Saffron types are graded by quality according to laboratory measurements of such characteristics as crocin (colour), picrocrocin (taste), and safranal (fragrance) content. Other metrics include floral waste content (i.e. the saffron spice sample's non-stigma floral content) and measurements of other extraneous matter such as inorganic material ("ash"). A uniform set of international standards in saffron grading was established by the International Organization for Standardization, which is an international federation of national standards bodies. Namely, ISO 3632 deals exclusively with saffron. It establishes four empirical grades of colour intensity: IV (poorest), III, II, and I (finest quality). Saffron samples are then assigned to one of these grades by gauging the spice's crocin content, which is revealed by measurements of crocin-specific spectroscopic absorbance. Absorbance is defined as formula_1, with formula_2 as absorbance (Beer-Lambert law). It is a measure of a given substance's transparency (formula_3, the ratio of light intensity passing through sample to that of the incident light) to a given wavelength of light.