System, including methods and apparatus, for optical detection. The system may comprise a light source to generate a beam of light, an optical element, and a detector. The optical element may include a light guide having a shaft and a tip, with the tip forming a beveled end of the light guide. The optical element may extend into the beam, such that the tip and at least a portion of the shaft are located inside the beam, and a window of the optical element is located outside the beam. Light of the beam incident on the tip may be transmitted longitudinally through the light guide and out the window, while light of the beam incident on the shaft may be transmitted transversely through the shaft and remains in the beam downstream. The detector may be configured to detect light received from the window.