When comparing the performance of LED downlights, lumen output alone doesn’t tell the whole story. On a recent hospitality project, the contractor wanted to substitute another light fixture that met the minimum lumen output requirement. It's a familiar story, but an incomplete one. In this case, however, the lighting designer showed that the would-be substitution did not, in this application, provide the uniformity of the original, specified fixture. Demonstrating this to the electrical engineer and architect confirmed that the original fixture should stay on the job.
Fixture manufacturers are continuing to improve their products by upgrading luminaire light engines resulting in higher lumen outputs. This is especially true with manufacturers of LED downlights. These improvements allow fixture manufacturers to offer smaller aperture downlights with lumen outputs that previously were only available in larger apertures. Most of the major manufacturers of downlights now offer a 6-inch downlight with 3000 lumen output or higher.
A common misconception among many lighting specifiers is that an equal lumen output from a substitute manufacturer of equal quality is an acceptable alternative. This is not always the case.
In the hospitality application, Luminaire A was specified as a 6-inch downlight utilizing a 2500 lumen light engine with delivered lumens of 2264. The application was a 20' x 40' room with a 9ft ceiling height and a 50 footcandle design criteria. However, Fixture B was submitted as a substitute. It utilizes a 3000-lumen light engine with delivered lumens of 2564. Comparing only lumen outputs, this might appear to be an acceptable substitution, but a closer look at the comparative performance tells a different story.
The Zonal Lumen Summaries of the two photometric reports shown in Figure 1, to the right, highlight the 0-20-degree zone of each luminaire. Luminaire B distributes 48.7% of the total lumens in to the 0-20-degree zone. For higher ceiling heights, this would be desirable. However, for this application with a 9-foot ceiling, it was not an acceptable substitute.
The calculation summary in Figure 2, to the right, reveals a maximum footcandle level of 81.4 FC for Luminaire B on the work plane compared to 60.6 FC for Luminaire A. This is due to the higher concentration of lumens in the 0-20-degree zone of Luminaire B. The Pseudo Color Rendering below also illustrates the better overall uniformity Luminaire A provides. The Average-to-Minimum Ratio and Maximum-to-Minimum Ratio also confirms that Luminaire A is the better solution.
In summary, both fixtures meet the design criteria of 50 footcandles maintained on the work plane. However, horizontal footcandle levels should never be the only measure of performance. Uniformity should always be included in the performance specification and photometric data should be closely examined. The Zonal Lumen Summary gave clues that further comparison was needed but the Calculation Summary and Pseudo Color Rendering revealed the facts.
Danny Streit, IALD, IES, LC is Design Principal of DHS Lighting Design in Nashville, TN, and has over 20 years’ experience in architectural lighting design and lighting control systems. He has successfully completed over 1300 designs in his career and has a history of providing innovative, functional, and sustainable lighting solutions for any design application. firstname.lastname@example.org
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