Is the color of OIII perfect cyan?

[Ann]

In another thread at Starship Asterisk, Chris Peterson has defined cyan as a color that contains no red, but is made up of equal amounts of green and blue. Or, as Chris put it, as far as I can remember: cyan is what you get if you remove red from white.

In the Helix Nebula APOD thread from October 4, 2012, Art wrote:

<<In physics, a forbidden mechanism or forbidden line is a spectral line emitted by atoms undergoing nominally "forbidden" energy transitions not normally allowed by the selection rules of quantum mechanics. Forbidden lines of nitrogen ([N II] at 654.8 and 658.4 nm), sulfur ( at 671.6 and 673.1 nm), and oxygen ([O II] at 372.7 nm, and [O III] at 495.9 and 500.7 nm) are commonly observed in astrophysical plasmas. These lines are extremely important to the energy balance of such things as planetary nebulae and H II regions. Also, the forbidden 21-cm hydrogen line is of the utmost importance for radio astronomy as it allows very cold neutral hydrogen gas to be seen.>>

The color that Art chose to illustrate OIII at 495.9 and 500.7 nm is #008080, which I interpret as no red, but equal amounts of green and blue.

I'd like a comment here. Should OIII emission be regarded as the perfect shade of cyan?

Ann

[Chris Peterson]

I'd like a comment here. Should OIII emission be regarded as the perfect shade of cyan?

There is no consistent way to map wavelength (a physical parameter) to color (a physiological and perceptual phenomenon).

The OIII emission is not a perfect shade of cyan because there is no consistent physical definition of cyan. When we say that cyan is the absence of red, it simply means that in a tricolor light source consisting of components that we see (broadly) as red, green, and blue, taking away the red component leaves a color we choose to call cyan. Since different display devices will have different curves for their red, green, and blue components, they will also display "pure" cyan differently- just as they will display "pure" red, green, and blue differently.

The only way two light sources can be considered to have the same color is if they have the exact same spectral output over the entire visible range- something that almost never happens in practice.

BTW, in the eye, an OIII emission stimulates all three receptors- so it doesn't represent a complete absence of red at the detection level.

[rstevenson]

Just to see what they said, I went to the Pantone site -- Pantone being the bee's knee of colour -- and looked up Cyan. Then I used my DigitalColor Meter to see what that particular shade of Cyan actually is on my screen. It comes up as 0.0, 65.1, 79.6 (in RGB %) or #00A6CB (in RGB hex). It's all in the eye of the beholder.

Rob

[Ann]

Thanks to both of you for your helpful replies. They are much appreciated.

Let me try to shift the question to the color of OIII emission. Should wavelengths of 495.9 nm and 500.7 nm be regarded as "midway between blue and green"?

Ann

[Chris Peterson]

Let me try to shift the question to the color of OIII emission. Should wavelengths of 495.9 nm and 500.7 nm be regarded as "midway between blue and green"?

Keeping in mind that people have their own ideas about what names go with what perceived colors, it's generally true that a pure 500nm source is usually described as "aqua" or "cyan", colors which most people consider lying between blue and green. Typically, a pure 500nm source will appear similar (to most people) to the color produced by an RGB display outputting only green and blue light.

This is confused mightily by the fact that color is determined by spectral ratios and intensity. A 500nm source will appear to change color radically as its intensity changes.