This Application Note describes how to develop a Tunable White luminaire for use with the Enlighted lighting system with sensor firmware v5.4. Luminaire manufacturers, electrical engineers, and Enlighted field application engineers will find this information helpful in adequately designing and deploying a Tunable White luminaire.
With the introduction of LED “white” light sources, a new market for adjusting these light sources to mimic the sunlight's color at different times in the day is born.
Specifying Tunable White “Color”
To control the ‘color’ of an LED light source, identify the method used to determine the color. The science of Colorimetry is well established and provides us with several methods. As defined in Wikipedia, “Colorimetry is the science and technology used to physically quantify and describe the human color perception.” The main takeaway is the CIE 1931 Chromaticity Diagram and the color space it defines:
The area of the diagram that is in color is referred to as the “gamut” or “color gamut,” representing all colors that are perceptible to the human eye. Any point (color) in the space (either inside or beyond the visible color gamut can be represented by a coordinate pair of decimal numbers (x,y). In the case of “White Light,” it is the area of the diagram indicated by the “black body locus,” which is the black line in the lower right third of the diagram. This is also referred to as the “black body line” or “black body curve” or just the “black body.” An x, y coordinate pair could specify any point on this curve, but that would be an overkill for Tunable White.
By Paulschou at en.wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=24903791
Another method of specifying points in the White Light region is “Correlated Color Temperature” or “CCT.” These lines cross the black body locus and extend into areas of the color gamut that would show an objectionable hue or color. The diagram below shows a closeup of the black body locus with the CCT lines indicated. CCT is measured in degrees Kelvin or “K,” and the basis of these values (and the black body locus) is that if an object were heated, it would start to glow; first a dull red, then a brighter red, and as the temperature continued to increase, would eventually glow orange, yellow, white and finally blue. A CCT value specifies the ‘color’ for Tunable White systems, as long as the actual color of the source is near the black body locus. From this, it can be inferred that a system’s light output can be represented by an intensity value and a CCT value to achieve the Tunable White function, and that is precisely how the Enlighted system operates.
Generating Tunable White Light
There are many methods to produce colors and white light in the color gamut, such as RGB, which is familiar to most of us from television and computer monitor displays. Unfortunately, the method using non-white primaries suffers from low efficacy (lumens/watt) and is not well suited for general illumination purposes. A simple process for general illumination is to use two different LED white light sources of different color temperatures and vary the relative intensity of these two sources to traverse the range of CCTs in between. The two sources are single points in the color space, and their combination will form a straight line connecting those points. The actual location on that line will be dependent on the ratio of drive given to each of the two sources, as shown below in the diagram. The colored background has been removed from the diagram for clarity:
To traverse the line and ‘tune’ to different color temperatures (CCT) of white light, adjust the drive level to each of the two equal wattage sources.
- At 100% cool (6500K in this example) and 0% warm (2700K in this example), “cool” white light with full brightness (6500K) is achieved.
- At 0% cool (6500K in this example) and 100% warm (2700K in this example), “warm” white light with full brightness (2700K) is achieved.
When the two sources are at 50%, the Tunable White light is at midpoint CCT (near 4000K) with full brightness. When both sources are driven to 100%, the Tunable White light would still be at midpoint CCT (near 4000K), producing an undesirable characteristic of the intensity varying as the CCT is changed.
A better approach is to have both channels equal 100% intensity so that the light output is relatively constant regardless of CCT selected. Note that this simplifies it as other considerations will affect the light output, for example, the efficacy of cool vs. warm LEDs and the effect of dimming. You cannot dim to a level that would change the warm to cool ratio since there would be a color shift as you dim, but these items can be addressed later. The ‘two-channel’ approach leads to the following system architecture:
While the two-channel method is a simple architecture, the implementation can be a bit tricky. The control signals for each of the drivers must be calculated from the Intensity and CCT values that represent the light output indicated by the “Color Mixing Algorithm.” The algorithm must consider the interaction between each single control input (intensity and CCT) and both dimming level outputs.
For example, as the intensity control input is varied, both dimming channels must be reduced or increased proportionately. If further reduction of either channel changed the color mix ratio (because one of the channels has reached its minimum level), the intensity reduction would be limited. Similarly, as the CCT control input is varied, the ratio of the two channels must be altered, but in doing so, the combined output of both channels must remain constant so that there is no intensity variation as CCT is altered.
