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.