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.