# Wavelength, Refractive Index and Absorption Coefficient as a function of colour

Hello everybody,

I would love some help on the following; I have a spectral date (As image 1 below) formed by the wavelength, refraction index and the absorption coefficient. Is there any way to relate this spectral date to a dominant wavelength? or in a few words extract the color from this spectrum?

This information is also found in the spectrum file, I believe it is also usable in some way to calculate the dominant wavelength:

The real dominant color of this object is part of the red spectrum.

Philip Koeck
If you look through the substance you should see mainly wavelengths above about 600 nm, so it should look red. The color is not a single wavelength, but a whole range, in this case longer than about 600 nm.

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Color is not a physical but a physiological entity. In this sense it's much more complicated than the physical description of light as an electromagnetic field.

Color is not a physical but a physiological entity. In this sense it's much more complicated than the physical description of light as an electromagnetic field.
So you're telling me color is not a physical property? These data were obtained by reading the spectrometer, that is, it is a measurable characteristic. Sorry if I sounded ignorant, it's not really my area of expertise

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What you measure are wavelengths, i.e., physical properties of electromagnetic waves but not color, which is a physiological quantity:

https://en.wikipedia.org/wiki/Color

What you measure are wavelengths, i.e., physical properties of electromagnetic waves but not color, which is a physiological quantity:

https://en.wikipedia.org/wiki/Color
Oh ok, I get it, it really makes perfect sense. But do you know if there is any way to determine the dominant wavelength from this spectrum I showed above?

Staff Emeritus
But do you know if there is any way to determine the dominant wavelength from this spectrum
You can see yourself on the plot where the curve peaks. But may I suggest that "dominant wavelength" may not be a useful concept in cases like this. The entire spectrum contributes to the effects, not just one frequency.

vanhees71
You can see yourself on the plot where the curve peaks. But may I suggest that "dominant wavelength" may not be a useful concept in cases like this. The entire spectrum contributes to the effects, not just one frequency.

Got it, so the peak wavelength represents the dominant, this is a very useful information.
Just to clarify, what I'm trying to do is a rendering simulation, and I would like to apply the "true" color (Or at least as close as possible.) of that specific material, in which one of the inputs to insert the color in the software is the wavelength. However, using this peak value found in the spectrum above, the software calculates the color displaying an orange tone, different from the real one. I will try to think more about the matter in trying to relate the values obtained.

Philip Koeck
Got it, so the peak wavelength represents the dominant, this is a very useful information.
Just to clarify, what I'm trying to do is a rendering simulation, and I would like to apply the "true" color (Or at least as close as possible.) of that specific material, in which one of the inputs to insert the color in the software is the wavelength. However, using this peak value found in the spectrum above, the software calculates the color displaying an orange tone, different from the real one. I will try to think more about the matter in trying to relate the values obtained.

The curve you show is an absorption spectrum. This means that the peak is the wavelength that is absorbed most! If you look through this substance at a source of white light you will see the light that's not absorbed, which is mainly 570 nm and longer, so the substance should look red.

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The curve you show is an absorption spectrum. This means that the peak is the wavelength that is absorbed most! If you look through this substance at a source of white light you will see the light that's not absorbed, which is mainly 570 nm and longer, so the substance should look red.
This is absolutely correct. Also you have not mentioned the illuminant which will further influence the perceived color. The graph mentions illuminant A which is a ~3000K tungsten lamp. This will also affect the perceived color.
So to quantify the perceived color you need to know the surface "finish", the exact illuminant, the absorbtivity of the surface as a function of wavelength and angle, and the illumination geometry. In addition you need to have the spectral response curves for the three chromophores in your eyeball.
Even then it is tricky. For instance you may need to be aware of metameric colors:
https://en.wikipedia.org/wiki/Metamerism_(color)
Frankly "dominant wavelength" in this context is not well defined.
It is in fact more complicated and requires greater specificity.

Philip Koeck
Staff Emeritus