You measure the 'darkness' of the outside of the beans, then grind them up and measure the darkness of the beans all ground and evenly disitributed. This may be similar (showing an even roast), or substantially lighter (demonstrating a fast roast where the inside is less developed than the outside – it then gives an average colour that is somewhere in between). This then gives you a measurement the spread of the roast.
While this method has some value, it still needs to be used in conjunction with taste testing - these numbers will never be able to guarantee good flavour, but may help with repeatability and getting new beans into the ballpark of consistent roasting.
But certainly, the fly in the ointment will be repeatable lighting conditions for home use

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