Originally published November 2004; revised May 2009
Indoor spaces with high ceilings, such as factories, warehouses, big box retail stores, gymnasiums and all-purpose rooms are often lighted by probe-start metal halide lighting systems. At higher ceiling heights, 350W and 400W units are common.
Probe-start metal halide lamps are compact, rugged, powerful light sources, well suited for illuminating large spaces with a crisp, white light. These systems are able to operate reliably in a wide range of ambient temperatures, with numerous fixtures specially designed to operate in demanding environments such as hazardous locations.
Probe-start metal halide lighting presents a number of disadvantages, however. These systems are not easily dimmable, experience color shift over time, and require four minutes to start and about 10 minutes for re-strike after shutoff. Most significantly, service life, light output and efficacy severely degrade over time. These systems are often deployed in basic-grade spun-aluminum dome fixtures, which present a typical 75% efficiency—meaning 25% of the light produced remains trapped in the light fixture. As a result of its lumen maintenance and typical fixture efficiencies, this standard metal halide system appears low-cost but in fact is not very economical relative to the best alternatives, as either more fixtures, or higher-wattage fixtures, are required to provide desired maintained light levels.
The inefficiency of these fixtures, in fact, led to a prohibition on manufacturing probe-start fixtures that do not meet a certain ballast efficacy standard, as mandated by the Energy Independence and Security Act of 2007, virtually eliminating probe-start magnetic-ballasted fixtures starting in 2009.
Advancements in lamp and ballast technology have resulted in two alternatives to this basic system that can significantly reduce energy consumption while providing other benefits. The first alternative is fluorescent T8 or T5HO hi-bay fixtures, which can replace probe-start metal halide fixtures in retrofit or new construction for energy savings up to about 50%. The second alternative is pulse-start metal halide lamp-ballast systems, which can provide up to 25% energy cost savings in existing applications and up to 30% in capital and operating costs in new construction.
In the lighting industry, one may hear the terms “high-bay” (also “hi-bay”) and “low-bay” (also “lo-bay”) lighting.
In the construction of some types of industrial facilities, a skeletal framework is used, which forms an interior subspace called a “bay,” which in turn marks the space as “high bay” or “low bay.”
An older definition designated hi-bay to mean >25 ft. off the floor, medium-bay to mean 15-25 ft., and lo-bay to mean <15 ft. Some manufacturers define hi-bay as being over 15 ft. or 20 ft. off the floor. IESNA categorizes spaces as either hi-bay (>25 ft.) or lo-bay (<25 ft.).
The terms hi-bay and lo-bay also refer to fixtures designed for these applications, although it is not uncommon to see hi-bay fixtures in lo-bay applications, and vice versa.
Fluorescent fixtures for high-ceiling applications offer single- or multi-point pendant mounting for retrofit or construction alternative to HID fixtures such as probe-start metal halide. Manufacturers include Lithonia, Holophane, Columbia Lighting, Cooper Lighting, Day-Brite, HE Williams, MetalOptics, Amerillum, Orion, Simkar, Intrepid, 1st Source Lighting, Ruud Lighting, Stonco, Guth Lighting, Hubbell and others.
- These fixtures may house 4, 6 or other number of lamps.
- The lamps are typically T8 or T5HO, although compact fluorescent models are available.
- Optics are available with narrow and wide distributions. Wide distributions are best for lower mounting heights and general lighting areas, while narrow distributions are best for aisle and similar applications. Some fixtures offer a degree of uplight as well as direct downlight.
- Some models are available that can operate in demanding environments.
- Models are available that offer emergency ballasting options.
T5HO lamps are about 5/8 in. in diameter, about 40% of the size of T12 lamps, and therefore enable better photo-optic control of the light produced by the fixture, increasing efficiency and providing uniform distribution of light output. T5HO lamps used for hi-ceiling lighting applications are typically 4-ft. 54W lamps. Because T5HO lamps are built to metric dimensions, a 4-ft. lamp is actually 45.8 in. long, a little shorter than T8 and T12 lamps.
