Infra-red reflectance array
A reflectance array is a complex sensor made up of a series of infra-red emitter and receiver pairs. The one shown in the figure below has three pairs, but more pairs give greater sensitivity. This type of sensor is designed to enable a mobile robot to follow a line on the ground effectively, but can also be used for other more general purposes such as range detection and differentiation between light- and dark-coloured objects.
The operation of the sensor relies on dark surfaces reflecting less IR radiation than light ones. The readings from the individual IR pairs need to be coordinated in order to determine the location of the line. Because it is a complex sensor, the manufacturer usually provides library code that performs these calculations giving a simple interface for incorporation into an application.