McIlvaine et al. (1992) describe issues related to lighting fixtures in cleanrooms:
Companies are making lighting more convenient and more adaptable to a user's needs. In order not to disturb the laminar air flow, tear-drop strip light fixtures mount directly to the T-Frame Suspension System and allow for 100 percent HEPA filtration coverage throughout the entire ceiling system. For Class 100 and better cleanrooms, integral filter-luminaire modules may be needed, or teardrop luminaires that are mounted to the grid support.
[Another firm] also manufactures fixtures to accommodate HEPA filters. The latest development is a fixture for the 2x2 filter design. Also about to be introduced is a new gel seal design. Integration of the lighting system with the filter as part of one module is an improved concept that allows higher lighting levels than with the older design in which tear drop lights were hung from the T-bars. Fixtures should be flush-mounted in the ceiling with the smooth side down and sealed to prevent air leakage.