The value of end-to-end interoperable systems for dynamic white lighting
Product Manager, eldoLED
There are many approaches to delivering tunable white, but the ever-improving DALI-2 standard is making the most significant advances to simplify delivery of tunable white luminaires and installing successful tunable white solutions at scale.
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DALI (Digital Addressable Lighting Interface) is a bidirectional communication protocol over a 2-wire bus for digital control of lighting systems. It was developed to facilitate the development, installation, and operation of scalable lighting networks.
A DALI network typically consists of a DALI bus power supply, control gear (e.g. fluorescent ballast, LED driver), and control devices (e.g. gateway, occupancy sensor, push button). Every DALI device that is connected to a DALI network is assigned an address, allowing each device to be individually queried, configured and/or controlled. DALI devices can also be assigned to one or more groups, allowing multiple devices to be operated and/or controlled in unison.
The original DALI standard, often referred to as DALI version-1, was developed to support fluorescent lighting in the 1990's but the standard has evolved over time to support additional control gear. Through standardization and compliance testing of control gear, DALI version-1 yielded a reasonable level of interoperability between DALI devices from different manufacturers. However, full interoperability for DALI version-1 devices was never guaranteed because compliance was based on self-certification and/or declaration by individual DALI device manufacturers. Moreover, control devices were not part of DALI version 1.
The DALI-2 standard was developed to:
- enhance the functionality of DALI networks relative to DALI version 1
- improve the interoperability between DALI control gear and control devices from different manufacturers through more tightly specified requirements and more rigorous compliance testing of both control gear and control devices.
Only DALI devices that have been independently validated and certified to meet the DALI-2 standard can carry the DALI-2 logo.
The DALI-2 certification program is developed and led by the lighting industry but is monitored and maintained by the Digital Illumination Interface Alliance (DiiA), a global consortium of lighting companies. Only DiiA member companies can obtain DALI-2 certification for their DALI devices and solutions.
For more information, see the DALI Quick Start Guide on the DiiA website.
DALI Control Gear: Device Type 6 vs. Device Type 8
DALI defines various types of control gear, i.e. devices that power luminaires and control the light output. The types of control gear that are relevant for LED lighting are Device Type 6 (LED control gear) and Device Type 8 (Color control gear), which are defined in Parts 207 and 209, respectively, of IEC 62836.
During commissioning of a DALI lighting network, each item of control gear on the network is assigned a unique short address. This enables individual addressing of control gear on the DALI network as well as assigning each item of control gear to one or more groups. A single DALI network can support at most 64 short addresses for control gear.
By design, Device Type 6 control gear can only accept a single intensity setpoint per short address while Device Type 8 control gear can accept intensity and color setpoints per short address. Consequently, a dual-channel DALI DT6 LED driver requires only one short address if both LED outputs are controlled in unison by a single incoming intensity setpoint. However, if independent control of each LED output is desired, a dual-channel DALI DT6 LED driver requires two unique short addresses. In contrast, the intensity and color-point of a multi-channel DALI DT8 LED driver can be controlled while still only consuming a single short address on the DALI network. Consequently, a DALI DT8 driver can reduce luminaire complexity and may also yield lower system cost given the limit of 64 short addresses for control gear in a single DALI network.
Figure 1: DALI Device Type 8 offers 3 different control types, each requiring only a single short address on the DALI network.
DALI Device Type 8 Control Types
DALI Device Type 8 supports 3 different control types, see also Figure 1:
- Color Temperature (Tc) - This control type is commonly adopted for Tunable White lighting applications. It offers both intensity control, via standard DALI dimming commands, and color control with CCT setpoints defined in Kelvin. Successful operation in a LED luminaire requires that the control gear can translate the target intensity and CCT setpoints into appropriate drive currents for each LED output of the LED driver.
- XY-coordinate - This control type offers both intensity control, via standard DALI dimming commands, and color control with color setpoints defined in (x,y) coordinates in the CIE 1931 xy chromaticity diagram, see Figure 2. Successful operation in a LED luminaire requires that the control gear can translate the target intensity setpoints and color coordinates into appropriate drive currents for each LED output of the LED driver.
