An apparatus for employing ambient light collected from an external light source to detect a user's fingers that are gripping at least a relatively transparent portion of a case. The ends of a plurality of waveguides are coupled at relatively unequal or equidistant positions to the interior surface of the transparent portion, where they are arranged to collect light from the exterior light source if the user's finger(s) are not gripping the mobile device at that position. If the light collected by the wave guides is blocked by one or more of the user's fingers, a profile can be determined for the placement, orientation (left handed or right handed), and size of the user's fingers and hand gripping the mobile device. Also, interactions of the fingers with the mobile device can be detected, such as lifting away, pressing, or sliding one or more fingers at the transparent portion for a short or relatively lengthy period of time. Additionally, the collected light can be provided by one or more external light sources, such as ambient light from remotely located sources, one or more illuminators within a mobile device such as a back light for a display, control or other element.