What precautions should be taken when using triac couplers for a long time?

When LED is energized continuously for a long time, its optical output degrades gradually. Owing to the above, trigger LED current IFT becomes greater than its initial value. By using data on LED deterioration over time, the initial value for IF must be set so that even after the designed time elapses, it exceeds the value for IFT(Max).
For details, also refer to Toshiba Application Note "Triac couplers-basic properties and application design", section 9.1 How to design the input-side LED current IF.