- RGBWAF - This control type offers individual control of the color channels Red, Green, Blue, White, Amber, and Free. Standard dimming commands are ignored; instead, dimming commands are defined by color channel. Note that the sequence of color channels is fixed. This control type has an optional link between color channels and standard DALI dimming commands.
Figure 2: The CIE 1931 xy chromaticity space, including the chromaticity of black-body light sources of various correlated color temperatures. Source https://en.wikipedia.org/wiki/Chromaticity.
A few remarks are in place:
- The first two control types, Color temperature and (x,y) coordinates, offer absolute target setpoints, which is beneficial if color consistency is required between luminaires from multiple manufacturers.
- A DALI DT8 LED driver consumes only a single DALI short address, regardless which control type is selected.
- The control type "Color temperature" is currently the only one for which DiiA has developed test sequence to certify compliance with the DALI-2 standard.
Figure 3: Dynamic white lighting explained by the example of a water faucet.
Dynamic White Lighting
A water faucet is a good analogy for what one tries to accomplish with dynamic white lighting based on light sources with two distinct color temperatures. With a faucet one can control the absolute flow of warm and cold water with the goal to achieve a comfortable overall water flow rate and temperature to suit the user's needs. For example, in summer, a user may prefer a relative cool shower while that same user in winter may prefer a warmer shower. Despite inherent differences in water heaters, plumbing, faucet designs, and environmental conditions, all users manage to adjust the various faucets in their environments to get the right amount of water at the right temperature. Dynamic white lighting is, in principle, not any different. Light with two different correlated color temperatures is mixed to yield the desired light output at the desired color temperature. For example, at night a user may prefer a low light level with a warmer color temperature while during the day that same user may prefer a higher light level with a cooler color temperature to mimic natural daylight, see also Figure 3.
Figure 4: To facilitate adoption of dynamic white lighting applications, we should aim for an intuitive relationship between control inputs and perceived color and light output despite the various nonlinearities in the overall system.
The challenge with lighting, though, is that there are various nonlinearities in the overall system that typically result in a non-linear relationship between a wall-pod controller on the wall and the light response that is perceived by the occupant of a room, see Figure 4. For example, the combination of wall-pod controller and driver may yield a nonlinear relationship between the setpoint provided by the user and the current that comes out of each channel of the LED driver. Furthermore, the light output of a LED light engine is dependent on the amount of current that is forced through the LEDs as well as the actual junction temperature of the LEDs. Finally, the human eye response to light, especially when color is expressed in correlated color temperature, is non-linear. A well-designed dynamic white lighting solution will take these nonlinearities into consideration and compensate for these to yield an intuitive (linear) relationship between the setpoint that is entered and the light output and/or color that is perceived by the user.
The two most common flavors for dynamic white lighting applications are Dim-to-Warm and Tunable White, see also Figure 5. Dim-to-Warm applications provide the end-user one control handle: light intensity. As the light intensity is decreased, the color temperature of the overall luminaire shifts, typically from cool to warm to mimic the behavior of an incandescent light bulb. Tunable White applications, in contrast, offer the end-user two control handles to independently control intensity and color temperature. The graphs in Figure 5 reflect the typical relationships between incoming control(s) and light output / CCT that both flavors of dynamic white lighting aim for.
Figure 5: Dynamic White Lighting applications typically come in two flavors: Dim-to-Warm and Tunable White. Dim-to-Warm offers end-users one control handle (intensity) while Tunable White offer end-users two control handles to independently control intensity and CCT. These graphs reflect the typical relationships between incoming control(s) and light output / CCT for both flavors.
Figure 6: Three different setups for achieving tunable white from a controls perspective.
Tunable White in DALI–Device Type 6 vs. Device Type 8
In Tunable White applications the user has access to two different control handles: intensity and color point, commonly expressed as correlated color temperature (CCT) in Kelvin. Tunable White applications typically accomplish this decoupling between intensity and color by mixing light from two different light engines, or two LED strings within a light engine, each with a unique color temperature. This means that the luminaire must have the ability to independently control the relative light output of the two LED light engines or LED strings.
