Lecture 27- The CIE system: XYZ chromaticity charts Flashcards

1
Q

What are the 2 classes optotypes fall into ?

A
  1. Spectrally neutral - defined by luminance contrast

2. Coloured- defined by both luminance contrast and colour contrast

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2
Q

What happens if you have a letter with low luminance ?

A

so will have a negative luminance contrast

-also a colour contrast

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3
Q

Explain the variation in the spectral luminance efficiency function of the eye you can expect ?

A
  • the average retina has 2x L cones compared to M cones
  • Spectral luminous efficiency function of the luminance contrast channel - which is responsible for our spatial vision- relies on only L and M cone signals
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4
Q

What is the most common spectral luminous effieciency function that corresponds to (2L + M ) (PHOTOPIC) ?
(graph in first slide)

A
  • IS indicated by the continuous black line on the graph
  • but due to variation of L:M ratio and also due to the absence of one class of cone photoreceptor either L or M in some subjects - which results in loss of red-green chromatic sensitvity - you can end up with 2 extremes
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5
Q

What are the 2 extremes (graph in 1st slide) ?

A
  • the spectral luminous efficiency function shown dotted in red- representing the V lamda response for a deuteranope - subject which lacks M cone pigment
  • Other extreme - dotted line shown in green- representing the spectral luminous efficiency function for a protanope- subject who lacks L cone pigment
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6
Q

What happens to the subject who don’t have the standard CIE lamda response ?

A

will see a different luminance contrast when presented with colour stimuli

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7
Q

What can the contrast be of subjects with no standard CIE lamda response ?

A
  • either be larger than observed in a normal trichromat

- or smaller than the corresposning perceived contrast in a normal trichromat

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8
Q

What is the stimuli defined by ?

A
  • Either luminance contrast alone

- or both luminance contrast and colour contrast

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9
Q

What are the luminance contrast of objects which are defined by light of same relative spectral composition as surrounding background ?

(grey objects shown in the diagram)

A

remains unchanged- with changes in spectral responsivity in the eye
therefore the use of ND( neutral density) test charts- perceived contrast is independent of both the spectral repsonsitivity of the patients eye and spectral composition of the illumiannt

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10
Q

What happens when coloured stimuli is involved ?

A
  • the effective luminance contrast ( combination of luminance as well as colour contrast) depends strongly on the precise V (lamda) function response of the eye
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11
Q

What is the lamda response of the eye is affected by ?

A

both the L/M ratio and with the class of CVD

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12
Q

How do we quantify hue sensation

and the strength of colour signals?

A

We use colour charts (i.e., The Munsell colour atlas)- consist of many patches of different reflectances which vary in both brightness as well as in hue and saturation

-or we measure tristimulus values and chromaticity coordinates

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13
Q

How is the mussel done?

A

test patch is judged against the nearest patch in the mussel system that matches both brightness as wells the colour of the test patch
-not a satisfactory system - hence invention of CIE

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14
Q

What does the CIE developed the X, Y Z system involve measurements of?

A

which involves the measurement of 3 signals -which required knowledge of spectral radiance distribution of. a patch
e.g green patch - measure the wavelength radiance distribution of this patch- as it contains more middle wavelength light - the largest signal produced by this light will be by M cones- with much reduced signals in L and S cones
-Same with Red - largest signal produced by the light will be L Cones
and less M and S cones

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15
Q

What do we do with the triplet of signals ?

A
  • plot in 3D space
  • each point in this space will define the luminance and chromaticity of the test stimulus
  • these signals relate to the signals generated in single cone photoreceptors- but not the single cone photoreceptors
  • good to combine these signals linearly to produce X Y Z value for instance Y value represents the sum of 2L +M cone signals- which means Y value will always be directly proportional to the luminance of the test patch
-3D space that plots three quantities
derived from linear combination of cone signals. 
 and is
350 450 550 650 750 Wavelength (nm)
therefore also proportional to luminance
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16
Q

How can we normalise any singles generated by any test patch of the CIE?

A

by dividing each of the tri stimulus values x,y,z by the sum of the 3
e.g x = X/ X+Y+Z
y = Y / X+ Y+Z
z= Z/ X+Y+Z

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17
Q

What is the advantage of this ?

A

the sum of little x , y, z is = 1

  • means that we don’t need 3 variables - because one of them is known as 1- (sum of the other 2)
  • e.g. z=1-(x+y)
  • we always lose some information when the we reduce the number of dimensions- we lose the absolute luminance
  • But we can now plot little y against the corresponding value of x - and end up with a chromaticity chart
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18
Q

What can we do with the chromaticity chart ?

A
  • can plot the colour and the saturation fo any patch with any wavelength spectra ldistubutin however we dont know the lumianance of the colour are as we lost raw 3 dimension through the normalisation
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19
Q

What are the useful properties of the CIE CHART ?

A

-Relates directly to the tri stimulus values ( X,Y,Z values)

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20
Q

How do you end up with an elipse ?

