An optical coherence tomography, referred to hereinafter as OCT, system (100; 170) for characterising first and second regions of interest below first and second surface areas of interest (102, 104) of a material, the OCT system (100; 170) comprising: first and second optical elements (108, 110) in use positioned at the first and second surface areas of interest (102, 104), respectively, the first and second optical elements (108, 110) being at least partially transmissive for electromagnetic radiation; a scanning system having first and second scanning portions (120, 122), the first and second scanning portions (120, 122) being positioned at the first and second optical elements (108, 110), respectively, to receive electromagnetic radiation that has interacted with the material within the first and second regions of interest; and at least one detector optically coupled to the scanning system; wherein the first and second optical elements (108, 110) are arranged such that respective reference radiation associated with the first and second optical elements (108, 110) is generated by reflection at interfaces of or at the first and second optical elements (108, 110), respectively, and the first and second optical elements (108, 110) are arranged or designed to be positioned such that an optical path length difference between the reference radiation associated with the first optical element and electromagnetic radiation that interacted with the first region of interest differs from an optical path length difference between the reference radiation associated with the second optical element and electromagnetic radiation that interacted with the second region of interest; and wherein the first and second optical elements (108, 110) are arranged such that an optical path length of the electromagnetic radiation through at least a portion of the first optical element to an interface of or at the first optical element differs from that through at least a portion of the second optical element to an interface of or at the second optical element by a length that is longer than an imaging depth of an OCT image of the OCT system (100; 170). first and second optical elements (108, 110) in use positioned at the first and second surface areas of interest (102, 104), respectively, the first and second optical elements (108, 110) being at least partially transmissive for electromagnetic radiation; a scanning system having first and second scanning portions (120, 122), the first and second scanning portions (120, 122) being positioned at the first and second optical elements (108, 110), respectively, to receive electromagnetic radiation that has interacted with the material within the first and second regions of interest; and at least one detector optically coupled to the scanning system; wherein the first and second optical elements (108, 110) are arranged such that respective reference radiation associated with the first and second optical elements (108, 110) is generated by reflection at interfaces of or at the first and second optical elements (108, 110), respectively, and the first and second optical elements (108, 110) are arranged or designed to be positioned such that an optical path length difference between the reference radiation associated with the first optical element and electromagnetic radiation that interacted with the first region of interest differs from an optical path length difference between the reference radiation associated with the second optical element and electromagnetic radiation that interacted with the second region of interest; and wherein the first and second optical elements (108, 110) are arranged such that an optical path length of the electromagnetic radiation through at least a portion of the first optical element to an interface of or at the first optical element differs from that through at least a portion of the second optical element to an interface of or at the second optical element by a length that is longer than an imaging depth of an OCT image of the OCT system (100; 170).