A through-beam or retro-reflective photoelectric sensor is an obvious choice since the sensor can easily detect when a dark object passes between the emitter and reciever unit, or when the beam of light between the emitter and a reflector is interrupted. A fiber optic sensor and two fiber optics made of plastic or glass fibers make up a fiber optic system. The sensor contains a light source (transmitter), typically an LED, and a photodiode (receiver). They rely on reflection, refraction, and scattering at the material surface; by measuring changes in signal intensity, frequency, and phase, they can identify and detect targets. They can detect very small objects, are particularly flexible to mount and are extremely resistant in harsh environments – even in high temperatures.