However between the primary source and the surrogate, the potential for corruption of the original data has been introduced, as a human operator has had to make decisions about focus and colour reproduction. Needless to say, because our ability to perceive colour is often flawed, and always limited by our ability to perceive the full light spectrum, every layer of reproduction adds a level at which errors are introduced. In addition, the eye is also incapable of fine focus beyond a certain level, and it is this further level of definition that is so essential in studying manuscripts in the digital medium. A scan from a surrogate is only as good as the surrogate, not the original. When photo labs make a reprint either from a print or from its negative they are rarely, if ever, able to reproduce the colour of the original print precisely, because they rely both on the unavoidably flawed judgement of the human developer's eye and also the use of chemical processes that are too inaccurately calibrated to be exactly reproduced.
Densitometers measure the amount of light reflected or transmitted.
The mechanism by which we perceive colour is still only partially understood, but one extraordinary ability of the brain, chromatic adaptation, which enables us to function normally in changing light conditions becomes a handicap when dealing with precise colour balancing. As an example, if you put on a pair of dark glasses, the lens will completely change the real colours reaching your eye. However, a split-second adjustment in the brain allows us to continue to recognise correctly and understand colours even through a very heavily tinted filter. However this does introduce the potential for inefficient use of the digital medium, since a photographer might consider that their ability to focus through a lens is sufficient for focusing under digital capture conditions. The accuracy of focus available to the eye as opposed to the Digital 'eye' is dramatically different, emphasized by the facility for enlargement of digital images.