In developing functional commercial spaces, specifiers are seeking fixtures that deliver the IESNA-recommended light intensity at the working level and can be configured into a layout that is compliant with ASHRAE 90.1 LPD limits. The optical system in a fixture determines the distribution pattern and placement of light. The type of lamp and the efficiency of the fixture determine the amount of energy necessary to generate those precious footcandles on the work plane.

Efficacy and Lamp Life of Commercial Lamp Types

At its most basic, the efficiency of a lighting system is determined by the efficiency of the lamp and the efficiency of the lighting fixture. When discussing the efficiency of a lamp, the term efficacy is often used. Lamp efficacy refers to the number of lumens, or amount of light, that a specific lamp type produces per watt of energy it consumes. This lumens per watt metric, often abbreviated into LPW, can be significantly different from one lamp type to the next. A higher lamp efficacy denotes that a greater amount of light is being produced per unit of energy consumed, indicating a more efficient lamp type.

In addition to lamp efficacy, lamp life should be considered when determining the total footprint that a light source leaves on a project. Rated lamp life describes the number of illuminated hours that one lamp is expected to last before needing to be replaced. Selecting a lamp type with a longer rated life will minimize the necessary maintenance and the material lamp waste created by the lighting system.

Taking a closer look at the many lamp types commonly found in commercial applications, including, linear fluorescent, compact fluorescent, halogen, high-intensity discharge (HID) and light emitting diodes (LEDs), reveals that there is significant variation in the efficacy and lamp life offered from one lamp type to the next. Identifying the lamp type that best meets the energy performance needs of the project is an important step in specifying good lighting.

Fixture Efficiency

Fixture efficiency describes the percentage of the total lamp lumens produced by the lamp inside the fixture that are then emitted by the fixture into the interior space. A naked lamp, with no fixture housing around it, is 100% efficient, because all of the light produced by the lamp is delivered into the environment.

Fixture efficiency is not an indicator of the quality of the light provided or the visual comfort produced by the luminaire. A naked lamp, for example, although efficient, would wreak havoc on the visual environment. The naked lamp would cause a glaring hot spot in the space and without any way to focus the light onto the work level, much of the light would be wasted illuminating the ceiling, corners, and other unusable places. It is an important note that a more efficient fixture does not necessarily provide more usable light.

Fixture efficiency is not an indicator of the quality of light produced or the visual comfort provided by the luminaire. Photo courtesy of Prudential Lighting

The importance of fixture efficiency, and its impact on the visual environment and the efficiency of the space, is often misunderstood when different fixtures are being evaluated. In the process of selecting a fixture for a commercial space, fixture efficiency should be a secondary or tertiary consideration. Once the visual tasks that will performed in the space have been addressed, and the IESNA-recommended light levels and location have been reviewed, and a lamp type that can produce the necessary lumens and meets the energy performance needs of the project has been identified, a specifier will start searching for the right fixture.

When comparing lighting fixtures, higher SC ratings can save money by reducing the total number of fixtures required to illuminate a space. Image courtesy of Prudential Lighting

If a fixture meets the LPD requirements of the setting, but cannot produce the recommended light level on the work plane, perhaps a more efficient fixture, with the same LPD, would be able to eke out enough additional lumens to bridge the gap. However, emitting more lumens, being a more efficient fixture, does not guarantee that the light levels at the work plane will increase, it simply means that overall, somewhere in the surrounding space, more light will be available.

If a fixture produces the right amount of light in the right spots, but has exceeded the LPD requirements for the space, selecting a more efficient fixture, with the same distribution pattern, may very well enable the design to provide the desired light with less energy.

Interestingly enough, selecting a fixture with a more advanced optical system can offer an additional benefit that will aid specifiers in meeting the ASHRAE 90.1 LPD requirements and, as previously discussed, can provide better, more controlled light distribution.

Spacing Criteria of Commercial Lighting Fixtures

The fixture spacing criteria (SC) rating, also referred to as the spacing-to-mounting height ratio, identifies the maximum fixture spacing that can exist, from one fixture to the next, while still providing reasonably even illumination at the work plane. This is a key metric that should be considered when selecting and laying out the fixtures in the space, because the SC rating can make a significant impact on the total energy footprint of the lighting system and can help a space meet the LPD requirements specified by ASHRAE 90.1.

To determine the ideal fixture spacing, simply multiply the SC rating by the distance between the fixture mounting height and the target working level. For example, a fixture with an SC rating of 1.4 is being considered for a space. The fixture will be mounted on a 9 foot high ceiling and the work plane is a desk that is 30 inches off of the ground (2.5 feet). The difference between where the fixture is mounted and the desk surface is 6.5 feet. Multiply the SC rating of 1.4 by 6.5 to determine that the fixtures can be spaced 9.1 feet apart, center to center, to maintain even working level illumination.

Fixtures with stronger optical systems can boost SC ratings from the fairly common 1.2 or 1.4 to an impressive 1.8. With a 1.8 SC ratio, the fixtures in the office scenario just described could be placed 11.7 feet apart, instead of 9.1 feet apart, dramatically reducing the number of fixtures that must be specified to evenly illuminate a space.

The SC rating of a fixture has implications on the project's ability to meet or exceed the ASHRAE 90.1 LPD requirements. Fixtures with larger SC ratings can be spaced further apart and illuminate more space with less wattage giving specifiers additional assistance for satisfying the ever-shrinking watts per square foot requirements.

Illuminating more space with fewer fixtures also saves money, because fewer fixtures will be required to provide optimum lighting levels, reducing material costs. Maintenance costs will also be reduced over the life of the building, because there are fewer fixtures to maintain.

The definition of good lighting may not have changed over the years. It still means that the lighting levels are appropriate to support the visual tasks that will be performed, that the visual environment is comfortable and evenly illuminated, and that the lighting is provided with an ever-shrinking energy footprint. In fact, it could be argued that the most significant change to good lighting over the last few years is that, with the development of advanced optical systems, it is becoming increasingly more possible for a specifier to provide it.

Manufacturing in Los Angeles since 1955, Prudential Lighting continues to create a wide range of fixtures, from elegant pendants to innovative LED luminaires, while collaborating with designers in realizing their visions for architectural lighting environments. Light creates life. Create with Prudential Ltg. www.prulite.com