Forensic Science
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Forensic Use of Light – an understanding of
light energy, its properties, its uses, and its
technological applications is fundamental in
the study of forensics
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Location/Collection of Evidence with Light
Observation of Evidence with Light
Analysis of Evidence with Light
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Both travel in the
form of waves
Both are frequencies
Our eyes and ears
cannot detect all of the
frequencies
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Light waves are much
faster than sounds
waves
Sound waves require a
medium to travel
through, but light
waves can travel
through a vacuum
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Wave – when some form of energy (light, sound,
water) is transferred by a disturbance in a medium
(light waves do not require a medium)
 Longitudinal (or compressional) waves – travel like
a flexible helical spring toy that somersaults down
steps (i.e. sound waves)
 Transverse waves –like ripples in a puddle (i.e.
light waves)
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Wavelength (λ) – the distance from the top of one
wave to the top of the next
 Larger waves have a longer wavelength
 Smaller waves have a shorter wavelength
 The unit used for light wavelength is the meter (m)
Wave Speed (c) – different wave mediums travel at
different speeds
 The speed of light is 3.0 x 1010 cm/s or 3.0 x 108 m/s
(speed is considered as though in a vacuum)
 The unit of speed of a wave is meters/second
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Frequency (v) – the number of waves that pass per
unit of time; wavelength and frequency are inversely
related
 Longer wavelength means shorter frequency and
vice versa
 The unit for frequency showing cycles per second
is the Hertz (Hz)
 Frequency and wave energy are synonymous
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Wave Formula (c = λν)
 Wave formula in words – speed equals wavelength
times frequency
 So the speed of a wave is a product of frequency
and wavelength
 Symbols
 c = speed of the type of wave (can be the speed of
light)
 λ = wavelength (meters)
 v = frequency (Hz)
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Amplitude – the height of the wave from the bottom
(trough) to the top (crest)
 Amplitude is also thought of as the energy the wave
carries
 Wavelength, speed, and frequency do not change just
because the height (amplitude) of the wave changes
High Frequency, Short Wavelength
Amplitude*
Amplitude*
Low Frequency, Long Wavelength
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The entire range of known light waves is called the
electromagnetic spectrum
Increasing Wavelength
Increasing Frequency
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There is an array of different light waves
(electromagnetic radiation or “radiation”) with
characteristic colors, invisibilities, wavelengths, and
frequencies
 Radio waves and microwaves
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Infrared light
Visible Spectrum
Ultraviolet light (black light)
X-rays and gamma rays
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Light (sunlight/electric light bulb) or white light
 Allows us to see
 Is actually a combination of all known colors
 Can be any color we see
An object absorbs most of the visible wavelengths and
reflects some of the wavelengths – this is what we see as
color (i.e. plants absorb all wavelengths (colors) except for
green)
Any object will absorb and reflect different light
wavelengths depending on its composition
 The chemical compounds the object is made of
 Or the chemical compounds of the paint on the object
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Investigators should use all forms
of light possible when gathering
evidence
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Regular white light
Ultraviolet (UV) light
Infrared (IR) light
A good forensic light source is
made up of a powerful lamp
containing all of the light
wavelengths
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Light (electromagnetic spectrum) behaves like
a wave in the way it travels; however, light
also acts like a particle in the way it transfers
energy to electrons
This is called the Dual-Theory of Light
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Photon – an energized packet of light energy
 In the late 1800’s/early 1900’s, scientists noticed
light behaving sometimes like a wave and
sometimes like particles
 This idea of particles was combined with the
Atomic Theory to create new light theories
(Quantum Physics)
 Photons of light are absorbed (energy gained) and
emitted (light is given off)
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In general, “light” refers to the portion of the electromagnetic
spectrum that we see
 Incandescence (hot) (i.e. light bulb)
 Luminescence (cold) (i.e. fluorescent lights)
 Chemiluminescence
 Thermoluminescence
 Photoluminescence
 Two Types that Absorb Ultraviolet Energy
 Fluorescence
 Phosphorescence
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Phosphors – any substance that causes an object to show
photoluminescence
 Causes certain objects to glow in the dark, or light up under
UV radiation
 Used in radar detection and plasma screen TV’s
 Examples: rare earth minerals, transition metals, nucleic acids
found in DNA, and biomolecules
 Because many different types of phosphors found in the
human body and other types of forensic evidence, UV lights
are irreplaceable in evidence collection
Semen under
UV light
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Emission
Absorption
Transmission
Reflection
Refraction
Dispersion
Intensity
Diffraction
Interference
Polarization
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Macro-View (magnifiers
and stereomicroscopes)
Micro-View (compound
microscope)
Microscopic Refractive
Index
Micro Colorimetry
Chromaticity Diagram
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Types and Techniques of Microscopy
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Bright Field Microscope
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Dark Field Microscope
Phase Contrast
Differential Interference Contrast
(DIC) or Nomarski Microscopes
Polarized Light Microscope
Fluorescence
Infrared/Ultraviolet Light
Digital Microscopes
Electron Microscopes
Other Microscopes
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A spectroscope uses prism or diffusion grating to
break apart incoming wavelengths; the specific
emission/absorption of wavelengths can be used
to identify the unknown element in the
composition of evidence
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Infrared (IR) Spectroscopy
Ultraviolet (UV) Spectroscopy
Reflectance Ultraviolet Spectroscopy (RUVIS)
Raman Spectroscopy
Microspectroscopy
X-ray Diffraction/Absorption
Atomic Emission/Absorption Spectroscopy
Microwave Spectroscopy
Types of Spectrometry that Don’t Use Light
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Nuclear Magnetic Resonance (NMR) – radio waves
and magnetic fields are used to penetrate unknowns and
collect information from hydrocarbons
 Valuable because it is nondestructive – evidence can
still be analyzed again later
 Can be used to analyze DNA and/or dangerous samples
such as explosives
 A form of NMR is used to determine time of death by
finding brain metabolite levels
 Electron Paramagnetic Resonance – uses microwave (not
radio) waves for similar purposes
 X-ray Fluorescence, Neutron Activation Analysis (with
infrared spectra), Inductive Couples Plasma – other
examples of the many technological advances that use
various forms of the EM spectrum to analyze and identify
forensic evidence samples
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Immersion Test – a transparent object (glass) is immersed
in several liquids with known refractive indexes to
compare the refractive indexes
 Glass appears to be “invisible” or disappear in liquid
that has the same refractive index
 If the liquid has a lower or higher refractive index than
the glass, the glass can still be seen (with a halo around
it)
 Examples
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Methanol RI = 1.33
Glycerin RI = 1.47
Clove Oil RI = 1.54
Pyrex Glass RI = 1.47 (would disappear in glycerin)
Lead Glass RI = 1.56 (would disappear in clove oil)
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Immersion Test – a transparent object (glass) is immersed
 Other Large Scale Uses of Refractive Index
 Refractometer – determines the refractive index of
various solids and liquids
 Used to determine the identity of unknowns in
forensics
 Can be handheld for fieldwork or larger for a
laboratory counter
 Also used to determine the density of liquids and
the concentrations of various components in the
liquids (sugar in urine, drugs in the blood, etc.)
