LED developments
LED developments
The light-emitting diode (LED) market for illumination applications is
developing fast. The year 2000 has seen a number of technical developments
as well as interesting applications. There has been some disappointing
news, too. Below a few highlights from the news flood:
LED technology
- In November 2000, Lumileds announced that it has developed a power
package that doubles the power output and efficiency of their AlInGaN
LEDs to as much as 25 lm per LED. Typical values for the Lumileds High-Flux
LEDs are now: Blue, 100mW, 5-8 lm; Green, 40mW, 10-20 lm; Blue/ Green,
70mW, 15.25lm. LEDs in the blue spectrum are particularly interesting,
since white LEDs are based on this technology.
- The Center of Photonics Research in Boston, USA, reports on developments in LED technology that promise an efficacy of 330lm/W. The source is a so-called photon-recycling semiconductor that generates light in two wavelengths, blue and orange. The emitted light is perceived as white. These efficiency numbers are a factor of ten - and more! - higher than LEDs on the market today. It should be stressed, however, that the road from the lab to a commercialized product may be very long.
Applications and products
- Osram OS has developed a low-profile LED module called Marker LED.
This product measures only 6 mm in height and consists of side-mounted
LEDs that feed light into a light-guide material that emits light evenly
from all over its surface (see Photo). The module can be used by luminaire
manufacturers for built-in products that provide orientation in walkways,
stairs, and seats (e.g. in cinemas and theatres). The energy consumption
of the module ranges from about one watt to a few watts (depending on
the size), and it generates virtually no heat. Any color LED can be
used with the product.
- Another module from Osram OS has its roots in the railroad sphere:
Osram developed a small, very narrow-beam unit with ten LEDs behind
thick, focusing lenses to be seen at long distances. The unit emits
light in a four-degree angle. The unit is designed to operate on 24V,
and in amber and yellow colors the unit only draws 1.2W with a typical
luminous intensity of 750cd (1.5W, 55cd for blue and green units). It
is now marketed by Osram for accent lighting for buildings and monuments.
- The oil company BP has chosen LED products when rebranding its 20000
US service stations. BP will use Lumileds' "Chipstrip" contour-lighting
system that replaces neon tubes. Chipstrip is basically a colored plastic
tube with colored LEDs inside. According to Lumileds, the Chipstrip
system will have about five times the life of neon lamps and is more
energy-efficient.
- Austrian lighting design firm Bartenbach Lighting Laboratory has
performed a large-scale test in which a whole room was lit by a mix
of 14 000 white and colored LEDs. Light levels of 600-700 lux were achieved,
adequate for a normal office. A computer was used to calculate the mix
of white, blue, blue/green, green, amber and red diodes to generate
a relatively warm light of 2500-3000K, with color rendering very close
to that of the best fluorescent lamps.
- In Sweden, the Swedish National Road Administration continues pilot installations with LED traffic guiding poles that reduce the need for traditional overhead lighting. Following a successful first pilot installation, the LED poles will now be tested along another 30 km of roads with high auto accident rates. Traffic safety is the main driver for the project, and overhead lighting is reduced in areas where it makes visual conditions worse in case of frequent fog, heavy snowfall, and rain. The reduction - and in some cases elimination - of overhead lighting leads to drastic energy and cost savings.
Short life
- The US-based Lighting Research Center (LRC) reports that the life
of white LEDs appears to be much shorter than had been hoped for. LED
manufacturers are already careful in promising long life other than
in applications where ambient operating temperatures can be kept cool.
Even so, LRC tests found that some LEDs had their light output depreciated
to 65% after only 4000 hours, and to as little as 45% after 6 000 hours.
Manufacturers and researchers alike claim that the problem is mainly
due to the epoxy package in which the LED itself is housed. The epoxy
is darkened when exposed to small amounts of UV from the LED.