Initial rated light output is based on peak output at an ambient temperature of 35°C (95°F), whereas T8 and T12 lamps are based on 25°C (77°F). Amalgam lamps extend reliability of light output across a wider temperature range between cold and hot. T5HO lamps operate on programmed-start or instant-start electronic ballasts; universal-voltage (120-277V and 347-480V) ballast, dimming ballasts and four-lamp ballasts are available. T5HO lamps are not interchangeable with T8, T12 and T5 lamps.
There are two recent developments of interest. First, 49-51W T5HO lamps are now available that can replace 54W lamps for energy savings and a boost in efficacy with no loss of light output. Second, amalgam T5 VHO lamps are now available. These lamps produce 7,200 lumens of initial light output, reaching 80% of light output about three minutes after startup. Using amalgam technology, light output is above 90% from 65°F to 170°F. Dimming, however, may not be recommended.
Fluorescent fixtures for high-ceiling lighting applications often include “Super T8” lighting systems. Super T8 lamps are 32W lamps that provide 3,100+ initial lumens instead of the 2,850 offered by standard 32W T8 lamps, and 95% lumen maintenance at 40% of rated service life. Examples include Philips Advantage, Sylvania Xtreme XPS and GE’s High Lumen Eco. Super T8 lamps can be operated on programmed-start or instant-start ballasts. For hi-bay lighting, they are often paired with high-ballast-factor ballasts (1.15-1.18 BF) to maximize system light output. For example, a system consisting of six 3,100-lumen T8 lamps operating on 1.18 BF ballasts produces nearly 22,000 lumens, still about a third less than a 6-lamp T5HO system but somewhat more than a 4-lamp T5HO system.
Note that amalgam T8 VHO lamps are now available that produce light output above 90% from 50°F to 160°F (10°C to 70°C). This lamp produces the same light output as the T5 VHO, but offers lower wattage, higher effiacy, shorter rated life, and ability to dim down to 20%. See the below table for a comparison.
|T5 VHO amalgam||T8 VHO amalgam|
|Mean lumens||6,480||6480 (3500K and 4100K); 6,550 (5000K)|
|CRI||85||85 (3500K/4100K); 82 (5000K)|
|CCT||3500K, 4100K||3500K, 4100K|
|Life @ 12 hrs/st on PS ballast||35,000||25,000|
|Light output >90%||65°-170°F||50°-160°F|
T5HO Versus T8
You may hear recommendations to use T8 fixtures for a better quality of light and less glare at fixture heights <20 ft., T5HO fixtures for quality light output and higher fixture efficiency at >20 ft., and either between 18 and 25 ft. However, while T5HO may produce “glare bombs” at lower mounting heights, both T8 and T5HO fixtures can be used in both hi- and lo-bay applications, depending on the application, and if correctly applied.
Otherwise, a T5HO system is not as efficacious as T8 lamps, but produces more light output for the same number of lamps. With more light produced from a smaller diameter lamp, T5HO lamps are much brighter than T8 lamps, which can become a lighting quality factor.
T5HO lamp operation is optimized at a higher ambient temperature than T8s; another thing to watch out for with T8s is high-BF ballasts, which produce more heat. This may make T5HO systems more desirable in industrial spaces with higher ambient temperatures at the fixture mounting height. Note that ambient temperature is less a function of heat around the fixture as it is heat within the fixture’s lamp compartment; for best results, specify fixtures with a good temperature design.
A final consideration is maintenance. To get the highest amount of light output from a T8 fixture, Super T8 lamps should be specified, but the owner must continue to order this lamp type to maintain lighting performance. The maintenance department should not be permitted to substitute cheaper and lower-lumen 32W T8 lamps, particularly if these standard T8 lamps are used in a connected office. Conversely, if Super T8 lamps are used in a connected office, then this can be seen as a maintenance advantage for using them in a hi-ceiling application in the same building or campus.
A 400W probe-start metal halide fixture, with a ballast factor of 1.0, produces 36,000 initial lumens. A 6-lamp Super T8 fluorescent fixture, with a ballast factor of 1.18, produces about 21,950 initial lumens. How can this fluorescent fixture replace the metal halide fixture to generate 52% energy savings and still produce comparable light levels?
