This disclosure provides apparatus and methods for scribing a substrate. In one aspect, an apparatus includes optics for focusing a scribe beam onto a substrate and a beam focus adjustment mechanism for adjusting the optics. A triangulation-based distance sensor determines a distance between the triangulation-based distance sensor and the substrate, with the triangulation-based distance sensor being positioned at a location offset from the scribe beam. Reflecting elements are positioned to reflect an incident beam from the triangulation-based distance sensor's source to the substrate and then back to the triangulation-based distance sensor's detector. The beam focus adjustment mechanism adjusts the optics based on the distance between the triangulation-based distance sensor and the substrate so that the scribe beam is focused at a desired position on the substrate.