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Even if an rgb camera is working in such an environment, it still can not obtain the spectral information needed to calculate accurate colour values.
If the rgb camera, scanner etc. is viewing materials that have known spectral reflection properties, then one can have a better chance of obtaining better colour results. The reason for this is if one knows the shape of the spectral curve coming from a material via transmission or reflection, then one does not have to measure the whole curve accurately but only a small part of the curve. That would be enough information to determine the whole curve. This is helpful when working with known materials such as photographic film and process inks. But the real world is filled with materials with unknown reflective properties and therefore an accurate measurement of the whole spectral curve is required.
Colour is a tricky subject because basically most people don't understand what it is. Colour does not exist in Nature. It does not exist in light. Colour is a perception in our minds from light stimulus to the eyes and that information being processed in the brain.
Where do colour values come from? They are not a direct measurement but are a calculation from measured information. Back in the 1930's researchers developed mathematical functions based on how a group of people matched colours. The Lab values that we used today are a direct mathematical construction of that original work.
So to get colour values, one has to calculate the X, Y and Z tristimulus values. This is done basically by using the three x,y and z colour matching functions determined from that work in the 30's and applying those functions to the measured spectral response coming from the object. This is done all across the visible spectrum. When you obtain the X,Y and Z tristimulus values, then these are used to directly calculate Lab values.
All of this is a strict mathematical operations and in general can not be shortcut by using RGB response curves.
If one could make a cameral that had filters that duplicated the x, y and z colour matching functions, then one would be able to get colour values out of a three sensor device. But having filters that can do this is not so easy to make and therefore, devices like spectrophotometers are needed that can divide the whole spectrum into many sections and measure them independently and apply the x, y and z colour matching functions.
With this method that spectrophotometers use, not only can one calculate the colour values but one can apply other functions to measure density etc.
The accuracy of the spectrophotometer will be related to how many sections can be measured along the visible spectrum. The more sections along the range that can be measured, the more accurate the calculation of XYZ will be. Graphic arts spectros might have 15 to 30 sections or zones while very expensive and more accurate spectros might have sections at each nm wavelength along the range which would be hundreds of zones. Basically graphic arts spectros will not be highly accurate.
Colour science has provided colour values that are not perfect indicators of how people see over the entire visual gamut but it is amazing at how well they have been for practical purposes.
So when we talk about colour values such as Lab etc. one needs to be aware of how these values were generated. There is a strict mathematical definition and shortcuts are only valid under conditions that do not violate the math.
