A head-mounted light source particularly suited to medical and dental applications includes a base unit with a source of illumination directed through a distal opening in the base unit, and a plurality of interchangeable beam-forming units, each separately attachable to the distal opening of the base unit to produce a different light spot size at a given distance. The preferred embodiment includes a wide-field beam-forming unit, an intermediate beam-forming unit and a high-intensity beam-forming unit, such that the diameter of the spot size progressively decreases from the wide-field to the intermediate to the high-intensity unit. The interchangeable beam-forming units are attached to the base unit through a threaded connection. Each interchangeable beam-forming unit may itself include a distal threaded connection for attachment of a UV or other filter. In the preferred embodiment, the base unit includes a clip adapted for attachment to an ocular mounting assembly.