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Finding the Diameter/Width of a Minute Object – Thomas
Young’s Double Split Patterns (interference/ diffraction of
light): an experiment that showed that specific light patterns are
dependent on the number of slits through which light is shown
 Using the known frequency of light and measurements of
interference patterns you can determine the actual width of
the solid that caused the light to split
 To find the diameter or width of an object: d = lL  10S
 d = diameter of a minute object in micrometers (µm)
 I = wavelength of the light (nm)
 L= distance from the light source to the screen (m)
 S = the average distance between bands (cm)
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Uses of Infrared Light (Thermal Radiation) – many
materials are sensitive to thermal or infrared (IR)
radiation (IR waves are longer waves in the EM
spectrum)
 Used in night vision goggles/equipment
 Many types of crime scene evidence are located
because they will absorb visible light and show IR
 IR luminescence is used for many types of
document analysis
 Illegal Alteration
 Erased Writing
 IR absorption or glow from different inks
 Revelation of charred document contents
 Used in conjunction with other technology
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Uses of Ultraviolet Light (Black Light)
 Mostly used in evidence collection
 Many bodily fluids (biomolecules) fluoresce when
illuminated by a source of UV light
 Detection of crime scene stains such as saliva, semen,
vaginal fluids, urine, and perspiration
 Many times latent fingerprints will fluoresce for detection
purposes
 UV light analysis is recommended as a first choice by the FBI
for examining and identifying biological evidence
 Also used for authenticating signatures, paintings, and ink
stains
 Used in the detection of trace evidence and illegal substances
 Used to see the light of luminol in order to find blood
evidence
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Forensic Light Source
 A powerful lamp with ultraviolet, visible, and infrared
wavelengths of light that has many components to enhance
visualization
 Direct lighting, such as a strong white light, is very useful to
reveal trace evidence
 Oblique or parallel lighting will also reveal small particles
 Used with all types of magnifiers and microscopes
 A multiple color band can penetrate many skin depths to
reveal details of a bruise pattern
 White light is normally used first, with other wave lengths,
chemicals, goggles, polarizers, and colored lenses are used
after the initial observation
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Cameras use refraction and polarization properties
(various lenses, including colored lenses) to capture
permanent proof of evidence and its analysis
 Used in evidence collection, observation, and
analysis to record results
 Used with
 Spectroscopy
 Scanning electron microscopes
 Fluoresce of IR or UV radiation
 Luminol (to record the chemicaluminescence of a
reaction with blood evidence)
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Location and Collection of Evidence with
Light – light, and all of its sources, are used to
locate evidence
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Examples
 Flashlight
 Ultraviolet or black light
 Light sources with all wavelengths
 Colored goggles or filters
 Many types of digital photography
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Observation of Evidence with
Light – once evidence has been
located and collected, light is used
to observe it
 Examples
 Magnifying glass
 Microscopes (stereomicroscopes,
polarizing microscopes, etc.)
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Analysis of Evidence with Light – science has made many
discoveries about light, and these are applied to analyze forensic
evidence
 Some Examples
 Microscopes of all types, including electron and ion
microscopes
 Spectroscopes – used to identify trace evidence
 Emission spectroscopy
 Gas and mass spectroscopy
 Infrared and ultraviolet applications
 Microwave, X-Ray, and nuclear forms also
 Mass Spectrometry – identification purposes
 Gas and liquid chromatography specializations
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Saferstein, Richard. Forensic Science: An
Introduction. New Jersey: Pearson Prentice Hall,
2008.
Bertino, Anthony J. Forensic Science:
Fundamentals & Investigations. Mason, OH:
South-Western Cengage Learning, 2009
Deslich, Barbara; Funkhouse, John. Forensic
Science for High School Dubuque, Iowa:
Kendall/Hunt Publishing Company, 2006
Texas Education Agency, Forensic Certification
Training, Sam Houston State University
Do an Internet search for a video using the
following: Flame Test 07.
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