Abstract: A laminar flow clean air enclosure has at least two spaced apart side walls and a planar filter extending between the side walls to define a work area. A transition duct couples air between the outlet of a blower and an entrance region near one end of the filter inlet to expand air leaving the blower outlet without introducing turbulence. A plenum chamber in communication with the entrance region and substantially the entire cross-sectional area of the filter inlet is formed so as to provide air at constant velocity to substantially the entire cross section of the filter inlet. As a result, the velocity of the air leaving the blower outlet is gradually reduced without turbulence to the desired exit velocity for the air leaving the filter. A perforated screen having a planar central region and an angled peripheral region covers the filter outlet.