Chapter Corner

Six Questions to Guide Tunable White Decisions

Posted in: Emerging Technologies, May/June 2017

Constant changes to LED technology can make keeping up with LED developments seem like a full-time job. Color tuning, and specifically tunable white, is one of today’s most intriguing control opportunities, but where do you start to ensure you’re making the best fixture and control decisions? Historically, fixtures have been purchased with a specific color temperature; however, emerging studies are confirming the human centric benefits of being able to change, or tune, a fixture’s correlated color temperature (CCT). Tunable white applications use more sophisticated LED drivers and light engines to control both the color temperature and the intensity of a fixture. The right solution for any project begins with asking the right questions. 
 
We have identified six key questions, along with a short explanation about why each question is so important
to achieve a successful tunable white installation. Explore these issues early in the project process to help guide you to the right decision, starting with the type of fixture you’re using. 
 
1. What are the control inputs to the fixture?
There are two types of fixture control: Warm & Cool control, and Intensity & CCT control. In all cases, we recommend the latter – tunable white fixtures that have one control input for intensity and one control input for CCT.
 
Fixtures with Intensity and CCT inputs are easy to integrate into a smart lighting system, while fixtures with Warm and Cool control are much more difficult to integrate. If the system’s sequence of operations requires specific intensities and CCTs, as well as automated dimming, daylighting, or user control, it is vital to know if the fixture can be integrated into the design. For applications where advanced control methods are required, only fixtures with intensity and CCT control will deliver the desired results.
 
2. What are the maximum and minimum CCTs for all fixtures in the space?
It is important to know if the fixtures offer the necessary CCT range for the application. For example, in a given space there may be multiple fixture types with different CCT ranges. If the defined sequence of operations includes a situation where tunable white control is desired simultaneously in all fixtures, the system performance will be constrained to operate within the overlapping range of ALL the fixtures in a space. In other words, if one set of fixtures has a CCT range of 3000-5000K, and another set of fixtures has a range of 2000-4000K, the functional tunable white range for the combined fixtures in that space is 3000-4000K. For proper design, it’s critical to understand these limitations up front.
 
3. Some fixtures may produce many more lumens at the high CCT than at the low CCT. What are the achievable lumen outputs at the highest CCT and the lowest CCTs?
As with the functional CCT range, the maximum light output across the range of CCTs determines how much light can be delivered in the application. If the design needs to meet a certain illumination level at a certain CCT, it is critical to know if the fixture, or combination of fixtures, can achieve that illumination level.
 
4. What are the x,y chromaticity coordinates for the maximum and minimum CCTs for each fixture in the space?
Different people in a space may experience different visual perceptions of the source color in multiple fixtures, despite the fact that the CCT is the same. Space occupants may become frustrated because they are unable to make the variety of fixtures match one another. This is a problem with fixture selection, not the control system’s ability to control the fixtures. By selecting fixtures with the same x,y chromaticity coordinates, you will be able to achieve consistency over a variety of fixtures.
 
5. Is the fixture a 2-color white, or a multi-color LED fixture (for each fixture)? 
Mixing 2-colors of white tunes across a straight line within the color space represented by a chromaticity diagram for the color mixing. Multi-color may approximate blackbody color mixing. The simpler, 2-color fixture, which tunes across a straight line, is unable to match the tuning capability of the multi-color fixture. This means the source colors may not always match. If the facilities team expects the light will follow the blackbody curve, they may be disappointed when they are unable to achieve such control. This again, is a problem with fixture selection, not the capability of the control system. Multi-color fixtures will provide greater flexibility.
 
6. Do the fixtures need to respond to a variety of automated features in the control?
This goes back to question number 1 – Warm and Cool control versus intensity and CCT control. Intensity & CCT control will allow for a better user experience and more control flexibility than a fixture that allows for control over a Warm and Cool LED engine.
 
The ability to independently control the intensity separate from the color is virtually assumed in a smart lighting system. But this is not the case when tunable white fixtures are specified. Standard controls that occupants have been accustomed to interacting within a space will not work with warm/cool fixtures. What does this mean? Space occupants will not have access to raise or lower their lighting level, daylight dim, or even activate demand response in their lighting system.
 
The fixture manufacturer does not think about the method of control, but codes and occupants assume that the system will be capable of these automated features. The needs of occupants are always changing, so even though they do not currently require the additional features (daylight dimming, timeclock control, demand response) the system should have the flexibility to adapt to their future needs.
 
Ask these questions early in the design process to limit surprises with system capability, and ensure clients
they have a lighting and control solution that is flexible, responsive, and taking advantage of the latest available LED technologies. For more information about tunable white control, manufacturers’ tools, such as the Lutron LED Center of Excellence, can provide more detailed information and design help.
 
Craig Casey is a leader in applying LED’s disruptive technology to lighting for Lutron as a senior building science engineer. He works at the intersection of theory and applied science, quantifying the human and energy benefits of leading-edge lighting/daylighting control systems.
 
He is active in the Illuminating Engineering Society’s (IES) Energy Management Lighting Controls Subcommittee, the Daylighting Terms Task Group, the Education Review Council and its Daylight Metrics Subcommittee, as well as the Architectural Simulation Subcommittee of the International Building Performance Simulation Association.