Apparatus and associated methods relate to a photoelectric sensor system with a primary photo-detector to receive an on-axis return reflected by a retro-reflective target, and a monitor photo-detector to detect off-axis light indicative of a diffuse reflective object blocking the optical path between an emitter and the retro-reflective target. In an illustrative example, the optical path may include a coaxial transceiver module that includes a beam splitter mirror to redirect the optical path. In some embodiments, a bifurcated lens may provide, for example, a lateral offset of the emitter axis through one lens, and the return axis through an adjacent parallel lens. The second photo-detector may provide, for example, an array of one or more detection surfaces to detect off-axis light associated with a diffuse reflective object, such as a white card, for example.
