Why is there a lack of consistency in portrayals of the visible light spectrum?

[vhbelvadi]

 

[jewbinson]

cool persuasive argument bro

 

[Claude Bile]

The term color needs defining here.

How do we define the color of an object?
By the transmission spectrum?
By the reflection spectrum?
By the spectrum we detect when we point a detector up at the sky?

The answer will be different depending on how you define color.

Claude.

 

[jewbinson]

Good point. I guess what is meant is "why, when we look up at the sky, do we see that it is blue?". The sky doesn't really have any "colour"

 

[vhbelvadi]

As always apparent colour!

 

[256bits]

Here is a picture of the Earth from space - Apollo 17

http://www.skyimagelab.com/ap17fulearim.html

The ocean looks blue.

 

[Drakkith]

We can go around in circles for days. Water WILL absorb small amounts of red light from a white light source that passes through it, causing it to look blue. It will also reflect light of all color if it hits it at the right angles. Trying to explain the color of water as being partially due to scattered blue light from the atmosphere is like trying to explain the color of the grass and everything else outside the same way.

I think this effect has been explained pretty well, so unless someone has something new to ask or add then I think this will be my last post here.

 

[cmb]

I may be presenting an opening gambit here that will get you thinking I'm not quite right, but I think I can evidence the following sufficiently to at least make a case;

The Sun is green [evidence: its peak wavelength is the same as the colour of green plants]
The ocean is green, it is full of the same chlorophyl pigment-bearing micro-organisms found in all other plants, and that match the peak emission of the Sun. [Take a photo of the sea with a ploarising filter set to vertical, so all the horizontal reflections are taken out of it.]
The sky is white, just the same colour as the clouds. The blue is an optical illusion - the blue receptors in the eye respond to an excess of UV stimulation making you think you're seeing blue. [evidence/thought experiment: take a white sheet of paper out in a rowing boat all painted sky blue. Dress head to toe in sky-blue clothing. What colour do you think the paper now looks? If the sky really was blue, why does white paper look white when shaded from the sun but otherwise under a fully blue sky? Answer - because the paper does not reflect the UV element of the spectrum, thereby avoiding the false stimulation of the eye making it think it is seeing blue.]

[Caveat - this is a straw-man proposition for you to cut down. I am not suggesting there isn't an element of blue in either the sea or the sky, because there is Rayleigh scattering going on, no doubt. But I do not think I have yet seen evidence to suggest it is the major part of us seeing blue.]

You may also ask - "hey, if the sky isn't actually blue, then why does my digital camera take the sky as 'blue'. Is my camera experiencing an optical illusion, too!?". Answer, yes. That's why there is a white balance setting - so that your camera can be made to take a picture like you expect it to look.

 

[Drakkith]

The Sun is green [evidence: its peak wavelength is the same as the colour of green plants]

The sun is white. The intensity of the output in the visual spectrum doesn't shift enough between colors to claim that the Sun is predominately green.

The ocean is green, it is full of the same chlorophyl pigment-bearing micro-organisms found in all other plants, and that match the peak emission of the Sun. [Take a photo of the sea with a ploarising filter set to vertical, so all the horizontal reflections are taken out of it.]

While I will agree that this has an effect on the color of parts of the ocean, as a whole the ocean is not that green.

The sky is white, just the same colour as the clouds. The blue is an optical illusion - the blue receptors in the eye respond to an excess of UV stimulation making you think you're seeing blue. [evidence/thought experiment: take a white sheet of paper out in a rowing boat all painted sky blue. Dress head to toe in sky-blue clothing. What colour do you think the paper now looks? If the sky really was blue, why does white paper look white when shaded from the sun but otherwise under a fully blue sky? Answer - because the paper does not reflect the UV element of the spectrum, thereby avoiding the false stimulation of the eye making it think it is seeing blue.]

I don't believe this is correct. First, IF the sky was perfectly transparent and didn't scatter light it would be black and you would be able to see the stars. Since it is not perfectly transparent it scatters light. The majority of the scattered light falls in the blue range of the spectrum, hence making it appear blue.

Second, in most people the eye blocks out UV light from reaching the retina. On top of that I believe that UV light that does strike the retina stimulates all types of color receptors equally, making UV light look white not blue.

