Visually comparing a digital capture on a monitor to the original object can be useful, provided due consideration is given to the environment in which this comparison is performed. If normal ambient lighting, consumer monitors, or an uncalibrated workflow are included, then visual comparison will be meaningless.
Visual assessment is especially useful as a guard against user-error or equipment failure. For instance, if a test target is dusty, numerical analysis may indicate there is excessive noise in the image; the software has no way of knowing that the target itself is actually dusty. Likewise, any manner of software or user error can result in numerical evaluations that are grossly incorrect. When numerical analysis shows a significant and sudden change from past results, the first step in troubleshooting is usually to make a visual assessment to determine whether the digital object or the evaluation of the digital object is in error.
In some cases, objective accuracy comes at the cost of a rendering that would be generally considered pleasant and representative of the intention of the original artist or the original presentation of the object. This is especially true in the case of transmissive and three-dimensional objects. A skilled technician can bring their visual vocabulary, experience, and understanding of the context and intent of the physical object to bear on the way it is rendered. For example, the imaging technician might need to render the object being imaged relative to the weight of the image content. If one is to image white paper that contains a light graphite drawing the exposure, tone needs to be darker than what the target indicates to make the content readable for the viewer. It is the experienced technician’s job, in consultation with curators and other stakeholders, to ensure that the rendering selected is optimal for the intended purpose.
A carefully calibrated print workflow can be used to compare a digital capture to the original physical object. This method has obvious limitations, namely the increased number of variables to control as the printing workflow itself needs to be done with high quality and meticulously calibrated hardware. This makes it less than ideal for truly critical color evaluation.
There are also significant benefits that cannot be easily achieved using a monitor-only comparison workflow. First and foremost, the print can be transported easily, allowing it to be brought to the object for comparison, if, for instance, the object has been returned to storage or is now located with the restoration team. Less obviously, a print is, like the original physical object, also a reflective object and therefore the disparity in perception between transmissive and reflective objects is not present.
It should never be assumed that because both a print and an object are physical objects that they will match in all lighting conditions. Color is a fickle perception. Changing the light source to one with a different spectral output means different objects that were the same color will appear as different colors, an effect called ‘Metameric Failure’ which limits the utility of print-to-object visual comparisons.