CAS PS 222 Lecture 8: Color and the Eye
18 May 2018
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How do we see in Color?
Why do some people see the same dress in different colors? – This gives us information about
how human vision works.
What is the physical stimulus? What are their properties?
How do we detect the different dimensions of the stimulus?
How do we process and perceive that stimulus?
The physical stimulus is light. Light as we know consist of a broad range of electromagnetic
radiation and we can only see a small spectrum of it.
Isaac Newton had an insight that color wasn’t a property of the world, our perception of color
came from how we processed different objects in the world. How does light give us the
perception of the different colors? – He had to carefully construct an experiment: Pinhole in his
wall and pass the sunlight through the prism which made a rainbow. So to figure it out, he took
the rainbow and focused the light and passed it through another hole. The hole was tiny enough
only to let a tiny portion of the light through. The point of this experiment was to see if the light
would be broken down further or it was a pure source of color. He found that it didn’t break up
anymore and still remaining the one color he was experimenting with.
This was evidence that there was pure light- lights that cant be broken into composite colors.
His insight was that what we perceive as white light is actually made up of a mixture of these
pure lights- he thought it was only 7. (that we know isn’t true).
The entire physical spectrum is made up of a range of wavelengths.
We now have a better understanding to conceptualize color. We define color on whats called a
color solid. We can break down the multiple dimension of color. Color varies on 3 dimensions:
- Hue: The category that distinguishes the different color categories
- Saturation: quality of dominance of hue in the color. How dark is the color? How deep is
the color?
- Brightness: Perceived light emitted. No discrete boundaries between the colors. Ranges
from the very dark to the very light: black and white
Complementary colors: Any two-spectral component that when added together, appear white
(or gray)- they appear opposite each other on the color solid (depending on the brightness)
Mixing paint changes the reflectance and the absorption of the surface. Adding light together
doesn’t do it. It is basically only reflectance. This drives the change in color.
How does light interact with objects?
Percieved color depends on two physical properties:
- The reflectance properties of the surface: which wavelengths does it reflect, but an object
cant reflect a wavelength if the illuminant doesn’t give out a certain wavelength.
- The spectral quality of the illuminant : the quality of the wavelength being reflected.
The arrows in the slide show the wavelengths of light emitted from the light source. For objects
like this, the object absorbs a large amount of light and a small amount is reflected. We can map
it out- with % of light reflected. What we find is that as we increase the wavelength, we find a
bump in the reflectance around the high end (long end) of the spectrum. So, to read it off the
graph, so since the longer wavelength, they’re in the redder side of the visible spectrum.
White, Gray and Black reflect all wavelengths but the only thing that changes is how much overall
is reflected.
If I have an object, that absorbs short wavelengths and long wavelengths only, what color is that
object? – Green, because it is absorbing the short and long wavelengths (blue and red zones).
Spectral quality of illuminant:
We have moved to planet that is orbiting a blue star that only emits short wave lengths. We see
that the pepper (earlier red). The light source emits a disproportionate amount of light in the
short spectrum. Very little long wavelengths of light to be reflected off the pepper. So because
there is very little long wavelengths, this will look Gray-ish. It is still reflecting a little bit of long
wavelength. Because there isn’t enough red light to reflect from the surface.
Foundation color: If you put makeup on your face, the foundation can look different in different
lightings.
How do we detect the physical stimulus?
- Photoreceptors: Cones.
- E.g. Theres a particular receptor response graph and its response to light of a particular
wavelength. We shine a light onto this particular cone, we get some response for a
particular wavelength. What happens if we shine a light into a cone that is 450 nm as
compared to that of 650 nm? – the response is indistinguishable. So this particular cone
has the same response to two different wavelengths of light. This is THE UNIVARIEANCE
PRINCIPLE.
- UNIVARIANCE PRINCIPLE: An infinite set of different wavelengths intensity combinations
can elicit exactly the same response from a single type of photoreceptor
- This means that if you had a single cone type you won’t be able to see color. A single
photoreceptor can’t respond differently to different wavelengths.
- If we only had one single cone type- we would not see color which is why color blindness
exists.
- Therefore, one type of photoreceptor cannot make very good color discriminations based
on its response aone.
Humans are trichromats- 3 cone types (S,M,L)
So why is it good to have 3 cone types? – if we shine a single source of light, and if we only had
the M cones, we would only know about the response of M. But if there was S and M, we would
get a reaction from S and M about the amount of light they absorb.
Document Summary
This gives us information about how human vision works. Light as we know consist of a broad range of electromagnetic radiation and we can only see a small spectrum of it. Isaac newton had an insight that color wasn"t a property of the world, our perception of color came from how we processed different objects in the world. He had to carefully construct an experiment: pinhole in his wall and pass the sunlight through the prism which made a rainbow. So to figure it out, he took the rainbow and focused the light and passed it through another hole. The hole was tiny enough only to let a tiny portion of the light through. The point of this experiment was to see if the light would be broken down further or it was a pure source of color. He found that it didn"t break up anymore and still remaining the one color he was experimenting with.
