When selecting the proper device, first make sure the object blocking the light will easily fit into the slot. Next, determine how accurately you need to identify the location of the object passing through the slot. Is this a clean or dusty environment? What are the temperature extremes? What kind of other contaminations are around? What type of electrical interface do you need? Will the sensor provide an Ic (on) current that can be used to accurately identify the location of the object? Photo darlingtons provide high gain while Photologic devices provide logical output states and high speed switching.