Moisture sensor

A moisture sensor comprising a light source;

• a light guiding layer;
• a coupling element for the coupling of light into the light conducting layer at an angle α between the coupled light ray and the surface of the light guiding layer at which the light of at least one wavelength undergoes total reflection in the light guiding layer, when the medium adjoining the light guiding layer has a moisture lower than a threshold moisture;
• and
• does not undergo total reflection when the medium adjoining the light guiding layer has a moisture larger than the threshold moisture;
• an uncoupling element for the uncoupling of the light from the light guiding layer;
• and
• a detector for the uncoupled light, characterised by
• a reflection hologram (3, 51) on a part of the light guiding layer (1) having such a holographically recorded structure that when a light ray (17) is incident at the angle a on the interface (13) between the reflection hologram (3, 51) and the light guiding layer (1), a light ray is reconstructed which runs essentially opposite to the incident light ray (17).