An optical system is for imaging a surface covered with a viscous substance using an illuminating beam having a spectral profile including an imaging waveband to which the viscous substance is transparent. The system includes a support frame, a flattening plate, a light source and a camera. The support frame has a front and a rear end, and has an abutment structure. The flattening plate is transparent to the imaging waveband, and is engageable with the viscous substance to flatten the substance into a substantially flat layer. The light source is operatively coupled to the support frame, and configured to project the illuminating beam towards the surface through the flattening plate. The camera is mounted onto the support frame, is sensitive to light in the imaging waveband, and has a field of view encompassing at least a portion of the surface illuminated by the illuminating beam.
