A new instrument has been developed to map the photovoltaic response of a semiconductor to very penetrating sub‐band‐gap radiation (for silicon, &lgr;=1.3 &mgr;m). The minority carrier density being nearly uniform throughout the thickness of the crystal, the photovoltaic response is dominated by the carrier diffusion length and thus reflects the quality of the material. A computer‐controlled scan of the semiconductor presents a map of the gross quality of the specimen. The actual probe is a small transparent electrode (water) illuminated by an optical fiber carrying 1.3‐&mgr;m light from an LED.