An apparatus for determining the identity, location, or level of one or more material phases or the location of an interface between two material phases within a defined volume having, a linear array of units configured to generate and detect electromagnetic radiation; an elongate enclosure containing the array of units, being at least partially transparent to the electromagnetic radiation generated by the units; the apparatus being configured to be at least partially submerged within the one or more material phases within the defined volume, the linear array of units being configured to generate transmission signals through the at least partially transparent elongate enclosure to the one or more material phases surrounding the enclosure at locations along the length of the enclosure, and to receive return signals through the elongate enclosure at locations along the length of the enclosure from the one or more material phases surrounding the enclosure.