Sky Color

[SteveE]

Seems kind of quiet around here, so I thought I'd throw this out to stimulate some uh ... spirited discussion, yeah that's it.

 

This XKCD cartoon raises an interesting spectralogical question:

 

https://xkcd.com/1145/

 

So, why isn't the sky violet?

 

Steve

[Andy Perrin]

Probably the same reason our cameras don't record the shortwave end of things: the sensor (retina) is not very sensitive to it compared to blue. Also the amount of violet in the sunlight is tailing off very quickly.

[colinbm]

The sky is more Violet to people that have had their cornea replaced.

[Andy Perrin]

Interesting, so it guess part of it must also be the lens filtering out the violet too.

[dabateman]

This is right in line with a camera spectrum sensitivity discussion I have been having over on the dpreview Sigma forum.

We all see color differently. And our cones are different. We get "standardize" based on being told what we are seeing is x color.

People whom can't see any color can see some as have been told that this tree is green. So when looking at that specific shade of grey will see it as green.

[Andrea B.]

We all see color differently. And our cones are different.

 

I just do not fully agree with this.

 

Humans all evolved to be much more alike than different (in spite of how we all look externally.) The pigments in our eyes' rods and cones are the same in all of us. The stimulation of those photopsins sends signals to the same part of the brain in all of us. So we all see pretty much the same colors.

 

Obviously I am leaving out color-blindness. And I'm assuming that we are each viewing some color in the same light. (At some point you have to account for the yellowing of the cornea with age. But this happens so slowly that the brain adjusts and still says that blue sky is blue.)

 

Measurements of human pigments has shown that the cone pigment peaks vary only by about 4-5 nm. That is really not enough to say that my red is different from your red.

 

Link to measurement paper goes here when I find it: https://www.ncbi.nlm...1279132/?page=1

 

---

The preceding represents my opinion. I'm not trying to issue a challenge!

[Andy Perrin]

Obviously I am leaving out color-blindness. And I'm assuming that we are each viewing some color in the same light. (At some point you have to account for the yellowing of the cornea with age. But this happens so slowly that the brain adjusts and still says that blue sky is blue.)

My mom has some beginnings of cataracts and we had an argument over whether a blue spruce is blue the other day. She couldn't see it anymore. My step-mom (10 years younger than her) could and I could. (My color issues are on the red end of the spectrum and don't affect my blue perception.)

[Andrea B.]

Yes, cataracts can certainly interfere with color perception. So add an assumption to my preceding that no cataracts are involved.

 

Interestingly, like your mom, I currently have a developing cataract on my right eye. The left eye was already fixed about 5/6 years ago and sees color like a 25 year old (as per the replacement lens transmission chart). The bad right eye sees blues as more of a cyan. If the blue is less saturated as in some natural objects, it tends to appear greyish blue thru my right eye only. So in addition to loss of acuity, it seems there is indeed lessening of some (not all) color perception under certain light. Greens and reds in nature still seem the same to both my eyes.

 

That was a broad observation based only on my own experience. There are different kiinds of cataracts.

Hope your Mom gets hers fixed soon. It is quite easy these days.

 

My good eye sees a blue sky. My bad eye sees a cyan sky. Makes sense given that the lens yellows with age. I can perceive violet where it exists with my left eye and scarcely at all with my right eye.

I've mentioned this stuff before. Getting out now. Don't want to bore all with repetitions!!!

I won't be this Living Example much longer as it is time to go get the right eye repaired.

 

But back to that violet sky? Or is it??

[JCDowdy]

.........The left eye was already fixed about 5/6 years ago and sees color like a 25 year old (as per the replacement lens transmission chart). The bad right eye sees blues as more of a cyan. If the blue is less saturated as in some natural objects, it tends to appear greyish blue thru my right eye only. ......My good eye sees a blue sky. My bad eye sees a cyan sky. Makes sense given that the lens yellows with age. I can perceive violet where it exists with my left eye and scarcely at all with my right eye.......

 

So, perhaps you are currently a pseudo tetrachromat who can perceive metamers that normally appear identical?

 

see: Double filters allow for tetrachromatic vision in humans

 

see also: Enhancing human color vision by breaking binocular redundancy

[Andrea B.]

Hi J.D. !!

 

I'm late getting back to this topic. Still trying to unpack the apparently infinite set of boxes we moved from NJ. (Or maybe the boxes multiply when I ignore them. Who knows?)

 

I don't think my color vision is unusual in any way.

But I don't know how I would test that.

Where do we find metameric pairs which can be used for testing? Can this be done online?

[Stefano]

A bit late... the UV version of this question is: why doesn’t the sky look green? Sunlight has less UV-green light, not all lenses transmit UV-green, and sensors are less sensitive there.

[dabateman]

Actually,

The peak wavelength of the sun is 500nm. So blue light dominates due to Rayleigh scattering and because it dominates the spectrum below 500nm. UV and violet are significantly less.

When I was testing stuff in sunlight, I was surprised just how much a giant 500nm light source the sun really is. Every other wavelength fall off in intensity.