Opto-electronic modules, which can be fabricated in a wafer-scale process, include light emitting and/or light sensing devices mounted on or in a substrate. The modules, which can include various features to help reduce the occurrence of optical cross-talk and help prevent interference from stray light, can be used in a wide range of applications, including medical and health-related applications. For example, performing a measurement on a human body can include bringing a portion of the human body into direct contact with an exterior surface of the opto-electronic module and using a differential optical absorption spectroscopy technique to obtain an indication of a physical condition of the human body.