Lighting designers have adopted a rule of thumb about color temperature: occupants prefer higher color temperatures in bright settings and lower color temperatures in dimmer settings. And that familiar warming of halogen as it dims down – it suits most of us just fine. Despite all the energy savings LED has to offer, some lighting designers and occupants have refused to give up that cozy shift in CCT. But now widely available in a variety of modi operandi, dim to warm LED technology brings that halogen-like dimming profile to LED. And today’s health headlines support it.
Though ongoing research has failed to confirm the public’s preference for bright and cool vs warm and dim, it makes intuitive sense. “When we want a darker, quieter light environment, we want it warm. It’s what we’re used to and what we’re programmed for,” according to Anne Militello, principal at Vortex Lighting. “We do want a warm shift at night. It not only feels natural, it’s better for us.”
Conventional LEDs normally maintain their color temperature all the way down the dimming curve, which can give a gray cast because it confounds our expectations. “When LEDs don’t shift in color, the light takes on a very strange coldness that people don’t really respond to,” said Dawn Hollingsworth, design director at Brian Orter Lighting Design. “Historically, look at the light sources we’ve used over time. The sun gets warmer as it gets close to the horizon during sunrise and sunset. We like that warmth. It’s that same color shift that takes us back to firelight or candlelight. It’s built into our DNA.”
Repeated research has shown that high light levels, in particular blue-shifted light, affect our circadian rhythms. Primarily through the suppression of melatonin, light affects our ability to fall asleep and our quality of sleep. Research on nightshift workers links late-night light exposure to obesity, diabetes, even cancer. Therefore, in healthcare and residential applications, it’s likely that transitioning to longer wavelengths in the evening, along with lower light levels, could be beneficial to sleep patterns and overall heath. (Research is also showing that blue-rich self-luminous displays like televisions and personal electronics should be avoided for several hours before bed.)
Conversely, brighter light and bluer light in the morning and the early part of the day – like a bright blue sky overhead – can help regulate normal circadian rhythms, making us sleepy at night but more alert and productive during the day.
The US Centers for Disease Control has declared insufficient sleep a public health epidemic, citing accidental injury (principally car crashes), hypertension, diabetes, depression, and obesity, as well as cancer. It may one day become commonplace to program patterns of both light level and color content in living and work spaces, particularly for older adults, young people and caregivers.
“When I design lighting for residences or hotel rooms, I feel it’s essential to use LEDs that allow for warm dimming,” said Militello. “At this time, the technology is cost prohibitive, which makes it a hard sell. But I won’t do a hospitality project without it, even if I have to sacrifice other parts of the design. If there’s anything I don’t like it’s a normal LED on a dimmer.”
Dim to warm modi operandi
Introduced in 2012, warm dim technology is available today in both replacement lamps and hardwired fixtures – mostly downlights and cove lights, but also sconces, pendants, etc. – plus a few outdoor fixtures. Generally, they start at about 3000K at 100% and then shift down to around 1800K at 1% dimming (though not all go to 1% output). The simplest versions use two white LED chips, one warm white and one cool white, that crossfade down the dimming curve.
Here is where the high price tag becomes apparent: multiple LEDs and complex electronics on a package that uses, pretty much, only half its luminous capacity at any time. And this is the simplest MO. Other solutions add an additional white or a red, amber, or blue string to fill in holes in the spectrum and maintain higher CRI.
Even more complex systems will use perhaps five different colors: red, green, blue plus white and amber. These are programmed to replicate the halogen dimming curve with consistently high CRI, or can be individually addressed to select, theoretically, any color of the rainbow.
Variable color LED lighting packages will employ a simple open loop control system or a closed loop system, which will provide feedback to the driver. Closed loop systems monitor current, junction temperature, ambient temperature, light output, or light color; or some combination thereof. This allows the driver to compensate and maintain highly accurate performance over life, in variable conditions. But this adds entirely new layers of cost, complexity and failure points.
Dim to warm LED fixtures and replacement lamps will often work on 0-10V dimmers but may be compatible with other dimming strategies: DALI, DMX, even legacy systems such as TRIAC and other phase-cut dimmers. Flicker and other problems have been common with dimmable LEDs since they were first introduced, and dim to warm is no exception.
“The manufacturers supply a list of the dimmers they’re compatible with, but what does that mean? How does it respond?” questioned Hollingsworth. “It may dim up and down but does it flicker? There’s no standard in the industry.” Some variable CCT protocols reject the dim to warm convention entirely: dual controls adjust color temperature independent of light level.
When combining such complex electronics, experts recommend testing a series of fixtures to evaluate fixture-to-fixture variations down the dimming curve. But this is not always easy. “Let’s say I test the fixture, and I was able to get the right dimmer. Who’s to say it’s the same fixtures with the same diodes and capacitors I’m going to get in a year?” Hollingsworth said.
Down the road, the next generation of the dim to warm technology may eliminate much of the complex electronics. Researchers from the Netherlands are testing LEDs with a thermoresponsive coating that automatically activates warmer phosphors at lower power.
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