Why is it not possible to get an accurate ΔE colour measurement on a metallic powder coating?
It is possible, but the result is limited to the geometry of illumination and view present in the spectrophotometer. Metallic materials exhibit geometric dependencies in their color that are not easy to capture and represent with spectrophotometric measurements (and therefore ΔE*). Sometimes such materials are measured at several combinations of illumination and view angle to try to characterize these properties. The other potential limitation is that the metallic highlights have reflectance factors that significantly exceed those of normal reflecting materials and this results in CIELAB coordinates outside the normally well-understood range (e.g. L* can be much greater than 100). While such values can be computed and used in a ΔE* calculation, the meaning of such values remains very uncertain.