Third, the amount of light scattered in the atmosphere is much less than the amount not scattered. Also, as I said above, if part of the blue light is being scattered out of the sunlight, then the non scattered light being reflected from the paper adds with the scattered blue light that also reflects off the paper and probably equals back out to look almost perfectly white again.

[Caveat - this is a straw-man proposition for you to cut down. I am not suggesting there isn't an element of blue in either the sea or the sky, because there is Rayleigh scattering going on, no doubt. But I do not think I have yet seen evidence to suggest it is the major part of us seeing blue.]

This effect is explained pretty well in the following link. I have never seen evidence to the contrary so I don't have any reason to not believe it.
See here: http://en.wikipedia.org/wiki/Diffuse_sky_radiation

 

[cmb]

This effect is explained pretty well in the following link. I have never seen evidence to the contrary so I don't have any reason to not believe it.
See here: http://en.wikipedia.org/wiki/Diffuse_sky_radiation

I'll overlay a colour spectrum over the 'spectrum of blue sky' given in the page you've just linked to.

More blue than green? I can't say I am entirely convinced?

For sure there is blue light scattering. I don't dispute it. That's why the Sun looks yellow - because the blue light is scattered out of the 'white light' (which peaks around green). I'm just interested to try to figure out how much of the blue we see is 'real' and how much is down to our perception of 'blue'. I'm certain it is not all from scattering, but 'how much of the perceived blueness' is scattering, I do not know.

The eye cannot see green very well. It generally sees 'red' & 'blue' ('L' and 'S' cone cells) and 'bright/dark' (rod cells). The 'M cells' that select for green don't seem to do a very good job and sensitivity is skewed towards responding similarly to the 'L' cells. The eye generally perceives deeper green (~520nm, like the Sun's peak colour) by seeing 'not red + not blue + bright' and the eye concludes that to be green. We can't see the 'Solar Green' colour directly, hence incandescent objects go from red to white to blue as they get hotter and we never perceive a green incandescence, but it seems absurd to say that there is never a 'green' incandescence. It is simply a continuum of spectrum, merely one we cannot perceive with our limits of colour vision. Red and blue are far enough from each other that we find them easy enough to distinguish. But green is a bit too close to either for our relatively poor colour acuity to discriminate.

You say you've never seen evidence otherwise. I'll put forward the paper experiment again - if you stand in the shade of a blue building and stand on a blue carpet, then if you look at a piece of white paper then surely it should look blue? Why does a piece of white paper not look blue under a blue sky, if the colours coming down from the sky are truly 'blue'?

 

[Drakkith]

Per the info when I clicked on that picture from the article:

Spectrum of blue sky somewhat near the horizon pointing east at around 3 or 4 pm on a clear day. Spectrum was taken using an Ocean Optics HR2000 spectrometer [1] with a high-OH solarization-resistant fiber optic light guide. this spectrum is NOT BY ANY MEANS IDEAL and was taken from inside a laboratory through probably 4 panes of window glass, thus completely attenuating virtually all radiation below 400nm also the end of the fiber optic was not coupled to any collimating optics thus there may be some slight skewing of the spectrum due to diffuse reflections off surrounding buildings and trees etc. Because the response of the CCD detector in the spectrometer is not linear the spectrum in the infrared region is also less than what is actually present in sunlight; the blackbody spectrum of sunlight continues much further into the infrared than is shown here. This spectrum is not calibrated for intensity.

Also, to my knowledge, the best response for the eye in sunlight is the Green/Yellow area, not its worst. (EDIT: I see what you mean about the response of the green cone cell being very close to red, but that is not part of the issue here.) Also, the effect of temperature on light emitted is very different than what you are describing. As the temperature increases, more light at a lower wavelength is emitted. For the visual spectrum an object continues to emit lots of red and orange light even as the temp increases and it emits shorter wavelengths. So we would never see green by itself because it is right in the middle and we see white instead. When it gets hot enough that it becomes Blue, the object is emitting much more light in the blue and UV range and the lower red/green portion has fallen off.

You say you've never seen evidence otherwise. I'll put forward the paper experiment again - if you stand in the shade of a blue building and stand on a blue carpet, then if you look at a piece of white paper then surely it should look blue? Why does a piece of white paper not look blue under a blue sky, if the colours coming down from the sky are truly 'blue'?

Are you sure it doesn't look blue? Your example has so many variables that I can't possibly say anything for sure on it. Ambient light isn't just dependent on scattered atmospheric light for one thing.

