MODULE No. 34: Digital Photography and Enhancement

Transcription

1 SUBJECT Paper No. and Title Module No. and Title Module Tag PAPER No. 8: Questioned Document FSC_P8_M34

2 TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 3. Cameras and Scanners 4. Image Enhancement 5. Image Enhancement as an Examination Tool 6. Image Enhancement as a Presentation Aid 7. Conclusion 8. Summary

3 1. Learning Outcomes After studying this module, you shall be able to- About digital Photography and enhancement techniques Learn about Image Enhancement as an Examination Tool Use and Importance of digital photography in Document examination 2. Introduction Cameras, whether they be conventional or the newer digital designs, are a necessity for document examiners. The clearest record of evidence examined can be obtained through the use of photography or digital imaging (scanners). Much has been written on conventional photography in textbooks and journals, so the focus of this chapter will not be on that area. Rather, the discussion will be directed toward the new imaging technologies. Where necessary, contrasts will be made between digital and conventional photography. In conventional photography different film speeds, exposure settings, and lighting conditions all impact on the quality of the photograph. In the preparation of charts for incorporation in a report or for presentation in court, a greater number of steps is required than is the case in digital photography. Conventional photography is widely recognized as having the capability to capture minute details and subtle differences in colour and contrast. It was the mainstay of forensic photographers and document examiners for many decades. Until the introduction of the scanner and digital camera, file copies of documents examined were traditionally made on a photocopier. However, now the resolution of even low-end digital cameras can be better than that of analog photocopy machines. There has been a general shift from analog technologies to digital. Even copiers now employ digital systems equivalent to those found in scanners. Furthermore, the digital copier often functions as a scanner as well. Both forms of photography can record the fine details in typewriter fonts, the type of printing process used on a computer-generated document, and the presence of torn paper fibres in an erasure. Digital imaging can be used to prepare work charts to aid in examinations and to create demonstrative charts for use in conjunction with reports and for testimony purposes. Today s digital cameras and scanners are an outgrowth of spy satellites used in the 1960s and 1970s and the U.S. space program, NASA. The needs of these programs led to the creation of the charge-coupled device (CCD).

4 Essentially, the CCD is a computer chip that converts light into electric signals and is the image sensor for most of today s digital cameras. The CCD has an array of light-sensitive diodes known as pixels. The resolution of the camera is a measure of the number of pixels. These pixels detect the quantity of light that hits them, but do not interpret the colour of this light. It is only through the use of one of many different filtering methods that the colour information can be interpreted. In most digital cameras, the colour information is an interpolation of the information gathered from a single CCD array. Only high-end 3 CCD cameras are capable of returning true colour information. Digital cameras offer advantages over conventional film cameras in some applications. Probably the most useful advantage is the immediacy of the image. 3. Cameras and Scanners The photograph can be viewed on, in most cases, a liquid crystal display (LCD) screen on the back of the camera as soon as the exposure is captured. This allows the user to ensure that the proper area of the target has been framed in the image, a problem not typically encountered in conventional film photography. It may be difficult, however, to observe small areas of blurriness on this very small image. Many of the newer models have a viewfinder, which to some extent lessens the problem. To assess the quality of the images, they can be immediately downloaded into a computer and viewed in a much larger format. If an image is deficient in any way, another exposure can be taken. The images are captured directly to a digital format that can then be processed with image-editing software on a computer. Also, various attachments enable a digital camera to be connected to a microscope to allow for instant microphotography. Typically, digital models are somewhat smaller than most 35-mm single-lens reflex (SLR) cameras and certainly smaller than the larger-format cameras. This can be a big advantage when the camera is taken into the field to photograph documents. Small portable scanners are now available as well and can be an alternative to digital and conventional film photography. There are several companies that are now beginning to offer digital SLR units that look and operate much like 35-mm SLR cameras, even using the interchangeable lenses. In fact, the settings found on many non-digital cameras, such as F-Stop and ISO, are now also present on digital models. Of course, one of the most practical advantages of digital units over film cameras is cost. Digital cameras do not require repeated purchase of film, developing chemicals, enlargers, and darkroom equipment.

