Optical sensors

An optical sensor comprises at least one bundle comprising a plurality of optical fibers and a plurality of separate fluid-tight longitudinally extending ducts; and fluid connectors for introducing fluids selectively into at least some of said ducts. Test cells are formed (a) by array members with a pattern of holes or recesses applied to the end of the bundle, or sandwiched between the ends of two bundles or (b) within the fibers in cases where a duct overlaps with the optical field of the fiber. Usually others of the ducts serve to remove fluid from the test cells.