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What is the current transfer ratio?

Internal circuit
Internal circuit

The current transfer ratio (CTR) refers to the ratio of the collector current at the output side Ic to the input current passed to the LED at the input side IF expressed as a percentage. It is defined by the following formula.
CTR=(IC/IF)×100 (%)
As is the case with hFE for transistors, the current transfer ratio is an important parameter for transistor-output photocouplers. The value of this current transfer ratio has temperature Ta, LED current IF, and transistor collector-emitter voltage VCE dependencies. In addition, when an LED is energized for a long time, its optical output degrades gradually. Therefore, in designing, such deterioration over time must be taken into account.
Refer to Toshiba Application Note "Basic Characteristics and Application Circuit Design of Transistor Couplers", Chapter  1. Basic Characteristics.