Fibers, like hair, are considered class evidence. Because fibers may form connections or associations, they have probative value. A suspect, for example, may deny ever being at a certain location or having contact with a specific individual. However, if police find fibers from several different sources at a crime scene, the suspects' statements become less credible.
Individuality is another factor affecting credibility. If a crime scene contains evidence that is unique or unusual in some way, it will help support the statement of one suspect over another. For example, if there are several blood stains on a wall and each one has a different pattern, this would be evidence of multiple people involved in the attack.
Class evidence is present when several items of an undetermined nature are found at a single scene. For example, if police find fibers from several different clothing brands at a crime scene, this would be class evidence. The fact that these fibers were found in conjunction with other evidence that can link them to separate suspects makes them relevant to the case.
Individuality evidence is present when there is proof that an item of evidence is associated with a particular person. For example, if police find a fingerprint at a crime scene that matches the print of one of the suspects, this would be evidence of individuality. Evidence such as this can be used to identify who was where during the crime.
Trace evidence typically lacks uniqueness; this is especially true with fibers, which are mass-produced in massive quantities. However, these connections can be broken, so their significance should be understood within the context of the case.
Documents, hair, fibers, fingerprints, dirt, and blood are all examples of physical evidence. Blood type, fibers, and paint are all examples of class evidence. Individual features are physical evidence traits that may be traced back to a common source with a high degree of certainty. These include hair types, blood types, and fingerprints.
Physical evidence is collected by crime scene photos, diagramming of the crime scene, and collection of physical evidence at the scene of the crime. Physical evidence can also be collected from the victim or other persons at the scene. This evidence may reveal more information about the case than what is apparent at first glance. For example, an apparently empty wallet contains a credit card receipt for items purchased at a particular store. That single piece of evidence leads police to identify the victim, locate possible witnesses, and start theming out potential suspects.
Credit cards are a common form of payment used in criminal activity. When investigators find a credit card in the possession of a suspect, it provides vital information about the person's identity and likely involvement in the crime. A credit card number can be used to track down the owner if he or she has been reported missing. A credit card receipt may show that the owner was not far away from the crime scene. This both rules him out as a suspect and gives an indication of when he or she might have left the area.
Cash is the most common form of payment for criminals.
Fiber evidence can be persuasive. A piece of cloth can be customized for a certain outfit. A fiber can be tailored to a specific piece of cloth. One of the most essential ways of fiber identification is optical microscopy. Microscopes use magnification lenses and optic systems to enlarge small objects so that their features can be seen with the unaided eye. Optical microscopes work on the same principles as conventional microscopes but use glass lenses instead of metal ones. Fiber-optic microscopes are similar to ordinary microscopes but use light beams instead of mechanical lenses to magnify images. Modern microscopes can also identify elements inside cells using stains and dyes that change color when they come in contact with certain chemicals found within living organisms.
Fibers are invisible to the naked eye. They are only visible under a microscope or with special testing equipment. There are three main types of microscopy used by crime scene photographers: light, electron, and fluorescent.
Light microscopy uses an optical system consisting of lenses and mirrors to magnify tiny objects such as fibers. The human eye cannot see objects smaller than about 200 nanometers (nm) so light microscopy needs to be combined with other techniques to visualize particles down to this size. Fiber bundles from clothing can be measured with a microscope to determine their denier rating, which is the number of fibers per square centimeter.
Individual evidence includes anything including nuclear DNA, toolmarks, and fingerprints. Class evidence can only identify a group: blood types A, B, O; fiber types cotton, wool, and linen; and paint types white, red, and blue.
Class evidence is evidence that can help identify multiple people, for example, blood types AB and O are both the same type, so they could be brothers or friends who were in a car accident. Blood samples from each person would have to be taken for them to be able to identify which one gave which type of blood. With class evidence you can usually make an assumption about how many people might be involved because there has been a recent event that causes multiple people to be of the same type. For example, if there was a fight at a party and several people had blood types A and O, then it could be assumed that more people might have blood types A or O than just those two. Class evidence does not need to be from the scene of the crime; it can come from anywhere blood is found. For example, if there was a murder and the victim's son found blood stains on his father's clothes, he could take the clothes to a lab to find out what type of blood it was.
Fibers are identified and compared physically, using microscopy, as well as chemically. Textile fibers, like hair, are among the most prevalent things found at a crime scene. However, even within the same garment, individual fibers may provide information about the person who wore it. For example, if a fiber is from a cotton plant, but not from the cotton grown in a particular region, this could be useful information in determining source country.
Fiber analysis can also reveal information about how a fabric was made. For example, certain techniques will produce different results when used with cotton than when used with wool. Lab technicians use this knowledge to determine the material of clothing worn by a crime victim or suspect.
Fiber analysis is an important part of any crime scene photography because so much of criminal activity involves textiles. The forensic scientist needs to know what to look for when processing a crime scene photo to ensure that no relevant evidence is missed.