Edit: I will agree that the response of the cone cells in your eye definitely determine how we differentiate color. But in the end the effect is the same. We "see" blue because that end of the spectrum is scattered more than the reddish end. In regards to the topic, I think we should avoid the issue of how color vision works and stick to wavelengths of light and the effects of the atmosphere on them.

 

[cmb]

We "see" blue because that end of the spectrum is scattered more than the reddish end. In regards to the topic, I think we should avoid the issue of how color vision works and stick to wavelengths of light and the effects of the atmosphere on them.

If you want to address a question related to 'colour' then you have to address how the eye sees it. 'Colour' is purely, and completely a human perception.

To be objective about it, you'd have to say something like;

"The light from the sky is similar to a black-body radiator with a flattened emissions' peak centred around 500-520nm, which is a wavelength perceived by humans as 'blue-green'." I don't see how you can be more objective than that.

The atmosphere does tend to filter (and scatter) the blue content of the solar radiation, as given in the figure below (sorry, I do not recall the reference I got this from. I added the colour spectrum for reference).

You can see there is a stack of near-UV and UV that is filtered and/or selectively scattered, but overall we still appear to end up with more green wavelengths at sea level than any other 'colour'.

Also, to my knowledge, the best response for the eye in sunlight is the Green/Yellow area, not its worst.

I surely didn't say it was the worst sensitivity. It is the worst region for discriminating wavelengths.

There is a very simple reason why the rods (that cannot perceive colour)* are most sensitive to ~500nm wavelength. It's because that is the wavelength most prevalent on the surface of the earth! In dark conditions, this is still true (the prevailing light, if any, is still the Sun's) so the eye has evolved to pick up every available photon, and it is obviously going to evolve tending towards a peak of sensitivity at the wavelength of the commonest photons.

*(we see only black and white in the dark because only the rods are sensitive enough to work there, which is why our brain isn't wired to use the rods to discriminate colour. So around the wavelength of the rods greatest sensitivity we discriminate colour badly. My interpretation of all of this is that the rods contribute significantly to our perception of green; if the rods detect 'bright' and the cones detect low 'red' and 'blue' we see that as green. But in white light there is also lots of red and blue, so the eye simply calls that 'white' as it can't resolve any differences for peaks around green.)

I realize it might be a 'shot-out-of-the-green' to realize 'blue sky' might be a perceptual illusion, but think it over and the idea might warm on you after a while!

(PS The sky looks blue to me, too!)

 

[Stcloud]

The sky is not blue, the grass is not green.. the colour is in us. Please see 'Physics Light'
and from there be directed to 'colour'.

 

[Drakkith]

The sky is not blue, the grass is not green.. the colour is in us. Please see 'Physics Light'
and from there be directed to 'colour'.

Yes and no. The sky scatters and absorbs certain wavelengths, and the way our eyes and brain process color results in us seeing it blue even though the wavelengths fall over a blue and greenish range. A green plant absorbs reddish and blueish light and so looks mostly green to us. This of course is assuming an average person with no injuries or abnormalities with their vision. A color blind person would not be able to discriminate colors as well, and the way things look would change. There is already a thread in process about color vision, so I REALLY don't want to get into the nitty gritty details in this one as well even though the two topics are related.

I realize it might be a 'shot-out-of-the-green' to realize 'blue sky' might be a perceptual illusion, but think it over and the idea might warm on you after a while!

Yes, most of the light that you see from a blue sky is in the blue-green area. It looks mostly blue because of the way our cone cells and brain work to give us color vision. HOWEVER blue light is scattered more than green light still, just as violet light is scattered more than blue. I have never claimed that more light reaches your eye in the blue range than the blue-green range.

You can see there is a stack of near-UV and UV that is filtered and/or selectively scattered, but overall we still appear to end up with more green wavelengths at sea level than any other 'colour'.

Yes, that is because more of the blue and violet light is scattered and absorbed than green and below. Note that the graph is talking about light from the sun hitting the Earth without being scattered or absorbed. Since more blue and violet light is missing, the sun appears yellowish when you look at it instead of white.

My interpretation of all of this is that the rods contribute significantly to our perception of green; if the rods detect 'bright' and the cones detect low 'red' and 'blue' we see that as green. But in white light there is also lots of red and blue, so the eye simply calls that 'white' as it can't resolve any differences for peaks around green.

