Digital Imaging
Digital imaging is the electronic recording, processing, enhancement, and storage of visual information. Its applications in forensic science range from documenting crime scenes to enhancing faint or indistinct patterns such as partial fingerprints. Although digital imaging is often considered to be synonymous with digital photography, digital imagery can also be obtained by conventional x-ray radiography, computed tomography (CT or CAT scans), magnetic resonance imaging (MRI), laser scanning, and infrared photography.
Both digital and film photography employ lenses to focus light rays into a sharp image, with the size of the image controlled by the focal length of the lens. Lenses with long focal lengths produce larger images than those with short focal lengths, although the magnitudes of long and short are relative to the film or sensor size. Focal lengths are generally given in millimeters. A diaphragm within the lens (in combination with shutter speed) controls the amount of light entering the camera as well as the depth of focus in the image. In place of the film used in a digital camera, however, a digital camera uses a light-sensitive electronic sensor. Two sensor types are commonly used: CCD, or charged coupled device sensors, and CMOS, or complementary metal oxide semiconductor sensors. Both types of sensors are composed of rows and columns of photosites that convert light into an electronic signal. Each photosite is covered with a filter so that it is sensitive to only one of the three (red, blue, or green) components of visible light. Digital image processing techniques can also be applied to film negatives or positives if they are digitized using a high-resolution scanner that operates in much the same way as a digital camera.
Two primary measures are used to characterize digital images: resolution and size. Resolution refers to the ability of a sensor to represent details, and is generally specified in terms of pixels per inch (ppi).
Image size refers to the total number of pixels comprising an image, and is typically given in terms of megapixels. A pixel is the smallest possible discrete component of an image, typically a small square or dot, and one megapixel consists of one million pixels. As of early 2005, the best commercially available digital cameras had resolutions of approximately 20 megapixels and many professional quality digital cameras had resolutions of 5 or 6 megapixels.
Regardless of its origin, once an image is available in digital form it can be modified or enhanced using digital image processing techniques. Common image processing techniques include contrast stretching to expand the tonal range of an image, edge detection to outline areas possessing similar textural or tonal properties, and unsharp masking to increase sharpness. Unsharp masking derives its unusual name from a film photography technique in which an original negative was combined with a deliberately blurred negative to produce a sharp print. Although these image processing techniques can do much to enhance subtle features of an image, they cannot create information that does not already exist.
One of the aspects that distinguishes forensic digital imaging from non-forensic digital imaging arises from legal considerations. Images that are destined for use in a court of law must be obtained and processed using carefully documented procedures if they are to be allowed as evidence. The documentation typically includes the name of the photographer, the date the image was obtained, the names of anyone who had access to the image before it was introduced in court, the names of anyone who enhanced or altered the image, and the details of any enhancement procedures. One issue that is a particular concern when an image is obtained with a digital camera is originality. Whereas traditional photography produces a film negative or positive that cannot be easily replaced without detection, digital cameras produce electronic files that can be modified and overwritten either accidentally or deliberately. It is possible to open a file, make modifications, and then save it with the same file name even though the image has been altered. Computer systems used to store forensic digital imagery must therefore be secure enough to prevent accidental modification of or deliberate tampering with original files.
The possibility of image tampering was raised during a 1995 murder trial in Seattle. The only evidence that linked the defendant to the crime scene consisted of a digitally enhanced image of a bloody palm print taken from a mattress pad. Prosecutors used a digital image that had been sharpened and filtered to remove the fabric texture, and the defense unsuccessfully claimed that the image could have been altered by the computer operator. The possibility of image manipulation was also raised in the O.J. Simpson murder trial, during which the Simpson defense suggested that photographs of him wearing a particular brand of shoes had been fabricated.
