PSYC 212 Lecture Notes - Lecture 22: Chromate And Dichromate, Dollarama, Color Vision

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20 Jul 2016
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Mechanisms of Colour Perception: 22:18
Covering the mechanisms of colour perception, continuing w/vision,
discussed already how the visual system is made up starting w/your eye,
thalamus, and primary visual cortex and extrastriate areas.
-Go back to the retina today and look at the diff. cones that make up your
colour vision, day time vision, and how they contribute to your perception of
colour.
-What we mean by colour, more imp., how light interacts w/material to
produce colour, talk about two diff. things that contribute to the perception
of colour like the nature of the light source itself and the and the reflectance
property of the material and that effetcst he colours you perceive from the
material.
-Describe the colour in a meaningful way to communicate it to other people,
otherwise we are dealing with a quality that is hard to convey, so how
colours are typically represented in various diff. professions and the primary
dimensions of colour, the dimensions on which colour varies, so that we can
basically manipulate it and measure physiological or psychophysical
responses to it.
Figure 11.1:
The easiest way to think of colour for most people is the quality of light
when it is split, if you take white light like sunlight, broad spectrum light and
pass it through a prism, because of the variation in the refraction of a prism
you will end up getting this rainbow pattern, in fact rainbow patterns are a
combination of the refraction and detraction of light and scattering.
If you pass white light through a prism and project it in a dark room, you
would see the splitting of the light into all these diff. colours, these are diff.
wave lengths, when you take full spectrum light or white light like from the
sun.
The sun is emitting light in many diff. wavelengths, by passing it through a
prism we are splitting it into separate wavelengths, light that has a short
wavelength will end up bending earlier than light with a longer wavelength,
that will bend later.
By passing it through a prism we have a pattern of refraction and we
perceive these diff. wavelengths as having diff. colours.
-First idea of colour- rep. of wavelength.
All light sources give off the same spectrum of light. That’s why images
taken under totally diff. light conditions come out looking the same:
-Keys you into the emission spectrum of the light sources.
A: False.
Diff. light sources can have totally diff. emission spectrum.
Emission Spectra of different light sources: Figure 11.2:
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-The 4 figures, each describes the relative intensity of the diff. wavelengths
making up one light sources.
-EX. Sunlight: Is basically full spectrum, emits light at practically every
visible wavelength to humans.
If you plot the energy of each wavelength, these are wavelength on X and
energy on the Y- you see power across the entire spectrum, the visible light
spectrum.
-If you take a tungsten light bulb:
These are now band, but you can still probably get them at dollarama.
They have a red shift, tend to emit more light in the longer wavelength of
the spectrum than in the short wavelength.
-If you look at fluorescent light bulbs:
Completely diff., work on a diff. principle.
They do not have a broad spectrum, they are giving light off in very fixed
intervals.
They have very limited band width of light.
-Helium-Neon laser beam- Generally emitting light at one length.
Many LDS can be designed to emit light very specific wavelengths.
This means that the emission spectrum has an important contribution to
make in your perception of colour but that alone is not the whole thing.
You can see a patch of this bag here that is red, this looks red under a
tungsten light that has this emission spectrum, but it also looks red under
sunlight and it will still look red under a fluorescent light bulb.
-Do you think the fact that tungsten emits much less light in the blue
wavelength than the sun light, makes it difficult to perceive the blue light
under the tungsten illumination-Is the perception of colour effected by the
wavelength of the emission spectra:
The answer is NO-FALSE.
jUST THE FACT that there is less blue light in this spectra, does not mean
that you will have a hard time seeing the blue light.
Because, what really happens is the material effectively filters light for you.
Emission spectra of the tungsten light:
-Does not emit a lot of light in the blue spectra, a lot for the red spectra.
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-The reflection spectra of a blue paper:
The blue paper, the way you measure these reflection spectra is you can
shine light of every wavelength and measure the power you get off of it.
If you shine light on the blue paper, you will get most of the blue light
reflected light and things not blue do not get reflected back.
The reflectance property of the blue paper acts like a filter, it absorbs these
wavelengths over here- the red ones, and it reflects back the blue
wavelengths.
-Will the blue bag look more blue if the light has more blue in it?????????:
-You would have to have a very biased light source to increase your
perception of blue ness.
-The resulting spectra- So now, the reflection spectra here is measured by
shining light of the same power at every wavelength and asking how much
are you reflecting back.
-This here is the power of the light at every wavelength from the tungsten
bulb.
-When you put the two together(tungsten light bulb+blue paper) you get
this.
Where basically, the blue light peak is the srrongest, one of the strongest
here.
Now, this alone won’t tell you whether you will perceive it as blue, but it
shows you relative to the adjacent wavelengths here, the blue reflection
from the paper is very strong, even though it is using a tungsten light that
has less blue wavelength.
The point of this is to bring up how we actually see colours in the natural
world especially.
Assuming we didn’t have screens, we would spend most of our time looking
at reflections of light of off material, not emission of light off light sources.
Reflection by material, Figure 11.5:
2 diff. types of reflection:
-One is the specular reflection- right off the surface, something that has only
specular reflection will bounce off all the light with no filtering of the light.
-Things like paper, actually have body reflection- The light basically enters
the superficial layers of the material and through scattering, it gets filtered
and so not all of the light ends up being reflected out.-some wavelengths are
filtered by the material property.
This here is a projected light source, whereas a computer screen is a
reflected light source.
The light that you absorb in your eye are only the lights that are bounced off
the object, if the object absorbs the light, you will not have the light to
absorb with your photoreceptors/photo pigments.
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