Method and device for non-invasively optically determining bulk density and uniformity of web material during in-line processing, by measuring and analyzing light scattered by volume segments of transported web material. Influence of process operating parameters of temperature, volumetric bulk material transport rate, type, and, physicochemical characteristics and properties, of the transported web material, on collecting and detecting the scattered light, are used for process correcting raw measured values of scattered light flux density/energy of scattered light exiting volume segments of the transported web material, for determining bulk density and uniformity. Pattern recognition and classification techniques are performed using the calculated bulk density and uniformity values of the web material. Bulk density is determined in the approximate range of from one to zero kilogram per square meter of web material, with corresponding measuring sensitivity of about one gram per square meter of web material.