5 The digital SLR models now address one of the biggest limitations of past digital versions compared with their film counterparts resolution. The newest digital models are capable of 10-megapixel or greater resolution. Though there is some disagreement as to the specific value, resolutions in the 6- to 10-megapixel range are generally considered equivalent to that offered by 35-mm film. Until the advent of these multi-megapixel cameras, digital images could not be enlarged to much more than an 8 10 size before they started to show pixilation a circumstance in which the individual pixel elements that comprise the image begin to show as noticeable squares. As the resolution capabilities have continued to rise, it is now possible to enlarge digital photographs with clarity equal to that of conventional film images. One photographic consideration regarding the use of digital cameras for forensic purposes is the need for macro-capabilities. Most mainstream point-and-shoot models are not capable of this type of close-up focusing. If the camera is capable of macro-focusing, it may have as an option a macro-flash unit, since the built-in flash will be useless at macro distances. The Forensic Document Examiner must recognize the difference between optical zoom and interpolated, or digital, zoom. Optical zoom is a function of the lens on the camera, whereas interpolated zoom is a function of software and results in deteriorated image quality. Another consideration is the storage formats available with the camera. Most cameras can store digital images in JPEG format, but this compression method discards varying amounts of image information, depending on the quality setting. A much more useful format for maintaining the image quality needed in forensic sciences is TIFF (Tagged Image File Format). While this convention also offers compression options resulting in data loss, its application in most digital cameras results in all of the information in each pixel of the image being recorded. The primary drawback of TIFF images is their size. A high-quality TIFF image can easily exceed 10 megabytes in size. This becomes important when considering the storage medium of the camera. A new format that is becoming popular for those seeking truer image reproductions is RAW. This format records the information as it appears directly from the CCD. Consequently, it is not subject to the colour interpolation performed in creating the other file types. Surprisingly, the RAW format often produces smaller files sizes. While some cameras have small internal storage capabilities, most storage is done with removable memory devices referred to generically as flash memory. These devices come in different formats, such as compact flash (CF) and secure digital (SD) and memory sticks, to name just a few.

6 Many contemporary computers and printers now offer direct input from the camera, its podium, or the memory medium. Another consideration in the purchase and use of a digital camera is the settings that can be adjusted prior to taking the photograph. Some cameras offer a variety of features, such as white balance, sharpening, resolution, image quality, exposure, and flash level. Similar options and adjustments are present in some scanner applications. Some of these options may alleviate the need to later employ enhancement techniques on the image with a computer imaging program. 4. Image Enhancement Since images captured with digital cameras and scanners are already in digital format, they can be downloaded into a computer and enhanced using a variety of image-editing programs. This process can be advantageous in many ways. With a digital camera or scanner and an image-editing program, a computer becomes a powerful tool in the document lab. There are a number of procedures and techniques that can be applied to a digital image. These techniques can be applied for two reasons. The first is to aid the Forensic Document Examiner in the forensic examination. For instance, by separating an image into its colour components, it may be possible to differentiate two inks that appear to be the same visible colour to the naked eye. Alternatively, enhancements may be applied to assist in presenting findings. For example, the enhancement technique described by Sperry and Tolliver can be used to improve faint images from electrostatic indentation impressions. 5. Image Enhancement as an Examination Tool Various image enhancement and digital handling techniques can play an important role in the examination process. The creation of virtual grids over a typewritten or computer printed image can aid in the detection of misalignments. By digitally overlaying and superimposing images using layers, the shapes of printed characters, stamps, and seals can be compared. The layering tool is also valuable when associating indented impressions with their source handwriting. Specialized stitching software allows multiple images, such as those that might result from infrared examinations, physical matching, or macrophotography, to be easily aligned and seamlessly stitched into a single image.

