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Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Steve McConner wrote ...
Yikes! I guess you figured out that black things can be burnt by visible light too, it doesn't need to be infrared.
I wonder if there's some law of optics that limits how fine you can focus it, given that the light is relatively diffuse and coming from a large area, compared to something like a 100W short arc lamp.
Yeah, but how do you know the burning is from the light and not from the heat dissipation. The black material looks like its right up against the LEDs. An old fashioned 100W incadescent light bulb will do the same thing too (unfocused) if you lay a black shirt over the bulb.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Dr. GigaVolt wrote ...
Steve McConner wrote ...
Yikes! I guess you figured out that black things can be burnt by visible light too, it doesn't need to be infrared. ...
Yeah, but how do you know the burning is from the light and not from the heat dissipation. The black material looks like its right up against the LEDs. An old fashioned 100W incadescent light bulb will do the same thing too (unfocused) if you lay a black shirt over the bulb.
Good question. But the incandescent bulb depends on internal temperatures in the thousands, while LEDs won't last long or work very well if their junction temperature exceeds 145 degrees C.
Notwithstanding the title of Ray Bradbury's book "Fahrenheit 451", says Wikipedia, the temperature to ignite paper is more like 450 degrees C. So any thermal conduction or convection between TDU's toasted targets and his lumeniferous LEDs would tend to cool the targets and heat the LEDs.
Registered Member #10
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
The cloth wasn't touching. I didn't want to get melted plastic in contact with the LED. Nor did I expect or want the LED junction temp to go above the rated 80C. To burn the CD case I needed to have a little polystryene foam surround to prevent the airflow from the LED fan from cooling the CD case.
Interestingly the LED surface is of a slightly soft substance more like silicone.
Note that it is about 5 times as hard to burn anything with a 100W LED as it is with a 100W incandescent. An incandescent radiates most of the heat as infrared like perhaps 90W (guess) and 5W visible. compared with a LED of 0W IR and 16W visible.
Registered Member #1497
Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801
I remember reading on the cree led website somewhere that their SMD LED's were sensitive to soldering via reflow, and didn't want the temperature to rise over a certain point, due to the clear epoxy they use. I'm guessing that the slightly rubbery surface is so that as the die heats, the bonded wire and connection(s) don't get strained due to expansion.
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