wk6: ND - Dichromacy Flashcards
What is the fundamental idea behind dichromacy?
Only need 2 colours in a colour mixture to match all colours
What does colour matching in a dichromat suggest about their vision?
Suggests they only have 2 chromatic/luminosity channels (compared to three in normal colour vision)
In comparison to the normal luminosity peak of 555nm in normal patients, how does this peak differ in protans and duetans?
Protans = peak is lower
Deutans = peak is higher
True/False: protanopes and normal subjects have similar sensitivity for long wavelength light
False!. Protanopes have a massive drop in sensitivity for long wavelength light. It’s DEUTERANOPES that have similar sensitivity to normals for long wavelength light
How good is colour discrimination for protanopes at long wavelengths
Pretty poor. At wavelengths over 540nm, only red (650nm) is used to make a match. This demonstrates how the discrimination at these wavelengths is poor. Note: this is basically the same for deuteranopes
How do you define “wavelength discrimination”? Use an example to explain
e.g. 540 vs 541 - px can’t tell difference, but 540 vs 543 px finally now can tell difference. This means their wavelength discrimination is 3nm at this wavelength
At what wavelengths do protanopes and deuteranopes have poor/no wavelength discrimination?
at wavelengths over 540nm (note significant patient variability)
What does it mean to have no wavelength discrimination?
colour always looks same no matter what wavelength you add
Comparing duetreranopes and protanopes, which have slightly better wavelength discriination?
deuteranopes. This means dueteranopes perform slightly better at practical tasks involving colour
At what wavelengths do protanopes and dueteranopes have their best wavelength discrimination? (2)
Protanopes: 490nm
Deuteranopes: 495nm
What is the typical wavelength discrimination value for normals?
<2nm required to detect difference over most of the spectrum
Define saturation
how far the colour is away from white
Define saturation discrimination
how much of a wavelength do I need to add to white for it to look tinged with colour
How do we measure saturation? (formula) How about saturation discrimination?
Lw + Llambda
deltaP = Llambda/Lw
How does saturation discrimination in normals work? (using 570nm as a reference point)
At 570nm: need to add a lot of yellow spectral light for white to be tinged yellow
Either side: don’t need to add as much colour
At what wavelength in protanopes does the saturation discrimination go to infinity? i.e when is saturation discrimination impossible and it just looks white. What is this referred to as?
495.5nm (492.5-497.2). This is called the “neutral point of dichromacy”, where no matter what, you’ll just see the light as monochromatic/white
At what wavelength in deuteranopes does the saturation discrimiation go to infinity?
500.4nm (495-506.4)
How do saturation discrimination thresholds for protanopes compare to normals?
Generally elevated thresholds across entire spectrum - so this means you have to add a lot more colour to the white to be seen
How can we summarise basic data on dichromat performance?
as confusion loci on a chromaticity diagram
Describe the orientation of the confusion loci on a protanopic CIE diagram
converge onto the “cone fundamental”, which is the location in colour space corresponding to the missing element in their colour vision
How many distinct colours/confusion loci do protanopes have?
17
Describe the orientation of the confusion loci on a deuteranopic CIE diagram
parallel confusion loci with no apparent convergence on “cone fundamental”
How many distinct colours/confusion loci do deuteranopes have?
27
As you move along the CIE diagram in protanopes and deuteranopes, what colours are perceived?
Saturated yellow on G-Y-R confusion locus becoming desaturated as the confusion loci move toward the NP confusion locus
Saturated blue in the B corner of the chromaticity diagram becoming desaturated for the confusion loci closer to the NP confusion locus
(NP = neutral point)