A commonly used schematic symbol for an optical isolator is shown below:
The Current-Transfer Ratio
The current-transfer ratio of a photo-transistor based optical isolator tells you how well it amplifies an input signal to an output signal.
For optical isolators with a single phototransistor driver, the current-transfer ratio is normally in the 20-120% range.
Some optical isolators have phototriac outputs instead of the normal phototransistor output. A phototriac is a triac whose gate is controlled by the incoming light source, essentially the same as what a phototransistor is to a transistor.
The outputs of phototriac optical isolators usually go on to drive power TRIACs which switch a mains powered load.
To achieve good isolation, different-net pins must be kept well away from each other to prevent arcing. This prevents many small-pitch packages from being used for optical isolators.