An interferometer sensor has a two-dimensional sensor head (1) comprising a polymer film (4) of substantially uniform thickness. An interrogating signal (12) is provided to the sensor head, the interrogation signal extending across the area of the sensor head and being incident normally to the sensor head (1). An optical sensing device (16) is arranged to receive an optical output signal from the sensor head at a location remote from the sensor head. The sensor converts ultrasound signals appearing over a two dimensional surface to an optical signal pattern, using a polymer interferometer sensing film. Spatial discretisation of the ultrasound signal pattern is performed by an optical sensing device. Such optical devices can be arranged having sufficiently high resolution to enable beam-steering imaging to be performed, including imaging outside the footprint of the sensor head.