This is incorrect. A rod cell is desensitized after it has been exposed to a large number of photons. It does not contribute to color vision at all. Rod cells aren't even wired the same way that cones are. They are grouped up with many rod cells converging onto a single nerve so that when multiple cells activate the signal is added together to give a greater response at the cost of resolution. This results in rod cells enabling to see in very low levels of light that we would otherwise be blind in.

White light looks white because it is stimulating all our cone cells at the same time. The combined input is filtered and interpreted by the brain and is perceived as white.

 

[cmb]

The sky is not blue, the grass is not green.. the colour is in us. Please see 'Physics Light'
and from there be directed to 'colour'.

Indeed. Colour is a human 'perception'.

But we can be a little more sophisticated than simply throwing our hands up and not looking further at particular scenarios.

I believe the Horizon programme (in the thread you reference) reports recent work on perception of, particlarly, 'grey' shades. Clearly, as I describe above, if the eye is making guesses about near-green colours because there is a hole in its perceptual ability there, then it may interpret a grey to be anywhere around green (or immediate neighbouring spectral colours), according to context, because that is how the eye works. Can't see green? Then maybe greens look white/grey, and white/greys can look green.

Another effect is where the eye sees a shadow under illumination of a solid colour. It's not evolved to see a full spectrum of such shadows, so again can get confused because it is expecting a blue-green dominated light spectrum, and sees shadow presuming the context is blue-green illumination.

The issue of the blue sky is different to these two. It appears to be over stimulation of the S cells by UV, because they have sensitivity right through to 400nm*, but is related to the first in that if it is 'only green' then the eye 'allows itself' to be easily fooled because it just doesn't 'do green' very well. It'll take other cues preferentially, if there are any, and allow them to over-ride a perception of green.

*(viz. if the eye is still 50% sensitive to 420nm than at 450nm, so if there is a stack of 400-420nm energy bundled into light that is strong in all wavelengths, then it'll tend to perceive that additional 'violet' as shading the white light towards strong blue. Caveat; again, I will repeat that this is a contribution to the effect of 'blue sky' but there is, agreed, a general shift towards blue-green due to scattering. It is the ratio of those two effects on the final perception of blue I do not believe is known or well understood.)

 

[Drakkith]

The issue of the blue sky is different to these two. It appears to be over stimulation of the S cells by UV, because they have sensitivity right through to 400nm*, but is related to the first in that if it is 'only green' then the eye 'allows itself' to be easily fooled because it just doesn't 'do green' very well. It'll take other cues preferentially, if there are any, and allow them to over-ride a perception of green.

*(viz. if the eye is still 50% sensitive to 420nm than at 450nm, so if there is a stack of 400-420nm energy bundled into light that is strong in all wavelengths, then it'll tend to perceive that additional 'violet' as shading the white light towards strong blue. Caveat; again, I will repeat that this is a contribution to the effect of 'blue sky' but there is, agreed, a general shift towards blue due to scattering. It is the ratio of those two effects on the final perception of blue I do not believe is known or well understood.)

Again, UV light is filtered out by other areas of the eye and is not seen by cone cells. People that have had the lens in their eye removed or replaced with a lens that does not block UV light have [STRIKE]reported that it appears white, not blue.[/STRIKE]

EDIT: Correction UV light does not appear to be white per this article: http://www.guardian.co.uk/science/2002/may/30/medicalscience.research However it is filtered out by the lens of the eye normally.

 

[cmb]

This is incorrect. A rod cell is desensitized after it has been exposed to a large number of photons. It does not contribute to color vision at all...White light looks white because it is stimulating all our cone cells at the same time. The combined input is filtered and interpreted by the brain and is perceived as white.

I'll happily take the correction on your advisement that this is so, but whether the green perception is significantly rods or M cones (that are not well-discriminating to the L cones), [edit:] and also however much filtering the eye does actually do on the UV content that hits the retina*. Whatever the nature of these fine details, ultimately it is self-evident (from the perceived spectrum of black-body radiators, and absence of green therein) we see green mainly by inference from the content of the rest of the spectrum, rather than direct perception of the peak of radiated emissions. Thus, we cannot directly perceive the peak of 'blue-green wavelengths' in the spectrum of the sky.

*(clearly, there are levels of UV emission that will make it to the retina, else there would be no need for UV safety glasses!)

