Currently, the world of crime scene investigation is largely centered around the recovery of fingerprints. Latent prints are those that cannot be seen without further processing. Patent prints are visible, but can often be enhanced by additional processing. The most common form of fingerprints that have evidentiary value are latent prints.
Latent prints are the result of moisture, oil, and other substances being transferred from a person's fingers to an object which occurs whenever you touch something. This is valuable to investigators because of the "ridge details" that are embedded in your skin. Nature gave us ridge details in order to provide more friction to grab things and hold on to them. Crime scene investigators use ridge details to match fingerprints. Most people have looked at their own fingerprints at some time or another. When you look at the fingerprints under magnification, you will notice that they are just a series of ridges. Some ridges end suddenly. Some split into two different ridges. Some look like very short lines or even dots. All of these are considered minutiae, or "points", and are used to compare fingerprints.
Recovering fingerprints from a crime scene typically involves the use of fingerprint powder and a brush. The composition of the powder depends on the manufacturer and type of powder. Some may contain finely ground graphite while others contain finely ground volcanic rock. They also come in a variety of colors, including fluorescent, to increase contrast with the background. In any case, the most important thing is that it is finely ground and very dry. Brushes also come in various materials from animal hair to synthetic compounds. The type of brush you use is dependent largely upon the surface that you want to process for fingerprints and your own personal preference.
Before processing any surface, you should document the scene and all of the evidence with photographs. Once you are ready to collect fingerprints, be sure to examine the surface of the object visually for patent prints that should be photographed in detail prior to processing. If there is any moisture on the surface, wait until it is dry before processing. Rain or dew will eliminate the ability to apply powder since the powder adheres to moisture. However, moisture will not damage the latent print, so waiting for the surface to dry will not reduce your ability to recover the print.
The application of fingerprint powder is straightforward. Dip the brush into the powder and shake off the excess. This is called "loading" the brush. Gently apply the powder to the surface with the brush. There are several techniques that investigators use for this. Some move the brush from side to side; others will spin the brush over the surface. This is also a matter of personal preference and will usually yield the same results. As the fingerprint powder makes contact with the latent print, the powder will grab' on to the moisture in the print making it appear. It is normally better to apply powder in very small amounts because you can always add more, but you can't remove what you have added. The exception to this is very dry prints. They tend to appear on the first swipe and disappear on the second. Prints dry out for several reasons but usually because of either age or heat. Theoretically, in the right conditions, latent fingerprints can last for decades, but realistically are very dependent upon the environmental conditions.
When further brushing does not improve the visibility of the ridge detail, you are done. Before lifting the print, there are a few steps that are highly encouraged. The first thing is to make sure you have a good photograph of the location of the print. Begin by taking a photograph of the general area where the print is located. Then use a dry-erase marker or similar instrument to write either a letter or a number next to the print and take a second photograph that includes the mark you made. Place fingerprint tape over the print by pulling out a section of tape and placing the end a few inches to the side of the print and the mark you made. Use your finger to hold the tape in place and use your thumb to gently roll the tape over the print and the mark. Then set the tape into place by firmly rubbing over the entire area to remove any air bubbles. Lift the tape off of the surface and place it onto a fingerprint card using the same technique that you use to apply the tape onto the object. The advantage of the dry-erase marker is that it lifts off with the tape and transfers onto the card. This not only helps you identify the location of the print in the photograph, but also shows the exact orientation. Write appropriate information on the card according to your agency's policy.
The most common problem with latent prints is small bubbles in the tape. This is usually caused by applying powder to a large section before lifting, which is an acceptable practice especially for automobiles. The problem with this is that as you apply powder to the surface, a certain amount of powder is carried through the air and collects on the surface you are processing. The solution to this is to gently brush any excess powder off of the print immediately prior to placing the tape over it. Another solution is to blow on the print to remove the excess powder which can be messier and less efficient, but less likely to damage a delicate print.
Latent fingerprint processing is time-consuming, tedious, and messy. But when it is done properly, it is the most productive tool for identifying perpetrators.