A method of estimating dimensions of an opening (36) in a body (26) comprises filling the opening (36) with a fluid that absorbs an electromagnetic radiation (40). The radiation (40) is transmitted through the body (26) and through the fluid in the opening (36). The intensity of the electromagnetic radiation, after transmission through the fluid, is determined at an area that corresponds to a radiation path through the fluid in the opening. The intensity is used to determine the absorbance of the fluid in the radiation path and consequently the length of the radiation path. In an alternative method, when the depths of the opening are already known, the intensity is used to determine concentration of a chemical in the opening.