An inspection apparatus includes 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 the depth profile and the surface mean.