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

[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[/color]
DarkCyan. Hue = 180 degrees[/color]
DeepSkyBlue. Hue = 195 degrees[/color]
DodgerBlue. Hue = 210 degrees[/color]
Royal Blue. Hue = 225 degrees[/color]
Blue. Hue = 240 degrees[/color]

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).