Sensors typically measure a specific physical effect, and provide the result of this measurement over an interface to a controller. The most common types include a gyroscope which detects rotation, normally defined as rotation around an axis (X, Y, Z); an accelerometer to detect linear acceleration, also normally defined along the axis (X, Y, Z); a magnetometer that detects strength and direction of a magnetic field, a common application is detecting earth's magnetic field when used as a compass; light sensors that detect differences in light intensities, e.g. to adjust the brightness of a display or detect an approaching object; and pressure sensors used to detect differences in air pressure in an enclosed space or altitude in the atmosphere. The common choice of input/ output interface for the low cost range of sensors is I²C. Some sensors come pre-calibrated from the factory, others need an initial calibration before being put to use.