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Types of fingermarks, their detection and enhancement
Types of fingermarks, their detection and enhancement
This video will cover the two different types of fingermarks and how they are detected and enhanced in order that they can be visualised and potentially identified. There are two different types of fingermarks. They can be either visible or latent. Visible fingermarks require no enhancement to be seen. They are generally recorded using photography and are commonly found to be deposited with the use of a contaminant. A contaminant could be blood, or paint, or it could even be a fingermark left behind in dust.
The evidential significance of finding a fingermark in blood at the crime scene is that the person who deposited the fingermark must have come into contact with the blood when it was wet, placing them at the crime scene either during or shortly after the incident. Latent fingermarks are invisible. They have to be detected either at the crime scene, or the item is removed to a mark enhancement laboratory. Either way, an enhancement process must be conducted to visualise these marks. The latent fingermark is the most common source of fingermark evidence, and also the one that is most problematic as it is present but invisible. Each primary ridge is associated with a sweat pore. This is the source of perspiration on the hands.
However, numerous contaminants can also be present in the latent fingermarked deposit including material from other glands, cosmetics, perfumes, and food residues. The three major glands responsible for the fingermark deposits are the eccrine, apocrine, and sebaceous glands. The eccrine glands are usually found throughout the body, but the highest densities are found in the palms and soles. The sebaceous glands are typically localised to regions containing hair follicles, as well as the face and scalp. The apocrine glands are primarily found in the axillary regions, ie the armpits. In most instances, only the eccrine and sebaceous glands contribute significantly to the latent fingerprint deposit. Fingermark enhancement methods have been developed to target the components within the fingermark.
The formation of the latent fingermark is dependent on a variety of factors. The first factor is the surface. A smooth surface will allow the fingermark ridge detail to be uninterrupted. Some examples of smooth surfaces would be paper or glass. However, if the finger is placed on a rougher-textured surface, the ridge detail becomes interrupted by the texture of the surface. Some examples of a textured surface would be a wooden door or textured wallpaper. As such, smooth surfaces would be better for preserving the fingermark pattern. The surface can also be porous, non-porous, or semi-porous. Porous substances such as paper absorb the water-soluble part of the fingermark deposit within seconds, offering it protection. This type of fingermark cannot be simply rubbed away.
None of latent fingermark deposit is absorbed if it is deposited on a non-porous surface such as glass. The fingermark deposit sits on top of the non-porous surface. So if you polish the surface, you would eradicate the mark. Semi-porous surfaces such as a varnished wood can absorb the water-soluble part of the fingermark deposit. However, this can take hours rather than the seconds for fully porous surfaces. Until absorbed, the surface would act as non-porous until the mark could be rubbed away. The time of contact also influences the fingermark quality. The longer the contact time, the better the quality of the resultant fingermark. The pressure of contact is another factor.
The harder the pressure of the contact, generally the better the quality of the resultant fingermark. Finally, the person depositing the mark also influences the quality. A person’s diet has been shown to influence constituents present in the fingermark, but also how long it has been since they have washed their hands could also affect the quality. However, the time, pressure, and person depositing the mark are unknown at the time of a crime scene examination. Any deposited fingermarks can also degrade. One factor is time. As the time since deposition increases the quality of the fingermark will decrease. Mechanical action such as rubbing a glass surface can also remove fingermarks. This is only possible on semi- and non-porous surfaces.
Dust accumulation, exposure to water, and exposure to light and heat will also have detrimental effects on a deposited fingermark. Therefore, the sooner a crime scene is discovered, the less chance any fingermarks at the crime scene have to degrade due to any of these factors. On shiny surfaces such as glass, the mark may be visible to the naked eye using oblique lighting. But very often, the surface is not so obliging, and enhancement techniques must be used. These are intended to enhance the contrast between the mark and its background. A common physical detection method is powders. These are traditionally black or silver but are now supplemented by coloured fluorescent powders.
They are applied using a brush with powder physically adhering to the fingermark which, visualises it. High-intensity light sources, such as lasers alone, or with fluorescent chemicals and powders, can optically detect fingermarks. Chemical detection involves using chemical reagents to visualise the fingermark. Powder suspension was used to enhance this fingermark on uPVC. Combinations of treatments can be used in succession such as DFO, a reagent which reacts with amino acids. This requires a green-colored light to fluoresce the fingermarks which have already been treated with DFO utilising a chemical and an optical detection method. There are many different enhancement techniques which have been developed over the years.
These are developed to target specific components found in either the eccrine or the sebaceous gland secretions in the fingermarks. As such, a fingermark may be treated with two different chemical enhancement techniques, one after the other. This allows the chemicals to react with both the eccrine and sebaceous parts of a deposited fingermark. When using chemical enhancement, firstly as we’re dealing with latent fingermarks, the scientist doesn’t even know if any fingermarks are there, never mind what parts of the fingermark are eccrine and which parts are sebaceous. Therefore, what guides the selection of a chemical enhancement reagent is the surface in which the fingermark has been deposited on.
You should now be able to describe what physical and latent fingermarks are, understand the factors which effect the quality of a fingermark, describe at least one technique for enhancing fingermarks on porous and non-porous surfaces.
The next set of videos present information about the detection and enhancement of fingermarks.
We will discuss the difference between visible fingerprints and fingermarks and latent, or invisible, prints and marks. The latter need to be enhanced in some way so that they can be visualized. Such enhancement makes use of the various secretions left in fingermark/print deposits and the nature of the surface that the marks are deposited upon.
There are a wide range of chemical enhancement techniques used by mark enhancement laboratories. Once enhanced, the marks are photographed and the photographs are then looked at by a fingerprint examiner. Chemical enhancement techniques have been used for many years, either on their own or in some cases sequentially, one after the other.
The UK fingerprint source book in the ‘see also’ resource section below provides a lot of detailed information about these techniques if you wish to read more about enhancement. There is a lot of material and it might be interesting for you to browse through some of it at your own pace.
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