Holography. Obtaining holograms in non-coherent light.
A hologram of an object in coherent light is obtained by illuminating the object with coherent radiation and recording the radiation reflected from the object using a reference beam on the recording material. Further, the recording material ( photoemulsion) is processed with chemical reagents and when it is illuminated by coherent radiation with the wavelength used for recording, a holographic image of the object is obtained.
The proposed method for recording a hologram consists in the fact that the object is illuminated by a source of incoherent radiation whose angular size does not matter and its size does not need to be set less than the angular aperture of the optical system., high image quality due to the small solid angle of incidence of the light wave on the recording material. The method is explained in Fig. 1, 2, 3. The diagram in figures 1 and 2 contains an object 1 illuminated by incoherent radiation, an optical system 2, a holographic Converter 3, a recording medium 4, and a mirror flat surface 5 ( a reflector). The diagram in figure 3 contains additionally coherent radiation sources 6 and 7, as well as frosted glass 8.
Object 1 is illuminated by spatially incoherent radiation ( see figure 1). Select a point on the object's surface. The radiation reflected from the object can be considered as independent point emitters that emit spherical waves. A spherical wave passing through the optical system 2 would focus in the focal point again. But, getting into the holographic Converter, in the form of some unfocused area , it is converted into a beam of parallel rays, passes through the recording material, and, reflected from the reflector, forms standing waves in the recording material. The amplitude in it corresponds to the brightness of the emitter. The spatial location is determined by the spatial-phase location of the wave beam in the recording layer. The standing waves formed in this way correspond to the location in space and the brightness of the recorded object. To get a brighter impression in the recording material from the recorded object, the resulting hologram is copied to another recording material using coherent radiation with the wavelengths that were used when recording the holographic Converter 3 and this carrier is already used at the stage of restoring the recorded object ( see figure 2). At the recovery stage, the resulting hologram is illuminated with coherent or white light. Parallel beams from the hologram, falling on the Converter ( transparency) go after the conversion in the opposite direction to the optical system 2 and the optical system is restored to the light wave , which builds an image of the object. A mirror plane is used as the reflecting surface. Polychromic radiation has a short coherence length. So for natural radiation, it is of the order of a micron. The holographic Converter 3 increases the coherence length. Its use causes an increase in the diffraction efficiency of object holograms. (Due to a decrease in the solid angle of the beams and their divergence. Therefore, the entire light beam collected by the lens passes through the Converter and is converted into beams with an identical beam path, and provides the ability to install a photo-recording medium with a selected gap value to the surface of the reflector).
Figure 3 shows a method for obtaining a holographic Converter ( transparency) 3. frosted glass 8 located in front of the optical system with the same focus as in figures 1 and 2 is illuminated sequentially by red, blue and green coherent radiation. Frosted glass can be considered as independent point emitters that emit spherical waves. Passing through the optical system, the rays fall on the recording medium. On the opposite side in the opposite direction, the recording medium is also sequentially illuminated by red, blue and green coherent radiation. As a result, an interference pattern is formed in the recording medium from the addition of the corresponding wavelengths. After exposure and processing, a holographic Converter 3 is obtained, which is then used to produce holograms in incoherent light.