 

[Drakkith]

I'll happily take the correction on your advisement that this is so, but whether the green perception is significantly rods or M cones (that are not well-discriminating to the L cones), [edit:] and also however much filtering the eye does actually do on the UV content that hits the retina*. Whatever the nature of these fine details, ultimately it is self-evident (from the perceived spectrum of black-body radiators, and absence of green therein) we see green mainly by inference from the content of the rest of the spectrum, rather than direct perception of the peak of radiated emissions. Thus, we cannot directly perceive the peak of 'blue-green wavelengths' in the spectrum of the sky.

*(clearly, there are levels of UV emission that will make it to the retina, else there would be no need for UV safety glasses!)

Ever heard of snow blindness? It is caused by sunburn to the cornea and lens (and other areas) of the eye by UV radiation.

And yes, the issue of color vision is very complex and isn't 100% understood. We don't see the green-blue peak as green blue because the M cone that corresponds to green is much further away at its peak than the S cone is.

 

[cmb]

Ever heard of snow blindness? It is caused by sunburn to the cornea and lens

Yeah, OK, I realized after I posted I didn't cover that 'purpose'.

But you should take a look at UV retinopathy, used to be a common cause of macular holes amongst welders using electric arcs before they figured out why. And there is also a particular condition that eclipse-observing astronomers tend to suffer, because although the Sun is obscured, the corona isn't.

see;

http://eclipse.gsfc.nasa.gov/SEhelp/safety2.html

The tissues in the eye transmit a substantial part of the radiation between 380 and 1400 nm to the light-sensitive retina at the back of the eye.

 

[cmb]

And yes, the issue of color vision is very complex and isn't 100% understood. We don't see the green-blue peak as green blue because the M cone that corresponds to green is much further away at its peak than the S cone is.

OK, so I think we're agreed that there is scattering of solar UV and blue light but, nonetheless, that the sky's spectrum peaks in the 'blue-green' region of the spectrum, although for complex reasons not entirely understood we perceive that as blue.

Is that the conclusion?

 

[D H]

I'll overlay a colour spectrum over the 'spectrum of blue sky' given in the page you've just linked to.

There are (at least) three things wrong with this:

1. Where in the world did you get this spectrum? Light with a wavelength of 500 nanometers is much closer to blue (435 to 490 nm) than it is to green (520 to 570 nm). Part of your problem is this lousy spectrum. The region between 490 and 520 (which coincides nicely with the peak of this spectrogram) is cyan. That region shows up as green in the spectrum you chose for your overlay.
2. The spectrum of the blue sky itself is not quite right. That spectrogram was made by shooting through four panes of glass. That is going to attenuate the high frequency part of the spectrum, and the author of the spectrogram did note that this is the case.
3. The peak of the spectogram of the blue sky without going through four panes of glass has a saturation of about 210^o. That saturation value is the tertiary color azure, halfway between cyan and blue. Some other names for colors with a saturation near 210^o: Dodger blue, cornflower blue, UN blue, and of course sky blue.

 

[cmb]

There are (at least) three things wrong with this:

1. Where in the world did you get this spectrum?

1. 
   
   The blue sky spectrum from the link the other poster made. The spectrum from wiki, under 'spectrum', for a K4 star. It matches every other spectrum I've seen so I saw no reason to question it. The spectrum of the blue sky is what I expected to see, also.
   
   Can you provide alternatives?

 

[DaveC426913]

Ever heard of snow blindness? It is caused by sunburn to the cornea and lens (and other areas) of the eye by UV radiation.

Reaaaaaally.
I'd always wondered what the actual mechanism was.

I assumed it was an optical overload thing.

 

[Sci-Fi]

Meh -.- it doesn't reflect the color of the sea But the sky appears blue due to REFRACTION of sunlight by water(moisture) molecules in the air.The sun light first goes through the molecules as their is a density difference between the water molecules and the air,it undergoes refraction and then it gets reflected internally(within the molecule) and refracts again and comes out of the water molecule.Its like refraction through prism,that creates a spectrum of 7 lights,but prism is angled to form 7 colored lights and due to preferable wavelength and shape blue light appears the most.
EDIT:Some people might replace the word 'molecule' with 'bubble' but the concept is the same.

 

[D H]

It matches every other spectrum I've seen so I saw no reason to question it.

