Colour vision I

why do we have colour vision

Helps in detection of objects from a background and helps to segregate objects similar in colour from a background

state the different ways colour vision aids in survival (3)

• finding food

• avoiding predators

• communication

explain finding food

• foraging/looking for fruits

• selecting fruits on the basis of colour (determining ripeness)

explain avoiding predators

camoflauge to blend into background to hide from predators and enemies

explain communication

assessing emotion and finding mates - ensures continuation of species

how many different cone types are there and explain each one

• 3 ; each with their OWN photopigment

• Short-wavelength sensitive (S cones) or sometimes just blue cones (relatively scarce – only 5-8%) - NONE in middle of fovea only green and red

• Middle-wavelength sensitive (M cones) or sometimes just green cones

• Long-wavelength sensitive (R cones) or sometimes just red cones

explain spectral sensitivity differences between the 3 cone types (3)

• each cone type has a different spectral sensitivity curve

• the peak of the curve varies for each cone type

• there is a variation in the distribution cone types across the retina

state the peaks sensitivities of each cone type (3)

• S-cones peak at 420 nm

• M-cones peak at 530 nm

• L-cones peak at 560nm

• slight overlap between the M and L peaks whereas S is further from these two

how many photopigments do rods and cones have

rods - 1

cones - 3

compare the sensitivity of rods and cones (2)

• Rods are 100 times more sensitive than cones in the middle of the spectrum.

• Cones are a little more sensitive for very long wavelengths

explain the principle of univariance (4)

• a SINGLE rod/cone cannot distinguish between different wavelengths of light (or the colour)

• this is because they only contain one photopigment - so no colour vision

• they can only perceive the intensity of the light

• cannot perceive anything of the wavelength (whether it is shortor long) except if it has been absorbed/hyperpolarized or not

what is the probability of any given photon being absorbed (3)

• higher probability of absorbance at the peak

• as the frequency of the wavelength resonates with the photopigment best at this point

• the probability changes with wavelength / depends on the wavelength

what needs to be done in visual system in order to extract information about wavelength (2)

• the visual system must compare quantum catches in different classes of cones

• essentially- more than one cone and different photopigments absorbing it

explain the cone mosaic / distribution of cones (2)

• in the direct middle of the fovea there are no blue / S cones only red and green cones

• moving away from the centre there are red, blue and green cones however still much less blue than red and green (many)

what is the peak density of blue/S cones

have peak density at 1~deg away from central fovea

why is the human vision system referred to as trichromatic (2)

• as there are 3 cone classes/types ; each containing their own distinct photopigments

• they can match any single coloured light (any single wavelength) with a mixture of 3 primary lights (R, G and B)… (in different proportions

state the equation that describes the trichromatic property of vision

• C(S) = C1(λ₁) + C2(λ₂) + C3(λ₃)

• Where C1, C2, C3 are called tri-stimulus values which represent the quantities of the colours

• Where λ _1, λ_2, λ₃ are primary colours.

explain why 4 lights may be needed to match one single colour (all scenarios) (3)

• Given any four spectral lights, by placing three of them on one side of a foveal matching field and one on the other

• or two on one side and two on the other

• it is always possible to cause the two sides of the field to match by adjusting the radiances of three of the lights

which scientists came up with the theory of moving ½ primary colours to the other side (2)

• Guild and Wright

• For any wavelength of light across the spectrum can mix up R,G and blue but for certain ones need to move one or two over

what is the XYZ colour matching functions

a linear transformation of the 1931 RGB Color Matching Functions in order to give them some mathematically convenient properties

what is the CIE 1931 chromaticity diagram (4)

• a reference chromaticity diagram that is based on the matching characteristics of a standard observer, and theoretical primary colours

• Shows all the colours that the human eye can see in a single space – different proportions of colour

• allows you to see what colour will be made when mixing any wavelength of colours together (blue and yellow would = white) (red and green = yellow)

• white in the middle

how is the white produced/made up of

when the theoretical primary colours are mixed in equal amounts = produce equal energy white.

explain how the proportion of each primary colour can be determined from the axis

• x and y axis

• The axes represent the relative amounts of two of the primaries in a match: x and y. 

• Because x + y + z = 1

• The amount of the third primary (z) is =1 – x – y

state the wavelengths of the Red, Green, Blue and Yellow colours

• red = 620

• green = 520

• blue = 480

• yellow = 570

• (nm)

on the CIE diagram where are the spectral colours located

the most saturated colours across the spectrum = indicated around the spectral locus(outermost boundary) NOT the middle as this is white

on the CIE diagram where are the pastel colours located

closer to the white/middle region

what are and where are the non-spectral colours on the CIE diagram

• Non spectral purples - are not on the wavelength spectrum

• they are not spectral colours

• made by mixing spectral colours together

what wavelengths of light can we see: visible spectrum

• We can see light of wavelengths between 380 nm and 780 nm:

• 380nm-Violet 

• 460 nm-Blue

• 480nm-Cyan 

• 520 nm-Green

• 580nm-Yellow 

• 600 nm-Orange

• 620 nm-Red

what are the 3 physically measurable variables (psychological attributes) needed to fully describe a coloured stimulus (to specify a colour)

• 1 - Dominant Wavelength (hue) Hue distinguishes colours

• 2 - Excitation Purity (saturation)

  Saturation distinguishes pale from vivid – strength of a colour - amount of whiteness

• 3 - Luminance (brightness)

• Brightness is related to the intensity of light

what is additive colour mixing and how is it done (3)

• Adding colours together can produce new colours.

• This happens when different colour lights are added together.

• Using 3 additive primary colours (colours that cannot be formed through mixing), other colours can be formed.

what is a real life use of additive colour mixing

This is the basis of the RGB monitors (TVs) etc. – RGB light of varying shade and intensity is emitted to create the full range

what are the different colour additions possible (4)

R + G = cyan

R + B = magenta

G + R = yellow

R + G + B = white

explain subtractive colour mixing (4)

• Light is removed

• E.g. stack of filters, mixing of pigments or paints

• The colour perceived is dependent on the wavelengths reflected to the eyes (the light that is reflected off the object)

• paint absorbs the light - made by mixing other paint colours together

define subtractive

Mixing colour so that wavelengths of light are selectively absorbed

this is used for paints specifically

what are the subtractive primary colours

• Cyan (-R)

• magenta (-G)

• yellow (-B)

• They absorb certain wavelengths.

explain the mixing of the above colours and state the dfferent combinations

• Y + M = R

• C + M = B

• Y + C = G

• C + M + Y = ~black

• Notice when 2 of these subtractive primary colours are mixed, a primary (additive) colour is produced !!

explain the clear difference between additive and subtractive colour mixing

• Additive – adding lights

• Substractive – paints – subtracting colours and therefore reflecting differently

 

what is the colour order system for paints and state the 3 vairables to specify colours

• Munsell system

• Hue specifies the colour (e.g. blue, green etc.) – wavelength

• Value specifies reflectance (brightness)

• Chroma specifies saturation.

define Complementary Colours

pairs of colours whose mixtures produce achromatic grey or white

define Metameric Colours (Metamers)

colours that match perceptually (look the same), but that have different spectral composition

example - one may be a pure blue, while one is made from mixing 2 primary colours etc.

Object –may peak at different wavelengths

Cone sensitivities absorb differently based on wavelength therefore get diff cone responses

Then response goes to brain to perceive color of object