LED retrofit lamps have started to show up on retail shelves. Many players in the LED industry are giddy about the anticipated growth over the next few years. Nevertheless, the road to success for LED lighting will not be completely smooth, as there are a number of pitfalls to navigate.
Bumps in the road ahead for solid-state lighting article gives a picture of the expected pitfalls. Some of the potential problems ahead are pricing, color quality, thermal management, regulatory, and consumer education. While most of these issues are not technical, the design engineer will nonetheless need to understand them.
While white LEDs are very efficient light sources, converting approximately one-third of the input power into light, the remaining two-thirds is converted into heat in the LED. Currently impossible to passively cool an LED that outputs 1,500 lm (the typical output of a 100-W light bulb) in the physical confines of the normal light bulb size form factor. So at 100 lm/W, about 10 W must be continuously and rapidly dissipated while keeping the LED well below maximum operating temperature (typically approximately 120ºC).
Thermal management will get somewhat easier in the future. As LED efficiencies improve, the thermal management improves by approximately the square of the efficiency, because the total power supplied to the LED decreases and the percentage of heat generated by that input power also decreases by the same amount.
Color quality may be the most difficult problem to solve. The industry has spent tremendous time and expense in measuring and controlling the color variability of white LEDs. But color temperature and tight chromaticity binning don’t tell the complete story, because two light sources with identical chromaticity coordinates may have very different wavelength spectra. LED spectrum is very different from the incandescent’s spectrum. If the spectra are too different, non-white surfaces will appear to be different colors under the two light sources. The Color Rendering Index or CRI is a measure of how closely the perceived color of a surface illuminated by a particular light source will be to the perceived color of the same surface under incandescent illumination. A CRI of 100 is a perfect match. A CRI above 80 for an LED is considered good.
In the short term, LED retrofit bulbs will make the initial splash, but in the long term there are great opportunities for custom LED luminaires. LEDs make possible much more complex form factors and consequently can create more interesting and useable illumination patterns than traditional bulbs and CFLs. Imagine a luminaire that not only is dimmable, but one that you can select the color temperature you desire.
When LED efficiencies reach the 150 lm/W range, it will become feasible to increase office lighting to 1,000 lux, as opposed to the 300 lux now typical in most office spaces.