Seriously? About the only consistent thing I've noticed with portrayals of the spectrum of visible light is an utter lack of consistency, particularly between 450 nm and 600 nm (blue to yellow), and particularly so those available on the internet. Just a smattering:

[PLAIN]http://www.antonine-education.co.uk/physics_gcse/Unit_1/Topic_5/em_spectrum.jpg

[URL]http://www.yorku.ca/eye/spectrum.gif[/URL]

[URL]http://withfriendship.com/images/b/9817/visible-spectrum.gif[/URL]

[URL]http://hosting.soonet.ca/eliris/remotesensing/LectureImages/visiblespectrum.gif[/URL]

[PLAIN]http://i231.photobucket.com/albums/ee127/jcrowmag/visible-a.jpg

[URL]http://3.bp.blogspot.com/_pF0P8-ezLug/S_JpwLquzoI/AAAAAAAAAFc/zOjSIBOwt9M/s1600/Our+perception+of+light.jpg[/URL]

There is no consistency here.

The spectrum of the blue sky is what I expected to see, also.

Compare that spectrum to a published one, one that is not taken through four panes of glass that attenuates the blue, and one that is taken with a calibrated device.

J. J. Michalsky et. al., "Shortwave, Clear-Sky Diffuse Irradiance in the 350 to 1050 nm Range: Comparison of Models with RSS Measurements at the Southern Great Plains ARM Site in September/October 2001". http://www.arm.gov/publications/proceedings/conf13/extended_abs/michalsky-jj.pdf

The graph on page 8 shows the peak is somewhere between 420 and 460 nm: indigo to blue. So why do we see a sky the color of the sky as azure? Simple: Our eyes don't see energy flux. We see photon flux. That shifts the color we see toward the red from the peak in the spectral irradiance plot.

Can you provide alternatives?

I just did with regard to the spectrum of the blue sky. Now for some colors. Note that green has a hue of 120 degrees. The sky is not green. It's blue with a tinge of green. So let's look at some colors, starting with some cyans (490 to 520 nm) and moving toward blue (440 to 490 nm).

MediumAquamarine. Hue = 160 degrees
DarkCyan. Hue = 180 degrees
DeepSkyBlue. Hue = 195 degrees
DodgerBlue. Hue = 210 degrees
Royal Blue. Hue = 225 degrees
Blue. Hue = 240 degrees

 

[cmb]

I really haven't seen such a variation of colours in spectra before, as you have found! But I have no contest that blue is <490nm and green is >520nm, and in between is blue-green. I've only pinned up here the work of others that one might readily find on the 'net.

In regards the paper you have found, this looks more authoritative for sure. But what is 'direct irradiance spectra' and 'diffuse irradiance spectra'? I guess the audience of that paper knows the difference, but 'fraid I don't.

The thing I still don't get, you see, is that if you take a sheet of white paper and go sit at the bottom of a well that only looks up to blue sky, with a spectrum as you point to on p8, does the paper still look white and not blue, if the only light coming down the well shaft to the bottom is light that comes from the sky, peaking around 400~450nm? Either it is blue, or it looks blue. If the former, then why don't white things look blue in that light, when shaded from all other direct reflections from the Sun, if the sky looks blue due to that light?

 

[cmb]

OK, I found a further paper; http://www.patarnott.com/atms749/pdf/blueSkyHumanResponse.pdf .

I'm happy with that and accept I was wrong to describe the sky as 'white'.

...the spectral irradiance of daytime skylight was shown to be a metameric match to a mixture of a monochromatic blue light plus white light (unsaturated blue light).

Still, I'd tend to think my basic point, that the blue observed is a constrution of colour perception rather than an actually observed 'peak wavelength', remains.

 

[D H]

In regards the paper you have found, this looks more authoritative for sure. But what is 'direct irradiance spectra' and 'diffuse irradiance spectra'? I guess the audience of that paper knows the difference, but 'fraid I don't.

Imagine looking through a paper towel roll at the Sun (don't do this at home!). The spectrum of the light you see shortly before you go blind is the direct irradiance spectrum. Specifically, the direct irradiance is that light that comes out from from a cone pointed directly at the Sun and having a radius of about 3 degrees. The diffuse sky irradiance is all the light coming from the sky except for that light coming directly from the Sun. In other words, the diffuse sky irradiance is the light from the clear blue sky (or the light filtering through a bunch of clouds, depending on the condition of the sky).

