What are the processes of holography?

Holography includes the following two processes:

Holographic recording of (1) wavefront

(b) Representation of objects before light waves

Holographic recording of (1) wavefront

Using interferometry to record the complex amplitude distribution of light waves scattered by an object on a wavefront plane is the holographic recording of wavefront. By interference method, the phase distribution of object light wave on a certain wave surface can be converted into light intensity distribution, which can be recorded with photographic negative. Because we know that the amplitude ratio and phase difference of two interference light waves determine the intensity distribution of interference fringes, and interference fringes contain the amplitude and phase information of object light waves. A typical holographic recording device is shown in Figure 2: the coherent light wave emitted by the laser is divided into two beams by the beam splitter, one beam is reflected and expanded and irradiated on the photographed object, and then the light reflected or transmitted by the object is projected on the photosensitive film, which is called object light wave; The other beam is directly irradiated on the photosensitive film after reflection and expansion, which is called reference light wave. Because these two beams of light are coherent, light and dark interference fringes are formed and recorded on the photosensitive film. The shape and density of interference fringes reflect the phase distribution of object light, while the contrast of light and dark fringes reflects the amplitude of object light. The information of object light is recorded on photosensitive film, and after development and fixing, a hologram with a grating-like structure is formed. So the hologram is just a record of the interference pattern between the reference light wave and the object light wave. Obviously, the hologram itself has nothing in common with the original object.

(b) Representation of objects before light waves

The reconstruction of the object light wave front uses the diffraction of light waves, as shown in Figure 3. Illuminating a hologram with a beam of reference light (exactly the same as the reference light wave used when recording a hologram in most cases), just like diffraction on a complex grating, the diffracted light wave will contain the original light wave, so when the observer looks in the direction of the object light wave, he can see the reconstructed image of the object. This is a virtual image with all the characteristics of the original. In addition, there is a real image called * * * yoke image. It should be pointed out that the three-dimensional structure of the real image formed by the yoke wave is not completely similar to the original.