7 In some instances, computer scanners together with image-editing software can be used to differentiate similarly colour inks, such as black ballpoint. Selective super saturation of individual colour channels in the digital image can accentuate subtle colour differences. By taking advantage of the various colour models used by graphics software, selective colour filtering can be used to isolate overlapping elements of a questioned document. These are just a few of the techniques currently being applied by Forensic Document Examiners in the examination process, with new and innovative processes constantly being developed. 6. Image Enhancement as a Presentation Aid It is not uncommon for a Forensic Document Examiner to find faint indented impressions on a questioned document. In some cases, these impressions can even be sourced to the handwriting that created them, or deciphered as important relevant information. The untrained eye of the layperson, however, requires that this type of impression be enhanced for viewing. The Tolliver and Sperry technique, described earlier in this chapter, is ideal for such situations. Similarly, an overlay and superimposition of indented and source writing can allow the viewer to readily appreciate the relationship. The overlay-and-superimposition technique is useful in many situations. It can be used to show the relationship between residual marks left behind after an obliteration relative to the original document, to compare typefaces, and to demonstrate a match between signatures in a cut-and-paste manipulation. In order to make such overlays easy to comprehend, the electronic images are often colorized so that one can easily discern one from the other. Adobe Photoshop allows the user to change the blending mode of each layer so that it affects the layers beneath in various ways. For Forensic Document Examiners, one of the most useful settings is multiply. Using this mode, areas of a layer that are white have no impact on layers below. Colour portions of the image, however, blend with those of the underlying layer. This allows the relationship between the two to be better appreciated. Distortions, such as those resulting from page curl when copying a book or general reproduction distortion, can be compensated for using graphics software. This can allow the layperson to more readily compare different reproduction generations of the same original. The techniques used to accomplish this can include simple transformations or more complex displacement mapping. These are often done solely for the benefit of presenting the evidence, as the Forensic Document Examiner may have little or no difficulty in appreciating the relationship, even on a distorted reproduction.

8 7. Conclusion Digital photography and scanning capabilities have added to the arsenal of equipment in modern forensic laboratories. Continued development and improvements of digital image capturing and the increasing versatility of enhancement software will increase their application to a greater variety of document cases. Conventional photography still has its place in document examination. For instance, capturing images of digitally generated or reproduced documents (created on a fax machine, computer printer, or digital copier) is sometimes best done using conventional photography. This approach avoids any interference between pixelation introduced by a digital image-capturing device and that of the document itself. For archival applications, conventional photographic prints are longer lasting than digital images printed on computer printers, which can fade or degrade much more rapidly. Some level of familiarity with conventional photography will aid in producing good quality digital images. Many of the settings now present on digital models, such as ISO, are simulated representations of adjustments important in conventional photography. Without a basic understanding of film cameras, the adjustments and options available on digital models may have little or no meaning to the Forensic Document Examiner. 8. Summary 1. In conventional photography different film speeds, exposure settings, and lighting conditions all impact on the quality of the photograph. 2. Digital imaging can be used to prepare work charts to aid in examinations and to create demonstrative charts for use in conjunction with reports and for testimony purposes. 3. Digital cameras offer advantages over conventional film cameras in some applications. Probably the most useful advantage is the immediacy of the image. 4. The photograph can be viewed on, in most cases, a liquid crystal display (LCD) screen on the back of the camera as soon as the exposure is captured. This allows the user to ensure that the proper area of the target has been framed in the image, a problem not typically encountered in conventional film photography.

9 5. One photographic consideration regarding the use of digital cameras for forensic purposes is the need for macro-capabilities. 6. Some cameras offer a variety of features, such as white balance, sharpening, resolution, image quality, exposure, and flash level. Similar options and adjustments are present in some scanner applications. Some of these options may alleviate the need to later employ enhancement techniques on the image with a computer imaging program.