Why isn't white light green?

  • Thread starter CWatters
  • Start date
  • #1
CWatters
Science Advisor
Homework Helper
Gold Member
10,529
2,297
White light is usually stated as having a uniform spectrum (eg all frequencies at same intensity). However the human eye is more sensitive to green light than other colours.

So why doesn't white light appear slightly green?
 
  • Like
Likes berkeman

Answers and Replies

  • #2
jbriggs444
Science Advisor
Homework Helper
2019 Award
9,302
4,005
It looks white because that's what white looks like. If you want to see something that looks green, you'll need to observe a different spectral distribution.

[Waits for @sophiecentaur to mention something about integrating to gray]
 
  • Like
Likes atyy
  • #3
831
293
There's also the factor that your optical system soon compensates for lighting colour, so stuff looks similar at dawn, noon and dusk, in the open and under foliage...

I used dark-green, 'industrial strength' UV-block goggles while doing HPLC detector lamp alignments and, initially, the the view through my big 'Froggles' was really, really weird. But, within ten minutes, my eyes had *mostly* compensated. Though blue-blind, I was otherwise functional. Removing my 'Froggles' made the world 'rose tinted' for ten minutes or so...
 
  • #4
CWatters
Science Advisor
Homework Helper
Gold Member
10,529
2,297
So the eye is only more sensitive to green in the absence of the other colours?
 
  • #5
DrClaude
Mentor
7,486
3,757
I don't think it is a question of eye sensitivity, but rather of brain interpretation. I think that anyone having done serious photography will be aware of the problem of withe things appearing white to the eye but not to the camera!
 
  • Like
Likes epenguin and atyy
  • #6
Drakkith
Staff Emeritus
Science Advisor
21,009
4,828
So the eye is only more sensitive to green in the absence of the other colours?
The light is sensitive to certain wavelengths, not to certain colors. The peak sensitivity happens to correspond to the portion of the spectrum commonly known as "green" though, and it changes slightly during dark adaptation.
 
  • #7
jim mcnamara
Mentor
4,096
2,578
Thread moved to Biology.
 
  • #8
jbriggs444
Science Advisor
Homework Helper
2019 Award
9,302
4,005
Suppose, simplistically that your brain is fed pixel values on three channels: Red with (scale of 0 to 60), Green (scale of 0 to 100) and Blue (scale of 0 to 60).

You get a reading of 60, 100, 60. This corresponds to the color that a chalk cliff produces under noontime illumination and the color that everyone has told you all your life is "white". Why would you see this signal as "green"?

Now suppose you get a reading of 30, 100, 30. This corresponds to the color that a tree leaf produces under noontime illumination and the color that everyone has told you all your life is "green". Would this not be the signal that you would see as "green"?
 
  • Like
Likes rbelli1
  • #9
CWatters
Science Advisor
Homework Helper
Gold Member
10,529
2,297
So why do some web sites say white light has a uniform spectrum? Does the eye/brain see a 60-100-60 spectrum as the same as a uniform spectrum?
 
  • #10
Drakkith
Staff Emeritus
Science Advisor
21,009
4,828
So why do some web sites say white light has a uniform spectrum? Does the eye/brain see a 60-100-60 spectrum as the same as a uniform spectrum?
Because they don't know what they're talking about. The response of the eye varies greatly over the range of the visual spectrum and "white" light can be one of many different different combinations of wavelengths and amplitudes. Hence why the 3 colors per pixel of your monitor can look like the same white as a fluorescent light or an LED light bulb, despite the fact that all of these will have a different spectrum when viewed with a spectrometer.
 
  • Like
Likes Nik_2213 and CWatters
  • #11
29
9
It is not just the wavelength. Retinal detection, nerve interpretation and transmission back to the brain, the peculiar distribution of optical interpretive functions of each brain will make one person's perception slightly different than another's. However, since my interpretation of rose red is consistent, I may be unaware that your brain interprets it differently, but also consistently. Therefore when we each see the same object that is "rose red", our two brains will process the information in such a consistent manner that we each recognize the object from our two differing viewpoints and interpretive patterns. FYI, women's brains generally tend to distinguish minimal hue differences more effectively than guys' brain do.
 
  • #12
White is a function of the brain's interpretation of light. You might as well have asked, "Why does green look green, or why does red look red." The only answer can be, "Because it does." Its as simple as that.
 
  • #13
CWatters
Science Advisor
Homework Helper
Gold Member
10,529
2,297
Can mark this solved.
 
  • #14
cmb
819
24
Green is a problematic 'colour' for our eyes because we can't actually see 'green'.

We see red, blue and 'bright stuff'. If we see 'bright stuff' but not red nor blue, then we interpret this as 'green'.

It is purely an interpretation.

It's why we cannot see black-body radiation in green colour. Nothing glows 'green hot'. There are no 'green stars'.

