Investigation of dynamic color deviation mechanisms of high power light-emitting diode
Introduction
Since Nakamura et al. first developed visible blue and green indium gallium nitride (InGaN) double-heterostructure light-emitting diodes (LEDs) in 1993, industry has focused on producing white-light LEDs with red chips or phosphors [1]. Four technologies for mixing light to produce white light exist. The first technology uses three-color LED chips to generate high color rendering index (CRI) and tunable color. Color deviation is due to the different decay rates of three color LED chips; however, the color deviation can be compensated by feedback control [2], [3]. The second technology uses a blue chip and yellow phosphor; the yellow phosphor is excited by a blue chip, producing white light by mixing of non-absorbed blue light. Due to the wavelength distribution between blue and yellow light, the CRI is worse than other technologies. Without proper mixing of blue and yellow light, the white light may be bluish or yellowish. Nevertheless, the low-cost of this technology has made it the most popular method for producing commercial white-light LEDs [2], [3], [4]. The third technology uses a blue chip and two color phosphors such as green and red phosphors. As green and red phosphors have different decay rates, the color deviation occurs after long-term use [2], [3], [4], [5]. The fourth technology uses an ultraviolet (UV) chip and three-color phosphors. This UV light excites the three-color phosphors, such as red, green, and blue, to generate white light with a high CRI. However, during usage, UV light may damage the package materials, such as silicone. The leak of UV light is a subject to be solved in this packing technology. Notably, color deviation is due to the different decay rates of three-color phosphors [6], [7].
For active light sources of devices, such as multi-color LED chips, the color deviation can be compensated by feedback control which composed of a detector, color evaluation system, and circuit control. For passive light sources of materials, such as phosphors, the endurance of material properties is the important thing to diminish color deviation. Because of the saving of energy, the solid-state lighting becomes a trend to replace conventional lighting. The high and stable quality of white light is demanded in the solid-state lighting. In order to obtain high color reliable white light LEDs, we must understand the mechanisms of color deviation. Therefore we demonstrated several experiments of LED lighting test, material verification, and LED aging test in this report.
Section snippets
Experiments
In order to investigate the color deviation mechanisms of different operation durations, this study divides operating time of white-light LEDs into three durations – transient (in a few seconds), short-term (<1000 h), and long-term (>1000 h) experiments [8], [9], [10]. The commercially available 1 W blue high-power LED chips in this study were grown on a sapphire substrate (0 0 0 1) sized of 1 × 1 mm2. The yttrium aluminum garnet (YAG) phosphor, silicate phosphor, and silicone commonly used in LED
Results and discussion
In the transient experiment, the white-light LEDs spectra were measured under gradually increasing electrical power with current increasing from 100 mA to 700 mA. Fig. 2a and c shows the spectra of remote packages of white-light LEDs composed of YAG and silicate phosphors, respectively. When the absorption of phosphor keeps constant, the greatly increase of blue intensity caused the same greatly increase of yellow intensity. These intensity increases did not influence the white-light color. For
Conclusion
This study identified the color deviation mechanisms of white light LEDs in different operation durations. The shift of blue peak exciting wavelength and heat influence were the main reasons of color deviation in transient duration. Color deviation was due to decayed phosphor quantum efficiency; however, color deviation due to transparent silicone must also be considered. The yellowed reflector and damaged silicone warmed color. After these studies, we provide two suggestions to obtain high
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