Teardrop light fixtures were the standard in cleanroom illumination for many years. When dealing with ISO Class 4 (Fed-Std-209E Class 10) or better cleanrooms, most of the available space in the ceiling is allocated to the air filters. The only locations left are directly underneath the channels of the ceiling grid. The fixture itself was designed to be aerodynamic in shape—or in the shape of a teardrop.

The purpose of this design was to minimize disturbance of the laminar airflow down into the room from the ceiling air supply.

The major problem often associated with teardrop fixtures is that they extend six to eight inches below the ceiling, reducing the effective headroom in the cleanroom space. Lower profile teardrops are available using smaller diameter lamps and extend only three to four inches below the ceiling.

Cleanrooms with an ISO Class 5 (Class 100) or above rating may have sufficient free area in the ceiling to accommodate 234 or 232 recessed fixtures. Recessed fixtures are usually supported by the ceiling grid and sit on a gasket flange or within a gel-filled channel to maintain an airtight seal. Total and reliable seals are required for recessed fixture housings regardless of cleanroom classification. Even a microscopic opening in the housing will permit the entry of the particles from the air supply above the ceiling grid to enter the cleanroom during fixture maintenance.

Recessed fixtures may also be combined with HEPA air filters when a space needs both a filter and a light. This fixture is typically used in small, pre-manufactured cleanrooms. The fixtures are recessed with open tops and with an acrylic louver in lieu of the lens to serve as both a lighting fixture and an air delivery module. They are used in the most restrictive class of cleanrooms, where the entire ceiling area must be covered with air supply modules.

The tops are generally designed to accommodate standard 42- or 48-inch. HEPA filters, with the air handling boots supplied and installed by the HVAC contractor. The housing must be completely free of microscopic openings or high velocity air moving through the fixture will pull contaminates from the plenum and deliver them into the cleanroom. These flow-through fixtures will typically have metal strips over each lamp, which provides structural rigidity, reliable lamp starting, optimum lamp output and lamp stability within the air stream.

Surface mount fixtures are generally used only when another configuration is not practical due to site-specific restrictions. In general, surface mount fixtures are only applicable to ISO Class 7 (Class 10,000) or above cleanrooms or to cleanroom utility chases (gray areas). Their shape cannot be made aerodynamic, and some turbulence of the airflow is unavoidable.

In order to accommodate the conflicting space demands of lighting and air filters in the ceiling, and as an alternative to teardrop fixtures, manufacturers have developed modular ceiling systems with integral lights built into the grid channels. These systems allow for uninterrupted laminar airflow, avoid hanging obstructions that can complicate move-in/move-out of large equipment and can provide foot-candle levels of 90fc. Typically these ceilings come in large assembled modules with prewired light fixtures every 2 feet. This kind of ceiling system can reduce the amount of wiring required between light fixtures.