Craig’s Lighting Articles, Lighting Design

Lighting 101: Some Light Reading

This article describes the photometric report and what electrical distributors can learn from it at a glance, part of a new series at tED Magazine (NAED) that aims to explore lighting fundamentals and attract new distribution professionals to the category. Originally published March 2023, reprinted with permission.

Light output is important when selecting an LED luminaire or replacement lamp. However, it does not tell the whole story of how the luminaire will perform in a space and what impact it will have on users. What we need to know is how the light is distributed.

This information is in the photometric report commonly available for specification-grade lighting products and typically found on the catalog sheet. Electrical distributors can use this information in projects where they have a role recommending lighting products.

“When comparing proposed ‘equals,’ distributors should check more than just whether lumens and watts are equal—that only tells part of the story,” said Kristen E. Mallardi, LC, MIES, Industry IIDA, Sr. Specification Sales Manager, C&I Sales for Acuity Brands Lighting. “The photometric charts on manufacturer specification sheets provide a visual of how their fixture distributes light, and this visually shows if the fixtures in question would perform equally. The fixtures shown in Figure 1 deliver the same amount of light, but they do it in vastly different ways.”

Figure 1A
IES LM79-08
Lumens: 7990.3
Wattage: 56.61
Efficacy: 141.15
Image courtesy of Acuity Brands.

Figure 1B
IES LM79-08
Lumens: 7990.3
Wattage: 56.61
Efficacy: 141.15
Image courtesy of Acuity Brands.

A luminaire’s photometric report should include light output, light distribution, and color and electrical characteristics. LED products are tested differently than traditional lighting. Rather than separately test lamps and luminaires using relative photometry, LED products are tested as integrated devices using absolute photometry. These items are covered in the Illuminating Engineering Society’s LM79 method. Manufacturer test reports should indicate that LM79 was used and identify the testing lab.

Regarding light output, a luminaire’s LM79 photometric report may include either total output in lumens or a zonal lumen summary table. This table reveals light output either measured in specific zones and then summarizes for all light emitted above and below the luminaire as well as the total. The catalog sheet may also include resulting spacing criteria for placing luminaires so as to maintain a uniform light level in a space. The distance between luminaires is simply calculated by multiplying the spacing criteria by the mounting height, or the distance between the workplane and the bottom of the luminaire.

Additional key items in the report are the luminous intensity table and polar luminous intensity graph, which together provide both numerical and visual representations of the luminaire’s light distribution.

The luminous intensity graph provides a lot of information at a glance. The center indicates the lamp position from which one or two shapes may extend to indicate the light pattern. The farther the shapes extend to the edge of the diagram, the higher the intensity of light in candelas. The light pattern with a solid line is distribution from a frontal view of the luminaire (0-180 degrees), while the dotted line shows the side view (90-270 degrees).

If the frontal and side distributions are symmetrical, the result would be a single curve. If we were to be viewing these shapes three-dimensionally, we would see an irregularly shaped bubble of light. Changing anything—light source, optics, etc.—would likely change the bubble’s shape.

Using the polar luminous intensity graph, important information is available at a glance:

Direct or indirect light distribution, whether the luminaire is direct (light emitted below the horizontal axis), indirect (above the axis), or direct/indirect (a mix of the two and to what degree). In Figure 1, we see both luminaires produce both up and down light.

Symmetrical or asymmetrical light distribution, whether it is symmetrical (light output is emitted in a roughly equal pattern on both sides of the luminaire) or, as is common with cove lights and similar luminaires, asymmetrical (light output is restricted to one side or the other). If the luminaire has symmetrical distribution, only half of the curve may be shown. In Figure 1, we can see that the luminaire on the top has a symmetrical distribution, while the luminaire on the bottom is asymmetrical.

Spot or flood distribution, whether the luminaire has a spot (narrow), narrow or medium flood (fuller pattern with a flatter bottom), or a wide flood (wide pattern and possibly a “batwing” shape where peak distribution is on each side of the center instead of direct above or below the luminaire) pattern. In Figure 1, the luminaire on the bottom has a batwing indirect distribution, producing wide flood uplight on the ceiling and a more focused pattern down onto the workplane. In spaces where uniformity is desired, batwing distributions are often used as they direct a high intensity of light from the optical axis, maximizing spacing between luminaires.

Likelihood of producing direct glare, based on what directions and angles light is distributed and at what relative intensities. This ensures that a luminaire being considered is not only the most efficient available but also delivers light with sufficient visual comfort for users.

“Visual comfort is all about reducing high-angle brightness, ensuring application uniformity, and managing contrast,” said Mallardi. “The glare zone is the range between 60 and 90 degrees from horizontal. Too much light in that glare zone causes discomfort glare. The photometric file will detail exactly how much light is being delivered in that glare zone.”

Accompanying the luminous intensity graph is the luminous intensity table, tabular data that enables a more in-depth and detailed analysis of light distribution and its impact on light levels and potential glare conditions using lighting design software. For this purpose, many manufacturers make the data available as downloadable standardized electronic files (“IES files”) on their websites.

While the luminous intensity graph provides a lot of information, it may take an experienced eye to consider two products as truly equal in lighting performance or a single product as being ideal for the job. For this level of evaluation, the luminous intensity table can be useful as it is more precise.

“With LEDs in particular, there can be great differences between optical performance fixture to fixture,” Mallardi noted. “Comparing optical performance using the polar candela distribution chart from the photometric file tells the most accurate story of fixture performance. Not all indirect bat wing distributions are equal. Batwings may range from 120 degrees to a full 180 degrees and that could make a huge difference in uniformity and spacing.”

Besides light output and distribution, two additional items are covered in an LM79 photometric test report. The first is electrical characteristics: input voltage, input current, and input power, which is used to calculate luminaire efficacy in lumens/W. The second is color characteristics: chromaticity coordinates, correlated color temperature (CCT), and color rendering index (CRI) and/or color fidelity and saturation metrics covered in IES TM30.

Overall, the photometric test report provides critical information for electrical distributors that sell lighting. Understanding and interpreting the report is foundational for personnel to gain expertise in evaluating, recommending, and comparing LED products.


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