The answer is lumen maintenance. In review, lumen maintenance is an expression of the fraction of initial light output that is produced by a light source over time—typically at 40% of lamp life, which provides mean lumens. This determines the design light level.
Probe-start metal halide lamps experience a higher level of lumen depreciation than T5HO and T8 lamps. For example, a 400W metal halide lamp can lose 35% of its light output at 40% of life, while a T5HO or T8 lamp will lose only 5-6%. As a result, a 6-lamp Super T8 lamp-ballast system produces 11% fewer mean lumens for 52% less energy.
|System||Initial Lumens*||Mean Lumens @ 40% Lamp Life**||Relative Mean Lumen Output|
|400W Probe-Start Metal Halide||36,000||23,500||100%|
|400W Pulse-Start Metal Halide||42,000||32,800 (magnetic ballast); 36,000 (electronic ballast)||140%; 153%|
|4-Lamp T5HO Fluorescent||20,000||19,000||81%|
|6-Lamp T5HO Fluorescent||30,000||28,500||121%|
|6-Lamp Super T8 Fluorescent||21,948||20,851||89%|
**Fluorescent lamp lumens are based on optical temperatures; adjust as needed.
**Note that pulse-start system light output declines at a significantly sharper rate than fluorescent after 40% of lamp life. To further the comparison, consider researching and comparing these numbers at end of lamp life rather than at the mean. Data source: Advance.
This article focuses on comparing a standard probe-start metal halide lamp-ballast system with relevant T5HO and Super T8 lighting systems. Note that when comparing wattages to do so based on system wattage (lamp/ballast) rather than solely on lamp wattage. A “400W metal halide” system, accounting for ballast losses, draws 458W, not 400W. Similarly, a 6-lamp T5HO system draws 324W based solely on lamp wattage but 351W when these lamps operate on necessary ballasts. Comparing system wattages can be important when determining cost savings resulting from a lighting retrofit, but in new construction, efficacy, covered on the next page, is often considered more important.
|System||Total Lamp Watts||Total System Watts||Relative System Wattage|
|400W Probe-Start Metal Halide||400W||458W||100%|
|400W Pulse-Start Metal Halide||400W||452W (magnetic ballast); 425W (electronic ballast)||99%; 93%|
|4-Lamp T5HO Fluorescent||216W||234W||51%|
|6-Lamp T5HO Fluorescent||324W||351W||77%|
|6-Lamp Super T8 Fluorescent||192W||222W||48%|
Data source: Advance.
Efficacy, in review, is an expression of relative lamp efficiency. Expressed in lumens of light output per watt of electrical input, this useful metric is similar to “miles per gallon.” As lumen output decreases over time, efficacy decreases because wattage says the same.
400W probe-start metal halide has an initial lamp-ballast system efficacy of 79 lumens/W. Although well below the efficacy of Super T8 with its efficacy of 99 lumens/W, it is only 7% less efficacious than T5HO with its efficacy of 85 lumens/W. However, initial efficacy is virtually meaningless because efficacy changes during operation. At 40% of lamp life, considered the design average, the efficacy of a 400W probe-start lamp-ballast system drops 40% to 51 lumens/W, while T5HO and Super T8 efficacies drop 5% to 81 lumens/W and 94 lumens/W respectively.
|System||Initial Efficacy (lumens/W)||Mean Efficacy @ 40% Lamp Life||Relative Mean Efficacy|
|400W Probe-Start Metal Halide||79||51||100%|
|400W Pulse-Start Metal Halide||93 (magnetic ballast); 99 (electronic ballast)||73; 85||143%; 167%|
|4-Lamp T5HO Fluorescent||85||81||159%|
|6-Lamp T5HO Fluorescent||85||81||159%|
|6-Lamp Super T8 Fluorescent||99||94||184%|
Data source: Advance.
