Abstract: A probing retroreflector for determining the location and dimensions of a target in conjunction with optical measuring systems, such as tracking laser interferometers. The probing retroreflector comprises a support, a retroreflector assembly coupled to the support and having a center point, a planar reflector coupled to the support for reflecting light towards and receiving light from the retroreflector assembly, and a sensor such as a slender probe coupled to an end of the support. The center point of the retroreflector assembly is located at a position which represents the virtual location of the center point of the sensor, so that the location of the point on the surface to be measured, which is sensed by the sensor, can be accurately determined by the optical measuring system.