The QTR-1A reflectance sensor carries a single infrared LED and phototransistor pair in an inexpensive, tiny 0.5" x 0.3" module that can be mounted almost anywhere and is great for edge detection and line following applications. The reflectance measurement is output as an analog voltage.
 
Specifications

•     Dimensions: 0.3" x 0.5" x 0.1" (without header pins installed)
•     Operating voltage: 5.0 V
•     Supply current: 25 mA
•     Output format: analog voltage
•     Output voltage range: 0  to supplied voltage
•     Optimal sensing distance: 0.125" (3 mm)
•     Maximum recommended sensing distance: 0.25" (6 mm)
•     Weight without header pins: 0.008 oz (0.23 g)