Abstract: The invention relates to a moisture sensor in which light is coupled into a light guiding layer by a light source, undergoes total reflection there as long as the surrounding medium has a moisture lower than a threshold moisture and does not undergo total reflection if the moisture of the medium exceeds a threshold moisture, having an uncoupling element to uncouple the light from the light guiding layer and a detector for the uncoupled light. In accordance with the invention, a reflection hologram is provided on a part of the light guiding layer which, when a light ray is incident at the angle with which the light propagates in the light guiding layer, reconstructs a light ray which extends substantially counter to the incident light ray. The invention furthermore relates to a corresponding moisture sensor in which a pane, in particular an aeroplane or vehicle windscreen, is used as the light guiding layer.

Abstract: An information medium consists of an essentially circular or polygonal disk having at least two pairs of sides parallel to one another which have the same distance to one another in each pair. To reliably read stored data from such an information medium at high speed, a hologram is disposed on at least one side of the disk at its center or at the intersection of two crossing symmetry lines which can be reconstructed with a reference beam incident to the side at a right angle to a pattern consisting of essentially concentric circles, with the reference beam forming the axis of said pattern.

Abstract: A method for determining the authenticity of a hologram with the following steps:a) The hologram (1) to be checked is illuminated with a restoration ray (2):b) The diffraction ray (3) of the first order of the wave field created at the hologram is imaged on a first photo-sensitive element, in particular a CCD target (5), and brought to interference with a first reference ray (7);c) The diffraction ray (4) of the zeroth order of the wave field created at the hologram (1) is imaged on a second photo-sensitive element, in particular a CCD target (6) and brought to interference with a second reference ray (10);d) The intensity distributions created at the different photo-sensitive elements (5, 6) are correlated with each other for at least two phase relationships between the restoration ray (2) and the different reference rays (7, 10) to obtain a correlation intensity distribution for the relevant orders of diffraction; ande) The correlation intensity distributions determined for the relevant orders of diffractio