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I understand that an RGB LED produces light in three constrained wavelength ranges and that any color beyond R, G, or B (orange, say) is due to the interaction of the cells in our eyes with the various wavelengths of light from the LED.

I am interested in building a spectrophotometer. It does not have to be highly precise, but I do want to be able to trust my data. If I shine emulated orange light through a sample and onto a detector, will the effect be in any way similar to shining actual orange light, as my eye perceives, or will the sample essentially be responding only to the R, G, B light?

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In reality, the R, G, and B that your LED’s are emitting are not single, narrow lines, but are instead rather broadband. See this question. To the extent that your LEDs’ spectra overlap, you can indeed make a spectrophotometer using it as a light source since you’ll have a continuous overall spectrum. But this would not be recommended because you’ll have wildly differing power flux at different nearby wavelengths, and this will complicate your analysis. However, if you are careful about taking reference measurements, and you don’t mind a couple of regions of low signal-to-noise ratio in the middle of your spectrum, then this can be done.

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If I shine emulated orange light through a sample and onto a detector, will the effect be in any way similar to shining actual orange light...

There is no "actual" orange light. There are only an infinite number of power spectra that appear orange to our eyes.

If you want "orange" in a spectrophotometer, then you almost certainly want a single, very narrow band of wavelengths that appears orange to our eyes. You won't get that by combining narrow bands of "red" and "green," even though that combination also appears as "orange" to us.


I'm not an expert, but I can't imagine how you would even begin to design a high-accuracy spectrophotometer that used anything other than a black body (i.e., incandescent) light source.

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You are able to get approximations of primary wavelengths based on RGB scale data but there are issues with specificity and accuracy. Multiple combinations of wavelengths will give you an RGB color you cant really distinguish. That being said there are a few apps on iPhone that use the screen pixels to shine light of approximated wavelengths and then measures the reflectance response with the camera- an optical spectrometer. Not 100% ideal but extremely clever and perfect in some circumstances.

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