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Theory of Optical Spatial Filtering and Its Application to Enhance Low Contrast Fingerprint Images Ludwig Scherlowski

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Theory of Optical Spatial Filtering and Its Application to Enhance Low Contrast Fingerprint Images Ludwig Scherlowski Rochester Institute of Technology RIT Scholar Works Theses Thesis/Dissertation Collections 1971 Theory of Optical Spatial Filtering and its Application to Enhance Low Contrast Fingerprint Images Ludwig Scherlowski Follow this and additional works at: http://scholarworks.rit.edu/theses Recommended Citation Scherlowski, Ludwig, "Theory of Optical Spatial Filtering and its Application to Enhance Low Contrast Fingerprint Images" (1971). Thesis. Rochester Institute of Technology. Accessed from This Thesis is brought to you for free and open access by the Thesis/Dissertation Collections at RIT Scholar Works. It has been accepted for inclusion in Theses by an authorized administrator of RIT Scholar Works. For more information, please contact [email protected]. THEORY OF OPTICAL SPATIAL FILTERING AND ITS APPLICATION TO ENHANCE LOW CONTRAST FINGERPRINT IMAGES by Ludwig Anthony Scherlowski Royal Canadian Mounted Police This thesis is submitted in partial fulfillment of the requirements for a Master of Science degree in Photographic Science at the Rochester Institute of Technology. ACKNOWLEDGEMENT I take this opportunity to thank my thesis advisor, Professor Mohamed F. Abouelata, as well as Dr. Gerhard W. Schumann and Professor John F. Carson, all of the Department of Photographic Science, Rochester Institute of Technology. Their continued assistance and advice throughout this pro ject was an invaluable aid. My appreciation is also extended to members of the Royal Canadian Mounted Police, who aided me when called upon to supply me with the various fingerprint patterns used throughout this project. j .t ti 2RQ i:L TABLE OF CONTENTS List of Figures iv Abstract v Introduction 1 Theory 7 Experimental 17 Optical Spatial Filtering Systems 17-20 Objects 21 Filters General 22 Filters Used 23-27 Annular Aperture Filter Result 29 Single Band Filter Result 30 Double Band Filter Result 32-41 Discussion of Results 42-46 Recommendation Section 47-48 Bibliography 49 m LIST OF FIGURES Figure Page 1. Enlarged Portion of Fingerprint 2 2. Image Distribution Due to a Spherical Wavefront 7 3. Optical Fourier Transform System 11 4. Expansion of Optical Fourier Transform System .. 13 5. Optical Spatial Filtering System 18 6. Optical Spatial Filtering System Which was Used in the Experiment 19 7. Blocking Filters Used Throughout Experiment .... 24-27 8. Unfiltered and Filtered Image Using . 50 mm Annular Aperture 29 9. Unfiltered and Filtered Image Using Single Band Cut Off Filter 30 10. Double Band Filter No. 21, Enlarged 5X 31 11. 32 12. 33 13. 34 14 35 1 Amplitude Spectrums of Original 15. Fingerprints Shown in 2, and 3 is 36 , the Filtered Print. fi ,y 17. 38 18. 39 19. 40 20. 41 21. Optical System Used to Check for Phase Variation in the Filters 43 22. Fringe Patterns Obtained 43 23. On Axis Filtered Image and Off Axis Filtered Image 44 iv ABSTRACT Image improvement, through the use of the Fourier transform property of lenses using various frequency block ing filters, was investigated to determine possible utiliz ation of this technique to enhance low contrast fingerprint image characteristics. Ten basic fingerprint patterns were degraded in contrast and then used as the object t(x,y) in an optical spatial filtering setup. A series of frequency blocking filters were used in an effort to improve the degraded object image. Visual comparison of the filtered images obtained by using a double band frequency blocking filter, which produced the most satisfactory images, shows improved overall contrast. Random ridge enhancement was achieved, which varied from pattern to pattern, but at the same time, degradation also occurred randomly from smeared- out and blemished sections. Although overall enhancement was not achieved, the enhanced partial areas are typical of the type used in identification work using fingerprints. I. INTRODUCTION The science of fingerprinting constitutes the only infallible means of positive identification of persons, known to man, since statistics show that the probability of finding two identical fingerprints is stochastically zero. The ridge patterns which can be seen on a newly born infant will remain unchanged throughout the life of an individual unless they are destroyed by accident or disease . It is not known who first noticed that the inner surfaces of the hands and fingers were covered with ridges or that these ridges tended to form definite patterns. It wasn't until 1858, that an Englishman, Sir William J. Herschel, began using fingerprints for identifying indiv iduals. This method of identification is now in general use throughout the world. The basis for this type of identification is based solely upon the ridge characteris tics and the sequence in which they occur. In tracing the courses of the individual ridges of the enlarged portion of the fingerprint shown in Fig. 