There are many parameters for optocouplers. To simplify the discussion, four of the major parameters will be reviewed. The forward current, IF is the input current to the infrared LED. When current is applied to the LED, infrared light is produced. The light passes through the electrically isolated material inside the coupler and is detected by the light sensitive phototransistor. The isolation voltage, VISO is the maximum voltage applied between the input and the output side that the optocoupler is able to withstand without allowing current from the output side to pass through the isolation barrier to the input side. This parameter is dependent upon the package and construction of the coupler. The collector emitter voltage, VCEO is the maximum voltage the optocoupler output transistor can withstand when in the “off” condition. Current Transfer Ratio (CTR), is the ratio of the output current to the input current. CTR is often used as a measure of gain, efficiency or capability of an optocoupler. It is the ratio of the collector current, IC to the forward current IF, or CTR = IC/IF × 100 and is expressed as a percentage.