Understanding Cleanrooms Lighting
It's no mystery that cleanrooms present numerous challenges to designers who specify lighting systems. Cleanroom lighting will vary depending on the room's use, its classification and ceiling air supply configuration. In an ideal situation, lighting systems should provide good visibility and be designed with contamination control issues—electromagnetic field generation and cleanability—in mind. But before that can be achieved, a thorough examination of the basic fundamentals, illumination requirements, available styles and fixture construction must be undertaken.
Considering the fundamentals, the purpose of lighting systems is to provide visibility. Issues such as initial costs, energy, and maintenance costs typically contribute to lighting design decisions in most facilities, but for a cleanroom facility, the cost of lighting fixtures typically represents less than 1 percent of the total budget. Energy costs associated with lighting are usually minimal compared to the cleanroom HVAC and process equipment power demands. The focus in cleanroom lighting then becomes proper illumination, coordination with air supply systems and reduction of contamination for the unique environments and processes that reside in the space.
The unit of lighting luminance in the United States is the foot-candle, which equals 1 lumen per square foot. Lumens are used to describe the light output (luminous flux) of the source. The foot-candle level required in cleanrooms has been historically high due to precision demands or the extremely small size of the object that requires attention. Illumination levels vary depending on the different process areas within the cleanroom. The engineer or lighting designer is encouraged to review each process area within the cleanroom to determine the type of task to be performed and the illumination levels desired.
The surfaces of most cleanrooms tend to be highly reflective. Most or all of the ceiling, walls and flooring are bright white with enamel-like surfaces. However, wall and flooring material color options are being reexamined to improve the user environment.
The air supply system further complicates lighting placement and spacing. In more restrictive cleanroom classes, most or all of the ceiling consists of air supply filter panels with no space left over for recessed lighting fixtures; however, a thoughtful design process must assure optimum visual comfort and efficiency to support the performance of tasks in the cleanroom. As we'll see, there are various lighting solutions—most utilizing fluorescent lamps.
Lighting systems can utilize a number of different lamp types. Incandescent, high intensity discharge and fluorescent lamps are most commonly used in residential, commercial and industrial facilities. Fluorescent lamps are nearly always used in cleanroom environments due to their energy efficiency, low maintenance and long life.
The key characteristics to be considered in fluorescent lamps are the color temperature, color rendering index (CRI) and lumen output. Color temperature describes how occupants within the room will perceive the light source. Color temperature is measured by Kelvin degrees, ranging from 9000K (which appears blue) down to 1500K (which appears orange-red). Typically, lamps are specified between 4000K, described as cool, and 3100K, which are described as warm. Lamps with temperatures around 3500K are considered “neutral” and often specified in cleanrooms.
Color rendering describes the effect a light source has on the appearance of colored objects. The CRI scale ranges from 0 to 100 with most lamps available from 60 to 90 CRI. In general, with higher CRIs, the color appearance of objects will be more accurate within the room. Lamps with CRIs of 85 are commonly specified for cleanrooms.
Lumen ratings can vary depending on the lamps used. Specifying lamps with higher initial lumens will reduce the number of fixtures required to achieve a desired foot-candle level. However, the number of light fixtures and their spacing should be reviewed to produce a uniform lighting level throughout the space.
Fluorescent lamps produce some energy in the ultraviolet (UV) spectrum, which can be detrimental to certain cleanroom processes. UV filtering can be accomplished with lens shielding or tubes to cover the lamps. However, this shielding converts the visible light to yellow color, which is not as pleasing to the occupant. While the shielding may reduce illumination levels by only 10 to 20 percent, the perceived lighting levels can seem less due to the color shift.
An increase in the illumination levels may be required to achieve the desired visual acuity. In some cleanroom applications, lamps that produce high levels of UV to reduce bacteria or other biological contaminants may be required.
Ballasts are required in lighting fixtures to provide the required starting voltage and operating current for the lamps. The two basic technologies used in ballasts are electromagnetic and electronic.
Electromagnetic ballasts use reactors and transformers to drive the lamps and were the standard for many years. Electronic ballasts are more commonly specified today due to better energy efficiencies; however, electronic ballasts produce harmonic currents, which may contribute to problems in the electrical power system. Ballasts should be specified with total harmonic distortion levels at 10 percent or less, unless detailed harmonic studies are conducted.
