Apparatus, methods and instructions for disinfecting air. The apparatus may include, and the methods may involve, a fixture. The fixture may include a germicidal light source. The fixture may include a fan. The fan may circulate air through a volume into which the germicidal light source propagates germicidal light. The light source may be configured to emit, upward from a horizontal plane, a beam that, absent reflection off an environmental object, does not cross the horizontal plane. The apparatus may include a shield that prevents light from the light source from crossing the horizontal plane. The sensor may face upward from the horizontal plane. The sensor may face downward from the horizontal plane.