Lecture 27- The CIE system: XYZ chromaticity charts Flashcards
What are the 2 classes optotypes fall into ?
- Spectrally neutral - defined by luminance contrast
2. Coloured- defined by both luminance contrast and colour contrast
What happens if you have a letter with low luminance ?
so will have a negative luminance contrast
-also a colour contrast
Explain the variation in the spectral luminance efficiency function of the eye you can expect ?
- 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
What is the most common spectral luminous effieciency function that corresponds to (2L + M ) (PHOTOPIC) ?
(graph in first slide)
- 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
What are the 2 extremes (graph in 1st slide) ?
- 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
What happens to the subject who don’t have the standard CIE lamda response ?
will see a different luminance contrast when presented with colour stimuli
What can the contrast be of subjects with no standard CIE lamda response ?
- either be larger than observed in a normal trichromat
- or smaller than the corresposning perceived contrast in a normal trichromat
What is the stimuli defined by ?
- Either luminance contrast alone
- or both luminance contrast and colour contrast
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)
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
What happens when coloured stimuli is involved ?
- the effective luminance contrast ( combination of luminance as well as colour contrast) depends strongly on the precise V (lamda) function response of the eye
What is the lamda response of the eye is affected by ?
both the L/M ratio and with the class of CVD
How do we quantify hue sensation
and the strength of colour signals?
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
How is the mussel done?
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
What does the CIE developed the X, Y Z system involve measurements of?
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
What do we do with the triplet of signals ?
- 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
How can we normalise any singles generated by any test patch of the CIE?
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
What is the advantage of this ?
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
What can we do with the chromaticity chart ?
- 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
What are the useful properties of the CIE CHART ?
-Relates directly to the tri stimulus values ( X,Y,Z values)
How do you end up with an elipse ?
- 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
What can we tell by the size of the ellipse ?
how sensitive the subject is to colour differences
-eye is very sensivite to colour differences
What is the definition of Y if its value is to also represent luminance in cd/m2?
- luminance- relies on spectral responsitvity functions of L and M cones- so Y has to be the sum of 2L +M