Luminaires

A completely packaged light fixture including lamps, holders, internal controls, reflectors, housings, ballasts and/or drivers is referred to as a luminaire. In essence, it is a manufactured electrical device to produce, control, and distribute light. The design and physical attributes of the luminaire are the final influencing factor on the total light that is emitted. The efficacy of light sources, the ballast factor, and the efficiency of the luminaire design all combine to determine the luminaire efficacy rating (LER) for fluorescent fixtures or the ultimate lm/W (lumens per watt) for LED lighting fixtures. LER is different from, and advantageous to, traditional lamp efficacy measurements because it takes into account all components of the luminaire system: luminaire efficiency, ballast factor, and input wattage.

The National Electrical Manufacturers Association (NEMA) has taken on the task of defining the tests and standards for determining the LER for fluorescent lighting fixtures in NEMA Standards Publication LE5-2001: Procedure for Determining Luminaire Efficacy Ratings for Fluorescent Luminaires. In this publication, they identify categories of lamps, ballasts, and fixtures that can be combined to create different luminaires. The procedures for testing and reporting are detailed so that equal comparisons can be made between different luminaires. They point out that LER values are published based on manufacturers' literature and are based on a specific lamp used in conjunction with a specific ballast in a particular luminaire. Since there are many variables between ballast and lamp performance, use of the LER data to compare different luminaire products requires attention to any differences in ballasts and lamps used in the tests. In this way, the focus of the comparison can be on the differences in fixture design and the optical properties that enhance or restrict light from leaving the fixture. The goal is obviously to get the most light out of the fixture for the least amount of watts. Hence a higher rather than a lower LER value is desired for fluorescent luminaires since it reflects the total lumens output compared to the watts input.

LED luminaires for commercial space applications are currently available that compare very favorably to fluorescent luminaires since they can provide equivalent light output using about half of the energy. The U.S. Department of Energy's Solid State 2011 index lists LED luminaires performing in the impressive range of 40-50 lm/W. The tests for this determination are more straightforward since there are fewer components to address. Hence, it is based on a straight lumens output compared to the total watts input for the lamps and driver.

Lighting Power Density

Energy codes, standards, and green building rating systems have taken all of this lighting efficiency information and applied it to buildings on a square footage basis. The common term of lighting power density (LPD) is a measure of the number of watts required per square foot of lighted area. Depending on the type of use in a building, the codes set a maximum number of watts that are allowed per square foot to be compliant. So, for example, low light areas such as parking garages and warehouses are allowed 0.3 and 0.8 watts per square foot respectively. Spaces that need higher light levels such as theaters and libraries can go to 1.6 and 1.3 watts per square foot respectively. In between is where many other commercial spaces fall such as offices, exercise centers, hotels, etc. which are allowed 1.0 watts per square foot. So for a typical office building, a 1,000-square-foot office area is allowed to consume 1,000 total watts for lighting.

Showing compliance with the LPD includes accounting for the energy used in the entire luminaire. Hence the LER or lm/W need to be shown based on published data for the lighting fixtures being used or specified. That means the ballast factor and luminaire design is accounted for in the LPD. So a 2 x 4 lay-in fluorescent luminaire with two lamps and a ballast may require 72 watts to power two 34-watt lamps and an electronic ballast. That will limit the total number of fixtures in a 1,000-square-foot office area to about 13 luminaires. Since LED lighting fixtures have inherently lower energy consumption as compared to an installed fluorescent lighting fixture, a comparable 2 x 4 lay-in LED luminaire could require as little as 44 watts to operate in total. That means as many as 22 LED luminaires are allowable in the same 1,000-square-foot office area. If that many aren't required, then the energy reduction increases by using fewer fixtures, reducing the actual LPD and saving the owner money on electrical lighting costs.

Light color can be a rather subjective thing to discern between different people, but standardized measurements and tests help determine generally accepted light quality characteristics. Image courtesy of RAB Lighting, Inc. Color rendering index (CRI): The CRI is a number between 1 and 100 used to describe the ability of a lamp to accurately render within the lighted space all the colors in the visible spectrum. For example, a CRI of 80 or above normally indicates that the lamp or luminaire has good color properties such that it would not significantly distort or diminish the true color of an object being illuminated. A low CRI rating would tend to distort the color of illuminated objects, making them appear too yellow or blue for example. Color temperature: The color temperature of a light source is a determination of its color appearance. Color temperature is used to describe the overall color tone of a white light source such as warm in appearance or cool in appearance. Correlated color temperature (CCT): The CCT numerically describes the overall color appearance of a lamp measured in degrees K or Kelvins. Common warmer light sources, similar to incandescent color, have a Kelvin temperature in the range of 2,700K to 3,000K. Somewhat cooler or neutral light sources commonly used in offices can range between 3,500K and 4,100K. Very cool color temperatures, often used to match daylight, are between 5,000K and 6,500K.

 

Light Quality Considerations

Energy savings are great particularly when determining the quantity of light needed, but what about the quality of the light desired for a building? Good lighting design begins with the development of a thoughtful statement of design intent for the various spaces within a commercial building project. This design intent obviously needs to be coordinated with everyone on the project team such as architects, interior designers, lighting designers, engineers, etc.

Establishing light levels and lighting quality to match the design intent should be based on professional consensus standards such as ANSI standards and recommendations published by the Illuminating Engineering Society of North America (IES). Most of these standards have been developed in the context of meeting or exceeding current energy code requirements such as the International Energy Conservation Code (IECC) or ASHRAE/IES 90.1. These standards are based on the premise that more light is not necessarily better light; rather it is all about the characteristics of that light. Is the light focused or dispersed? Is it used to create contrasts or uniformity? Are there a mix of lighting needs within a space such as ambient lighting and task lighting that need to be addressed? What effects related to the color of the light are desired?

Color in particular is an important and controllable aspect of electrical lighting. Although perceptions of color can vary between different people, the IES describes several key metrics that are used to define color traits when applied to lighting. These include a color rendering index (CRI), color temperature, and the correlated color temperature.

Once the fundamental questions of preferred color and other light qualities are answered it is then a matter of determining the best lighting strategies to help achieve the stated design intent. Some strategies are related to the building such as using light colored finishes which need less light than dark colored finishes. Similarly, open spaces, interior glass, and low partitions allow for artificial light to flow from one area to another and could reduce the number of luminaires required. Other strategies have to do with the use of daylighting in the building and how to coordinate electric lights and controls with daylighting zones in a building.

One important consideration in all lighting designs is glare. Light sources that are too focused or too numerous will create the condition of too much light (excessive luminance) causing most people to have a negative response from visual discomfort to the point where they will squint or look away. It also detracts from our ability to see detail accurately to the point that some people will try to thwart the excess light by covering or shading it thus causing potentially unintentional consequences, not the least of which is wasted energy. The other aspect of glare is too much contrast. If our eyes are adjusted to a certain light level and a light source is introduced of a notably higher light level, then we have the same negative reaction. Hence, the preferred situation in most commercial building settings is to achieve a uniformity of lighting across a particular space. However, using a combination of lower level ambient lighting with a higher level of localized task lighting has been done quite effectively without producing glare. This can be a particularly helpful strategy for spaces that have a lot of computer screens since it can eliminate reflections on the screen which make them difficult to read.