An apparatus, for interferometric microscopy of a sample comprising activatable optical labels characterised in that the apparatus comprises: an activation source arranged, in use, to selectively activate fluorescent response of an optical label in the sample; an optical source configured to excite the activated optical label in the sample such that fluorescence is emitted from the activated optical label in a plurality of directions; an optical system having a beam splitter device and a plurality of detectors, the optical system configured to produce a plurality of images of the optical label in a first dimension and a second dimension substantially orthogonal to the first dimension based on an interference of emitted fluorescence of the optical label in a first direction and emitted fluorescence of the optical label in a second direction, the second direction being different from the first direction, the beam splitter device configured to interfere the fluorescence in the first direction and the fluorescence in the second direction to produce at least three output beams (1030, 1032, 1034), the plurality of detectors configured to detect optical energy from the at least three output beams of the beam splitter device, each detector from the plurality of detectors being uniquely associated with an image from the plurality of images at a time; and a processor configured to calculate a position of the optical label in a third dimension based on the plurality of images, the third dimension being substantially orthogonal to the first dimension and the second dimension; the interference between the emitted fluorescence in the first direction and the emitted fluorescence in the second direction being caused at least in part by a difference in path length between the emitted fluorescencein the first direction and the emitted fluorescence in the second direction. an activation source arranged, in use, to selectively activate fluorescent response of an optical label in the sample; an optical source configured to excite the activated optical label in the sample such that fluorescence is emitted from the activated optical label in a plurality of directions; an optical system having a beam splitter device and a plurality of detectors, the optical system configured to produce a plurality of images of the optical label in a first dimension and a second dimension substantially orthogonal to the first dimension based on an interference of emitted fluorescence of the optical label in a first direction and emitted fluorescence of the optical label in a second direction, the second direction being different from the first direction, the beam splitter device configured to interfere the fluorescence in the first direction and the fluorescence in the second direction to produce at least three output beams (1030, 1032, 1034), the plurality of detectors configured to detect optical energy from the at least three output beams of the beam splitter device, each detector from the plurality of detectors being uniquely associated with an image from the plurality of images at a time; and a processor configured to calculate a position of the optical label in a third dimension based on the plurality of images, the third dimension being substantially orthogonal to the first dimension and the second dimension; the interference between the emitted fluorescence in the first direction and the emitted fluorescence in the second direction being caused at least in part by a difference in path length between the emitted fluorescencein the first direction and the emitted fluorescence in the second direction.