A
  • if you start with the grey background in the middle and move away until you see coloured sitmulis- you end up with a elipse - this is the chromatic threshold contour for a normal trichromat
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21
Q

What can we tell by the size of the ellipse ?

A

how sensitive the subject is to colour differences

-eye is very sensivite to colour differences

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22
Q

What is the definition of Y if its value is to also represent luminance in cd/m2?

A
  • luminance- relies on spectral responsitvity functions of L and M cones- so Y has to be the sum of 2L +M
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23
Q

What is the spectrum locus?

A
  • all the monochromatic spectral colours plot along the spectrum locus
24
Q

What is the line of non-spectral colours

A

line of purple/ magenta colours produced by combining pure red and blue lights

25
Q

What are the BB and daylight loci?

A

both BB which can vary in temp can have a number of different spectral power distribution and so does day light depending on the phase of daylight

  • represented by red curves for BB
  • blue dotted line for daylight locus
  • any source if light with a particular temp 0 would plot on this red light
  • this spectra loci is useful in determining the extent to which a particular illuminant approximates either BB light or a daylight at a particular temperature
26
Q

What was the justification for the introduction of the CIE (u’,v’) chart?

A

transformation of the =e XY chart

  • if we could transform little x and y - by distorting the chart - to ensure the chromatic threshold ellipse measured in the XY diagram is elongated in one direction so equal distances in different directions do not respond to the same colour signal strength
  • if we could alter distort the little x and y mathematically so to transform this ellipse into a circle- would be an advantage- this is why the CIE ( u’, v’ ) was introduced in 1976
27
Q

what are the mathematical transformation? Want to go from x and y and z to u’ v’ ?

A

non linear

u’=4x/(x+15y+3z) v’=9y/(x+15y+3z)

28
Q

What equation do use to get to x ,y ?

A

x= 9u’/(6u’-16v’+12) y= 4v’/(6u’-16v’+12)

29
Q

What is the ‘Colour Temperature’ of an illuminant? CT (measured in K)

A

colour temp will be the temp of the BB which has the same chromaticity as the Real source of light

  • for the majority of tungsten sources - this is very appropriate because tungsten sources have chromaticity which are very close to the BB locus
  • so this would be the colour temp of the source
30
Q

What is the ‘Correlated Colour Temperature’ of an illuminant? CCT (measured in K)

A

BB temp which matches nearest in colour that of given source of light

31
Q

What is the Colour Rendering Index (CRI)?

A

the chromaticities that different patches and surfaces are going to have depend on both the reflectance of the patch and the spectral power distribution of the illuminant
for e.g- if you have light which approximates day light- want to know how good the reproduction of colours is for different spectral reflectances different colours when using a particular light source - that is done in comparison with a standard day light source

32
Q

How do we go about calculating the colour rendering index ?

A
  • CIE selected a number of representative patches
  • each of these patches has a certain spectral reflectance
  • chromaticity of each patch will depend on the spectral reflectance of the patch and also the illuminant
  • can use a day light illuminant or any other illuminant derived from a different lamp
  • so when we plot the chromaticity of a particular path in the standard illuminance
  • now when we change to the real illuminant we will get a shift in the chromaticity of the test patch
  • can do this with each of the 14 patches
33
Q

How to compute the CRI ?

A
  • the smaller the shift the better the colour rendering of this patch with a new illuminant
  • this means the shorter the separation between them
  • then work out the magnitude for the separation for each of the 14 pathces
  • so is difference in chromaticity between that measured with a standard illuminant and that measured with a test illuminant
  • Then need a scale - so when the difference add up to 0 due to the spectral power distribution off the test illuminant is the same as the standard illuminant then we would like the colour rendering to be 100% because the chromaticities will remain unchanged
34
Q

What do you get with the worst illumiancne ?

A

you would get large differences

35
Q

How do you calculate CRI ?

A

sum/mean of all difference can be weighted and scaled and then subtracted from 100 (would be the worst)
-End up with a scale which ranges from 100 to 0 or even negative - which is intended to characterise the colour rendering index of that source

36
Q

What can colour rendering indices range for various sources can range from ?

A

close to 100% down to 10% or even become negative as is the case for sodium lights

37
Q

What is the colour gamut of a visual display employing three primaries of known chromaticity?

What are typical SPDs for the three primary colours employed in visual displays?

A
  • if we dont know the chromaticity of the colours we can always measure what the chromaticity is.
  • once we know the chromatcitiy- we can plot these in the xY chromaticity chart and we end up with a triangle as shown in the diagram
  • for this particular display one can reproduce any of the colours shown in the triangle- this is the gamut of this visual display
  • the spectral power distribution of the primaries can be quite spiky for R,G,B due to the phosphorus and the LED lights involved
  • doesn’t make any difference to the eye - due to metamerism- the L,M ,S cones have broad spectral repsonsivitites and appropriate amounts of these 3 primaries can reproduce the same cone signals in the eye as we would expect to get at daylight at a particular temperature - 6500K- illustrated by grey dot - when this happens the eye cannot tell the difference between the white on the display and daylight at 6500K in terms of natural daylight -the signals are the same and that is because of metamarism.
38
Q

What are typical luminances for each primary colour?

