As mentioned the LED chip in your LED light bulb is sensitive to high temperatures such that overly high temperatures of > 85°C will significantly shorten the life of your LED bulb as well as the lumen output or brightness. For example, the lumen maintenance for a BRIDGELUX ES BXRA-W0802 with a case temperature of 55°C or 85°C is approximately 94% over 6,000 hours, whereas at a case temperature of 105°C the lumen maintenance is approximately 91% of the original brightness. This equates to a decline to 70% brightness by approximately 20,000 hours at 105°C versus approximately 40,000 hours at 85°C. At case temperatures greater than 105°C the LED will fail within a relatively short time, potentially approximately 2,000 hours.

Therefore, due to the importance of LED case temperature thermal management techniques in the form of passive and active cooling methods have been developed to maintain the temperature of the LED chip temperature within normal operating temperatures, typically between at 55°C and 85°C. On LED manufacturer data sheets this is typically referred to as LED case temperature (T_S).

In our laboratory tests we have found that LED heat sink temperature is generally equal to LED case temperature below approximately 60°C in lower power LEDs for 6-8W. Although there are some exceptions with older less efficient models with poor heat sink design. In general a poor heat sink design will have difficulty conducting heat away from the LED chip leading to a much higher LED case temperature compared with the LED heat sink temperature with a differential of up to 20°C.

In higher power LED light bulbs (10W+) such as 10W 500lm+ we measure case temperatures of approximately 70°C, whilst heat sink temperatures are approximately 62°C. What this tells us is that as the LED chip gets hotter the ability for the heat sink to transfer heat away from the LED chip is reduced. LED bulbs with a heat sink temperature of greater than 70°C are probably reaching the limits for safe LED functioning. 

Therefore, proper thermal management and heat sink design is paramount and will separate the “winners from the losers” in this emerging industry. 

References:

Bridgelux LM80 Report: 6500 Hour Test Data, Bridgelux RS LED Array BXRA -W3000, November 2, 2011. Innovative Circuits Engineering Inc.