Abstract: Centering devices are provided for use with Brinell hardness measuring instruments, particularly electro-optical probes, which include one or more illumination sources in a housing having an aperture at one end which is aligned with an optical axis of the probe to provide the optics a view of the specimen surface opposite the opening. In one embodiment, a spider-like centering structure is provided protruding through the probe end opening. The centering structure directs light from source in the probe directly into an indentation in a specimen surface engaging the centering structure and prevents light from crossing the indentation and reducing edge contrast. A mask further prevents spillage of light from each source onto the specimen surface adjoining the indentation and the source so as to heighten contrast differences between the bright surface of the indentation and the dark surrounding specimen surface.