Angle of rotation ε. Angle through which the sample is turned about its vertical axis from any arbitrarily established position counterclockwise (+ε) or clockwise (-ε) viewed in the direction of illumination. If retro-reflective materials or devices have a marking (e.g. TOP), this marking governs the starting position. The angle of rotation ε lies in the range -180° < ε ≤ 180°.