A
  • WE CAN measure them - shown in the diagram in the slide -the blue primary colour increases as you increase the appleid voltage until you end up with a maximum luminance 10cd/m2 - for blue
  • maximum for red - 30-40cd/m2
  • maximum for green 80-90cd/m2
  • these are typical values for the average display
39
Q

How does this gamut change with luminance?

A

gamut- the triangle

  • anything below the luminance of the maximum luminance of the blue can reproduced anywhere in this triangle
  • but if we want the blue light of this chromaticity and we wish to reproduce this on this display
  • that blue light cannot have a larger luminance than the luminance of the blue primary
  • if you still want blue light but of large luminance we have to saturate that blue by adding some red and green to it
  • we then get reduced gamut
  • the same happens if you want to represent red light of a chromaticity close to the red primary colour- we can have this chromaticity up to the luminance of the primary colour but any higher luminance we have to reduce the gamut
  • This happens same for green when you want to represent a higher luminance you need to reduce the gamut
40
Q

How much ‘red’, green’ and ‘blue’ light does one need to mix together to produce an ‘equivalent’ daylight?

A
  • we end up with the point in the centre which is the brightest white
  • we can’t have any higher luminance than this as this is the highest
  • but will only be one colour
  • if you want another colour - you have to drop the maximum luminancce level that you wish to represent on that display
41
Q

What is the relationship between chromatic saturation and cone contrast signals?

A
  • cone contrast generated by many colours in the chromaticity diagram relate to the situation of colour which is the distance away from background chromaticity
  • for every position along these directions we can plot the corresponding cone contrast signals generated by the colour
  • for a particular
42
Q

What happens as we move further away from background chromaticity?

A

the more saturated the colour becomes

43
Q

What happens as we get to the yellow colour in the CIE?

A

In yellow colour - there is no L and M cone signals generated by the yellow - it is undetected as we move along towards the yellow region of the spectrum locus by L and M cones
-but strongly detected by S cones- this is what corresponds to perceived yellow , constant L and M and a changing S signal

44
Q

What happens when we get to the blue colour ?

A
  • not producing any change in the steady state L and M cone signals which are generated as you move towards the short wavelength region of the spectrum locus
45
Q

What happens when we move to green ?

A
  • strong positive green contrast
  • negetaive red cone contrast
  • and absolutely 0 for the S cone contrast so the S cones are not involved in mediating colour vision along the red-green colour axis
46
Q

What happens when we move to red?

A
  • have a positive red cone contrast
  • negative green cone contrast in the same direction
  • virtually no S cone signal
47
Q

What are cone contrasts ?

A

Cone contrasts are directly proportional to chromatic displacement (CD) (i.e., chromatic saturation

  • this chromaticity chart shows how far you need to move away from the chromaticity of the background to see any of these colours
  • sos if you have to move further than the average normal trichromat which is what the chromaticity illustrate - then you have reduced chromatic sensitivity - because you need a larger colour signal strength to perceive a threshold colour difference
  • these thresholds which can be measured directly - are a good measure of chromatic saturation therefore they are directly proportional to the subject severity of colour vision loss
48
Q

WHAT IS ‘ADDITIVE’ COLOUR REPRODUCTION?

A
  • everything shown so far in display is additive colour
  • add red and blue to produce different purple / magnet colours
  • add red to green to produce a range of yellows
  • can add blue to green to produce cyan colours
  • is employed in virtually every display
49
Q

What is the advantage of the additive colour system ?

A

is that you can represent virtually any colour within this triangle

50
Q

What is the Maximum luminance for full gamut reproduction is limited by?

A

the smallest luminance (usually the blue).

51
Q

What does the the maximum luminance of the 3 primaries added together corresponds to?

A

the highest luminance of white

52
Q

What is subtractive colour reproduction ?

A

subtract from white certain colours

  • reduce the amount of light in certain regions of the spectrum so that white light becomes coloured light
  • this involves a subtractive process
  • so e.g a

cyan pigment would absorb all red light

magenta pigment- would absorb green light

yellow pigment- would absorb blue light

this is the CMYK system
-K stands for key - and is intended to denote the black pigment which is also added- need to reproduce blacks of low luminance using the additive colour system

53
Q

What produces black via the additive colour system ?

A

adding the cyan magenta and yellow pigments together

  • but because of the spectral absorption functions of most available pigments are not as sharp and complete there is always some light reflected that should be black
  • in order to produce good black with very low reflectance - needs to add in addition to these 3 pigments is black
54
Q

What does the subtractive colour system employ ?

A

-a cyan colour which is the mixture green and blue with complete absence of red
-magenta colour and yellow colour to then reproduce the colours of interest through a subtractive process by removing colours from white light
-

55
Q

Compare RGB and CMYK

A
  • red plus green plus blue = WHITE
  • green and blue = cyan
  • blue and red= magneta
  • red and green= yellow
  • can reproduce primary colours using the subtractive colour system
  • adding all the colours result in black as no light is reflected