CTR is the Current Transfer Ratio. The most critical parameter for optocouplers is the transfer efficiency, typically measured in terms of current transfer ratio or CTR. This represents the ratio between a current through the phototransistor and the current through the LED. This ratio number is generally noted as a percentage or as a ratio of two numbers. VCE(MAX) represents the output transistor’s maximum collector-emitter voltage rating, which limits the supply voltage in the receiver circuitry. IF(MAX) represents the input LED’s maximum current rating, which can be used to calculate the minimum value for its series resistor. Normal operation is much less than this number. This is given in data sheets as a single number. Typical IF may also be provided in the data sheet. Isolation voltage is the difference in voltage between both sides of an optocoupler. This number is typically given in kV’s (, 000Vs) and can be either a DC, AC Peek-to-Peek or AC RMS value. Output Device is the choice of phototransistor (for an analog signal) or Photologic® (for a digital signal).  Housing configuration is dependent on the physical and environmental conditions of the application. For component packaging, hermetic or plastic discrete devices may be used depending on the environmental conditions for the optoisolator.