optocoupler characteristic
It is an interesting point. LED degradation is a fact, but my experience tells me it is misperceived by people (also partly due to marketing wars made by companies offering other technologies). I appreciate resources in internet, but some of them are biased by marketing purposes. My humble thoughts are:
Advantages of Optocouplers:
+ LED degradation is true, but light output never goes down to 50% of initial value after three years. For degradation data there are several sources, which are tested acc to standards (like MIL-STD-883). Some suppliers show some accelarated test data E.g. Page 11 of designer guide Agilent Technologies
+ Good CMR and EMI performance
+ ESD robust
Disadvantages:
- Comparably expensive technology (but simple photransistors are not)
- Many suppliers. The quality is not same for all. Use good ones e.g. TOSHIBA, AGILENT, SHARP
Personally I have a board (DC motor control, where high voltages -up to 600V- and harsh noise conditions are included) being 10 years old. It is still functioning without any problem.
PS. if you are still concerned, I recommend you to put a 40% margin on to your initial forward current.
Regards