In bright field, front light illumination, light reflected by a flat surface is collected by the optics. This is the most common situation, in which non-flat features (e.g. defects, scratches etc.) can scatter light outside the maximum acceptance angle of the lens, showing dark characteristics on a bright background (the bright field - see pictures.
Bright field, front light can be produced by LED barlights or ringlights, depending on the system symmetry. In both cases LED light can be direct or diffused by a medium (sometimes the latter is to prefer to avoid uneven illumination on reflective surfaces).