Spectrum graph from an image

I'd like to plot a graph (wavelength - intensity) from an image of color like this, how I can do?

Every color should correspond to a specific wavelength of the visible spectrum and to and intensity (from black to pure color). To make the problem easier I can use also only one row of pixels.

Can you help we with this problem? Thanks

Answer 1

[Ameer Hamza]

You will need spectrumRGB() function available in this package: https://www.mathworks.com/matlabcentral/fileexchange/7021-spectral-and-xyz-color-functions?focused=5172034&tab=function to convert wavelength to RGB values. Download it and place it in MATLAB path, then run the following code

rgbValues = spectrumRGB(400:0.2:650); % go from 400 to 650 nm wavelength
image = repmat(rgbValues, 200, 1, 1); % 200 is the height of image in pixel
imshow(image)

[Francesco Panico]

Thanks Ameer Hamza it's a very usefull tool. How I can do the opposit process ? From the image to the wavelength ?

[Ameer Hamza]

The answer to this question is not quite simple. The inverse mapping is not one to one. Several colours in nature are combination ( interference) of several wavelengths. For most rgb values, the result will not be a single wavelength, but a range of wavelength. Refer to this link for more details: https://www.mathworks.com/matlabcentral/answers/240430-rgb-to-wavelength-is-it-possible#comment_308370

Answer 2

[Image Analyst]

There are lots of ways to make a color, not just one. Have you ever heard of metamerism? Why don't we make an assumption that your spectrum is just monochromatic spikes (like lasers). Then you can simply say that the left color is 400 nm and the right color is 700 nm, and simply make up a lookup table giving the wavelength for some input RGB value.