Forensic Use of Light

murmerlastUrban and Civil

Nov 16, 2013 (3 years and 7 months ago)

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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


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



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



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Low Frequency, Long Wavelength

High Frequency, Short Wavelength




Amplitude*


Amplitude*


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The
entire range of known light waves is called the
electromagnetic spectrum



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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


R
adio
waves and
microwaves


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


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



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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



Types
and Techniques of
Microscopy


Bright
Field Microscope


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


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


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



㄰1


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


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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