Fluorescent and metal halide lighting systems operate as the light-producing component within a light fixture. The light output and efficacy numbers previously discussed, therefore, must account for the impact of the fixture.
Many probe-start metal halide light fixtures found in the field offer low efficiencies of about 75%, while the best T5HO and T8 (and HID) hi-bay fixtures offer efficiencies as high as 91-92%. (For best results when choosing fluorescent, select fixtures with optics that are specifically designed for the specific lamp type, whether it be T5HO or T8.)
When one considers the impact of fixture optics, the basic-grade 400W probe-start metal halide fixture produces the lowest amount of maintained light output of all the options, and has a maintained efficacy of less than half the Super T8 option.
|System||Fixture Efficiency||Fixture Mean Lumens @ 40% Lamp Life||Relative Mean Lumen Output||Fixture Mean Efficacy (lumens/W)||Relative Fixture Efficacy|
|400W Probe-Start Metal Halide, basic-grade dome||75%||17,625||100%||39||100%|
|400W Probe-Start Metal Halide, high-performance dome||92%||21,620||123%||47||121%|
|400W Pulse-Start Metal Halide, high-performance dome||92%||30,176 (magnetic ballast); 33,120 (electronic ballast)||171%; 188%||67; 78||172%; 200%|
|4-Lamp T5HO Fluorescent, high-performance reflector||92%||17,480||99%||75||192%|
|6-Lamp T5HO Fluorescent, high-performance reflector||92%||26,220||149%||75||192%|
|6-Lamp Super T8 Fluorescent, high-performance reflector||91%||18,974||108%||85||218%|
Source of fixture efficiency numbers: Lighting Wizards, Inc.
Probe-start metal halide lamps take 4 minutes to start and 10 minutes to restart after being turned off and then shortly after turned on again. Pulse-start lamps take 2 minutes to achieve full brightness on a magnetic ballast and less than 1 minute on an electronic ballast, while taking 4 minutes to hot re-strike. Because of safety concerns, HID systems are not compatible with switching controls such as occupancy sensors.
Fluorescent systems, however, start almost instantly, opening up significant controls possibilities. Line-voltage occupancy sensors have significantly reduced their installed cost, making it economical to install one sensor per fixture for intermittently occupied spaces. (This type of strategy, for example, can be used to satisfy the Commercial Buildings Deduction’s bi-level switching requirement.) Fluorescent systems are also relatively easy and inexpensive to dim, enabling daylight harvesting with skylights or flexible light level selection in all-purpose spaces. These opportunities further extend the potential for energy cost savings.
In review, the rated service life of gaseous discharge lamps is an average. At rated life, half of a large population of lamps is expected to fail, distributed according to the lamp’s mortality curve. Lamp life is particularly important in hi-bay applications because the fixtures can be difficult to reach for maintenance.
At first glance, probe-start metal halide appears to offer very good service life compared to fluorescents. However, service life is rated based on the anticipated switching cycle, or “hours/start,” as the frequency of switching lamps on and off significantly impacts service life. Fluorescent lamps are typically rated based on 3 hours/start, while metal halide lamps are typically rated based on 10 hours/start. Fluorescent service life improves on an apples-to-apples basis of 10-hour switching cycles. At 10 hours/start, Super T8 leads the pack with a 28,000-hour service life compared to 24,000 hours for T5HO and 20,000 hours for probe-start.
Note, however, that fluorescent lighting enables the introduction of occupancy sensors, which may switch the lamps more frequently and thereby reduce lamp life. For these applications, programmed-start ballasts can be specified to optimize lamp life.
|System||Rated Service Life @ 10 Hours/Start (hours)||Relative Service Life|
|250W Probe-Start Metal Halide||15,000||75%|
|250W Pulse-Start Metal Halide||20,000||100%|
|400W Probe-Start Metal Halide||20,000*||100%|
|400W Pulse-Start Metal Halide||20,000||100%|
|4-Lamp T5HO Fluorescent (Programmed Start Ballast)||24,000**||120%|
|6-Lamp T5HO Fluorescent (Programmed Start Ballast)||24,000**||120%|
|6-Lamp Super T8 Fluorescent (Instant Start Ballast)||28,000||140%|
*OSRAM SYLVANIA has introduced a 250W pulse-start metal halide lamp rated to 20,000 hours.
