The fluorescent lamp that is most often used for both general ambient and specific task lighting is the "energy saver" four-foot, cool white, fluorescent tube (F40T12/CW/SS).
In the current trend to install new fluorescent lamps that are significantly more efficient in light output and energy consumption, F32T8 fluorescent lamps are generally used as replacements for F40T12 lamps. The T8 lamp, combined with a high-frequency electronic ballast, provides a rich source of lighting that delivers a high lumen package, a high CRI (color rendering index) rating and exceptional energy efficiency. [Eley et al., 1993; IES, 1993]
A brief comparison of the two lamps will demonstrate their different operating characteristics and performance. Understanding their technical differences will help the laboratory lighting designer to select the appropriate lamp to install for a particular use.
The 34WF40T12 lamp was originally produced in response to demand for a more energy-efficient fluorescent lamp during the "energy crisis" of the 1970s. Krypton was addedas a fill gas to the standard F40 lamp, creatingthe so-called energy-efficient fluorescent lamp. Today, standard lamps typically employ neon, argon, and sometimes xenon as a fill gas. Because krypton altered the standard F40 lamp's electrical operation just enough to produce measurable energy saving, it was embraced by the lighting community as the panacea for energy problems. [Lindsey, 1991; IES, 1993]
Unfortunately, time and experience revealed reduced lamp life, poor color rendering, and low light output resulting from the following technical problems:
|Rated lamp wattage||34W||32W|
|Actual energy use||37W||29W|
|Effective lumens **||2,376||2,668|
|Lumens per Watt||64.2 L/W||92.0 L/W|
|Rated Life||20,000 hrs.||20,000 hrs.|
One laboratory situation particularly demands T12 lamps. If there are E-Prom or computer chip boards being manufactured or assembled in a laboratory, it is critical that lamps be ultraviolet shielded. An F40T12 Gold lamp is available that emits virtually no UV; unfortunately, this lamp has a very low lumen rating, with effective lumens around 1,800 lumens/watt. This low rating will necessitate a larger number of lamps to meet the luminance requirements of the area than would be needed if another type of lamp was used; however these T12 lamps insure that there will not be any damage to the components being assembled.
More efficient T8 lamps could be used in this specialized environment; however, the extra cost for installing fixtures with UV shielded lenses might offset the energy savings from using the more efficient T8 lamps.
** Effective lumens are determined by multiplying the rated lumens of a lamp by the ballast factor or the particular ballast that is operating the lamp.
The 34WF40T12/CW lamp has an initial rating of about 2,750 lumens. The magnetic core and coil energy-efficient ballast operating this lamp has a ballast factor of 0.88:
0.88 * 2,750 lumens = 2,376 lumens.
This equation tells the designer that the lamp/ballast combination will produce about 2,376 lumens. Rule of thumb indicates that only about 50 percent of this light will reach the work surface.
The F32T8 lamp has an initial rating of about 2,900 lumens. The electronic ballast has a ballast factor of 92 percent.:
0.92 * 2,900 lumens = 2,668 lumens.