A range finder is any device capable of measuring the distance between two points. Optical methods to measure distance have a long history. There are several methods used to measure distance-using light.These include instruments based on the rangefinder principles such as fixed base and fixed angle. Other technologies include theodolites that use stadia tacheometry, tangent tacheometry and wedge attachments to theodolites. Finally there are devices such as self-reducing tacheometers, subtense bars and plane table alidades.