The invention relates to a lyophilised form of dabigatran of formula its use as a calibrator in the assays for the determination of pharmacodinamic effects of dabigatran etexilate as well as such assays per se. The apparatus and method allows calibration of color images taken with commercially available digital cameras in arbitrary illumination. This supplies high network strength in terms of section loss variations and greatly improves the OSNR of the WDM signal. Each second spherical surface is in contact with another flat surface at a point offset from the circumferential center of the flat surface in the other circumferential direction.