For transistor-output photocouplers, when LED current IF is passed to the input side, collector current IC flows at the output side. The transfer ratio of Ic to IF is called the current transfer ratio (CTR) and defined in the following formula:
As is the case with hFE for transistors, the current transfer ratio is an important parameter for transistor-output photocouplers.
Also refer to Toshiba Application Note "Basic Characteristics and Application Circuit Design of Transistor Couplers", section 1-1. Current Transfer Ratio.
For IC-output photocouplers, photorelays, and triac-output photocouplers, however, the threshold value current for input current IF reversing the output state for operation to be performed at the output side is defined as the threshold input current, and for photorelays and triac-output photocouplers, it is defined as the trigger LED current. When using any of the above photocouplers and phororelays, a value equal to or greater than the "threshold input current (max.)" or "trigger LED current (max.)" written in the data sheet must be set for the driving LED current setting.
Also refer to Toshiba Application Note "Basic Characteristics and Application Circuit Design of IC Couplers", section 2. 3. 1. Threshold input current IFLH, IFHL.