Web inspecting method and apparatus.

摘要

A web e.g. a fibre web as used in the manufacture of textiles is inspected e.g. for thickness uniformity or the like by transillumination, where the web is subject to ambient illumination, the transillumination being effected in modulated fashion whereby to distinguish measuring illumination from ambient illumination. Modulation may be effected by time-chopping a beam or by a narrow spectral band filter to improve signal-to-noise ratio.