The field of optical sensors is a very broad with many subdisciplines. Optical sensing systems require a light source, detector, optics, and various optical components. Some sensing systems utilize external light for the sensing scheme. Light intensity measurements can sense changes either due to the sensor configuration or due to change in the emission of the light spectrum. Changes in light absorption can be induced by a variety of reasons and can be utilized for sensing by using a light source and photodetector along with optics and electronics. Color change can be measured using a series of detectors with narrow-band optical filters. A variety of methods exist that can detect change in refractive index, such as using prism-coupled, fiber optic, or interferometric sensors. Refractive index of active optical materials can also be altered by magnetic field or electric field. Optical interferometers are used for a variety of sensing applications. Interferometric sensors include the Michelson, Mach-Zehnder, Fabry-Perot, and Sagnac interferometer sensors.