On 6 Nov 2005 07:43:26 -0800, "Shaun Eli"
wrote:

Here in NY they've started replacing traffic light bulbs with LEDs.
They're certainly bright enough (obviously it's a cluster, not just one
LED), and they have several advantages-- longer life, they tend to fade
a bit with time rather than simply fail, and they won't all fail at the
same time which is what obviously happens with one regular bulb.

Since the outside temperature in NY can reach close to a hundred
degrees in the summer daytime, obviously some LEDs work just fine at
higher temperatures.

The existence of LED-based traffic signals that work in
summer time temperatures does not contradict what has been
said earlier about the effect of high temperatures on LED
performance and life. Data sheets available from any
reputable LED manufacturer, such as Lumileds, include data
on the decrease in light output with increasing ambient or
junction temperature. The better data sheets use junction
temperature and also include data on the thermal resistance
of the LED package between junction and case. While few
manufacturers have data on LED life as a function of
junction temperature, some data is now available from
various independent lighting product evaluation
organizations.

100F, the temperature you quote is less than 38C, which is
not a very high temperature as far as an LED is concerned.
The issue in the design of LED systems is to insure that the
junction can dissipate its waste power, where the ultimate
heat sink is 25C or 38C.

--
Vic Roberts
http://www.RobertsResearchInc.com
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