1. Science of Crime Scenes
Chapter 5.1
Science of Crime Scenes

2. 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

3. 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

4. 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

5. Electromagnetic spectrum
Science of Crime Scenes

6. 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

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

8. 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

9. 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

10. 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

11. Phosphorescence and fluorescence
Science of Crime Scenes

12. 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

13. 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

14. 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

15. Alternate Light Source (ALS)
Science of Crime Scenes

16. 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
Dried blood, bloody fingerprints, semen, GSR, fibers, semen, lubricants
Semen, GSR, ninhydrin-treated fingerprints
Selective absorption for stained bloodmarks
Science of Crime Scenes

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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