For many years, the standard for lighting high bay applications such as warehouses, industrial and manufacturing facilities has been the 400 Watt Metal Halide fixture. Advances in ballast technology and the introduction of the T5 High Output Linear Fluorescent lamp have led to many new luminaries that are targeted specifically for high bay applications. The following article details the many benefits that the T5 HO high bay fixture offers over traditional 400 Watt Metal Halide high bays.

Energy Efficiency

With the rising cost of energy, facilities and operations managers are doing everything possible to lower total power consumption. One of the main reasons many companies turn to T5 HO high bay fixtures when retrofitting their lighting system is the T5’s ability to provide better light while consuming much less energy than traditional 400 Watt metal halide fixtures. T5 HO fluorescent lamps consume 54 Watts of power and output up to 5,000 lumens per lamp. Typical 4-lamp fixtures consume a total of 231 Watts vs. the 454 Watts that a 400 Watt Metal Halide consumes. In most cases, maintained foot candle readings are almost always better per energy dollars spent.

Light Source	Conventional Lumens per Watt
Low-pressure sodium	165
5000K T5 fluorescent	104
4100K T8 fluorescent	90
Clear metal halide	85
5000K pure triphosphor fluorescent	70
3500K triphosphor fluorescent	69
50 Watt high-pressure sodium	65
2900K warm white fluorescent	65
Daylight fluorescent	55
35 Watt high-pressure sodium	55
Deluxe mercury vapor	40
Standard incandescent	15
Tungsten halogen	22

Lumen Maintenance

All lamps have some depreciation of the light output as they age. T5 fluorescent lamps only lose 5-6% of their lumen output between the rated initial spec and the mean spec. By contrast, Metal Halide lamps can have as much as 35% depreciation of lumen output during the same time period. This difference becomes more pronounced as the lamps continue to age.

No Strike/Re-strike Delay

HID lamps require some time to turn on. This is called the strike time. If the lamps have been on for some time and power is cut off, they would also need time to re-strike. Fluorescent lamps however, turn back on instantly without affecting the rated life of the lamp. This allows them to be used with occupancy sensors, photo cells and advanced lighting control systems that can further reduce operating costs by turning off the fixture when not in use. Modern T5 ballasts also allow for multiple switch legs that can individually control the pairs of lamps in a fixture. These can be wired into separate circuits or control systems to allow for dimming options.

More Usable Light

A linear fluorescent lamp shines light out radially in every direction from the center of the tube. Light that is cast out of the top of the lamp towards the fixture bounces off of the reflector and comes back towards the work plane. For larger T12 and T8 lamps, the size of the tube prevents most of this light from reaching the work plane because it bounces back to the tube. Since T5 tubes are extremely thin (only 5/8” diameter), more light can exit the fixture. This leads to more efficient fixtures and less shadows.

Better Color Rendition

Color rendition index (CRI) is the measure of the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source. T5 lamps have a CRI of 85 while metal halide lamps, on the other hand, have a CRI of 65-70. The lower CRI negatively impacts visual acuity and can strain the eye. The following table provides a comparison of the CRI for several light sources.

Lamp Type	CRI
T5 Fluorescent	85-98
T8 Fluorescent	75-98
High color-rendering metal halide	80-93
White high-pressure sodium	60-85
Standard metal halide	60-70
Pulse-start metal halide	65-70
High-pressure sodium	27
Low-pressure sodium	5

Color Temperature

Modern fluorescent lamps are available in a wide variety of color temperatures. These range from 3000K to 6500K. Specialty lamps extend the spectrum even more to suit specific purposes.

Consistent Color Output

Fluorescent lamps maintain a more uniform color output throughout their life. Metal Halide or other HID sources can have their output color shift as the lamps age.