L6: ALS and presumptive tests

Question 1

what is ALS

Answer 1

american sign language [JKJK]

als = alternate light source

equipment used to produce visible and invisible light at various wavelengths

Question 2

why/how is ALS used in forensics

Answer 2

different wavelengths of light can be used to detect or locate different types of trace evidence

UV-visible-IR rays used for forensic purposes

different light sources correspond to different regions of the visible spectrum
- visible spectrum 400-700nm
- deep purple = UV light [below 400]
- blue/purple = 450nm
- green = 520nm
- yellow = 630nm
- red = 720nm
- white - all colours combined

Question 3

3 things that happen when light strikes an object

Answer 3

1. light reflection
2. light transmittance
3. light absorbance

the manner in which visible light interacts with an object is dependant upon the frequency of the light, and the nature of the atoms of the object

Question 4

when does light reflection occur

Answer 4

occurs because frequencies of incoming light waves don’t match natural frequencies of the vibrations of the atoms in the object

light waves are REFLECTED back to us, at the same angle they struck the object with

Question 5

when does light transmittance occur

Answer 5

occurs because frequencies of incoming light waves don’t match natural frequencies of vibrations in the object

light waves pass THROUGH the object

Question 6

when does light absorption occur

Answer 6

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 from of energy

Question 7

which of the 3 things that happen when light strikes an object explains ALS theory in forensics

Answer 7

light absorption [and fluorescence]

Question 8

explain absorption and fluorescence [ALS theory]

Answer 8

• forensics interested in re-emission of absorbed wavelengths
• fluorescence is the emission of light by an object that has absorbed light or other electromagnetic radiation
• radiation energy is absorbed by an object
• electrons become excited from the ground energy state to high energy levels
• electrons eventually relax back down to the ground energy level
• energy is lost during the relaxation process
• remaining energy is emitted as light
• stokes shift

Question 9

explain stokes shift

Answer 9

the difference in the peak height between the excitation [absorption] and emission [fluorescence] wavelengths

• [from google] ‘fluorescent molecules absorb light at one colour (wavelength) and emit it at another. the difference in colours is called the stokes shift’

Question 10

3 different filters

Answer 10

• exciter filters
• barrier filters
• narrow band pass filters

Question 11

exciter filters

Answer 11

allows the passage of a single band of wavelength while blocking all others

band width

ex. 450nm ALS blue light - band width 425-475nm

basically an ALS flashlight

Question 12

barrier filters

Answer 12

isolates the desired fluorescence while blocking all other reflected wavelengths

in the form of goggles or camera lens filters

Question 13

narrow band pass filters

Answer 13

allows for further isolation of desired fluorescent wavelength

narrows the bandwidth

eliminates the fluorescence of the background

Question 14

what is chemiluminescence

Answer 14

‘glowing reaction’

Question 15

in which presumptive test does chemiluminescence occur

Answer 15

luminol or blue star [blood locator]

Question 16

how do luminol and blue star work to locate blood

Answer 16

• used to locate blood stains that may not be visible [clean up messes]
• liquids in spray bottles
• chemicals in these sprays react with hemoglobin in blood which reveal the location of blood due to chemiluminescence which occurs as a result of the reaction