PROBLEMS IN HOLOGRAPHY WHEN OBJECT IS TURNED INTO IMAGE
- 2. holography Holography is proposed by D.Gabor in 1948, it initially aimed at solving the resolution problem of electron microscope with coaxial holography (Inline Hologram) . Because the invention of coherent light source (laser) (1964), holography was able to be realized.
- 3. Holography When you look at the front side of the object with your eyes, you see the light reflected in that direction only. When you move your head sideways, you see a different light field that is reflected in that direction. This is the result of the three-dimensional structure of the object. The hologram is simply the recording of this information in the form of interference patterns created with the light field. This is usually done with the help of lasers. The holographic image is simply the reconstruction of the image from the interference patterns. holographic images are virtual images that are recreated from some information, recording the a)the object is transformed into a photographic record. b) Reconstruction: the hologram is transformed into the image.
- 4. object is transformed into a photographic record
- 5. Reconstruction: the hologram is transformed into the image
- 6. PRACTICAL PROBLEMS IN HOLOGRAPHY Light source stability Exposure time Photographic emulsion Limited dynamic range
- 7. Light source Historically, the extremely short coherence lengths of optical sources available before the advent of the laser seriously constrained the types of holograms that could be recorded. Today the availability of high-quality laser sources has vastly alleviated this problem. However, the experimenter must still take some precautions, for the coherence of lasers is not perfect. For example, it is good practice to measure the distances of the reference beam and the object beam travel from source to photographic plate and to equalize the lengths of these paths as closely as possible.
- 8. The process of recording a hologram is an exercise in interferometry. As with any interferometric experiment, if clean and sharp interference fringes are to be recorded, it is essential that all path-length differences for interfering light be kept stable to within a fraction of an optical wavelength during the duration of the exposure period. The higher the power available from the laser source, the shorter the required exposure time and the less severe the stability requirements become. STABILITY
- 9. EXPOSURE TIME The exposure time required depends on a multitude of factors, including *the transmissivity or reflectivity of the object *the distances and geometry involved, and *particular film or plate used to record the hologram. Pulsed lasers with pulse durations as short as a few nanoseconds have been used in some instances, and CW (continuous wave) exposures as long as several hours have been used in some cases.
- 10. Photographic emulsions Some of the most stringent experimental requirements are associated with the recording of holograms of three dimensional scenes Photographic emulsions with extremely high resolution are required in such cases One of the most commonly used emulsions for this use, Kodak Spectroscopic Plate Type 649F, has a resolution better than 2000 lines-pairs (cycles)/mm and an equivalent ASA speed of about 0.03.3 It is invariably true that high-resolution emulsions are extremely insensitive.
- 11. Limited Dynamic range some significance is the limited dynamic range of photographic recording materials. The amplitude transmittance vs. exposure curve is linear over only a limited range of exposure. It is desirable to choose an average exposure that falls at the midpoint of this linear region.
- 12. for example ,a object transparency with a rather coarse structure, there may exist significant areas on the hologram with exposures falling well outside the linear region. As a consequence of this nonlinearity degradation of the reconstructed images can be expected. The dynamic range problem can be largely overcome by a technique first demonstrated by Leith and Upatnieks Leith And Upatnieks(1962)Applied Laser light To Holography And Introduced an Important off-Axis technique
- 13. The object is illuminated through a diffuser, which spreads the light passed by any one point on the object to cover the entire hologram. Thus a bright spot on the object will no longer generate a strong Fresnel diffraction pattern on part of the hologram, but rather contributes a more uniform distribution of light.
- 14. Attendant with the advantageous reduction of dynamic range of the exposing light pattern is another advantage: since each object point contributes to every point on the hologram, an observer looking at a reconstructed image through only a portion of the hologram will always see the entire image. As might be expected, the virtual image appears to be backlighted with diffuse illumination.
- 15. Application https://www.youtube.com/watch?v=0ics 3RVSn9w https://www.youtube.com/watch?v=4N 0Ewb_OVsU https://www.youtube.com/watch? v=yVzk7bbQOA8
- 16. THANK YOU