what is the purpose of a recessed filter
Parliament (cigarette) Parliaments are one of few brands of cigarettes on the mainstream market to feature a recessed paper filter. Cigarette filters became common in most cigarettes in the 1950s. In the 1940s and 1950s, the brand was marketed for its unique filters, with advertisements reading, "Only the flavor touches your lips", and "Tobacco tastes best when the filter's recessed".[3] The brand also claimed that the recessed filter prevented tar from making contact with the smoker's mouth, unlike standard filters.[4][5][6] However, this recessed would also explain the popularity of the cigarette in another way: the sleeve would be suitable for snorting cocaine.[7]
X-ray filter For medical purposes, X-ray filters are used to selectively attenuate, or block out, low-energy rays during x-ray imaging (radiography). Low energy x-rays (less than 30 keV) contribute little to the resultant image as they are heavily absorbed by the patient's soft tissues (particularly the skin). Additionally, this absorption adds to the risk of stochastic (e.g. cancer) or non stochastic radiation effects (e.g. tissue reactions) in the patient. Thus, it is favorable to remove these low energy X-rays from the incident light beam. X-ray filtration may be inherent due to the X-ray tube and housing material itself or added from additional sheets of filter material. The minimum filtration used is usually 2.5 mm aluminium (Al) equivalent, although there is an increasing trend to use greater filtration. Manufacturers of modern fluoroscopy equipment utilize a system of adding a variable thickness of copper (Cu) filtration according to patient thickness. This typically ranges from 0.1 to 0.9 mm Cu.