The Meaning Of Lighting

Mark S. Rea, Ph.D., professor and director of the Lighting Research Center at the Rensselaer Polytechnic Institute, believes the lighting industry has a problem. Solving it first requires an admission that it exists.

To a large extent, Rea believes, the lighting industry has traditionally promoted itself as a category with an emphasis on illuminance—light level, measured in lumens and foot-candles—as the product. This puts the focus on light, viewed as a commodity prone to value engineering toward lowest cost, and not enough on lighting, which is light applied to a space in a way that adds real value.

At stake is the question of whether lighting can be bought and sold based on a perception of its true value. If the lighting community can change the way it engages the end-user, the end-user will, in turn, perceive a higher value in the profession and what it provides.

“Clients care deeply about different design dimensions, but they never care about illuminance levels,” Rea said. “Yet, illuminance level is functionally our only measurable quality metric in an application, and we apply it irrespective of the design dimensions important to the client.”

What Rea said isn’t entirely new. For many years, the Holy Grail has been a lighting-quality metric that would help distinguish lighting systems built based on quantity (lumens and foot-candles) and those designed to support a broad range of needs.

The reason this approach has stagnated is that—while the evolution of best practice has nudged design toward quality—achieving quality is highly dependent on each application. We can define lighting quality, but a high-quality solution for one application might not be right for another. This is a common refrain in the lighting industry: “It depends on the application.”

Another approach was the Light Right survey, an online tool that enabled facilities’ operators to survey space users about their lighting perceptions and needs. Lighting quality, as applied to each space, would reveal itself through users articulating their needs and wants. While this approach showed promise, the industry didn’t entirely get behind it.

Rea’s solution offers even greater promise. He proposes the industry adopt a new systemized approach where the client would define quality, which would include a prioritization of project needs. The lighting practitioner would then engage the client in a conversation about how to satisfy these needs using more precise metrics.

“To create high-quality lighting, it is first necessary to identify the design dimensions that the client cares about,” Rea said. “Safety, security, productivity and health can be more or less important, depending on the client. The combination of dimensions that meet the client’s needs constitutes high-quality lighting for that application.”

The next step is to deliver the resulting solution in a way in which its quality is metrically quantifiable. This creates the potential for a client conversation in which evidence-based decision-making leads to the right lighting solution, which is more deeply valued.

The human eye responds to light at different levels, distinguishing photopic vision (eye light receptors active under high light conditions, allowing color perception), scotopic vision (eye light receptors under very low-light conditions), and mesopic vision (covering applications with both conditions). Rea proposes redefining the lumen based on which type of vision is predominant for the application as a way to determine true light-source efficacy.

Furthermore, eye sensitivity to various wavelengths of light relates to how bright a scene appears, resulting in another metric. For example, the spectral emission of sources, such as metal halide and white light-emitting diodes (LEDs), engages the eye in such a way that a scene appears brighter than under high-pressure sodium. People tend to feel safer with greater brightness, so metal halide or LEDs can be used in outdoor applications to achieve similar lighting results using significantly less energy.

The overall energy efficiency that results from the precise placement of light through optical control is another metric. By placing light only where it’s needed (and when it’s needed) using good optical control, the resulting application efficacy can generate significant energy savings.

Rea further calls for the adoption of metrics for circadian lighting—incorporating brightness, color and duration of exposure to light—to support application goals related to health. He also calls for adoption of new supplementary metrics, such as the gamut area index to more accurately evaluate the color rendering properties of light sources—notably LEDs, for which color rendering index by itself often falls short.

“Maximizing value to clients and society is a lot more than meeting a recommended illuminance level at the lowest price,” he said. “Knowing how to maximize benefits through a palette of light spectrum, amount, distribution, duration and timing requires metrics and professional skills to apply those metrics. Like becoming a good physician, good lighting specifiers must learn those skills.”

To learn more, read Rea’s book, “Value Metrics for Better Lighting” (SPIE Press), and visit the Lighting Research Center website at

About the Author

Craig DiLouie

Lighting Columnist
Craig DiLouie, L.C., is a journalist and educator specializing in the lighting industry. Learn more at and .​

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