Abstract:
A holographic sensor for recognizing moisture on a glass pane of a motor vehicle is provided, having a casing, a carrier layer, a diffractive element and an adhesive layer, the diffractive element being fixed, preferably glued, on the carrier layer. The diffractive element may be embodied as an embossed hologram.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a holographic sensor, in particular for recognizing moisture on a glass pane of a motor vehicle. 
   BACKGROUND INFORMATION 
   Sensors for recognizing moisture on a glass pane of a motor vehicle are discussed, for example from German Patent No. 199 33 640, but these sensors may be very large because of their complex, optical elements. Since the optical elements must be situated on the windshield of the motor vehicle in the region swept by the wiper, and are consequently in the visual range of the driver, the appropriate casing may distract the driver. This applies in particular because the relatively large casings require complex fixing elements, which are additionally provided on the glass pane. This may be complex and may be very costly in the series production. 
   A holographic sensor for recognizing drops of water on a glass pane is discussed in European Patent Application No. 999 104. In this reference, a hologram film is glued to or into the windshield. 
   SUMMARY OF THE INVENTION 
   A holographic sensor may have the advantage that the optical diffractive element is fixed on a carrier layer, which is glued to the windshield of the motor vehicle. A sensor assembly, including the diffractive element, may be manufactured in this manner as a compact structural element, which only needs to be glued to the windshield in the series production of motor vehicles. Through the compact design of the sensor, in particular by integrating all the components of the sensor within the casing, tolerances may be met more precisely, for example between the radiation transmitters and receivers and the optical structures of the diffractive element. 
   It may be advantageous if the carrier layer may be directly attached, preferably glued, to the glass pane of the motor vehicle. This may be easily overcome on the conveyor belt in the series production of motor vehicles, in particular since this requires only moderate tolerances. 
   Furthermore, it may be advantageous if at least one transmitter and/or at least one receiver are situated in the casing and the casing is connected to the carrier layer, thereby resulting in maximum dimensional stability of the individual components to one another. The sensor may be produced in such a manner and easily transported to the vehicle manufacturer without impeding the dimensional stability and the quality of the sensor. 
   It may be advantageous if the carrier layer is connected to at least one fixing element to which the casing is fixed. In this way, the sensor assembly may be glued to the glass pane of the motor vehicle, and for example in the case of repair, the casing may nevertheless be separated from the carrier layer having the diffractive element. Repairs to the optical or electronic components may consequently be made with less effort. 
   Furthermore, it may be advantageous if at least one transmitter and/or receiver are situated in the casing, able to emit or receive the radiation having at least one frequency f, and if the fixing element, at least in parts, is essentially transparent for the radiation of the at least one frequency f. The fixing element may then be attached with its entire surface to the glass pane and bears the carrier layer on which the diffractive element is provided. The casing is consequently connected to the glass pane of the motor vehicle with a very large adhesion surface area, resulting in a good bond of the sensor on the glass pane and preventing the sensor from falling down, even in extreme conditions. 
   Furthermore, it may be advantageous if the fixing element is designed as one piece with the carrier layer since a separate carrier layer may thereby be dispensed with and the cost of the sensor is reduced. 
   The holographic sensor may have the advantage that the diffractive element is designed as an embossed hologram, which is cheaper and easier to manufacture than optical holograms. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic illustration of a holographic sensor. 
       FIG. 2  shows a schematic illustration of a section of a casing of a holographic sensor according to an exemplary embodiment of the present invention. 
       FIG. 3  shows an exemplary embodiment of a sensor according to the present invention having fixing elements. 
       FIG. 4  shows another exemplary embodiment of the sensor according to the present invention. 
       FIG. 5  shows yet another exemplary embodiment of a sensor according to the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a holographic sensor in a schematic cross-sectional representation. Sensor  10  is essentially made up of a first diffractive element  12 , which is glued to a glass pane  14 . Diffractive element  12  is designed here as a holographic film. A transmitter  16 , capable of emitting radiation having a frequency f, is positioned over diffractive element  12 . In operation, transmitter  16  emits radiation having frequency f in the direction of diffractive element  12 , which deflects this such that the radiation is completely reflected on the glass-air interface facing away from the sensor when glass pane  14  is dry. 
   After this complete reflection, the radiation reaches the glass-air interface of glass pane  14  facing the sensor, and is reflected there through another diffractive element  18 , for example a retro reflector, and another complete reflection in the direction of diffractive element  12 , which focuses the radiation toward a receiver  20 . 
   Diffractive element  12  is designed in particular as a film or thin layer element, which using holographic structures deflects the radiation of transmitter  16  in one direction, for example through diffraction or refraction, and which is capable of coupling or decoupling the radiation in or from glass pane  14 . 
   Transmitter  16 , receiver  20 , and diffractive element  12  are enclosed by a casing  22 , which is fixed to glass pane  14  using fixing means  24 . 
   In  FIG. 2 , a cross-section of a holographic sensor  10  according to the present invention is shown in schematic representation, but further diffractive element  18  was left out for reasons of clarity. A printed circuit board  26  is situated in casing  22 , the printed circuit board bearing transmitter  16  and receiver  20 . Casing  22  is essentially shell-shaped, whose interior space having printed circuit board  26  is sealed by a carrier layer  28 . On the side facing the interior space of casing  22 , carrier layer  28  bears diffractive element  12  so that the sensor casing, along with diffractive element  12 , may be glued to glass pane  14  via an adhesive layer  30 . This is may be an advantage in series production since diffractive element  12  need not be mounted separate from the remaining components of sensor  10 . 
   In  FIG. 3 , a variation of the sensor according to the present invention from  FIG. 2  is illustrated. Here, carrier layer  28  is rigidly connected to fixing means  24 , to which casing  22  is fixed, in particular clipped, so that the electrical part of the casing containing printed circuit board  26  may be separated from the optical part containing diffractive element  12  and carrier layer  28 , but may nevertheless be mounted as a whole on glass pane  14 . Here, carrier layer  28  and fixing element  24  are rigidly connected to one another and are directly injected against one another in an injection molding process, for example. 
     FIG. 4  shows another variation of a sensor according to the present invention. Fixing element  24  extends in this case over the entire sensor region enclosed by casing  22 . In addition, fixing element  24  is of course transparent for frequency f of transmitter  16  or of receiver  20  in the regions in which the radiation from diffractive element  12  must pass through carrier layer  28  and fixing element  24 . Carrier layer  28  is provided directly on fixing element  24  on the inside of casing  22  and bears diffractive element  12 . Adhesive layer  30  is provided on the side of fixing element  24 , which is in contact with glass pane  14 . 
   In  FIG. 5 , a third variation of a holographic sensor  10  according to the present invention is shown. Here, diffractive element  12  is glued directly to fixing element  24  via an adhesive or also only through adhesion, so that carrier layer  28  and fixing element  24  are designed as one piece. 
   Conventional holograms manufactured by optical or mechanical means, i.e., stamped holograms, may be considered as diffractive element  12 . 
   In particular, a plurality of diffractive elements  12 , for example, one each for transmitter  16  and receiver  20 , may also be provided. 
   The remaining components, such as fixing element  24  or casing  22 , may be made from plastic, in particular as an injection-molded part. In principle, carrier layer  28  may of course also be pressed onto glass pane  14  and held on this via clamping or spring elements.