A fiber interface configuration for a chromatic point sensor optical pen is provided wherein a detector aperture element provides an aperture that is smaller than the light-transmitting core diameter of an optical fiber that is connected to the optical pen. The detector aperture element is fixed relative to the chromatically dispersive optics of the optical pen, the optical fiber abuts the aperture element, and the optical fiber core is aligned to the aperture. The aperture element and the end of the fiber may be inclined relative to the axis of the fiber, to deflect spurious reflections away from the optical signal path. The configuration provides high measuring resolution without using a tapered optical fiber, and provides interchangeability of the optical fiber connected to the optical pen.