The thing I still don't get, you see, is that if you take a sheet of white paper and go sit at the bottom of a well that only looks up to blue sky, with a spectrum as you point to on p8, does the paper still look white and not blue, if the only light coming down the well shaft to the bottom is light that comes from the sky, peaking around 400~450nm? Either it is blue, or it looks blue. If the former, then why don't white things look blue in that light, when shaded from all other direct reflections from the Sun, if the sky looks blue due to that light?

First off, I doubt that you have ever conducted this experiment. If you had, you would know that the only color you see is the tiny but brilliant patch of blue you see when you look straight up the well. (This is why you cannot see stars from the bottom of a well during the day.) Everything else is black and white. There just isn't enough light coming down the well for your cones to be active.

Here's an alternate experiment: Walk in and around your house with that white sheet of paper in hand. Look at that paper in a nicely sunlit area outside and it will look white. Look at it indoors in a room with soft white lights and it still looks white. It looks white in a room with bluish industrial florescent lights, and in a bathroom with the heat lamp turned on, and in a dressing room with the makeup lights on.

It looks white because in your mind you know that that piece of paper is white, so your mind automagically compensates the signals coming from your eyes that say that the paper is a bit off-white in some regard.

 

[D H]

Still, I'd tend to think my basic point, that the blue observed is a constrution of colour perception rather than an actually observed 'peak wavelength', remains.

The peak wavelength in a spectrograph is a high frequency blue, maybe even indigo. Our eyes don't see energy flux. They see photon flux. The peak in the photon flux is a lower frequency blue, azure. That is more or less what we see.

 

[shiva999]

thanks

 

[sophiecentaur]

Seriously? About the only consistent thing I've noticed with portrayals of the spectrum of visible light is an utter lack of consistency, particularly between 450 nm and 600 nm (blue to yellow), and particularly so those available on the internet. Just a smattering:Compare that spectrum to a published one, one that is not taken through four panes of glass that attenuates the blue, and one that is taken with a calibrated device.

J. J. Michalsky et. al., "Shortwave, Clear-Sky Diffuse Irradiance in the 350 to 1050 nm Range: Comparison of Models with RSS Measurements at the Southern Great Plains ARM Site in September/October 2001". http://www.arm.gov/publications/proceedings/conf13/extended_abs/michalsky-jj.pdf

The graph on page 8 shows the peak is somewhere between 420 and 460 nm: indigo to blue. So why do we see a sky the color of the sky as azure? Simple: Our eyes don't see energy flux. We see photon flux. That shifts the color we see toward the red from the peak in the spectral irradiance plot.I just did with regard to the spectrum of the blue sky. Now for some colors. Note that green has a hue of 120 degrees. The sky is not green. It's blue with a tinge of green. So let's look at some colors, starting with some cyans (490 to 520 nm) and moving toward blue (440 to 490 nm).

MediumAquamarine. Hue = 160 degrees
DarkCyan. Hue = 180 degrees
DeepSkyBlue. Hue = 195 degrees
DodgerBlue. Hue = 210 degrees
Royal Blue. Hue = 225 degrees
Blue. Hue = 240 degrees

You guys are all doing it again. How can you possibly be demonstrating colours on a TV or printed display and try to kid each other that those are the "colours" of spectral / monochromatic sources? They are all desaturated matches to those spectral lines and made up of a mix of two or three non-spectral and quite broadband primaries.
A monochromatic source of light has a colour, of course, but the only colours you will ever see in real life (excluding those that are produced from a spectrometer or a gas discharge tube in a darkened room) will not be of a single wavelength.
Just do yourselves a favour and look at the RGB values of the colours in a scene that you would describe as 'colourful'. Very few of them are actually very saturated ((255,0,0) or (255,255,0) etc

Also, the 'colour' is more than just the 'angle' on the colour chart - it's the distance from the white point, too. That is needed in order to describe the saturation. Why else do we refer to 'colour space' (implying more than one dimension)?

All those curves with colour bands behind them are just 'hints' as to what the actual monochromatic sources will look like (near matches). Moreover, everyone's appreciation of just how good a match they are will be different.
Although wavelength has (matches to) a colour, you cannot assign a wavelength to a colour.

Three colour printing is even worse for the same sort of reason. when you want one really bright or 'special' colour, you have to produce it with a purpose-made pigment (spot colour).