Photo Alteration

A common sight at the scene of a forensic investigation is one or more photographers. Recording the details of the scene prior to the removal of evidence is an essential step to the subsequent reconstruction of the course of events. Whether using traditional photographic film or the recently developed digital photographic capability, photographs can be manipulated or altered to enhance the information.

However, as a caveat, the ability to add or remove details digitally from photographs requires the authentication of the photographic file to ensure that the photographs produced of a crime or accident scene, in fact, represent reality.

The camera was invented in 1839. By the next decade, photographers had already begun to manipulate photographic images. Initially, the manipulation was part of the exploration of the artistic potential of the new medium. Soon, the informational power of the photograph became recognized.

The ability to produce photographs that reveal more detail than do traditional photographs, especially at longer distances or using small cameras, has increased the information that can be gathered.

With new technology, the ability to alter a photographic image is easier than ever before. For example, in a traditional photograph, the difference in skin tone between a face and the neck or shadows that point in different directions can be clues that an image has been manipulated. However, these visual discrepancies can be eliminated in the digital image. Thus, the ability to generate false or misleading information has become routine.

In the days before digital technology, photo alteration was accomplished in the darkroom during the development and printing of the photograph. In a technique called dodging, the light shining through the photographic negative onto light-sensitive paper was obscured. Because less light strikes the paper, that region appears lighter in the developed image. In contrast, the technique of burning allows an increased amount of light to strike the photographic paper. The result of burning is to make the region appear darker in the print.

The traditional techniques of dodging and burning are used to enhance or disguise aspects of the photo. As well, details can be excluded from an image by the use of cropping, where only the selected portion of the image is printed. Photographs can also be enlarged to selectively print portions of the image. Enlarging cannot be done indefinitely, however, since the eventual inability to separate the informational components of the image from one another produces a blurry picture.

The coming of digital photography revolutionized the ability to alter photographs. The laborious darkroom manipulations of preceding times could be accomplished by a few commands in specialized photographic software.

In traditional photography, the reflected light from the subject enters the camera through the lens and is focused onto the surface of a light-sensitive emulsion. The emulsion records the image, which can be beamed onto light-sensitive photographic paper. The paper is subsequently treated with chemicals to make the image appear. It is during this latter printing process that the alteration of the photograph can be accomplished.

In digital photography, the reflected light that enters the camera is focused onto a chip that is known as a charged coupling device (CCD). The surface of the CCD contains an array of light-sensitive photo diodes. Each diode represents a pixel (the basic unit of programmable color in a computer image). Each photo diode is hooked up to a transistor, which sends an electrical signal (whose voltage corresponds to the light intensity that registered on the photo diode) to another chip. The second chip converts the electrical signal to digital information—1s and 0s—that can be interpreted by computerized photo manipulation software programs.

Colors are assigned a code sequence between 0 and 255, 0 is black and 255 reveals the most intense shade of red possible by the software. These coded assignments are in turn converted to sequences of 0s and 1s. Black, for example, is 00000000, while the most intense red is 11111111. Shades in between are mixtures of the eight-digit sequence of 0s and 1s.

Digital photo manipulation involves the alteration or elimination of the digital 1s and 0s. Changing an eight-digit sequence is trivial. When the digital information is reconstructed into an electronic image, the result can be an altered color.

In addition to color change, a myriad of effects are possible, including color enhancement, elimination of regions of the image, increased contrast, correction of a blurred image, and the merging of other images with the original image (a photographic version of the "cut and paste" operations in word processing). Images of missing children or crime victims also can be digitally manipulated to create an aged appearance, and have proved useful in identifying victims years later.

As digital photo manipulation software has increased in technical sophistication, and people have become more adept at using the software, the task of detecting manipulated images has become very challenging. Digital photographic manipulation is now so sophisticated that it can sometimes be impossible to discern whether people or objects in a photographic were actually there when the photo was taken. This has spurred efforts, especially in the military and intelligence communities, to establish a system of image verification. In this regard, the United States Air Force Research laboratory in Rome, New York, has developed a technique called digital watermarking. Akin to the watermarking of paper currency to establish authenticity, digital watermarking embeds an encrypted image over the actual photo image. The encrypted image is invisible to the naked eye, but can be detected by specially designed image scanners. The lack of the digital watermark is evidence of a altered image.