Below is my contribution to the November issue of tED Magazine. Reprinted with permission.
LEDs have further differentiated themselves from traditional light sources by offering dramatically expanded color capabilities. These capabilities enable distributors to better serve existing customers and build new markets. Accomplishing this requires understanding LED technology, metrics used to evaluate color, and knowing what the customer wants and needs.
The LED advantage
Visible light is energy residing along the 400-700 nanometer band of the electromagnetic spectrum. The size of these wavelengths corresponds to specific colors from violet to red. Combining these wavelengths produces white light. Separating them via a prism produces a rainbow.
The eye perceives color in an object because that color is present in both the object and the light striking it. The object absorbs all colors except a given color, which reflects to the eye. While daylight offers a full spectrum source, electric light sources are engineered as mixes of wavelengths at relative intensities, typically focused on red, green, and blue (RGB). The spectral makeup is expressed in the source’s spectral power distribution (SPD).
As with traditional light sources, with LEDs we have a choice of specific colors or white light. In the case of color, this is accomplished with LEDs emitting light in a narrow spectral band. For white light, blue or ultraviolet LEDs coated with a phosphor producing a deep blue peak and high irradiance in the 470-630 nanometer range.
With LEDs, however, colors can be mixed to produce virtually any color needed, allowing dynamic effects. White light color appearance can be adjusted with relative ease using controls. Combining white and color LEDs allows virtually any SPD to be created, opening possibilities in targeting light to human physiology, plant growth, and environmental needs. And with advances in LED technology, building owners no longer have to choose between excellent color quality and high efficiency.
Tunable-white LED lighting, matched with appropriate controls, enable CCT adjustment across a given range to satisfy variable preferences for applications demanding color flexibility. Image courtesy of USAI Lighting.
Color appearance
Manufacturers describe the color quality of light sources using metrics based on standardized measurements. The most popular are correlated color temperature (CCT) and color rendering index (CRI).
Measured in kelvins, CCT is the color appearance of a light source relative to an ideal reference light source. Color appearance is generally classified as visually warm (about <3000K, or yellowish white), neutral (about 3500K, white), or cool (about >4000K, or bluish white). A light source heavily laden in blue and deficient in red wavelengths will saturate blues in the space while muting reds.
A challenge for LEDs is the manufacturing process inherently involves variations in CCT. The result is potential color variation between LED products. To address this issue, manufacturers test and bin their LEDs according to deviation from CCTs based on x, y coordinates on the CIE 1931 Chromaticity Diagram, using a standardized method. The smaller the bin, the tighter the control of color variation, though gaining this consistency may impose a higher cost. Some manufacturers maintain extremely tight deviation as a point of differentiation for their products.
“Color consistency from credible LED manufacturers has improved significantly since white LEDs were first produced,” said Andrew Kites, Global Product Manager, Philips Lighting. “Some manufacturers have gotten much more skilled at producing LEDs that are closer to the center of the ANSI bin for that CCT, reducing waste in manufacturing from out-of-spec product, reducing LED costs, all while improving color consistency.”
Advances in control and driver technology enable manufacturers to provide custom SPD (using RGB+ LEDs), luminaires to produce both high-quality white and color (White+), and designers and users to adjust CCT in the field (White/White+). This extraordinary potential is opening new markets. Additionally, dim-to-warm LED products are growing in popularity for applications where users expect their lighting to dim to a warm glow similar to incandescent.
“It’s always important to listen to the customer,” said Bonnie Littman, President and CEO, USAI Lighting. “The better we can understand their preferences for color, the better we can serve them and provide the right product. There’s no ‘one size fits all’ when it comes to lighting, and there’s no reason someone should be relegated to static white light if that’s not what they want or need.”
She pointed to several examples where coming up with a customer-specific color solution became a point of differentiation for her company. Outdoor lighting on the Gulf Coast that provided nighttime visibility without disrupting the nocturnal habits of sea turtles. Experimentation with different CCTs in classrooms. Optimal SPDs for high-end retail. As the industry’s understanding of light and health develops, this capability may prove integral to circadian lighting, as spectrum is a major factor in circadian response. And some manufacturers are already looking beyond health to well-being, mood, and satisfaction via personalized lighting solutions.