If you take the peak emissions wavelength of the Sun that penetrates the atmosphere to ground level, it actually peaks around 'green'. If we perceived black body radiation as the colour of the maximum emission wavelength, then, yes, we could see 5600K day-time white light as 'green'. We don't, so it isn't! Black body radiation goes from red to yellow then blue. We can't see black-body green radiation.
 
  • #15
Drakkith
Staff Emeritus
Science Advisor
21,009
4,828
Green is a problematic 'colour' for our eyes because we can't actually see 'green'.

We see red, blue and 'bright stuff'. If we see 'bright stuff' but not red nor blue, then we interpret this as 'green'.
I'm not sure what you mean here. We can certainly see a green object or a green emission line. Are you just talking about black-bodies and their thermal radiation?

Black body radiation goes from red to yellow then blue.
Don't forget white after the yellow and before blue. The Sun is, after all, a WHITE object!
 
  • #16
cmb
819
24
I'm not sure what you mean here. We can certainly see a green object or a green emission line. Are you just talking about black-bodies?
Yes, black body radiation.

My understanding is that it is mainly red and blue that we see as 'colours' and outside the fovea there are scarce few green cones and that job is done by the rods (black-white) with a similar spectral sensitivity in the 'green' wavelengths. So we don't have good acuity in green, unless it is 'just' green.

Green mixed with other colours is not well resolved.

If we do see much with green retina cells then I'll have to go look that up, I didn't think it was a big contribution.

A broadband colour which has a strong red peak we see as red, a strong peak in blue we see as blue, but a strong peak in green we see as white. The rod doesn't really pick up colour, so seeing green is as much about brain perception than the actual wavelengths getting into the eye. Else, where is green in the black-body temperature spectrum, if that's not true?

(The cones also are particular on how much light is falling on them, needs to be enough but not too much. When the light is low the cones don't work well, the more sensitive rod cells take over and we see in black and white in the dark. You might not have noticed. Walk into a dark room that you can't see in straight away, let your eyes adapt, and notice that you can't see any colours.)
 
  • #17
Drakkith
Staff Emeritus
Science Advisor
21,009
4,828
My understanding is that it is mainly red and blue that we see as 'colours' and outside the fovea there are scarce few green cones and that job is done by the rods (black-white) with a similar spectral sensitivity in the 'green' wavelengths. So we don't have good acuity in green, unless it is 'just' green.
On the contrary. We are most sensitive to light in the green area of the spectrum and are least sensitive in the blue area, on account of the relatively few S-cone cells compared to M and L cones (S sees blue, M sees green, L sees red). Note that L cones actually have peak sensitivity in the yellow area of the spectrum, not red. But we often say that the L cone is the 'red' cone for reasons of simplification.

Rod cells are actually desensitized after exposure to bright light and do not play any role in color vision. My understanding is that they aren't even connected to the areas of the visual system and the brain that are responsible for processing color, but I could be wrong.

A broadband colour which has a strong red peak we see as red, a strong peak in blue we see as blue, but a strong peak in green we see as white.
That is true, but that's only because we have cone cells sensitive to wavelengths on both sides of green, not because we aren't sensitive to green or have few M cones. If we had another cone cell that was sensitive to wavelengths beyond blue, then a blueish-white object, like a very hot star, would probably look more violet and less white (or perhaps some other color that might result from having that extra cone cell).

The peak wavelength isn't actually what matters. It's more about how intensely each set of cone cells are stimulated by incoming light. This is why a small set of individual wavelengths can appear the same color as a broad spectrum source. Fluorescent lights are a perfect example of this. They commonly emit in only a handful of narrow wavelength ranges, yet they appear to be the same color as a broadband source.

For a thermally-based broadband light source the 'shape' of the spectrum (how intense each wavelength is) is set purely by its temperature, as is the peak. So while the peak may appear to be what sets the color of the source, it's not. It's just that both the peak and the shape of the spectrum are set by the temperature and cannot vary independently of each other.

(The cones also are particular on how much light is falling on them, needs to be enough but not too much. When the light is low the cones don't work well, the more sensitive rod cells take over and we see in black and white in the dark. You might not have noticed. Walk into a dark room that you can't see in straight away, let your eyes adapt, and notice that you can't see any colours.)
Yes, I am aware of how dark adaption works and the fact that low-light vision is purely black-and-white.
 

Related Threads on Why isn't white light green?

  • Last Post
Replies
3
Views
3K
  • Last Post
Replies
2
Views
2K
  • Last Post
Replies
4
Views
4K
  • Last Post
Replies
14
Views
5K
  • Last Post
2
Replies
28
Views
11K
  • Last Post
Replies
21
Views
23K
Replies
15
Views
4K
  • Last Post
Replies
6
Views
2K
Replies
2
Views
8K
Top