Abstract: A fibre optic sensor, particularly for ascertaining fluid levels, utilizes two cylindrical lightguides with regular perturbations to emit and/or accept light in a radial direction. Each waveguide is coupled to one (or more) light source and/or to one (or more) light detector. A light source such as a light emitting diode or laser diode is coupled to the lightguide which emits radially, illuminating a planar dielectric surface, which in the presence of a fluid with a low index of refraction, e.g. air, behaves as a mirror, coupling light via total internal reflection to the lightguide which accepts this light radially and channels the light to a light detector. In the presence of a fluid with a high index of refraction, for example water, at the planar dielectric surface the fluid dielectric interface becomes primarily transmissive, with relatively little light being coupled to the lightguide which accepts light radially.