- High output fluorescent lamps can now effectively compete against high intensity discharge lighting.
- High performance T8 and T5 fluorescent lamps can reduce operating costs in many applications.
- Metal halide lamps offer higher efficiency and lower costs, but come with some limitations.
Lighting accounts for about 20% of the total energy consumed in commercial buildings. Nearly every commercial building is artificially lit and lighting is a target for energy savings through the use of energy-efficient light sources and other advanced technologies. Many factors can affect lighting choices, including the type of building, occupant traffic, and occupancy schedules. The following serves as a guideline to help you evaluate these choices and select the best fit for your application.
Fluorescent versus High Intensity Discharge (HID)—High Bay Applications
Historically, HID lights such as metal halide and high-pressure sodium have dominated indoor high bay applications. These lights require fewer fixtures for an equivalent lumen output than fluorescents, thereby lowering capital and installation costs. However, the development of high output (HO) fluorescent lamps has raised the height level at which fluorescents can effectively compete against HID lighting. Now, there are many commercial and industrial fluorescent applications at the 20 to 25 foot ceiling level, and in some cases even higher. Key factors for consideration include the following:
- Evaluate initial lumen output, as well as lumen degradation. HID lights typically lose 35% to 40% of their lumen output, while fluorescent lights may lose 15% to 20%. Over the full service life, a fluorescent light of equivalent initial lumen output will deliver considerably more lumens than a comparable HID light.
- HID lights are a more concentrated light source than fluorescents, yielding similar lumen output with fewer fixtures and lamps.
- Fluorescent lights have more effective dimming capability. Most HID lights do not yield much energy savings when dimmed.
- Fluorescent lights can be used with occupancy sensors. HID lights with restrike times of 5 to 10 minutes are not effective with occupancy sensors.
- Fluorescent lights have a better color rendering index (CRI), with ratings in the 80 to 90 range, compared to 60 to 65 for the better HID lights.
- Four lamp and 480-volt fluorescent ballasts are available, making retrofits easier.
- Lamp life is comparable at 15,000 to 20,000 hours.
T12, T8, and T5 Fluorescents
Fluorescent lights achieve efficiency by using the optical properties of mercury vapor in combination with phosphors and buffer gases. Standard T12 tubular lamps have traditionally dominated the commercial market. However, higher performance, higher efficiency T8 and T5 lamps can reduce operating costs in many applications.
T12—The T12 is the least expensive to purchase, although it is not as efficient as the T8 or T5. T12s typically use only magnetic ballasts, which do not operate the lamp as efficiently as electronic ballasts. T12s can provide good color quality or high efficacy (lumens per watt), and newer, high-efficiency models are available. However, they still cannot compete in efficiency with other fluorescent choices. The T12HO (High Output) lamp provides more lumens, but is not as efficient as a typical T12.
T8—A smaller diameter fluorescent, the T8 has improved efficacy, higher ballast factor, longer operating life, and higher CRI compared to a T12. While T8s have higher performance levels and lower operating costs, they typically cost more to purchase and install. Interchangeable with T12 fixtures, they can be easily retrofitted, although T8s generally use only electronic ballasts. The total energy savings of a T8 compared to a T12 can vary. Estimates range from 10% to 30%, depending on the specific application.
T5—Developed in Europe and introduced to North America in the 1990s, the T5 is even smaller in diameter than the T8 and up to 15% more efficient. T5s are a good choice for cove lighting or display cases because they operate more efficiently at higher temperatures, with peak light output at 95°F. The higher output T5HO has up to twice the lumen output of standard T5s. Although they can be used in ceiling heights over 15 feet, they will not operate as efficiently if the tube temperature drops below 95°F.
Note that standard T12 magnetic ballasts were phased out of manufacture in June 2010 to comply with ballast efficacy standards set by the U.S. Department of Energy (see 10 CFR 430.32(m) Energy Conservation Standards).
Metal Halide versus Other HID and Fluorescents
Metal halide (MH) lamps offer much higher efficiency than mercury vapor lamps and better light quality than mercury or sodium lamps. Compared to fluorescents, a metal halide application would require fewer fixtures, perhaps lessening capital cost expenditures. Metal halide arc tubes are either ceramic or quartz.
- Whiter light than the yellowish hues of sodium lamps
- Fewer fixtures than fluorescent systems for similar lumen output
- The availability of pulse start, which can increase efficiency by 20% and lamp life by 50%, as well as provide better color quality and faster restrike capability
- Ceramic metal halides (only available for 150 watts and below) have greatly improved color capability, making them good competitors to halogen incandescent lamps for retail applications like spot and track lighting
- Not as efficient as sodium lamps (both HPS and LPS)
- Electronic ballasts are relatively new for HID, so proceed with caution, particularly above 150 watts
- Sometimes shift color erratically, compared to fluorescent and other HID lamps
- Can take up to five minutes to start and up to 20 minutes to restart after loss of power
- Can only be dimmed to about 50% and dimming provides only minimal energy savings
- Fluorescent color rendering capability is higher than metal halide
- If the arc tube within the metal halide lamp does not have a protective shroud, failure mechanism may be explosive and cause shattering
Making the Right Choice
While energy efficiency is a key consideration, it is clearly not the only factor when evaluating lighting choices. Your type of building, ceiling height, the specific application, as well as other factors come into play in choosing the right equipment to minimize operating costs and optimize lighting performance.