One way to implement a Tunable White application is to deploy two single channel drivers, each connected to one of the two light engines, see Figure 6. Although this approach is conceptually straightforward, the main limitation is that the controller must include the heuristics/algorithm to translate the setpoint from the user (typically defined in terms of light output and CCT) into the correct current setpoints for each driver. This approach, often referred to as control-side implementation, may be acceptable in small installations where a controller is connected to one luminaire type. However, this approach becomes quickly unmanageable in larger installations that combine multiple luminaires types, possibly from different vendors. Also, from a DALI perspective, each single channel driver consumes one short address on the network, reducing the number of Tunable White luminaires that can be part of a single DALI network.
A dual-channel DALI Device Type 6 driver can address some of the limitations of the previous approach. In particular, the algorithm that translates the incoming intensity and color setpoint into the correct currents going to each LED light engine can be stored inside the driver. In this driver-side implementation, the controller is agnostic to how the mixing between light engines is accomplished, in other words, it only needs to send an intensity and color setpoint to the luminaire. However, a dual-channel DALI Device Type 6 driver still consumes two short addresses on the DALI network, as a Device Type 6 control gear can, by definition, only accept a single setpoint. Furthermore, the color setpoint in this approach is expressed relative to the full CCT path that the luminaire is capable of. In other words, a DALI color setpoint of, say, 128, doesn't necessarily translate into the same color setpoint for different luminaires, each with their own CCT path. Consequently, controlling multiple luminaires, with DALI DT6 drivers and different CCT paths, is still not supported.
A Tunable White implementation that relies on a dual-channel DALI Device Type 8 LED driver offers additional benefits relative to a Tunable White implementation with a dual-channel DALI Device Type 6 LED driver. First, by design, a DALI Device Type 8 driver accepts both intensity and color setpoints while consuming only a single short address on the DALI network. Second, if the control type is either Color Temperature or XY-coordinate, the driver accepts absolute color setpoints. Consequently, Tunable White luminaires from various vendors, each with different CCT paths, can be easily combined into a single DALI network. After all, each luminaire is designed to translate the incoming absolute color setpoint into the correct currents for each LED light engine. Finally, each DALI Device Type 8 driver knows what path it is capable of. During commissioning, the control path that is made available to the end-user can then be chosen such that a target color setpoint will never exceeds the path of any individual luminaire on the network.
eldoLED DALI Device Type 8 implementation
Figure 7: DALI-2 DT8 System Flow.
The continued progression of the DALI-2 standard, and associated certification framework through DiiA, advances the state of the art for Tunable White lighting applications. It's also clear that without additional tools, system design and implementation will remain difficult and time consuming, particularly in installations with multi-vendor solutions and/or different LED light sources in use. What initially looks like two variables (color and intensity) is in reality a problem with n variables, where n is the number of luminaires. A successful project, therefore, depends on selecting end-to-end components that work together – they're interoperable. DALI-2 is intended to be applied across a range of components. With DT6 or, better yet, DT8 selection of a control set should be straightforward. The selected driver should work with the control set and have embedded color science algorithms (e.g. LightShape) that can be programmed and applied to luminaire and LED specifications. With detailed luminaire and LED information, LightShape can be deployed at the front-end of a project. Balancing the light from each luminaire becomes practical and advantages the installation and commissioning process that often engages contractors, engineers and system specialists. With everyone working from the same, consistent position, projects can proceed smoothly, on-time, and on budget.
What to look for in Tunable White drivers
- Deep dimming performance
- Flicker performance – color separation
- What is available path when dimmed down? If minimum dimming level is relatively high, the path may shrink significantly when the luminaire is dimmed; in some cases, the luminaire may not be able to maintain constant color during dimming. Need to demonstrate this with an example.
- Driver side vs. controller side implementation; single solution that works for any arbitrary light engine, design; no need to program controllers for every different type of luminaire.
- Intuitive design regardless of what LEDs are used; can use different LEDs while still getting the same overall performance.
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