Apparatus and method for transmittance mapping of an object which is at least partially transparent to deep ultraviolet radiation. The method comprises directing a wide-band deep ultraviolet radiation so as to illuminate different areas of an array of successive areas of the object; using an optical detector positioned on an opposite side of the object with respect to the radiation source detecting the wide-band deep ultraviolet radiation that emerges from the object; and processing signals from the optical detector to determine the transmittance of the radiation through the different areas of the array of successive areas of the object.