SparkFun SEN-00246 Optical Detector / Phototransistor
This sensor uses an infrared emitted diode combined with an infrared phototransistor to detect the reflected infrared signal. Ideal for sensing black-to-white transitions or can be used to detect nearby objects (.5-1cm).
Documents:
Martin Macmahon –
This emitter / detector pair is very effective and easy to use. Range isn’t great, but it works very reliably within it’s range.