A Guide for Specifying Commercial Lighting

Selecting effective and efficient fixtures for productive workspaces
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Sponsored by Prudential Lighting
Jeanette Fitzgerald Pitts

Photometric reports summarize and graphically illustrate how a fixture distributes light with a candela chart and in a polar graph called a candela distribution curve. Looking at these reports, a specifier can quickly compare the distribution patterns of multiple fixtures and determine which fixture seems best equipped to meet the illumination needs of the space. Designers can also use these reports to deduce whether or not a fixture has the potential to produce glare or unwanted striations.

The Candela Chart

The candela chart is an at-a-glance summary of the light intensities (measured in candelas) distributed by a fixture into specific areas of the surrounding space. The candela chart organizes the candela values in terms of the horizontal and vertical coordinates of their location around the fixture. The horizontal coordinate refers to a unique viewing position, called a horizontal viewing angle, which exists along an imaginary circle around the horizontal axis of the fixture. The vertical coordinate refers to various heights, or vertical angles, along a vertical plane located at each unique horizontal viewing position.

This candela chart displays the light intensities of complete vertical planes as measured from five distinct horizontal viewing locations.

Imagine a hanging, 2-by-4 linear direct/indirect troffer fixture. Visualize the horizontal axis of the fixture, running through the fixture end-to-end. Now draw an imaginary circle around the horizontal axis of the fixture, so that one end of the fixture rests at 0 degrees and the other end rests at 180 degrees. Now imagine a person walking around the fixture on the imaginary circle. Every few steps, the person stops to admire the fixture. The location where they stop to view the fixture is an example of a horizontal viewing angle. A candela chart will typically contain between three and five unique horizontal viewing positions on that circle, denoted as angle measurements. Five horizontal viewing angles commonly seen in a candela chart are 0, 22.5, 45, 67.5, and 90 degrees.

Batwing pattern and spherical pattern as observed at a 90° horizontal viewing angle

Image courtesy of Prudential Lighting

Now, imagine a ladder at each of the five horizontal viewing angles on the circle, so there are five ladders on the imaginary circle. Each ladder represents a vertical axis and it is positioned so that the fixture is in the center of the ladder, with rungs extending in equal distances above and below. The bottom of the ladder denotes 0 degrees on the vertical axis, the top of the ladder is 180 degrees on the vertical axis, and the center of the lighting fixture should be at 90 degrees. There can be as few rungs or as many rungs on the ladder as necessary, depending upon the level of detail contained in the candela chart. Some candela charts will step up the vertical axis in 5-degree increments, others in 2.5-degree increments, and so on. The vertical viewing angles referenced in the candela chart refer to the various rungs on the ladder placed at each unique viewing position. All of the measurements taken from one ladder, or vertical axis, represent the vertical plane at that specific horizontal viewing angle.

If the fixture emitted a perfectly symmetrical pattern of light in all directions, the entire lighting distribution pattern could be detailed from one vertical plane of measurements taken at one horizontal viewing position. However, most fixtures do not emit a symmetrical pattern of light in all directions, so more measurements need to be taken, which accounts for why most candela charts contain between three and five horizontal viewing angles and the measurements from their associated vertical planes.

The candela chart displays the horizontal viewing angles across the top as the column headings and the vertical viewing angles (in increments between 0 and 180 degrees) as the row headings. For example, look at the candela chart shown on the opposite page. The unique horizontal viewing angles (0, 22.5, 45, 67.5, 90) are charted across the top as the column headings. The viewing angles, staggered in 5-degree increments between 0 and 90 degrees, are the row headings. The various candela measurements taken along the vertical plane at each unique horizontal position are contained within the chart.

Commercial fixtures today commonly distribute light in either a spherical pattern or a batwing pattern. A spherical pattern of light resembles a ball stuck to the fixture and the batwing pattern of light resembles a bat, with wings spread, going headfirst into the fixture. It is by plotting the information captured on the candela chart into a candela distribution curve that the pattern of light emitted by the fixture is realized.

The candela distribution curve illustrates that this fixture distributes light in a batwing pattern.

Image courtesy of Prudential Lighting

The Candela Distribution Curve

The measurements from the candela chart are plotted onto a polar graph to create a candela distribution curve. The candela distribution curve graphically illustrates the lighting distribution pattern of the fixture. This circular (or semi-circular) graph places the light source at the center and maps the various light intensities (in candelas) in incremental steps up the vertical plane (0-180 degrees) for a single horizontal viewing angle. The plots for multiple horizontal viewing angles can be laid over one another onto one distribution curve to better illustrate changes in the direction and intensity of light output that a person would experience as they walked around the fixture from one horizontal viewing angle to another.

Look at the candela distribution curve shown at left. This one polar graph contains three vertical viewing planes taken from three horizontal viewing positions (0, 45, and 90 degrees). Notice that the graph illustrating the vertical viewing plane at 90 degrees has a distinct batwing shape.

A Few Quick-Read Characteristics

There are a few important characteristics about the lighting fixture and how the lighting fixture will perform in a space that are quickly revealed by referencing the candela distribution curve. It can be an easy way to compare the lighting distribution of multiple fixtures of interest. First, the distribution curve will illustrate if the fixture is direct (emits light only between 0 and 90 degrees on a vertical axis), indirect (emits light only between 90 and 180 degrees on a vertical axis), or direct/indirect which emits light both above and below the fixture (0 to 180 degrees on the vertical plane).

 

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Originally published in Architectural Record
Originally published in November 2012

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