by Markus Appel @ edn.com:
Optocouplers, also known as opto-isolators, are components that transfer electrical signals between two isolated circuits by using infrared light. As an isolator, an optocoupler can prevent high voltages from affecting the side of the circuit receiving the signal. Transferring signals over a light barrier by using an infrared light-emitting diode and a light-sensitive product, such as a phototransistor, is the main structure of an optocoupler. On the first page, datasheets provide the main product description, its features, suggested areas of applications, ordering information, and agency approvals, as shown in Figure 1 for the VO617A optocoupler with phototransistor output. Following pages provide key technical specifications, operating conditions, and graphs showing the behavior of the product.
Guidelines for reading an optocoupler datasheet – [Link]