Ballast and lamps can produce electromagnetic fields that can be in both the low and high frequencies. Both low and high frequency electromagnetic fields can interfere with sensitive equipment. High frequency fields are normally termed “RFI noise” and can be transmitted on the power conductors and radiated out into space. The RFI that is transmitted on the power conductors can be blocked by an RFI filter installed by the fixture manufacturer, or purchased from an electrical distributor for field installation. Such filters are effective for both magnetic ballasts and the more energy-efficient electronic ballasts. The RFI that is produced by the lamp and radiated through space through the lens is generally of a lower energy level than that produced by the ballast, but can also be disruptive.
Low frequency magnetic fields may also be radiated by the fixture into the cleanroom space and could interfere with sensitive equipment. Low frequency fields are often expressed in units of milligauss (mg) and diminish in strength with the square of the distance from the source. In general, lighting fixture outputs will be under two to three milligauss at a distance of two feet or more.
Cleanroom users often express concerns about electromagnetic interference from lighting fixtures. Requests to remotely locate ballasts or provide special shielding should be based on performance specifications and available testing data. The engineer should request specifications on the frequency ranges and field strength levels that would cause interference with cleanroom process equipment. Proposed fixtures can be tested in the field with handheld meters or in a laboratory setting prior to purchase or installation.
Foot candle (fc) requirements vary throughout cleanroom facilities, from 30 fc in mechanical/electrical rooms to 100 fc or more in the cleanroom. Some cleanroom applications that involve food inspection and grading require at least 150 foot-candles. The engineer should work with the cleanroom staff to determine the proper foot-candle requirements in each of the areas within the space.
Revision B of Federal Standard 209 (now 209E) originally contained a requirement for 100 fc in main cleanroom areas. In practice, 100 fc has been viewed as excessive and detrimental to eye comfort levels in most cleanroom applications. In addition, it can be difficult to obtain 100 fc in ISO Class 4 (Class 10) and lower cleanrooms, since most of the ceiling space is dedicated for air filters. Attempts to obtain 100 fc would reduce the available cleanroom ceiling space for filters and reduce overall energy efficiency. Surveys of existing cleanrooms have revealed adequate lighting levels in the 60 to 80 fc range. These illumination levels appear to accommodate user comfort and most types of cleanroom processes.
Cleanroom fixture types
The basic cleanroom fixture types are teardrop, recessed, surface mount and integral ceiling grid. The selection of fixture type usually depends on the cleanroom classification.
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.
Contamination control is the primary design goal of any cleanroom. Any potential source of contamination into the cleanroom must be thoroughly evaluated and minimized. Lighting fixtures should be subjected to contamination evaluation. Unfortunately, there are no established standards to prequalify fixtures for a particular cleanroom class. The National Sanitary Foundation (NSF) does provide listing and testing of materials used in food, pharmaceutical, medical or other FDA applications. Fixtures used in these applications should be required to carry the NSF listing mark. Manufacturers often advertise fixtures as being suitable for cleanroom use with no data or standard qualifications to substantiate their claims.
The materials and construction types of cleanroom fixtures vary widely. Since no accepted standard exists for specific cleanroom classifications, it is up to the engineer or lighting designer to carefully evaluate any proposed fixture. In general, it is recommended that fixtures used in ISO Class 5 (Class 100) to ISO Class 3 (Class 1) cleanrooms be constructed of powder-coated steel, anodized aluminum or stainless steel.
In NFS/FDA applications, stainless steel is typically required.
In addition, the fixtures should have airtight seals around the lens and frames. Gasketing materials should be closely reviewed for outgassing and resistance to cleaning solvents. The overall fixture construction should feel solid, tightly joined or sealed in all areas, should not flake or shed particles when lightly scratched and be smooth to the touch.
Doorframes for recessed fixtures are commonly available in a variety of materials, construction details and closure methods. The frames may be of aluminum, carbon steel or stainless steel and may be formed in one piece or assembled from extruded components. Extruded aluminum doors are specified most often, when the cleanroom process will permit, due to their cost advantage. If the cleanroom activity requires frequent wash down, such as in NSF/FDA or animal research clinics, a one-piece stainless steel door is generally specified and door closure is accomplished with countersunk screws.
Lens material is generally acrylic, but glass is occasionally used. The lens should be inverted, with the smooth side down for positive sealing and ease of cleaning. A uniform prismatic structure is most common, but a combination structure providing symmetric/asymmetric light distribution is available for special applications. The lens projects light down and away for general illumination to supplement specific task lighting.
Cleanrooms present numerous challenges to designing and specifying lighting systems. The mission-critical nature of cleanroom processes requires thorough analysis, specification and placement of the lighting fixtures.