Alternate Light Sources Flashcards
What are alternate light sources?
Equipment used to produce visible and invisible light at various wavelengths
Describe the relationship between wavelength, frequency, and energy.
Shorter wavelengths = higher frequency = more energy
Longer wavelengths = lower frequency = less energy
What type of light is considered the forensic spectrum?
UV, visible, infrared
What is the electromagnetic spectrum?
The range of all electromagnetic radiation
What type of radiation is not visible to the naked eye?
Below 400 (UV), above 700 (IR)
What colour of light is 450 nm?
Blue/purple
What colour of light is 520 nm?
Green
What colour of light is 630 nm?
Yellow
What colour of light is 720 nm?
Red
What are the three things that could happen after light strikes an object?
Reflection
Transmission
Absorption
What does the manner in which visible light interacts with an object depend on?
Frequency of the light and the nature of the atoms of the object.
Describe light reflection.
Occurs because frequencies of incoming light waves don’t match natural frequencies of the vibrations of the atoms in the object.
Describe light transmittance.
Occurs because frequencies of incoming light waves don’t match natural frequencies of vibrations in the object; light passes through the object.
Describe light absorption
Occurs because frequencies of incoming light waves match the natural frequencies of vibrations in the object; light is absorbed by object and converted to another form of energy.
Describe the forensic aspect of the ALS theory.
Interested in the re-emission of absorbed wavelengths or the fluorescence.
Wavelength of light that is emitted is of a longer wavelength than the absorbed radiation
Describe the science behind fluorescence.
Radiation energy is absorbed by an object
Electrons become excited from the ground energy state to high energy levels
Electrons eventually relax back to ground energy level
Energy is losy
Remaining energy is emitted
Stokes shift
What is Stokes shift?
Refers to the difference in the peak height between the admission (fluorescence) and excitation (absorption) wavelength
What is an ALS used for at a crime scene?
Locate evidence
Visualize evidence
Enhance evidence and details
What are the benefits of using an ALS?
Non-intrusive
Doesn’t degrade, alter or damage evidence
What types of evidence fluoresce? (7)
Sweat
Saliva
Semen/vaginal fluids
Trace (hairs/fibres)
Volatile organic compounds
Bone/teeth
Fingerprints
What type of ALS is considered to be the most versatile for crime scene work?
450 nm ALS
What are the three types of filters used with ALS?
Exciter filter
Barrier filter
Narrow band pass filter
What is an exciter filter?
Used on flashlight; allows passage of a single band of wavelength while blocking all others
What is a barrier filter?
Used on goggles, camera lenses; isolates the desired fluorescence while blocking all other reflected wavelengths
What is a narrow band pass filter?
Allows for further isolation of desired fluorescent wavelength from a busy background that also may fluoresce.
