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CATEGORY: Topics » Daylight Harvesting
By Lighting Controls Association, on November 21, 2011
Last month at LightingControlsAssociation.org, we published an article about how to establish daylight zones in sidelighted (windowed) building spaces. We looked at an industry rule of thumb and then how the latest generation of energy codes and standards address it.
In review, when designing an energy-saving daylight harvesting control system, a critical decision is to establish lighting control zones, identifying lighting loads to be separately controlled. Before this decision can be made, however, we must first determine the daylight zones.
A daylight zone, also called the daylight area (expressed in square feet) is defined by the ASHRAE/IES 90.1-2010 energy standard as “the floor area substantially illuminated by daylight.” In other words, it should consistently receive significant quantities of daylight.
By identifying daylight zones, the lighting control system designer identifies areas where daylight harvesting control is appropriate. The designer can then make further decisions about how many control zones are appropriate for the given daylight zone, and their configuration.
In this article, we will examine methods for establishing daylight zones in toplighted building spaces, such as spaces with skylights, roof monitors and clerestories.
Rule of Thumb for Toplighted Daylight Zones
For toplighted spaces, a rule of thumb is that a daylight zone can be established as the skylight length or width plus 1/2 the ceiling height on each side, and a second zone as the skylight length or width plus the ceiling height on each side.
 Image courtesy of the Lighting Research Center. If a space is uniformly lighted using skylights as shown below, with properly spaced skylights covering about 3-5% of the floor area, the entire space may be considered a daylight zone suitable for daylight harvesting control.
 Image courtesy of Acuity Brands Controls. Energy Codes/Standards And Toplighted Daylight Zones
As stated in last month’s article, daylight zones are increasingly being determined by codes and standards, notably the 2009 IECC model energy code, ASHRAE/IES 90.1-2010 model energy code, ASHRAE 189.1-2009 model green building code and California’s Title 24-2008 (state energy code). These codes and standards all require that daylight zones be established around toplighting apertures, and general lighting in these zones be separately controlled from other lighting. The dimensions of the daylight zone are defined and adjusted based on elements in the space that would limit daylight availability, such as tall obstructions (e.g., walls and stacks).
Codes and standards may recognize one or more of the following types of toplighting:
Skylights
Let’s begin with skylights. Below is a short reference to how the latest generation of codes and standards establish daylight zones, or daylight areas, in spaces toplighted using skylights. Note that CH = ceiling height (floor to ceiling), and OH = obstruction height—the height of permanent obstructions (to daylight distribution) such as walls and permanent storage stacks.
The below graphic illustrates the daylight zoning requirements. In each case, a wall restricts the daylight zone in the north because the distance between the skylight and the wall is less than the ceiling height (IECC 2009) or 0.7*CH (ASHRAE 189.1-2009).
ASHRAE/IES 90.1-2010 and Title 24-2008 use a different system that addresses how likely the permanent partition will block the daylight. If the distance between the skylight and the obstruction is less than 0.7*CH and greater than 0.7*(CH–OH), then the front of the partition (facing the skylight) marks that boundary for the daylight zone.
Further, IECC 2009 and ASHRAE 189.1 reduce the given daylight zone dimension to 1/2 the distance to the nearest skylight or vertical fenestration, while the daylight zone in ASHRAE/IES 90.1-2010 and Title 24-2008 is reduced by the outermost boundary of any nearby primary sidelighting zone or roof monitor daylight zone.
Let’s look at a sample problem:
We have 10 skylights that are each 18 ft. from each other, providing illumination in a space with a ceiling height of 20 ft., and bounded on all sides by ceiling-height walls 10 ft. away. Under IECC 2009, what are the dimensions for each skylight daylight zone?
Under IECC 2009, 1/2 distance (D) between skylights (9 ft.) < CH (20 ft.) and D to any nearby partition, so the zone around each skylight would be the skylight length or width + 1/2 D between it and its nearest skylights (9 ft.). The exception is the dimension facing the walls, which would be limited by the distance between the skylight and those walls (10 ft.), as D to these nearby partitions < CH (20 ft.).
Now suppose we added a 15 ft. tall warehouse stack 10 ft. from the edge of a skylight. Under ASHRAE/IES 90.1-2010 and Title 24-2008, would that stack limit that skylight’s daylight zone?
The distance between the skylight and the stack (10 ft.) > [0.7*(CH-OH)] (3.5 ft.) and < 0.7*CH (14 ft.), so the stack would limit the boundary of the daylight zone extending toward the stack to 10 ft. In this scenario, any obstruction over roughly 5 ft. in height would impose a limitation on the dimensions of the daylight zone.
Now suppose the northern row of skylights is 20 ft. from a windowed wall with a primary sidelighted daylight zone of 15 ft. Under IECC 2009 and ASHRAE/IES 90.1-2010, would there be any limitation to the daylighting zone?
