In a photocoupler, deterioration over time of the LED rather than the photodetector is dominant. The optical output of the LED deteriorates as the length of time the LED emits light increases. Therefore, by using data on the deterioration over time of the LED in use, the optical output reduction rate is estimated. In designing circuits, the rate of change of optical output is calculated from the usage environment for the set to be used, and the total light-emitting time, and reflected in the initial value for the LED initial forward current (IF).
For details, also refer to Toshiba Application Note "Basic Characteristics and Application Circuit Design of Transistor Couplers", section 2-4 Design Example for Interface Circuit Using Transistor Coupler.