Abstract: Method and apparatus for determining the amount, if any, of residue remaining at the bottom of an aperture in a layer of dielectric or insulator material. A layer of electrically conducting material is positioned adjacent to the aperture bottom, an electron collector is positioned adjacent to the mouth of the aperture, and a voltage difference (optional) is impressed between the conducting material and the electron collector. The aperture bottom is illuminated with a light beam with photon energy greater than the electron work function of the conducting material, and a portion of the photons that comprise the light beam reach the conducting material and produce photoelectrons by photoemissive action. A photoelectron current is sensed by the electron collector, and the cleanliness of the aperture bottom is determined from the value of the current.