**Philips Lighting has re-rated its T5HO lamps with programmed-start ballasts to 25,000 hours at 3/hours/start, which would increase for 10 hours/start.
Data source: Advance, with notations by Lighting Wizards.
In review, color temperature indicates the color appearance of a light source and the light it emits. For general lighting in many industrial spaces and warehouses, 4000K is considered suitable. In big box retail stores, color temperature is typically on the warmer side of neutral-white (3000-3500K), but can vary based on preference.
Typical probe-start metal halide lamps provide a 3000-4000K color temperature. As metal halide lamps age, however, chemical changes occur in the lamp which can cause a shift in color temperature of 200-600K over time. If group relamping (replacement of all lamps in a system at periodic intervals) does not occur, replacement lamps mingling with older lamps can result in noticeable poor lamp-to-lamp color consistency over time; some lamps may appear white while others may appear bluish, pink or purple. Additionally, when metal halide lamps are dimmed, they may shift to a higher color temperature, from white to blue-green; when a clear lamp is dimmed to 50% of rated power, color temperature can increase by 1500K, according to the Lighting Research Center.
HID lamps can experience a color shift during dimming and also a reduction in color rendering ability. Metal halide lamps are most susceptible to changes in lamp color characteristics.
T8 and T5HO experience negligible color shift during operation (although dimming may make the lamps appear uniformly cooler) and therefore maintain consistent color lamp to lamp. These lamps also offer a broader color temperature range from a neutral-white range up to a very cool 5000K.
|Probe-Start Metal Halide||3000-4000K|
|Pulse-Start Metal Halide||3600-4000K|
|Ceramic Pulse-Start Metal Halide||3000-4200K|
|Super T8 Fluorescent||3000-5000K|
Data source: Advance.
In review, color rendering, expressed on the Color Rendering Index (CRI), is the ability of a light source to make colors in the space appear “natural.” According to IESNA, in a manufacturing space, an >80 CRI rating may be suitable, although a CRI >90 may be desirable for tasks where matching or distinguishing colors is critical. In a warehouse, a CRI of at least 60 is suitable, with a CRI of at least 80 desirable where color is important. In big box retail stores and supermarkets, light sources should have a >80 CRI.
T5HO and T8 lamps provide 82-85 CRI compared to 65 for probe-start metal halide lamps. (Note that metal halide lamps may suffer a reduction in CRI when dimming; for example, when a clear metal halide lamp is dimmed to 50% of rated power, the CRI value may decline from 65 to 45.) To achieve a 90+ CRI, some fluorescent models are available but the higher color rendering is achieved at the expense of light output, disqualifying these lamps for many hi-bay applications. Other choices include daylight, ceramic metal halide and incandescent, although incandescent is generally undesirable due to its short service life and very low efficacy.
|Probe-Start Metal Halide||65 CRI|
|Pulse-Start Metal Halide||65 CRI (clear); 70 CRI (coated)|
|Ceramic Pulse-Start Metal Halide||80-90+ CRI|
|T5HO Fluorescent||82-85 CRI|
|Super T8 Fluorescent||85 CRI|
Data source: Advance.
Lighting Quality and Aesthetics
Lighting quality and aesthetic issues that are important to consider include color, glare, shadows, uplight, uniformity, vertical distribution and fixture appearance.
Metal halide lamps are point sources, while fluorescent lamps are linear sources. As a result, fluorescent fixtures are less likely to present “glare bombs” than metal halide fixtures, while increasing vertical light levels and providing softer light distribution, which minimizes shadows. However, whether metal halide or fluorescent is used, these aspects are highly dependent on good fixture design. On the other hand, metal halide hi-bay fixtures with clear prismatic domes are often seen in big box retail stores, selected partly for their aesthetic appearance and ability to provide dramatic highlights and a uniform uplight pattern on the ceiling. Wherever metal halide is selected, pulse-start metal halide should be considered.
