Figure 1: The human eye’s sensitivity as a function of electromagnetic radiation wavelength.
High-brightness LED makers take the human eye’s response into major consideration when designing their products. They appreciate that it is of little use if a device is turning electrical power into wavelengths of light that are barely visible. That is why white LEDs absorb blue light from the central blue device and then re-emit it across a broad range of visible wavelengths comprising some green and red, and a lot of yellow.
Figure 2 shows the spectrum from a high-performance white LED – in this case an OSRAM OSLON SSL device – superimposed on the human eye’s sensitivity function (Vλ). Note the peak at 442 nm that corresponds to light directly emitted by the blue LED that hasn’t been subject to Stokes shifting by the phosphor (see TechZone article “Whiter, Brighter LEDs”).
This peak occurs at a point where the human eye’s sensitivity is poor, so it fails to contribute significantly to the device’s apparent “brightness”. For today’s high-efficacy LEDs, this “wasted” light is not a big problem because they produce a secondary peak centered on 555 nm – right where the eye’s sensitivity is highest. (This isn’t just a happy coincidence; manufacturers have spent a lot of research dollars “tuning” their phosphors to ensure the light emissions occur at this optimum point.)
Figure 2: Relative output from OSRAM OSLON SSL white LED compared with the human eye sensitivity function. Note the peak at 442 nm (due to non–Stokes Shifted light from blue LED) occurs where eye’s sensitivity is poor but the secondary peak coincides with highly sensitive region. (Courtesy of OSRAM.)
The key parameters of white LEDs
Engineers are spoiled for choice when it comes to choosing a high-performance white LED for their lighting projects. Companies such as Cree, OSRAM, and Seoul Semiconductor produce highly-efficient devices at prices that continue to drop.
The manufacturers have also made comparing devices simple. Checking the datasheet reveals key facts such as the luminous flux (in lumens) and efficacy (in lumens/watt). The luminous flux is a measure of the visible energy or apparent brightness of a light source and as such takes into consideration the sensitivity function of the eye. (Luminous flux differs from radiant flux, which is a measure of the total power of the electromagnetic radiation emitted from the light source, including infrared, visible light, and ultraviolet.)
From a purely scientific consideration, there are several ways to measure the luminous “efficacy” (i.e., the ability to achieve the desired result) of a light source. However, as we are concerned with mainstream lighting here, the most useful definition from an engineering perspective is the ratio of luminous flux (lm) emitted to the electrical power used to power the light source (W).
Luminous efficacy (lm/W) is a key parameter for LEDs because it allows a direct (and typically flattering) comparison with other forms of lighting such as incandescent and fluorescent bulbs (see TechZone article “Material and Manufacturing Improvements Enhance LED Efficiency”). The measure also takes into account how much power is wasted generating heat and nonvisible electromagnetic radiation rather than visible light.
Armed with such information, an engineer is able to select a white LED to suit almost any application.
Cree’s XLamp® XP-G2, for example, is typical of the remarkable devices now at the lighting engineer’s disposal. Using a drive current of 350 mA, the XP-G2 can produce 122 lm and an efficacy of 124 lm/W. OSRAM and Seoul Semiconductor manufacture similarly impressive devices. It is no surprise that LEDs with an efficacy in excess of 120 lm/W are making rapid inroads into the mainstream lighting market. Traditional incandescent bulbs are hard pressed to reach an efficacy of 10 to 18 lm/W. LEDs also perform a little better than compact fluorescent lights (CFLs) (35 to 60 lm/W), and on a par with fluorescent tubes (80 to 100 lm/W).1
Efficiency, not efficacy
LEDs are a fledgling technology and the market is highly competitive, so chip vendors are falling over each other to announce technological improvements that increase the efficacy of their LEDs (see the TechZone article “LED Efficacy Improvement Shows No Signs of Slowing”).
Nevertheless, there is still plenty that can be done. It turns out that the theoretical limit for white LEDs is around 263 lm/W.2 (Incidentally, the highest possible efficacy of any light source is 555 nm green light – the wavelength to which the human eye is most sensitive – which can reach 683 lm/W.3)
Remote phosphor is one technology that is gaining in popularity. The technique builds on research by Rensselaer Polytechnic Institute that revealed moving the phosphor away from the blue die improves efficacy by up to 30 percent (see TechZone article “Remote Phosphor Offers Alternative to White LEDs”). Commercial solutions are available to put the concept into practice in the form of phosphor discs or spheres. One such product is ChromaLit from Intematix.
The phosphor disc must be used with a royal blue LED that acts as a source of photons. There are plenty to choose from, but things can get confusing for a designer who is only familiar with white LEDs.
Royal blue LEDs are highly efficient – approximately 50 percent of the input power is converted to photons – but, as noted above, the human eye just does not see most of the emitted radiation. Consequently, the luminosity is very low, of the order of 10 to 15 lm, and efficacy (in terms of production of visible light per watt of electrical energy) is similarly limited.
However, the poor luminosity of royal blue LEDs, used in both conventional white LEDs and remote phosphor designs, is of minor consequence as the device itself contributes little to the overall illumination of the lighting fixture. Rather, the LED acts as a photon pump, emitting a stream of radiation that is absorbed by the phosphor and re-emitted at wavelengths to which the human eye is most sensitive. It is the luminosity of the phosphor that is most important, not that of the LED photon pump.
The datasheet for a device such as Cree’s royal blue XLamp XT-E LED (Figure 3) does not include a figure for luminous flux. Instead, the output is defined as a radiant flux of 475 mW and there is no mention of efficacy; rather, the manufacturer cites an efficiency of 501 mW/W. Similarly, OSRAM’s royal blue OSLON SSX LED, is rated with a radiant flux of 160 mW and an efficiency of 346 mW/W. (Note that efficiency here is a dimensionless (percentage) measure of power in over power out.)
Figure 3: Cree’s royal blue XLamp XT-E LED is suitable for remote phosphor lighting applications.
Figure 4: Excitation and emission spectra of YAG phosphor compared with output of royal blue, green, and red LEDs.
Figure 5: Shift in dominant wavelength against junction temperature for blue, green, and red LEDs. (Courtesy of Seoul Semiconductor.)