Fixture type. Indirect-lighting fixtures, which are suspended from the ceiling or mounted on a wall, distribute 90 percent or more of the light upward so that it is reflected off of ceilings and walls. In contrast, direct-lighting fixtures project 90 percent or more of the light downward. Lighting fixtures are also available that combine indirect and direct lighting (see Figure 1).
Figure 1: The ups and downs of lighting
Lighting fixtures may send all, or most, of their light downward (A), upward (B), or provide a mix of up and down light (C).
Many designers warn against using only indirect lighting—it can lead to ceilings with too much light and create a comparatively gloomy effect underneath. That is what led to the introduction of indirect/direct fixtures in the mid-1980s. By adding a portion of direct lighting to an indirect fixture, lighting designers can provide more lighting variety while retaining the low-glare benefits of indirect lighting. Occupants who work at a desk or computer screen benefit most from the low glare of the indirect portion; and those walking about or passing through appreciate the depth and contrast that the direct component adds.
In the past few years, manufacturers have introduced furniture- or partition-mounted indirect fixtures for open office environments. These can make indirect lighting viable in lower spaces. However, there can be problems with the application of furniture-mounted indirect fixtures. One veteran designer says that these designs work best with static offices; otherwise the fixtures make it difficult to reconfigure offices. In its Green Floors project, Natural Resources Canada found that mounting fixtures on wall partitions limited design and modification flexibility. The agency's designers also found it difficult to control lighting according to space occupancy, because the partition lights were shared by two offices. Low placement of the partition lights provided better light distribution but created glare problems for tall occupants walking by.
Lamp type. Indirect and indirect/direct fixtures typically use T8 or T5 lamps with electronic ballasts. T5 lamps are thinner, more efficient, and offer a higher intensity of light output than their T8 predecessors. The high intensity of T5 lamps means that rows of indirect fixtures can be placed as much as 12 to 15 feet apart on ceilings as low as 9 feet (some manufacturers claim that the fixtures can be used on ceilings that measure 8 feet 6 inches or lower) and still provide uniform ceiling illumination levels. With T8 lamps, the standard spacing is 10 to 12 feet, and ceilings have to be at least 9 feet 6 inches high—higher than most conventional office ceilings. Wider spacing means that fewer fixtures need to be used in a given space, and the overall cost for an installation can be reduced accordingly.
T5 lamps are available in two types: standard output and high output (HO). The HO versions put out almost twice as much light as a T8 lamp of the same length, and therefore the number of single-lamp T5 fixtures required in a given space can be cut almost in half compared with single-lamp T8 units. Thus, indirect lighting systems that use T5 HO lamps can be less expensive than a T8 indirect system, despite the fact that T5 lamps themselves are still more expensive than T8s. The cost advantage should continue to grow as T5s become more popular and the manufacturing volume increases. The lamp price should eventually fall below that for T8s, because their small size means that they inherently require less material to manufacture. Single-lamp fixtures also boast an advantage over two-lamp fixtures, because light distribution is easier to control with one lamp than two. T5 lamps can also offer better performance than T8s in enclosed fixtures and warm spaces because they are designed for a higher operating temperature. An advantage occurs at the end of lamp life as well—a single T5 lamp is easier to dispose of than a pair of T8s, because there is less material involved.
Some designers warn, however, that the HO models may be too bright, and care must be taken to avoid the creation of hot spots on the ceiling, which would lead to the very glare that designers are trying to avoid. T5 HO lamps are also less energy efficient than standard T5 lamps—standard T5s offer an efficacy of about 104 lumens per watt, whereas T5 HOs and T8s come in at about 92 lumens per watt. In addition, T5 lamps come in lengths that are shorter than the standard lengths for T12 and T8 lamps. For that reason, T5 lamps cannot be retrofitted into existing T12 or T8 luminaires.