“Research is ongoing to determine the appropriate light levels, spectral content, and lighting design that provides support for human circadian biorhythms,” Kites said. “The research points to humans generally having a biological response to both blue and red wavelengths.”
“With all of the promising LED products on the market now to support circadian health, I see this time as an exciting moment for the lighting industry to have a meaningful impact on workplace and healthcare environments,” Littman noted. “By mimicking the daily color temperature cycle of natural daylight, these technologies we’re creating can help minimize disruptions to the natural circadian rhythm, thus supporting overall health, well-being, and healing.”
Paul Scheidt, Product Marketing Manager, LED Components, Cree, however, says he has not yet seen a product that demonstrates a comprehensive understanding of physiological response to lighting. “The industry is not here yet,” he said. “We have identified the right variables for circadian lighting—color and light amount. However, we do not know where or how you set these controls to create direct biological impact, such as mood and energy levels. No one has a ‘mood’ knob on their light. Today’s controls are color and light amount. As an industry, we are still at the beginning of understanding the notion of mood and human preference for lighting.”
Color rendering and TM-30
While CCT is useful, it does not indicate whether the light source renders colors how most people would expect them to appear. Two sources with the same CCT may render various colors differently due to differing SPDs. A balanced SPD, particularly RGB, generally means the source offers good color rendering. A simpler and more direct way to evaluate color rendering is the lamps CRI rating. If two sources have the same CCT, one can meaningfully compare CRI to choose the right source.
Manufacturers test their sources and calculate CRI based on how closely they render eight standard color samples compared to an ideal reference source with the same CCT. The CRI rating is the average of these values. The less deviation from the reference source, the higher the CRI. Traditionally, about 80+ CRI is considered “good” for typical commercial applications requiring social interaction, about 90+ for color-critical applications such as higher-end retail. While the standard has endured, it has not been updated in many years, and its limitations are more pronounced with LED technology. In particular, a source may have a high CRI while ineffectively rendering saturated reds commonly found in applications like retail, supermarkets, etc. For this reason, some manufacturers began publishing R9 values to indicate color rendering for saturated reds for sources serving these markets.
The core problem of CRI’s deficiencies remained, however, particularly in light of CRI being used in specifications such ENERGY STAR and the DesignLights Consortium, and in regulations such as California Title 20 and Title 24. In 2015, the Illuminating Engineering Society (IES) published TM-30, a method for evaluating color rendition that introduces two new metrics. First is the Fidelity Index (Rf). Based on 99 color samples instead of 8-14, it was designed as a more accurate alternative to CRI. Second is Gamut Index (Rg), which expresses average color saturation. To determine which colors are saturated or muted, graphics are provided. While more comprehensive and precise than CRI, adoption has been slow.
“Right now, the whole industry is still in the process of educating lighting designers,” said Scheidt. “For the most part, the lighting designers who have heard of TM-30 and understand it really like it and see the benefits of getting more information about the light ahead of time, without having to do trials.”
He added that TM-30 is useful for applications where color is important, such as museums, hospitals, car dealerships, retail, and some offices.
Selling with color
Traditionally, the key to selling with color is to know the customer and the application, understand best practices, and recommend lighting products that will satisfy the need for an appropriate cost. LED is no different, though it can accommodate a broader range of needs, thereby creating new markets. It provides a more powerful tool to explore and understand lighting’s impact on people than traditional sources ever could.
Scheidt said the first step is to do no harm. “It’s fairly simple,” he said. “If the color is bad, then people are not going to like the product and you will have more returns and unhappy customers.”
After that, he pointed out, listen to the customer to find out what they need. “Distributors do not always need to recommend the best color performance or the best color consistency into everything,” he added. “It’s about understanding which customers are going to care about color and which ones aren’t.”
“The only consideration you should need to make is the customer’s preference,” advised Littman.
To produce the right solutions, distributors further need to understand LED technology and the metrics used to evaluate products. “Customers new to LED lighting will look for recommendations, and distributors have the opportunity to help educate the market,” Kites said. “Spectral knowledge and color-tunable systems are new and exciting to the lighting industry, and will bring more challenges and opportunities to the market. The more we know and understand how these systems can positively impact our customers, the bigger the opportunity to bring value to our customers.”
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