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 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 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 .