The patented FretFX  array is mounted along the top-edge of an instrument fretboard. It's durable semi-rigid body is water clear as to offer no visual distractions. As seen in the photo below, the array is so low in height and smoothly radiused that even placing your fingers directly on it will in no way hamper any fretting of the instrument. Normal hand positionings or over the neck frettings are completely un-affected.
FretFX are virtually non-existant physically when playing your instrument while at the same time emitting bright stage lighting to give you great ACTIVE fret markers.