lecture 5 - chemistry of colour Flashcards
what colours are the three colour cones in the human eye?
blue, green and red
what happens to the three cones in the human eye when seeing white?
all cones are stimulated equally
what happens to the three cones in the human eye when observing orange?
stimulation of only two cones
where can you find the 3 colour cones in the human eye
the retina at the back of an eye
what does the CIE system base colour on
the coloured cones in the human eye
what is the drawback of the CIE system?
colour can only be described with this system, not quantified - due to the fact not all colours are represented equally in the diagram
what do we use the Munsell system for in forensics?
used for the quantification of the colour of soil - when its wet/dry
what is hue?
the actual colour
what is value (in the Munsell system)
how light or dark a colour is - vertical axis - ranges from black (0) through to white (10)
what is chroma (in the Munsell system)
how strong or weak your colour is - horizontal axis - varies continuously and ranges from neutral at the centre to maximum saturation at the outer edge
name the 5 primary colours used in the Munsell system
red, yellow, green, blue, and purple
when labelling using the Munsell system what’s the order we use?
hue, value, chroma
what are dyes soluble in
the solvent they are contained in - it has an affinity for the substrate it is deposited on
what are pigments
particles suspended in a solvent - with no affinity for the substrate
what is opacity
the ability of your paint to stop the transmission of light
what do pigments produce that causes opacity
scattering of light - the greater the scattering of the light the greater the opacity of a paint
what is optimal scattering
ensuring the diameter of the pigments particles are half that of the wavelength of incident light that is directed at the pigment particles e.g. white pigment particles need to be around 200-400nm as that’s half the wavelength of incident light for white
what is the definition of paint
a liquid that oxidises in the air or a coloured powder in a glue-like medium
what are the 5 things that make up paint and why do we use them?
- water
- pigment - for colour and opacity
- binder - to help bind the pigment to the wall
- surfactant & antifoam - to reduce bubble
- thickener - thickens the paint once its on the wall
to maximise scattering and reflection what 3 things do you need to ensure?
- pigment strength is high
- the pigment RI is higher than the binder RI
- the size of the pigments particles are half the wavelength of incident light
give an example of an inorganic pigment
Titanium dioxide (TiO2)
name the 3 crystal structure Titanium dioxide exists in
Rutile
Anatase
Brooktite
which of the three crystal structures of Titanium dioxide is not used in the production of white paint
Brooktite
which of the three crystal structures of Titanium dioxide produces the best opacity in white paint
Rutile as its RI is the greatest
why is paint that contains Titanium dioxide photocatalytic (self cleaning)
sunlight will react with the TiO2 in the white paint and oxidise any dirt found in the paint
what are luminescent pigments used in and how do they work
fluorescent inks and paints e.g. found in emergency exit signs. They work by absorbing light of a specific wavelength and emitting light of a longer wavelength
what are pearlescent pigments used in and how do they work?
pearlescent paint used on cars. They work due to the differences in the RI of the pigment (high RI) and the mica (low RI)
what are thermochromic pigments and how do they work?
pigments sensitive to temperature - they change colour when heated up or cooled down
name 4 different dyes used in forensics (only one acid)
- solvent black
- gentian violet
- acid yellow/black/violet
- indigo
name an example of an azo dye
solvent black
what is solvent black used for in forensics?
the detection of oil based fingerprints
Name a basic dye used in forensics
gentian violet
what is gentian violet dye used for in forensics
the detection of oil based fingerprints on the adhesive side of tape
Name the three acidic dyes used in forensics
acid black/ yellow/ violet
what is acid black (and yellow & violet) used for in forensics
the detection of fingerprints in blood as they bind to the haem group of blood
Name a carbonyl dye
indigo dye
give a use of indigo dye
blue fibres - mostly found in jeans
name the three less common dyes and their uses
- Methyl violet - the main dye found in ballpoint pen ink
- Basic red 40 - used to further enhance white superglue fumed fingerprints
- Copper phthalocyanines - used in pigment paint and the detection of fingerprints
name the three types of pigments
inorganic (TiO2), organometallic (contain a transition metal complex), and natural (lapis lazuli)
name the 3 general components found in ink
- Colourants - such as dyes and pigments
- Solvents - the vehicle for the colourant
- Additives
what does the type of solvent used in inks depend on
the age of the pen - older inks used oil based solvents and newer inks use glycol based solvents
name 3 additives to inks and what they are used for
- fatty acids - lubricate the ball in the pen
- natural resins/ synthetic polymers - help to dissolve your inks within the solvent to make the ink go further
- organic additives - prevent corroding
- glycols and Formamide - ensure a pen will not dry out
name the three ways we analyse dyes in inks and what they do
- UV-Vis spectroscopy - helps us differentiate between inks
- IR spectroscopy - determines the composition of inks
- Raman spectroscopy - helps us differentiate between inks
name the 4 ways we analyse dyes in fibre in forensics
- ATR-FTIR - non-destructive
- HPLC and TLC
- UV-VIS attached to a microscope - non-destructive
- UV-Visible spectroscopy and Raman spectroscopy - UV is only tentative whereas Raman is confirmatory
name the three ways we analyse pigments in paint forensically
- Fourier transform infrared (FTIR)
- Raman and FTIR spectroscopy
- X-Ray fluorescence
