If the peak of the first set of waves coincides with the peak of the second, constructive interference would occur. Two waves of the same wavelength are said to be in phase with each other when their peaks and valleys appear as parallel curves.
Holo- graphic
Wave attenuation occurs where the positive peak of one meets the negative trough of the other. This addition can also occur if one of the two wave sets falls behind the other by a full wavelength or many wave numbers. The circular stripes act like the horizontal parallel lines in the grid described earlier.
The circular stripes cause a real image of the original object formed by real concentration of light. The sum of all the virtual point images will reconstruct a perfect replica of the original object for the viewer, even in the absence of the object itself.
Types of Holograms
If this difference is exceeded, the fringes produced by the lowest frequencies of the light source wash out those produced by the highest frequencies. Any disparity of path lengths will cause some edge shift even if the coherence length is large;. But if the material or emulsion is thick or relatively transparent to the illuminating radiation, these interference fringes are produced not only on the surface, but also through the volume of the emulsion.
Photography in one sense limits holography, due to the lack of stability exhibited by the emulsion during chemical processing. A major benefit of the conventional silver-based photographic process is the large amplification of the signal which is received chemically.
Use of Photographic Emulsion
While the utility of holography would be enhanced if the lengthy photographic processing stage could be dispensed with without incurring other disadvantages, there is no doubt that photographic addiction will remain for some time to come. As the market for hologram recording grows, we can look forward to further product development coupled with improved processing methods. Wavefront reconstruction due to diffraction effects in photographic emulsion is certainly not limited to recordings of photographic density variation.
If only the density variation is used, the maximum reconstruction efficiency is about 4%; but if the developed silver is removed by bleaching, the efficiency can be increased to 40% or more. The photographic materials may have their shortcomings, but they will be the basis for holography for some time to come.
Holography with Dichromated Gelatin
Illumination of the hologram and subsequent processing does not remove any material, but the image appears to be made in terms of gelatin swelling where water absorption takes place; therefore, the amount of swelling is reduced to increase exposure. Gelatin can be sensitized and used repeatedly without destroying previously recorded information. Such an arrangement seems ideal for holographic interferometry, but unfortunately the hologram cannot be corrected and is slowly removed by the reference beam when it is used for reconstruction.
The hologram is stored in a photochrome by either a positive or negative process, according to the laser wavelength used. Reconstruction can be performed with the helium-neon laser, but the efficiency is very low.
Display Holo-
After processing the holographic plate, the true real image of the object (not the virtual image) is reconstructed with a light beam parallel to the original reference beam, but in the opposite direction. Thus, the light is bent only towards the true image of the thin horizontal slit of the main hologram. When the observer places his eyes in the real image of the slit, he sees the virtual image of the object reconstructed in only one color, the color that reaches his eyes.
Another advantage of rainbow holograms is their high reconstruction efficiency because most of the diffracted light is scattered in the direction of the observer's eyes. However, horizontal parallax, and thus stereoscopic vision, will be usual as long as the observer does not try to observe from the outside the real image of the slit.
Commercial Ventures
V. Holography
Current techniques would require a transmission bandwidth of approximately 106 MHz, which cannot be accommodated in the current radio band of the electromagnetic spectrum. There is also a problem with recording and playback, one of the biggest successes so far has been the transmission of the letter B, which stands for Bell Laboratories, U.K. The input data had to be transferred to film and the output data photographically copied from the cathode. ray tube and treated before reconstruction. Copies of holograms may be required for display purposes to meet the needs of the commercial world.
Even the general nature of the reference wave can be changed so that the original plane reference wave can be converted into a copy of the spherical reference wave. For practical mm-sized contacts (near-contact scanning), light with a high degree of spatial and temporal coherence is required to preserve hologram fringe information under illumination.
Data Storage
Each hologram can be considered as collectively storing a series of data and possessing the optical properties of a powerful magnifying lens. Second, a large number of holograms can be stored in such a way that the real image from each hologram is displayed in a prescribed position. Third, the image resolution can be as good as the diffraction limit imposed by the size of the hologram.
Fourth, the hologram recording is relatively impervious to dust or scratches, as the information stored in the hologram is not localized but can be focused, scattered and distributed. This allows the data to be stored at a high density, while the readout can have a relatively low density.
Reading Out
Finally, high uniformity of material sensitivity is required for reliability and lower error rates. The physical dimensions of the storage medium will be determined primarily by the resolution of the material (space-bandwidth product) and geometric factors such as thickness. The linearity of the record and the light scattering property of the material affect the signal-to-noise ratio of the read data.
The entire system lifetime will be determined in part by the available number of write, read and erase cycles. The emphasis of the experiments was on determining the best format of the digital data, the effects of non-linearities due to the recording medium and the level of noise in the reconstructed image.
Engineer-
One of the attractive applications of the technique is technical inspection, where simple holographic interferometry is too sensitive. In the recording phase, a double exposure hologram is created with a fixed geometry, but the illumination wavelength is changed slightly or the path length is varied by changing the refractive index of the object area between the two exposures. These lines represent the set of points that are equidistant from a solid plane, which can coincide with the plane of the hologram.
If the exact shape of an object is known, say in the case of a hydraulic piston, the shape of the contours is also known, and any change in the contours can be used to detect a defect. If the inspected object has some intentional irregularities in shape, the resulting complex contours may not show small enough errors to be picked out by qualitative observation of the contour pattern.
Testing by Holography
High quality holograms are made of the entire computer board and a photograph of the reconstructed image is used for analysis. Holograms of the entire PC board may not show the desired joint detail, but they can be used to show faulty joints and irregularities in material composition. Although it is quite impossible to detect such faults by standard visual inspection, a faulty joint can be easily detected from a double exposure hologram.
The special feature of this method is that it enables test measurements to be carried out with minimal disturbance of the object. Another alternative is to produce a time-averaged vibration hologram at the resonance of the plate as it vibrates at ultrasonic frequencies.
Vibration Analysis
A long exposure of, for example, a person moving past the camera will cause smearing; similar exposure to a swinging pendulum will produce images of the pendulum at both ends of the swing because the pendulum spends more time at the ends of the swing than at any other point. The interference pattern between the superimposed hologram images provides an accurate measure of the amplitude of the oscillation of the object's surface. The rays reflected from the nodes are undisturbed and appear as the brightest parts of the hologram image (the intensity is equal to the original illumination intensity).
The antiknots appear darker due to the washout of the hologram's fringe component. A clear image of the shock wave produced by a bullet in flight is provided by this hologram.
Non-
Because most non-optical detectors can directly record both the amplitude and phase of the object wave, the reference beam and illuminator can sometimes be eliminated. In conventional acoustic holography, the detector array is often replaced by the water surface of the test tank. Unfortunately for medical work, the most satisfactory scanning methods for producing the hologram are unacceptable due to the long time required.
It is very common in ultrasonic holography to use an electronically injected reference beam that allows manipulation of the hologram. 34;interaction edges", which are very clearly visible and provide a vivid representation of the geometric changes produced on the object between the two exposures.
