I’m asking this question from the perspective of a hobby computer scientist as well as someone who has a base level understanding of optics/light from middle school physics.
As I understand it, humans have 3 distinct cones for detecting lights in the broad red, green, & blue wavelength ranges. The red cone is also able to detect wavelengths shorter than peak blue light (wavelengths corresponding to purple) so that a mix of triggering red & triggering blue cones by purple light lets us see purple. Here’s an image corresponding cone detection spectrums as I understand them: [https://www.unm.edu/\~toolson/human\_cone\_action\_spectra.gif](https://www.unm.edu/~toolson/human_cone_action_spectra.gif)
When displaying images containing purple on my digital computer, my screen isn’t displaying purple wavelength light, but rather a mix of red light in the typical red wavelengths & blue light in the blue wavelengths to trigger both cones & trick me into seeing purple. However, I can still take pictures of purple things that originally trigger my red cones with wavelengths on the complete opposite end of the visible light spectrum & display them on my screen to trigger my red cones with red light instead. How does my digital camera translate purple light to the correct mix of red & blue to display in RGB image formats when these formats have to trick my red cones to seeing purple by displaying light on the opposite end of the visible light spectrum?
In: Physics
Basically, it’s because the camera is doing the same thing.
The CCD sensor in the camera is monochrome by its nature, so there are RGB filters embedded in the device (roughly analogous to the cones in your eye) to capture color information. The amount of light in RGB falling on a given pixel is encoded in an image file. Every pixel records the amount of red, green, and blue light that was falling on that pixel. When your computer displays the image, it lights up the pixels in your monitor, mixing the light to make a color according to those levels.
You already know that if you took a spectrum of the computer screen, it would be different than the real object. This is because the RGB channels only give an approximation of the spectrum, but there is enough color information there to make it very difficult for our brain to tell the difference. It’s good enough.
EDIT: One other point of clarification: purple is not a spectral color, it’s a sensation of the brain. If the camera captures the correct mix of red/blue light to give the sensation of purple, and that information is conveyed to the monitor, your brain will perceive purple when you look at the screen because it’s a good enough approximation.
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