A spectrometer such as the X-Rite i1 can be used to make a spectral measurement of a specific point on a object. This can be useful where the object may contain material which suffers strong metameric failure, or for forensic or scientific analyses. However, it has four significant limitations in mass digitization programs: speed, location specificity, and illumination angle, and the requirement for contact.
Speed: Measurements using a spectrometer are tedious time consuming. Each measurement only takes a moment, but can only be taken after the location has been carefully selected and the spectrometer precisely positioned.
Location Specificity: Each measurement is inherently linked to a specific location on the object, so the exact location on the object must be carefully recorded. Moreover, it’s most useful to select an area of homogenous tone since it can be difficult to correlate the location in the PDO with sufficient precision to isolate a particular brush stroke or detail. This eliminates (or at least greatly challenges) use in areas of high frequency detail.
Illumination Angle: Most spectrometers provide illumination which is fixed at 45°. Those with “flexible” angles of illumination are locked to a selection of three or four hard-wired angles (e.g. 30/45/60°). There are systems called goniospectrometers, and allow for a full spectral reading at any arbitrary angle of illumination, but they are incredibly slow to use; they are useful for research, but are not suitable for use in a mass digitization program. In practice, nearly all such measurements made in Cultural Heritage environments are made with a spectrometer that illuminates the object at 45°. This is in stark contrast to the actuality of digitizing real world objects, which often require non 45° illumination to avoid issues like glare and reflections.
Requirement for Contact: Most commercial spectrophotometers require direct contact, or extremely close proximity, with the surface being measured. This limits their applicability for objects where handling protocols may forbid direct contact (or high risk of direct contact). Spectrophotometers capable of taking measurements at a distance are available, but are manufactured for the scientific-research market and carry commensurate price, size, and technical acumen.