Epistar: Lighting Class Warm White LED with High CRI and High Lighting Efficiency Already Been Achieved — LEDforum 2009 Report (8)

Boosting advantages such as high efficiency and long lifespan, LEDs are playing an increasingly important role in today’s lighting industry. In the future, with the  packaging and chip technologies keep evolving, the production cost of LEDs will be lowered and the luminous efficiency enhanced, which will help the new light source find its place in even wider applications, said Huang Zhaonian, the Vice President of Taiwan’s Epistar Corporation, at the LEDforum 2009.

Huang noted that as an energy-saving light source which eliminates the hazardous waste to the environment, LEDs have received widespread attention and been considered as one of the leading roles in the future lighting industry. However, to achieve the target of replacing traditional lighting sources, more innovative technologies are yet to be developed to make LEDs meeting the existing lighting requirement of users. Epistar expects that new technologies will accelerate the development and promotion of solid state lighting in the market, and subsequently contributes to reduction in global carbon emission.

Requirements on lighting class LEDs

Mr. Huang, a specialist in development and production of nitride-based epitaxy and crystal processes, pointed out that for residential lighting applications, the color temperature of 60W incandescent lamps is 3000K, and the color rendering index (CRI) is close to 100; while the 21W fluorescent lamps with a color temperature of 3000k provides a CRI of around 83. The 50W MR16 halogen lamp, with the same color temperature of 3000K, offers a CRI close to 100. To eventually replace these three types of light sources, LED lighting must provide comparative performances in these indexes while not sacrificing the luminous efficiency, which will be a challenge for the manufacturers.

The luminous efficiency and light quality are two important performance indexes for lighting class LEDs, Mr. Huang noted. And the factors influencing the luminous efficiency of LED lighting include the original efficiency of the LED, the IC efficiency, the efficiency of the lighting fixtures and the heat management of the lamp, etc. The quality of light, on the other hand, is affected by the light form, the reliability and endurance of the LED.

Currently, the luminous efficacy of LED components had been substantially improved. For the whole structure of an LED lamp, when it is powered on, the efficiency of the power driver is 90%, that of the electronic circuit is 95%, the LED IQE efficiency is 90%, and the LEE efficiency is 90%, etc. The ENERGY STAR criterion also requires the conversion efficiency of phosphor for LEDs to reach 73%. Assuming a 90% efficiency for the heat diffusion and 92% for lighting fixture and optical efficiency, the actual output of LED lamp would be significantly reduced when the energy loss are added together. Therefore, besides increasing the luminous efficacy of LED components, the manufacturers must pay more emphasis on other parts of the whole lamp in order to provide LED lighting products with higher specifications.

The three innovative technologies for lighting class LEDs

Based on Epistar’s experience and technology, Huang pointed out that there are three innovative methods in production of LEDs for lighting applications; one is the AC LED, and Epistar has introduced relevant products based on this method. The AC LED is produced by combining several LED chips together, and it can be driven directly by the AC power, eliminating the need for external current conversion device. For the AC LED, its size and the number of constitute LED chips could be customized to meet the specific requirements. And the current luminous efficacy of such LEDs has reached 56-65 lm/w. The AC LEDs can be used in 4W AC LED candle lamp (for replacement of 40w incandescent bulb), 5W AC LED bulb (for replacement of 60W incandescent bulb) and 15W AC LED bulb (for replacement of 50W PAR 38 bulb).

And the second method is called HV LED, a new process by positioning the LED matrix on a single wafer. The HV LED features adjustable voltage and current, which enables a flexible design. The advantage of this type of technology is that the LED is based on single chip, thus brings the single light output effect. The wire bonding and the driving current on single chip are also reduced, which improves the optical design and simplifies the LED packaging procedures. In addition, the better current diffusion technology also provides higher luminous efficiency than AC LED. The cost and the loss in current conversion are also lowered, making the technology another innovative LED lighting solution. 

The third innovative technology for lighting class LED is hybrid white light LED. This type of LED provides remarkably improved performance in CRI and luminous efficacy. The process is to use blue GV LED combined with special phosphor and red light LED to form a new type of hybrid white light LED.

Comparing the CCT, CRI and luminous efficacy of the three types of LED, it could be found that the CRI of the cold white and warm white LEDs based on blue light LED and yellow phosphor is 70 and 48 respectively, lower than the 89 for white LED based on blue LED combined with yellow and red phosphor, and also lower than the 85 for hybrid white LED, Huang emphasized.

The luminous efficacy of LEDs will continue increasing in the next few years, he projects. At the same time, the white light AC LED has already achieved a luminous efficiency of 65lm/W. And the new hybrid white light LED has realized a high efficiency warm white light with a CRI of 90 and a lighting efficiency of 106.8 lm/W.

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