The most critical factor is the direction of the light source. A critical horizontal light that strikes the ceiling surface at an acute angle accentuates any ceiling irregularities and may result in an unsatisfactory appearance. Fixtures located close to the underside of the ceiling, such as surface mounted or semi-recessed and cove lighting are generally unsatisfactory. In such cases, light grazes across the ceiling and emphasizes variations as small as .005 of an inch. This causes shadow problems that accentuate slight unevenness of joints and gives an unsatisfactory appearance to the ceiling.
Exterior lighting from high windows and window walls can result in similar problems to the ceiling.
The selection of recessed lights and skillful use of shielding of semi-recessed or surface mounted lights as well as the use of valances or draperies can help minimize the problem of troublesome, low angle, side grazing light. Ceiling units shall be so arranged that units less than one-half width do not occur unless otherwise directed by reflected ceiling plans or job conditions. All tile joints and exposed suspension systems shall be straight and in alignment. Tile shall be neatly scribed against abutting surfaces and to all penetrations or protrusions when mouldings are not required. Tile surrounding recessed troffer lights and similar openings shall be installed with a positive method to prevent movement or displacement of these tiles.