Physical Considerations

Clearly, the area and attributes of the physical space in which a lighting fixture will be used can exert a strong influence over ultimate system selection. This is especially true of retrofitting applications where an existing infrastructure may limit cost-effective options due to physical obstructions and other considerations. Here are some factors to consider to ensure the appropriate fixture and lamp selection for your project.

Ceiling Obstructions & Fixture Selection

Ceiling obstructions such as air ducts and sprinkler heads are important to consider when looking at fluorescent light fixtures, which are larger than their HID counterparts and therefore may represent an installation challenge. Cost considerations, especially the long-term avoided operation costs desired in retrofit applications, should take into account physical obstructions and the costs associated with existing infrastructure modifications.

Environmental Cleanliness

An environment high in airborne dust and dirt will cause any lighting fixture to lose its efficiency due to accumulation. In this regard, fixtures with up-light components, such as a prismatic high bay, will be more affected by dirt accumulation than one with only down light, such as an aluminum high bay. This is especially a concern with linear fluorescents, in which the entire length of the lamp can become coated, thus reducing light output. Regardless of luminaire type, all fixtures (HID and fluorescent) should be cleaned each time re-lamping is performed.

Ambient Temperature Affects

Ambient room temperatures can have a significant impact on the light output of fluorescent lamps, unlike HID sources, which are not affected by temperature variances. With this in mind, metal halide lighting provides a good fit in commercial freezer environments or non air-conditioned spaces, especially those found in very hot climates. Color Temperature, Color Rendering (CRI) - Better visual acuity, perceived brightness, and lower energy costs continue to be critical factors in effective lighting environments. Research also has shown that substantial increases in worker productivity occur in well-lit, color-corrected environments. These characteristics are primarily resident in the light source. Fluorescent lamps, for example, offer a more diverse choice of color temperatures and color rendering than those found in metal halide.

End-of-Life Light Levels

Unfortunately, in most applications, end users do not perform planned lighting maintenance. As a result, light levels in these environments typically depreciate to well below the intended design levels over the life of the lamps. This is another area in which fluorescent lamps perform exceptionally well by offering greatly reduced lumen depreciation. This also results in improved (more consistent) light levels over the life of the job.

Planned Lighting Maintenance

HID fixtures typically have one lamp per fixture, while T8 and T5HO (fluorescent) fixtures are more often used in four or six-lamp configurations. The number of lamps used per linear fixture can affect the time required for cleaning and lamp changing during planned lighting maintenance events.

Power Interruptions

Metal halide lamps require between two to 15 minutes to startup and return to full output in the event of a power interruption, or if they are completely shut off. By comparison, fluorescent systems startup and return to full output nearly instantaneously.

Occupancy Control

If your space provides opportunities for occupancy sensor control of your lighting, you may want to consider a linear-fluorescent system. Their nearly instantaneous ability to attain full-light output when powered on makes them superior to metal halide lamps. As a result, they are especially useful in warehouses and distribution centers. With regards to metal halide systems, please note that in order to be of practical use, they must operate at 50% power in the 'low-mode' of a 2-level system. By contrast, linear fluorescent lamps can actually be at zero power in the low-mode, which results in greater opportunities for energy savings.