1. An inspection apparatus comprising: an imaging unit producing image signals; a processing unit for receiving the image signal; the imaging unit producing a stack of images of an article at different focal lengths in response to the processing unit; the processing unit generating a depth map from the stack of images; the processing unit analyzing the depth map to derive a depth profile of an object of interest; the processing unit determining a surface mean for the article from the stack of images; and the processing unit characterizing the article as degraded or contaminated in response to comparison of the depth profile and the surface mean, wherein: if the depth profile is below the surface mean, the article is characterized as degraded, and if the depth profile is above the surface mean, the article is characterized as contaminated depending on secondary factors.