A sensor may be used to measure a degree of tilt of a sample. The sensor may include an apparatus having a light source, first, second, and third optical elements, a lens, and an aperture. The first optical element may supply light from the light source toward the sample, and may supply light input into the first optical element from the sample toward the second optical element. The second optical element may supply light toward first and second sensing elements. An aperture may be arranged on a focal plane of the lens. A light beam incident on the first sensing element may be a reference beam.