Lighting : Lighting October 2016 - Vol 36 Issue 5
October/November 2016 | LIGHTING MAGAZINE 21 rendering index, CRI). Hence, for the purpose of general illumination, this paper will discuss the luminous efficacy of white LEDs. There are two common white LED technologies: colour mixing (some call it direct emitting) and phosphor- converted. The former is achieved by mixing the light from red, green and blue LEDs and the latter usually uses a blue LED sources and a phosphor to convert some of the blue radiation into greens, yellows and reds. Even though there are various ways of colour mixing (such as adding an amber LED) and phosphor conversion (such as using different colours of phosphor coating or using UV LEDs instead of blue LEDs) to better adjust the output spectrum, they are still the same fundamental technologies; the paper will consider their theoretical maximum values. RGB white LEDs The efficacy of RGB LEDs is strictly constrained by each underlying LED emitter. By using the CQS Calculation Spreadsheet (v9.0.3) jointly designed by Dr Yoshi Ohno and Dr Wendy Davis in 2013, the maximum luminous efficacy of radiation (LER) achieved by this method is around 403 lm/W (3000K, Ra=85). Note different local maximum efficacy could be achieved when starting from different initial wavelengths, according to the instructions for the spreadsheet. The Full Width at Half Maximum (FWHM) data shown in Table 1 and Figure 2 is from the example used by Dr Yoshi Ohno3. The result is actually a little higher than the US Department of Energy (DoE) efficacy projection due to wider green diode FWHM. Phosphor-converted (pc) white LEDs The most common architecture is the combination of blue LEDs and yellow phosphor coating/dip. Some of the blue photons excite the phosphor that has Figure 1. The relative sensitivity of the human visual system to different wavelengths2. a broad emission in the yellow and red regions, producing white light. The CQS spreadsheet was used to calculate the theoretical maximum efficacy of pc-LED. Many variables need to be considered, so instead of using a blue pump-yellow phosphor LED, a blue pump-red+green phosphor LED was used, coinciding with US DoE’s solid-state lighting multi-year plan. The theoretical maximum efficacy (LER) achieved is around 390lm/W Figure 2. The output spectrum of the RGB white LED.
Lighting August 2016 - Vol 36 Issue 4
Lighting December 2016