Science of Crime Scenes Chapter 5.1 Science of Crime Scenes

Science of Crime Scenes Detecting Discovery of contact traces depends upon: The nature of the surface onto which the material has been transferred. For instance, fabric surfaces will retain fibers better, but are poor collectors of fingerprints. The time and weather since the contact. The way evidence has been handled since the commission of the crime, and once collected, the methods employed to avoid losing traces on it. Science of Crime Scenes

Science of Crime Scenes Detecting The most important detection tool at a crime scene is the eye of the investigator itself Physical and chemical tests that develop (or visualize) latent evidence are important but They are of use only in specific areas, suggesting these areas were already “detected” and indicated these areas be further processed Their detection properties are identical to their enhancement properties, and are specific to the matter and substrate searched Science of Crime Scenes

Science of Crime Scenes Light and objects Electromagnetic radiation refers to energy in the form of waves Humans and animals are visually sensitive to certain kinds of radiation; this is referred to as light Different kinds of matter react with different waves of light The matter absorbs more or less of the radiation causing one or more responses, some of which are useful in crime scene investigations Waves can be described by their wavelength (λ) the distance between corresponding points on two adjacent waves Light can also be measured in terms of its frequency (ν) the number of waves that pass a given point in one second, measured in cycles per second or hertz Science of Crime Scenes

Electromagnetic spectrum Science of Crime Scenes

The ultraviolet-visible region Contains ultraviolet and visible light These two regions are spoken of together (“UV-Vis”) Both UV and visible light have similar effects on matter When a molecule absorbs this light, electrons are shifted within the material Some are absorbed Others emit photons as excess energy (such as luminescence) As the frequency of visible light decreases, the light changes from violet to red at the lowest frequencies Science of Crime Scenes

Science of Crime Scenes The UV-Vis range Science of Crime Scenes

Science of Crime Scenes Human vision The human eye is most sensitive at around 550 nm It can be deceived by other wavelengths (colors) where it is not as sensitive For instance, a fingerprint in blood on a dark surface may not be clear to the unaided eye but will appear clearly on a camera The CCD (or the film) is more sensitive than the eye with these wavelength Science of Crime Scenes

Science of Crime Scenes Infrared region Below the red region of visible light is the infrared region (“infra” = below) This type of light causes bonds between atoms in a molecule to vibrate Every substance absorbs light in the infrared region The vibrations of the molecules are different for every substance Science of Crime Scenes

Science of Crime Scenes Luminescence The emission of light by a substance that has not been heated Two types: Fluorescence: Emits light only when the excitation source is on Phosphorescence: Emits light after the excitation source is off The wavelength of the emitted fluorescence light is longer than that of the exciting radiation The substance absorbs a specific range of the energy Most of the energy not absorbed by the substance is re-emitted Compared with the exciting radiation, the fluorescence radiation has lost energy Therefore, its wavelength will be longer than that of the exciting radiation Science of Crime Scenes

Phosphorescence and fluorescence Science of Crime Scenes

Excitation and filters The exciting wavelength is controlled using a band-pass filter Allows certain bands (or wavelengths) to pass and blocks or attenuates wavelengths outside that range Part of the light striking the object is absorbed by the specimen and re-emitted as fluorescence Science of Crime Scenes

Science of Crime Scenes Barrier filters To enable the comparatively weak fluorescence to be seen, the light emanating from the object is filtered out by a secondary filter placed between the specimen and the eye The second filter is called a barrier or emission filter and it blocks any reflected excitation light and transmits only the fluorescence from the object of interest Science of Crime Scenes

The Forensic Light Source Alternate light sources (ALS) Lamps provide narrow bands of wavelengths over the spectrum but with a strong intensity on each of the desired bands Control of bandwidths down to 5nm Use goggles as barrier filters (and for safety—the lamps are over 300 watts) Science of Crime Scenes

Alternate Light Source (ALS) Science of Crime Scenes

Science of Crime Scenes Useful bandwidths Wavelengths Application Emission filters Goggles All (white light) General searching, particularly visible fingerprints, stains, 3D impressions, 2D impressions on smooth surfaces 400–420 460–480 490–520 530–570 560–580 580–600 600–650 620–650 Clear Yellow Orange Red 350 Traces on UV luminescent surfaces, especially fingerprints, semen, GSR, fibers, paint chips, lubricants 415 - 450 Dried blood, bloody fingerprints, semen, GSR, fibers, semen, lubricants 450 - 550 Semen, GSR, ninhydrin-treated fingerprints 550-600 Selective absorption for stained bloodmarks Science of Crime Scenes

Science of Crime Scenes Reflections A trace on a flat, dark, shiny surface will likely produce a diffuse reflection of the incident beam The background will create specular reflection The incident light will arrive at 45° and enter the eye or camera positioned parallel to the trace Due to specular reflection, some diffuse rays emitted from the trace will reach it and, hence, the trace will appear light on a dark background Science of Crime Scenes

Fingerprint Detection Latent fingerprint secretions are not visible under normal lighting conditions Screening with a strong white light at 45° and at low angle/oblique incidences Then more specific lighting techniques could be applied on areas of interest Fingerprints are not photoluminescent on their own; any potential luminescence is caused by contaminants on the finger the luminescence could be improved with various excitation and emission filters Photographs should be taken before any further physical (powder) or chemical treatment Science of Crime Scenes

Science of Crime Scenes Blood Dried blood absorbs at an optimal wavelength of 415 nm Absorption at 415 nm is recommended for a colored or luminescent substrate It appear bright while the blood will appear black Because the human eye is not highly sensitive in this violet region, darkness is mandatory Science of Crime Scenes

Science of Crime Scenes Semen Dried semen shows a very strong luminescence with excitation varying from 300 to 500 nm Its respective emission varies from 450 to 540 Because the substrate can interfere, this protocol can be useful: UV-A excitation, white goggles: If the substrate is luminescent, go to the next step, or else the stains will appear bright blue. LPF 415 excitation, yellow goggles: If the substrate is luminescent, go to the next step, or else the stains will appear bright yellow. CS Blue, yellow goggles: If the substrate is luminescent, go to the next step, or else the stains will appear bright yellow (see also with orange goggles). Excitation at 500 nm, orange goggles: If the substrate is luminescent, go to the next step, or else the stains will appear orange. Excitation at 550 nm, red goggles: The stains will appear bright red. Science of Crime Scenes

Science of Crime Scenes Fiber, Hair, Glass Fibers have optical brighteners added to them Their dyes may also be fluorescent dyes Hairs may also fluoresce due to bleaching or dyes The coating layers for many types of glass are luminescent Caution should be exercised: the CSI should not collect only those that emit as many fibers may not luminesce at all Science of Crime Scenes

Science of Crime Scenes Gunshot Residue Unburned or partially burnt propellants may show weak luminescence Despite the huge number of particles deposited after a discharge, their weak luminescence requires a dark environment to detect them A suggested protocol: UV-A excitation, white goggles: If the substrate is luminescent, go to the next step, or else the particles will appear bright blue. LPF 415 excitation, yellow goggles: If the substrate is luminescent, go to the next step, or else the particles will appear bright yellow. CS Blue, yellow goggles: If the substrate is luminescent, particles will appear bright yellow (see also with orange goggles) Science of Crime Scenes