The present invention provides a calibration fixture that enables more accurate calibration of a touch probe with respect to a camera, for example on a CMM. The camera is mounted such that its optical axis is approximately or substantially parallel to the z-axis of the probe. The probe and workpiece move relatively along a plane defined by an orthogonal x and y axis, and optionally a rotation R about the z-axis and/or and the z-axis. The calibration fixture is arranged to image from below the contact surface of the probe and, through a 180-degree prism structure, transmit light from the probe contact point to the camera along the optical axis. Alternatively, two cameras see the fiducial position relative to the CMM arm and the probe position, respectively, when aligned on the fiducial. The fixture may divide an integrated assembly having an optical block and a camera assembly.