A portable apparatus having a source, a detector and a filtering element placed a known distance from the source and/or the detector. One embodiment of the apparatus is a headlamp having an illumination source, a camera, and two eye pieces. The headlamp has an adjustable headband for positioning the headlamp on the head of the user and a protective cover for interacting with the camera, eye pieces, and/or illumination source. The protective cover preferably includes multiple sets of filtering elements such that motion about or along an axis changes the filtering elements interacting with the camera, eye pieces, and/or the illumination source.