I am building a plant light using 24 10 watt white LEDs and 80 5 watt red LEDs. I plan to throttle back my current sinks if the heatsink/frame gets too hot. What do you think the maximum case temperature I can use to have a reasonable lifespan for the LEDs?

 

Hello,

The lifetime versus temperature should be given in the datasheet of the leds.
The hotter the leds are, the shorter the lifetime.
Perhaps forced cooling would be an option.(using a fan on the heatsink).

Bertus

 

For long life I shoot for no more than 60-65 deg C measured on the top of the aluminum pad they are mounted on as close to the plastic housing as possible.. (basically right on the +/- stamping in the led posted below)
LED Junction temps (usually about 20deg C hotter than the heatsink) should never be more than 85 deg C if you want to get the "rated" 50K hours out of them..

Thats assuming an led like this http://www.mpja.com/1000-Lumen-10-Watt-Super-Bright-Cool-White-LED-Module/productinfo/31325 OP/

 

Also, the LED is less efficient at higher temperatures. Again, consult the data sheet for the LED's you are using.