Hi-bay fixtures with linear sources can improve vertical footcandles, important in applications such as big box retail, warehouses and some sports facilities.
Fluorescent hi-bays often present 4-6 times more lamps to maintain, with the primary cost-adder being labor. As lamps fail, fixtures exhibit lamp outages, which can affect space appearance, not to mention produce less light. Typically, a lift or similar mechanism will be required, as pole changers do not work with linear fluorescent lamps.
On the other hand, if a metal halide lamp fails, a significant space will not have a sufficient light level. With fluorescent fixtures, when a lamp fails, the space will still receive light from the remaining lamps. Similarly, fixtures usually contain more than one ballast, so if one ballast fails, the other may continue operating. Lamp life with fluorescent systems can be maximized with programmed-start ballasts, especially important if occupancy sensors are present which can result in frequent switching. If maintenance is an extremely critical issue, consider induction lamps, which can provide up to a 100,000-hour rated lamp life and retained performance in extremely cold conditions, albeit for a much higher installed cost.
Another maintenance issue is lamp replacement when Super T8 lamps are used. It is critical for maintenance personnel to replace Super T8 lamps with Super T8 lamps and not standard 32W T8 lamps because this will result in a reduction in light levels.
Disadvantages of Fluorescent
Fluorescent fixtures are not for all hi-ceiling lighting applications:
* Extreme mounting heights, which may lend themselves better to 1000W metal halide lamps.
* Unconditioned spaces with wide temperature ranges.
* Severe environments such as hazardous locations, corrosive environments, etc. for which a suitable fluorescent fixture is not available.
* Environments where the aesthetic of a dome-shaped fixture is desired; for these spaces, one can still consider domes fitted with compact fluorescent lamps.
* Spaces where a retrofit or upgrade alternative is not economical. In a retrofit, this will depend on product purchasing, installation labor and local energy costs. In a new construction project, note that a good fluorescent hi-bay fixture costs more to install than a basic-grade hi-bay metal halide fixture, but these initial cost savings are wiped out within months due to higher operating costs.
As always in lighting, the choice of the best system will often depend not just on the economics of initial and operating cost, but also on environmental considerations and what level of performance the owner is looking for from their lighting system.
Hi-bay fluorescent lighting enables owners to take advantage of all the control systems already enjoyed in office settings—scheduling, daylight harvesting, bi-level switching, occupancy sensors and dimming.
Fluorescent lighting starts almost instantly and therefore is highly compatible with automatic switching strategies such as automatic shutoff using occupancy sensors or control panels with time clocks.
Besides scheduling, occupancy sensors represent a major controls opportunity that can be used to maximize energy savings during a fluorescent upgrade, particularly in warehouses and similar spaces that are often under-occupied.
Line-voltage occupancy sensors have slashed the cost of occupancy-sensing by about two-thirds, according to Platts/McGraw-Hill, making it economical to consider installing a sensor for each fixture in intermittently, infrequently occupied areas. The sensor is installed directly onto the fluorescent fixture or electrical junction boxes. Occupancy sensors are available with lenses specifically designed for hi-bay applications, providing reliable coverage from a range of mounting heights, and some are available with narrow-view lenses for warehouse aisles. When using occupancy sensors, which can result in frequent switching, consider programmed-start ballasts to maximize lamp life.
Fluorescent dimming can be accomplished in two ways. First, fixtures can be wired with multiple circuits to vary light levels, enabling bi-level or multi-level switching. Unlike hi-lo HID ballasts, energy savings proportional to light output reduction. Second, the fixtures can be equipped with dimming ballasts for continuous dimming. Unlike HID dimming, the lamps can be dimmed to 10-20%. Both bi-level switching and continuous dimming can be instituted to generate energy savings resulting from occupancy-sensing (with occupancy sensors), scheduled demand reduction (with a scheduling device such as a control panel with a time clock), and/or daylight harvesting (with a photosensor). Bi-level switching and continuous dimming also enable flexibility to adjust light levels for multiple uses of a space.