This images below depict various defects in substrates and coatings that can be visually observed.

Chip on the edge of a doublet lens:

Peeling at the center of a coating, and a breakage:

Crack on the edge of a singlet lens:

A large breakage:

Cracks on the edge of a singlet lens:

Cracks on an outer optical element:

Crack in a lens caused during the grinding process:

Scratches on a coating caused by poor handling:

Contaminations in raw material:

Peeling coating:

Digestion beginning of the reflective coating on the edge of a mirror:

Digestion beginning of a reflective coating at the edge of a mirror: