Abstract: An apparatus for measuring the geometry of an optical fibre having at least one coating, the apparatus comprising a container for containing a coated optical fibre in a fluid, means for directing light transversely through the fibre in the container and means for permitting light deviated at the coating interfaces or surfaces to exit the container whereby the light can be detected by a light detecting means by substantially dark field illumination. The invention also provides a method of measuring the geometry of an optical fibre having at least one coating the method comprising the steps of: (a) disposing the coated optical fibre in a fluid; (b) directing incident light transversely through the coated fibre; and (c) detecting transmitted light from the coated fibre which is deviated at the coating interfaces or surfaces so as to detect the coating(s) by substantially dark field illumination.