1, it is seen that some end abruptly, while others fork. Sometimes the limbs of such a fork join together again almost at once and form an enclosure. Occasionally there are short independent ridges. Thus, when two impressions are being compared and it is found that the ridge characteristics occur in sequence ISLAND '. BIFURCATION' LAKE fclDGt ENDING Fig. 1. Enlarged portion of fingerprint, showing individual characteristics and the random manner in which they may be found. in both impressions it can be assumed without doubt that the impressions were made by the same finger. In order that a positive identification can be made, a certain num ber of these characteristics must appear in sequence. Generally, for presentation in the courts, a law enforcement agency will insist that a fingerprint technician be able to demonstrate at least ten characteristics in sequence before a positive identification is made. It does not necessarily follow that the entire finger print pattern must be complete; only a partial impression is needed. This impression must contain a minimum of ten ridge characteristics from which positive identification can be made. This means that in some cases only a small portion of the overall fingerprint pattern is required. The area where this type of fingerprint is most often en countered is at scenes of crimes, although they may be found in a number of other areas, such as identification of the dead; from plane crashes, fires, drownings and any other case where identification by other means has failed. The types of fingerprints found in the course of investigations are grouped into three classes: (1) Latent or invisible; (2) Visible; and, (3) Moulded Impressions. (1) Latent or invisible impressions are a result of the transfer of the thin film of sweat and body grease from the finger surface to the object touched. The surfaces on which this type of fingerprint may be found is almost with out limit. Generally, hard smooth surfaces are excellent. Reproduction on these surfaces is achieved by brushing the surface with special fingerprint powder using a special brush. Once made visible these prints can be lifted and photographed or just photographed directly on the surface where they are found. The lifters are a transparent material, similar to scotch tape. As the name implies they are used to transfer prints which have been made visible from the surface on which they were found, to a backing material which is nothing more than a support for the lifter. The color of the support is such that maximum contrast will be obtained between the support and the fingerprint impression. Latent impressions on paper may also be made visible by treating the paper with silver nitrate solutions. In this case the sodium chloride from the sweat reacts with the silver nitrate solution to form silver chloride. On exposure to strong light, a brown colored image is produced. Although there are other treatments which may be used on paper, these will not be discussed. It will be sufficient to say that after such treatments the developed images are photographed. (2) Visible prints are those which do not require any form of development to make them visible. They may be found in dust, blood, or some other such material. Due to the unstable nature of these materials, these fingerprints are almost always photographed where found. (3) Moulded fingerprint impressions are those which are found in soft material, like plasticene, wax, soft putty, etc. These impressions are visible and do not require any form of development. The procedure as in the preceding two classes, is the recording of the fingerprints on film. As a result of this wide range of conditions under which fingerprints may be found, law enforcement and other agencies are daily confronted with fine detail, low contrast images. These images when photographed in the conventional one-to-one manner, on fine grain film and processed to pro duce the best reproduction often lose enough of the detail, to render them unusable or quite difficult to compare to master copies. In other words, some of the information which existed in the original fingerprint is lost in the tone reproduction process. The fine details in the fingerprint reproduction are usually necessary details in order that a positive identif ication may be made. Therefore, it is important to retain these fine details. One of the uses of optical spatial filtering is to reduce the noise in the original image; thereby increasing the signal to noise ratio in the reproduction. Thus in creasing image contrast as well as edge enhancement. The purpose of this thesis is to determine whether or not a significant visual improvement in contrast and edge enhance ment can be obtained by the use of optical spatial filtering on first generation negatives of ten different fingerprint images . II THEORY Just as complex electronic wave forms can be broken down by a spectrum analyzer into frequency components, so a photographic image can be optically broken down into a pattern of frequency components. In both cases the output signal is a Fourier transform* of the input signal.
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