Abstract: An electro-optical device 10 comprises a body 12 of a semiconductor material, such as silicon. A light source 14 is formed integrally in the body. The device comprises an associated light detector 16 and an optical path providing part 19 having a refractive index and extending between the light source 14 and the detector 16, to provide an optical path 18 having a path length. A sensor 20 cooperates with the optical path providing part 19 and is configured to modulate light emitted by the light source 14, by changing at least one of light absorption characteristics in the optical path by exposing a medium in the optical path to the emitted light, the path length and the refractive index.