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Digital Hologram Image Reproduction
Making digital holographic reproduction , how to eliminate the influence of the zero-order image is a hot research topic , direct transmittance interference because the zero level the like occupy most of the light energy . Currently , the major impact of the phase-shift technology to eliminate the zero-order light . But use the four phase mutually perpendicular to the reference light of the recorded hologram . A typical phase shift technology digital holographic method
In addition to the phase shift technology also can use other methods to deal with zero-order light problem . Hybrid hologram approach , the image subtraction party digital hologram technology also need to collect multiple images , it is difficult to conduct real-time reproduction . Can be used to eliminate the zero - level method of image processing based on the Laplace operator , the algorithm has a certain degree of complexity . Zhoucan Lin and other orthogonal wavelet transform applied to digital holography to eliminate the impact of the zero-order image , and simply a hologram .
  
Study of the moving objects in digital hologram
Real-time measurements using digital hologram is an important research direction of the digital holography . Although many optical methods vibration problems , but are not a good solution to the problem of power effect , [ 2] discussed the digital holography is applied to real-time measurement of the vibration mode of the object , a can be used for real-time monitoring of the vibration mode of the object equipment .
  
The equipment consists of two parts: the part of the recording the time average holographic pattern ; real-time reproduction of another part of the hologram . Recording and reproducing the use of two different optical structures, one said prevail Fourier transform off-axis recording Another called the extended Fourier transform reproducing . The latter process , in the reproduction like before can unwanted diffraction component filtered out. Usually pulsed laser and low-speed CCD digital hologram of moving objects .
Lasers and high speed CCD of a moving object of the digital holography, the experiment shown in Figure 2. The objects used in the experiments is a spherical balloon CCD per second can be shot 4000 holograms, adjacent to the two holograms time interval of 250μs, the hologram size is 512х512 pixels, the split rate of 8 bits. Now can also use digital holographic organic microbial detection research in this area is also a hot topic.
The like , so that the object is reproduced from a digital hologram due to the impact of the zero-order light and the laser speckle is not very clear, using optical super-resolution digital hologram is an effective way to improve the reproducing resolution of the image . The resolution of the optical system is determined by the diffraction-limited aperture optics beyond the diffraction limit of the technique is referred to as super-resolution technology .
The super-resolution digital hologram method , a multiple exposure hologram recording is the complex amplitude of the object (amplitude and phase) , the intensity of object is not usually hologram , which can complex amplitude by the object to increase the optical system superimposed bandwidth. Although each object with limited bandwidth , center frequency . To lightwave angle on the object to change the center frequency of the bandwidth by changing the incident , but the angle of the reference light at the same time also needs to be changed , so as to enable the complex amplitude of the superposition of the effect of increasing bandwidth .
The use of two-dimensional vertical cavity the spontaneous laser arrays (VCSEL) to achieve the object of the super-resolution digital holography . Two-dimensional vertical cavity spontaneous laser array can be regarded as a non- coherent spherical wave light source , the basic principle is to use different tilt spherical wave illumination multiple superimposed object digital hologram . Different spatial bandwidth by the lens with respect to the different position of the illumination light source is formed by the Mach-Zehnder interferometer are recorded , and finally form the total digital hologram , when the digital reproduction , but also by adjusting the focusing procedure which each center frequency the bandwidth adjustment. The focusing process used in recording , so you want to apply reproduced Fourier transform . The whole experiment is simple and do not need to change the structure of the system , and almost all of the spatial frequency of the interference pattern can be recorded .
The current problems of the digital hologram
The development of a digital hologram does not solve the problem still exist today many such has an important influence in the size of the object, diffraction or scattering properties of the recording of the digital hologram; CCD size small, low resolution, as a space of the recording medium of the CCD resolution rate up 100line/ram resolution 5000line/ram lower, decided the digital hologram only directly record the smaller size of the object and its low-frequency information than the space of traditional silver halide dry plate. In order to as many records spatial information of the object, it is desirable recording distance as small as possible, but the decrease in distance will lead to sampling conditions are not met and reproducing like separation difficulties, to reduce its resolution; large amount of data, a large amount of computation, the figures show the resolution of the device may also be improved, it is difficult to recording and reproducing a moving object, the color of the object restore more difficult, it is difficult to achieve large area display.
The largest digital hologram difficulty is that the resolution is too low, this result is that our traditional digital hologram is the use of the CCD directly receiving coherent lightwave doing although you can not make the information is lost, and can dynamically record the information of the objects under , but far from the CCD resolution and holographic plates compared to the resolution of the interference pattern thus obtained is very low, it would seriously affect the reconstructed image quality. therefore we envision if CCD direct multi-angle shooting objects, the information thus obtained as the object information, and then enter the computer coding processing to produce a Fresnel hologram, and then enter the spatial light modulator (SLM) using a laser to reproduce objects. In Figure 3 is constituted with a high-resolution high-quality CCD a schematic diagram of the real-time hologram display; (1) to enter the computer processing, as addressing signals to modulate the LCD via the LCD readout light can be obtained in real-time three-dimensional reproduction of an object like.
Although part of the object will be lost, but can greatly improve the resolution, and lost part of the information through computer programming process to make up for, but this will inevitably lead to the increase of the amount of computation doing, which no doubt gave fast algorithm a new test. In the second step this concept can also be directly processed by a computer to get one like holograms to microfilm, can be to reproduce objects like white light.
Charge Coupled Device (CCD) is a semiconductor solid 1970 Bell Labs invented device has the advantages of small size , light weight , high sensitivity , long life , low power consumption and dynamic range , it is a simple structure of silicon photoelectric conversion function , storage capabilities, and scan functions are combined , so it does not need to warm-up , easy to use , the ability of anti- vibration and shock as well as damage caused by anti- ability , and are not easily by the lag and the impact of the smear . CCD detector in real time the two-dimensional optical reproducing an image signal is converted to a timing electrical signal , which has a simple, fast , compatible with the electronic signal processing system , easy to interface with computer advantages , the drawback is the spatial resolution of the optoelectronic receiver and lower gray levels .
With the continuous development of science and technology , we firmly believe that these difficulties can be gradually overcome , will be not possible to estimate the prospects for the application of digital hologram : the use of computer integrated two-dimensional image as a three-dimensional hologram , and lithographic molding technology combination of can achieve light machines , electrical integration of the mechanized production of synthetic hologram ; the embossed hologram application to the packaging industry , not only decorative effect , but also has the most innovative security features , the system can also create a new image , electric and operators of mechanized production of integrated synthetic hologram ; embossed hologram can be applied to the packaging industry , technical , commercial advertising and design industry , large-scale exhibitions , decoration and beautification of people's lives . If we can image acquisition , encoding , together in a device controlled by the computer to achieve , and that is the real integration of digital hologram . Kind , holographic television , hologram film and digital display technology breakthroughs will come soon .
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