Lighting Design

Focal Point’s Matthew Blakeley on Quantifying Lighting Quality (Part 1)

This is the first of a two-part article addressing advances in light sources and light quality, authored by Matthew Blakeley, Vice President – Product and Business Development for Focal Point as an exclusive contribution to LightNOW. This first article focuses on the evolution of light sources and measurement tools that can accurately describe the quality of the color rendition in a space. The second article highlights recent research, which provides a baseline definition of a quality light source that’s preferred by humans, independent of cultural background and familiarity with the habitual built environment.

Light-Emitting Diodes or LEDs are now the common light source installed in commercial spaces. They offer many advantages in terms of efficacy and energy savings (lumens/watt), long useful life, controllability, and integration with building automation systems. As inherently electronic devices, they are an enabler of the IoT, helping transform luminaires from simple light sources to data collection points that support smart building management.

But have LEDs improved the quality of light that all of us live, work, and play under in our daily lives? And how can that light quality be accurately measured?

Until the invention of the Edison blub in 1879, mankind had spent millennia under natural light sources including sunlight, fires, and candles. We became accustomed to that warm glow which the incandescent bulb also produces. One of the complaints often associated with fluorescent lamps and LEDs is their lack of warmth. This is due to the light spectrum distribution of those light sources, which contains more short-wavelength light, thus resulting in a greener and bluer tone. On the positive side, this directly correlates to their higher luminous efficacy which has made them preferred light sources in commercial buildings, starting with fluorescent tubes gaining popularity in the mid-20th century followed by LEDs in the early 2000s.

To achieve efficacy levels that compared to that of fluorescent tubes, hence making them commercially viable, the light spectrum of LEDs had to be tweaked. To maximize the amount of light produced more green content was incorporated, the spectrum that the eye and therefore luminous matching function is most sensitive to.

As a result, the perceived color of objects and natural elements lit by LEDs is different from that lit by incandescent light sources; skin tones don’t appear as healthy, colors are not as vibrant.
The most common way to measure light color quality is CRI – the Color Rendering Index which was developed in the 1960s. It is generally accepted that an 80 CRI LED produces average light, good enough for most commercial environments, while LEDs with a CRI of 90 or above have a higher color quality.

While CRI is an easy-to-understand rule of thumb, it also has limitations. It is a measure of fidelity only: it describes how a light source differs from a reference illuminant, a blackbody radiator, without describing the direction of the change in either chroma or hue. For example, consider a light source with a 20% oversaturation of yellow content and another with a 20% oversaturation of blue content. These two light sources with very different color saturations, assuming the other colors are the same, would both have a similar Color Rendering Index value.

In 2015, the IES launched TM-30-15, an updated technical memorandum that addresses improvements in color control with LEDs. Incremental improvements were made with the current version, TM-30-18.

The main takeaways are that TM-30 measures both shifts in color fidelity and in color gamut from a reference illuminant and provides a color vector graphic which helps visualize the changes in hue and saturation. It divides the color spectrum of the light source in 16 hue bins, clearly denoted on the graph, and for which various data points are reported on when using the IES TM-30-18 calculator.

Going back to the oversaturation in blue versus yellow example, TM-30 would provide a clear depiction of the differences between the light sources, as well as measurements relative to local chroma shift, local hue shift, and local color fidelity for each of the 16 hue bins.

Using TM-30 as a tool to describe the quality of a light source is more accurate than using CRI alone and can support the selection of light sources that will contribute to human comfort and well-being, attributes directly linked to productivity in commercial buildings.

author avatar
Craig DiLouie

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