity of drive circuits and results in the same thermal degradation rate.
Conclusions
This study proposed a white LED package for real–time color temperature tuning. The package was composed of source LED chips with wavelengths of 372 nm (UV), 410 nm (purple) and 465 nm (blue). The red and green phosphors were used for the emission of red and green light respectively. The color mixing of blue light from a blue LED with red and green light from phosphor emission has color properties close to CIE standard illuminant D65. The optimum data for the test color properties are (0.3347, 0.3384), 5398 K, 81 for chromaticity coordinates, CCT and CRI, respectively. Using ODR to block and recycle the UV light solved UV leak and enhanced luminous efficacy. The applied current of each source LED chip enables CCT tuning in the range of 3137~8746 K. The electrical measurement of the I-V curve confirms that the white LED package can have similar applied voltages for source LED chips and avoid drive circuit complexity and the degradation rate problem caused by the different epitaxial material systems. This study solved the problems of non-uniform radiant power and CCT distribution resulting from the wavelength dependant reflectance of ODR and positioning of source LED chips, by roughening the opposite side of the ODR substrate. The range of emitting angle of uniform CCT distribution can be enhanced to ± 70 degrees. In this angle range, the variation of CCT is 6284~7499 K; and uniformity of CCT distribution is obviously enhanced.
Acknowledgement
This work was supported by the National Science Council of the Republic of China under contract No. NSC 95-2221-E-011-124 and NSC 96-2221-E-011-050-MY3.
References and Links
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