IAEEL newsletter 3-4/97
Costs of LED traffic lights cut by half
Costs of LED traffic lights cut by half
Costs of light-emitting diode (LED) traffic lights are plummeting. Red LED lamps now cost half as much as they did a little more than two years ago, and green lamps, which were not on the market back then, are now offered for the same price as red lamps cost in late 1995. New problems with LEDs, such as a cut-off voltage that was too high, have been identified and solved.
In 1997, the City of Stockholm, Sweden, completed a technology procurement program for three-color LED traffic lights. By mid-1998, the city will have 2100 of these LED traffic-signals in operation, thus replacing about 6 000 tungsten halogen lamps.
Prices have been pressed considerably. Back in 1995, the IAEEL Newsletter reported that the cost for a red LED signal retrofit kit was $230. By late 1997, quotes for an order of 2 000 units are now at $145 for yellow and red, 20-cm (8") lamps, which is a little more than half the price that the lamps, with a much less sophisticated power supply, cost two years ago. The green lamps, which did not exist in 1995, are now offered for $250, down from $360 in July 1996, according to Sven Persson at Stockholms Entrepenad, the body responsible for maintaining traffic lights in the Swedish capital. Prices for green lamps are thus about twice as high compared with prices for the other colors, partly because up until fall 1997 there was only one supplier, Nichia of Japan. During the summer of 1997, however, Toyoda, also Japanese, entered the market.
Improved performance
The diodes and optics are steadily improving. The present 153-diode lamps are being succeeded by 90-96 diode lamps which are claimed to be "at least" as bright as the present lamps, though independent testing will have to confirm this. Power consumption will fall from 12 to 8 watts, including the power supply but not the equipment that controls a number of traffic light poles in an intersection.
In view of the low power consumption of LEDs, it would theoretically be possible to supply power with photovoltaics instead of from the mains. However, Sven Persson does not foresee such a development in the near future because solar cells tend to get stolen unless the PV poles are very tall. Stockholm City's policy is to replace lamps with LED kits each time a replacement is due, and the city is likely to soon adopt a plan for all-out replacement, according to Lars Söder at the Street Department.
Overcoming problems
It has not been smooth sailing the whole time, though: Because the power consumption is so low, stray currents in the power lines sometimes turn on a LED signal when it is supposed to be turned off. This problem was solved by a later specification that ensures that the current is cut off if the voltage drops below 150 V. Other recently developed specifications have focused on reducing harmonic distortion and increasing the power factor (at least 0.95 for the latter). "We are trying to be fore-runners in that respect too, says Söder." Conventional traffic lights have reflectors that can produce a confusing "phantom light" when the sun is low. LEDs do not have reflectors, so that problem was eliminated. But a new one took its place ‹ the so-called "washout effect". In direct sunlight the LEDs look pale or even white. This does not happen very often since Stockholm has kept the screen on all poles, so at least it is no worse now than before, although there is no overall solution to the washout problem. If the diodes are on a circuit card, it must be black.
Color blindness
LEDs have a more narrow spectrum than filtered incandescent light, and there has been some concern that this would result in reduced visibility for color-blind people, most of whom are men. Gösta Werner at the Swedish National Testing Laboratory does not believe there is much of a problem. "Purple-blindness, which is relatively common, has already been taken care of by the standard for red light which has not been changed by LEDs." Sven Persson adds that one of his co-workers with impaired green vision liked the first yellowish green diodes better, though they did not meet the standards. But he could still easily see the more bluish green lights which meet the standard and are now mushrooming up everywhere in Stockholm.
Fredrik Lundberg