For testing an IR LED cluster lamp design for safe current, run one for at least half an hour, preferably an hour at 40 mA per LED. Measure the hottest measured interior temperature with a non-contact thermometer. Subtract this from the exterior ambient temperature for "ambient temperature rise".
Add this to the highest external ambient temperature that you want this cluster lamp to work at. Do not figure external ambient less than 35 C unless you are personally controlling the usage of every unit. If you want to sell thousands of cluster lamps, figure external ambient temperature can get to 40 C. Should you know of a reason for exposure to even higher external ambient temperature, then consider that. For operation in direct sunlight, add to your external ambient however much temperature rise specific to sun exposure occurs - which you should measure.
Now that you have highest significantly foreseeable internal ambient temperature at 40 mA, add 27 C to this. This is your highest significantly foreseeable junction temperature.
There is more calculating work to do. Subtract your above high-side-foreseeable
external ambient temperature from this highest significantly foreseeable
junction temperature. This is the temperature rise of the LED
junctions above the external ambient, at 40 mA.
(There is a slightly simpler way to figure this, but I consider it helpful to consider high-side figures for junction temperature.)
Next, choose a maximum safe junction temperature. This is:
85 C for a conservative design (good chance low or at least moderately low
failure rate at 100,000 hours.)
95 C for a moderate design (fair chance at least moderately low failure rate at 100,000 hours.)
100 C for a moderately aggressive design (failure rate at 100,000 hours has significant chance of being moderate to high - datasheets for these LEDs often specify "absolute maximum" junction temperature of 100 C.)
The next step is to subtract your high-side-foreseeable external ambient temperature from your choice of maximum safe junction temperature. This is your "safe" temperature rise of junction over external ambient.
The last step is to divide this "safe junction/ambient rise" by the above figure for 40 mA. Multiply the result of this division by 40 mA to get maximum safe long-term continuous current through each LED.
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