Detection and preservation Flashcards
Which light has highest energy and which has lowest energy
Violet light has the highest energy (shortest wavelength)
Red light has the lowest energy (longest wavelength)
How is light reflected (Giving colour)
When light falls on a surface, a photon of light is absorbed if the energy of the photon exactly matches the difference in energy between two of the energy levels of the molecules of the surface substance. If light of a particular colour or energy does not match the difference in energy, it is reflected.
What is photoluminescence
After a molecule absorbs light and is raised to a higher energy level, it tends to relax back to the lowest level or “ground state” by giving off energy as heat, usually through collisions with other molecules. In some molecules, however, the excess absorbed energy is given off in the form of light. This is photoluminescence.
What sources may produce fluorescence
1) native constituents in latent print residue
2) Foreign substances picked up by the hand and transferred through deposition
3) Intentional chemical enhancement
4) Substrate (background) fluorescence
Define absorption
The absorption method exploits the colour difference between an object and its background surface. In our work the object is the fingerprint deposited on the
surface. One way to enhance the difference between the two is to use a band of light to blacken or darken the object as much as possible.
This method is useful if the visible fingerprint has a contaminant with characteristic absorption properties, for example a fingerprint in blood.
Define refraction
When light passes from one substance to another it is bent or ‘refracted’. The ‘angle of incidence’ is the light’s angle before bending and the ‘angle of refraction’ is the angle after bending. Every substance has a characteristic bending power known as its ‘refractive index’.
Define reflection
Light reflection involves two light rays: an incoming ‘incident’ ray and an outgoing ‘reflected’ ray. Diffused reflection is used in fingerprint work where the part of the surface is rough (ridge detail for example), and the rest of the surface is dark, flat or polished. If the substrate surface is dark, the incident light falling between the ridges will be absorbed by the surface. If the substrate surface is flat and polished the incident light between the ridges will be reflected at the same angle towards the normal as the incident light (specular reflection).
Describe the wavelength and barriers used for Panacryl
Wavelengths: 415-450 nm
Barriers: yellow-orange
Describe the wavelength and barriers used for Rhodamine
Wavelengths: 490-530 nm
Barriers: Orange-red
Describe the wavelength and barriers used for Ninhydrin and Indanedione
Wavelengths: 505-530nm
Barriers: orange-red
Describe the wavelength and barriers used for DFO
Wavelengths: 505-590nm
Barriers: Orange-red
Describe the wavelength and barriers used for enhancing blood
Wavelengths: 415nm
Barriers: Yellow-orange
Describe the wavelength and barriers used for blood treatments
Wavelengths: 530nm
Barriers: Red
What factors influence choice of development
Seriousness of the crime
What will provide best contrast
The type of latent print residue
Type of substrate
Environmental conditions
Types of fingerprints:
Patent - Visible prints made by contaminants
Latent - Hidden
Plastic - formed when FRS is pushed into a substrate forming a mould
Take-away
