11.2: Properties of Light

Light has many important properties including rectilinear propagation, the ability to travel in a straight line; refraction, the ability to bend when passing through glass or water; and most important for our purposes, reflection, when light bounces off a surface; absorption, when light is absorbed by a surface; and conversion, when light fluoresces. Reflected light defines the color of the surface of an object. Remember that light is necessary for photographic documentation. Reflection and absorption combined become an essential aspect of observing and photographing forensic evidence. Combined, they comprise the properties of contrast. Contrast is the visual ratio of light and dark areas of the object being visualized or photographed.

Figure \(\PageIndex{1}\): Reflection off a white surface . Author's collection

Figure \(\PageIndex{2}\): A black surface does not reflect light - Author's collection

Contrast can be used in creative photography to enhance the photograph so that it is more appealing to the viewer. High contrast can create a dramatic visual effect by emphasizing the difference between light areas and dark areas. Low contrast can soften the image providing a moody atmosphere for the viewer. Color contrast can be vibrant and alluring to the viewer by using contrasting colors. Contrast in forensic photography is used solely to sharpen the entire image and to ensure the entirety of the image can be viewed apart from its background or surroundings. This is accomplished by correcting, removing, or enhancing lighting or color.

Figure \(\PageIndex{3}\): Contrast on a developed thumbprint. Author's collection

Adding additional lighting can compensate for the absorption aspect, but this might provide too much light in areas that are reflective. Therefore, it might be necessary to add or subtract light on the same object being photographed, which takes a keen eye, skill, and paying a great deal of attention to detail. As every forensic technician will know, there will be those occasions when the object that you want to clearly visualize or photograph is on a multicolored surface or is being overwhelmed by the color of its background. The answer to this problem is color definition through reflection or subtraction, and monochrome photography.

Your author was tasked by the county prosecutor’s office to review forensic evidence from a cold-case homicide that occurred in 2001. Unfortunately, most of the evidence collected from the scene had been damaged or otherwise contaminated and therefore was no longer available for analysis. Fortunately, the crime scene technician had taken care to document everything using 35mm emulsion film. While examining the photographs, your author came across a bloodstained footwear impression that had been deposited on a blanket that had been used to cover the decedent. The blanket had a checkered pattern that was of large dark blue and yellow checks. The problem was that the shoeprint straddled the blue and yellow checks but was mostly in the blue field. The blanket was also likely the source of the contamination that destroyed much of the stored evidence and therefore had been the first thing to be discarded. The technician had taken over 60 photographs of the shoeprint, but none of them were of examination quality because of the overwhelming saturation of the blue background. Your author’s immediate thought was, had the technician only placed a blue filter on the lens of the camera and then photographed it with monochrome (black and white) film, the result would have been a proper examination quality image of the impression. The blue filter would eliminate the yellow background and since the impression itself was red, the impression would have had more contrast than the blue background. The contrast would be further enhanced by monochrome photography as it would turn the blue background grey while the bloodstain would remain dark. This can also be done using a forensic light or ALS.

Figure \(\PageIndex{4}\): Light reflecting from a red object. Author's collection

Figure \(\PageIndex{5}\): Color separation using like-colored filters. Author's collection

Take for example the thumbprint on the following photographs developed using black magnetic powders on a red surface. As you can see, the red background overwhelms the developed print. The application of a red filter on the camera lens eliminates the white of the scale and enhances the dark print.

Figure \(\PageIndex{6}\): Thumbprint developed with black powder on a red surface. Author's collection

Figure \(\PageIndex{7}\): Developed thumbprint on a red background with a red filter. Author's collection

Photographing the developed thumbprint in monochrome turns the red background grey, which further enhances the dark print.

Figure \(\PageIndex{8}\): The same thumbprint photographed in monochrome.

Using the color filter also allows the technician to ensure the object of the photograph is sharp and in focus instead of relying on the scale for focus.

This technique can also be used on multicolored surfaces. In the photographs below, you will notice that the thumbprint, also developed with black powder, is on a background with green, red, and white. In this case, the majority of the print is on a red background. Therefore, by using a red filter we can eliminate the white and green background and enhance the black print. Photographing the print in monochrome highlights only the print. Notice that the green filter did not enhance the print quite as well as the red filter did.

Figure \(\PageIndex{9}\): A series of photographs demonstrating using red or green filters to remove background colors on the substrate and monochrome photography. Author's collection

It is important for the forensic technician to have color filters for their cameras that cover the visual light spectrum, not only for color subtraction, but to block the excitation light produced by the alternate light spectrum by using the opposite color of the light wavelength being used.

Figure \(\PageIndex{10}\): Colored lens filters. Author's collection

Your author remembers the color wheel by common associations of color. Traffic light colors are red and green, which happen to be opposing each other not only for the flow of traffic, but also on the color wheel; Madri Gras colors are yellow and purple; and the Denver Broncos wear orange and blue as their team colors. Therefore, if you are using a wavelength in the blue spectrum, you would use an orange filter. If you are using a wavelength in the green spectrum, you would use a red filter. If you are using a wavelength in the violet spectrum, you would use a yellow filter.

Figure \(\PageIndex{11}\): Opposing colors. Author's collection

Barrier filters for ALS and forensic lights come in the form of colored goggles or plates for the operator, but remember if you are photographing the visualized object, you must have the same color barrier filter on your camera lens as well. It is also important to understand that the Alternate Light Source or forensic light is best used in the dark, and therefore it is important to control the aperture and shutter speed of the camera. You will need a tripod or copy stand for your camera and an off-camera shutter release mechanism. You will need to set your camera to the manual mode on “bulb” setting. This will allow the camera to open the shutter until you decide you have captured enough light to preserve the image. Your aperture should be between f11 and f14, and your ISO should be set no higher than 400.