A device for measuring reflected ultrasound and optical signals may include: an optical source; an optical assembly comprising at least one lens, configured to focus reflected optical illumination from a target onto a detector; and an ultrasound transducer aligned to transmit and receive ultrasound radiation co-axially with the reflected optical illumination and wherein the ultrasound transducer at least partially obstructs a path of the reflected optical illumination. An obstruction may be distant from a focal spot of the optical assembly. The device for measuring reflected ultrasound and optical signals may be particularly useful for characterizing fluid behind an ear drum to diagnose otitis media.