Controlling Tunable White Light
The two common methods for controlling the Two-channel generated Tunable White include:
- Warm/Cool (W/C) method – independent channels
- Intensity/CCT (I/C) method – linked channels
The decision of which method to choose depends on the driver selection; several driver technologies are available that partition the “Color Mixing Algorithm” in different ways.
1. Warm/Cool Method
From a hardware standpoint, this is the simplest method as it follows single-channel dimming. Combine the two single channels in a single luminaire and have the sensor control one channel as “Warm” and the other as “Cool” in the system. The Color Mixing Algorithm is implemented within the Enlighted sensor based on information provided by the luminaire manufacturer.
Advantages of Warm/Cool Method
- Familiar components - use the same drivers as monochromatic fixtures.
- A wide portfolio of drivers is available.
Disadvantages of Warm/Cool Method
- Requires twice as much driver ‘wattage’ as needed for the luminaire rating (each channel must be able to drive up to 100%, but the combination of both channels can never exceed 100%. Since drivers are priced at $$/W, more watts will result in a higher cost.
- Two drivers take up more space, and
- Input wiring is complicated.
2. Intensity/CCT Method
Drivers and mixing units are available to receive intensity/CCT control inputs. They either provide a mixing function to be used with an external driver or provide a dual-channel driver capability in a single unit. The Color Mixing Algorithm is implemented within the driver/mixing unit based on the luminaire manufacturer's information.
Advantages of Intensity/CCT Method
- Lowest component count – single, dual-channel driver (optional energy metering and on/off control eliminating the need for an external control unit)
- Driver ‘wattage’ is tuned to the luminaire's requirements – the wattage is split between the two channels as required by the requested CCT.
Disadvantages of Warm/Cool Method
- Less familiar driver SKUs
Block diagram of the Intensity/CCT Method using a separate driver and current splitter:
Block diagram of the Intensity/CCT Method using dual-channel driver:
The Enlighted system supports Analog (0-10V) drivers and Digital 2-wire DALI Type 8 (DT8) methods of representing these values. An advantage of using the 2-wire solution is that the wiring is simplified because the values of both channels, as well as the power to the sensor, are all communicated over the same two wires. Additionally, the 2-wire solution also simplifies the system's setup, since the values for the CCT of the LED sources used can be pre-loaded into the driver and automatically be read by the sensor and propagated through the rest of the Enlighted system.
Refer to the following line diagrams:
- SU-5 and CU-4 in Tunable White Fixture with 2-channel LED Driver
- Tunable White - 2-wire with Powered Driver
- Tunable White - 2-wire Sensor with Non-Powered Driver
Fixture Manufacture Table (FMT)
The Enlighted system supports Analog (0-10V) drivers and Digital 2-wire DALI Type 8 (DT8) drivers. The FMT (Fixture Manufacturer Table) is only required for Tunable White if analog (0-10V) drivers are being used.
When a DT8 (digital) driver is used, the color information typically provided in the FMT is provided by the driver from values stored in the driver by the fixture manufacturer, so the FMT is not required. Enlighted Manage system automatically detects that the driver is a DT8, Tunable White, and enables the Tunable White controls. The CCT values stored in the driver know the proper range for the controls, eliminating the need for an FMT.
The Enlighted sensors with firmware v5.4 and above require Fixture Manufacturer Table (FMT) to test Tunable White fixtures in the production line (End of Line testing) when they are built to control Tunable White fixtures after they are deployed in the field. You can upload an FMT to a sensor in Manage. Refer to Configuring Tunable White for instructions on how to upload the FMT.
If the FMT is not uploaded to the sensor when the Tunable White fixtures are being built in the factory, the sensors might fail the wiring test, and visual cues need to be used to verify proper wiring – the sensor will exercise (min to max intensity) both the warm and cool channel of the fixture independently.
Controlling Tunable White Fixtures
The Enlighted’s Manage can be used for controlling the color temperature and intensity of the light to fit any setting or use case. Additionally, the system’s astronomical clock implements sunrise and sunset CCTs following natural daylight patterns. Enlighted Manage (EM) can also integrate time-of-day calculations to match or enhance occupants’ circadian rhythm patterns. For more details on controlling color temperature, refer to the Configuring Tunable White article.