Current Transfer Ratio (CTR) - is the ratio of the output current to the input current for an Opto-Coupler
CTR=\frac{I_{c}}{I_{f}}
I_{c} = Collector Current
I_{f} = Input current
More information can be found here:
Current Transfer Ratio (CTR) - is the ratio of the output current to the input current for an Opto-Coupler
More information can be found here: