What Are T5 and T5HO High Bay Lights?

Comparison of T12, T8 and T5HO lamp dimensionsT5 and T5HO fluorescent high bay fixtures and other T5-based specialty lights are built around a linear fluorescent tube lamp.  Like other types of lamps, the number in the name of the lamp reflects its size.  The “T5” name reflects the diameter of the tube in eighths of an inch.  A T5 lamp is five-eighths of an inch (5/8”).  Similarly, a T8 fluorescent lamp is eight-eighths (or one inch) in diameter and a T12 fluorescent lamp is 12 eighths (or 1-1/2 inches) in diameter.  One of the features that make a T5 bulb and T5-based high bay light fixtures more efficient is the fact that the lamp is 37% narrower tube than a T8 and over 55% narrower than a T12 lamp.

This article describes the background and technical specs of the T5 and T5HO lamps as well as some considerations you should keep in mind when relighting your facility with these types of lamps.


The standard T5 lamp was developed in Europe and then introduced to the North American market in 1996. While some lighting designers began specifying the T5 luminaries immediately for various applications, market penetration wasn't great; one reason being that both the lamp and ballast were, and still are, more expensive than T8 systems. Then in 1998, the T5HO (high output) lamp was introduced to the North American market, providing twice the lumen output as standard T5 lamps.

Lumen Output

While the standard T5 and T5HO lamps are the same diameter and length, the 4-ft T5 is rated at 2,900 lumens, similar to the lumen per watt output of a T8 lamp. On the other hand, the T5HO lamp is rated as high as 5,000 lumens, offering twice the maintained light output of a T8 lamp. This means that on some projects a designer can use fewer fluorescent high bay fixtures or lamps, thus providing certain savings on installation and long-term maintenance.

T5HO High Bay with Enhanced ReflectorFor High Bay applications, the T5HO fluorescent tube allows more of the lumens that come out of the lamp to be directed towards the work surface.  Since a linear fluorescent lamp outputs light outward in every direction, modern high bay fixtures employ advanced reflectors above the lamp to direct the light that emanates from the top side of the lamp down towards the work surface.  A narrower tube allows more of this light to exit the luminary.

Another important advantage of T5 and T5HO lamps are their low mercury content. The lamp has a coating on the inside of the glass wall that stops the glass and phosphors from absorbing mercury. This barrier coating drastically reduces the amount of mercury needed from approximately 15 mg to 3 mg per lamp. Minimizing the amount of mercury also brings another very important advantage: since mercury absorption causes the lamp's light output to depreciate over its life, the coating helps to keep light levels much closer to initial output. This is why the T5 and T5HO lamps have only 5% depreciation in the first 40% of life.

T5HO Lamp Ballast Characteristics

The ballasts for the entire T5HO family are unique for a number of reasons.  T5HO lamps are the first family of linear fluorescent lamp types that only use electronic ballasts.  Additionally, the vast majority of ballast manufacturers only provide programmed start ballasts for T5 fixtures.  Programmed start ballasts extend lamp life over the traditional fast start units.

With power ratings of 24, 39, 54, and 80, the T5HO lamp produces as much as twice the lumen output of the standard T5 lamp, and nearly twice the light output of a T8 or T12 system. Each T5HO lamp requires its own dedicated ballast model. The typical T5 system provides from 83 to 94 lumens per watt. A number of T5HO ballasts have features, in addition to a dimming function. For example, one model can operate one, two, three, or four F54 lamps, while allowing remote mounting and a high-low switching option.

Temperature Considerations

One result of enclosing a smaller lamp with a relatively high output in a smaller fixture is heat and lots of it. For that reason, the T5HO lamp provides peak light output at 35°C (95°F) air temperature, whereas the T8 and the T12 lamp provide peak light output at a 25°C (77°F) ambient air temperature. Thus, in indirect luminaries, where there is little or no air circulation, the T5HO lamp can have a higher lumens-per-Watt efficacy than a T8 lamp of about the same wattage because of this thermal characteristic.

Temperature can also come into play as a lamp ages. Because of the lamp's small diameter and high-frequency operation, temperatures can rise at the lamp base ends as the lamp nears end of life.  This is because the cathode emission material gets depleted over time. This can cause a rise in voltage and result in cracking in the bulb wall. For this reason, all T5HO ballasts have what is called “end-of-life circuitry.” This feature shuts off power to the lamp when the ballast senses a rise in voltage across the lamp.


It's easy to see that both the T5 and T5HO lamps have increasing uses in both office and industrial applications. Now readily available, the T5HO systems are a viable option for virtually any application with ceiling heights higher than 15 ft, including warehouses, industrial buildings, and gymnasiums. Compared to a conventional 400W metal halide system, T5HO fixtures offer energy savings of nearly 53%, assuming equal hours of operation. Additional savings are possible where occupancy-based switching is used.

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