Under IECC 2009, the answer would be yes. The windows are 20 ft. away from the skylight, and 10 ft. < CH (20 ft.) and there is no intervening partition, so the daylight zone would be halved to 10 ft.
Under ASHRAE/IES 90.1-2010, the answer is also yes. The windows are 20 ft. away, so the outermost edge of the primary sidelighted daylight zone is 5 ft. away. As 5 ft. < (0.7*CH, or 14 ft.), the northern daylight zone for these skylights would extend 5 ft. toward the windows.
What if, for theoretical purposes, we add a ceiling-height vertical obstruction 2 ft. away from the skylight? Would the daylight zone be limited to 2 or 5 ft.?
Anytime the limitations are in conflict, common sense dictates that the closer one applies, in this case the obstruction 2 ft. away, as it would block any light from extending the extra 3 ft.
Roof Monitors And Clerestories
Now let’s move on to roof monitors and clerestories, which are recognized by ASHRAE/IES 90.1-2010 and ASHRAE189.1-2009.
ASHRAE/IES 90.1-2010 specifically recognizes roof monitors, which it defines as “vertical fenestration integral to the roof.” Below is a summary of the ASHRAE/IES 90.1-2010 requirements:
The below graphic illustrates the daylight zoning requirements.
Let’s look at a sample problem:
We have a roof monitor with a width of 10 ft. and a monitor sill height of 20 ft., meaning the daylight zone would extend 20 ft. from the vertical glazing. A 14-ft.-tall vertical obstruction, placed 15 ft. from the monitor, protrudes into the daylight zone, and the outermost edge of a primary sidelighted daylight zone is 18 ft. away from the glazing. Under ASHRAE/IES 90.1-2010, what are the dimensions for this daylight zone?
The daylight zone would normally be 10 ft. wide and extend from the glazing 20 ft. (the MSH), or an area of 200 sq.ft. However, the primary sidelighted daylight zone is 18 ft. away, which is less than the MSH, so that would limit the zone to a depth of 18 ft. The distance to the vertical obstruction (15 ft.) > MSH – OH (6 ft.) and < MSH (20 ft.), so it too would limit the zone to a depth of 15 ft. along the front face of the obstruction. Basically, in this scenario, any obstruction over 5 ft. in height would impose a limitation on the dimensions of the daylight zone.
Finally, ASHRAE 189.1 recognizes clerestories, roof monitors and clerestory roof monitors. In review, ASHRAE/IES 90.1-2010 defines clerestories as “that part of a building that rises clear of the roofs or other parts and whose walls contain windows for lighting the interior.” Below is a summary of the ASHRAE 189.1 requirements.
The below graphic illustrates the daylight zoning requirements.
Once the daylight zones are established in a space, we can then decide whether daylight harvesting control is warranted, how many control zones we will need (including what loads will be covered by each controller), and what control method or methods—switching, dimming, etc.—we will use. Each of these areas may be covered by separate code/standard requirements.
By Craig DiLouie, on October 12, 2011
When designing an energy-saving daylight harvesting control system, a critical decision is to establish lighting control zones, identifying lighting loads to be separately controlled. Before this decision can be made, however, we must first determine the daylight zones.
A daylight zone, also called the daylight area (expressed in square feet), is defined by the ASHRAE/IES 90.1-2010 energy standard as “the floor area substantially illuminated by daylight.” In other words, it should consistently receive significant quantities of daylight during the day.
By identifying daylight zones, the lighting control system designer identifies areas where daylight harvesting control is appropriate. The designer can then make further decisions about how many control zones are appropriate for the given daylight zone, and their configuration.
In this and a second article to be published next month here at LightingControlsAssociation.org, we will examine methods for establishing daylight zones based on prevailing energy codes and standards. This month, we will cover the most common type of daylighting—sidelighting, or daylight entering a space through vertical fenestration such as windows. Next month, we will cover toplighting (e.g., skylights) applications.
Energy Codes/Standards And Sidelighted Daylight Zones
Daylight zones are increasingly being determined by codes and standards, however, notably the ICC’s 2009 International Energy Conservation Code (IECC) (model energy code), ASHRAE/IES 90.1-2010 (model energy code), ASHRAE 189.1-2009 (model green building code) and California’s Title 24-2008 (state energy code). These codes and standards all require that daylight zones be established adjacent to sidelighting apertures, and general lighting in these zones be separately controlled from other lighting. Regional and national design firms working in multiple jurisdictions and project types may find themselves determining daylight zones using up to four or more definitions that have many similarities but also significant differences.
Basically, each code or standard defines the dimensions of a daylight zone, and then identifies elements in the space that could limit these dimensions, such as tall obstructions (e.g., walls) and other daylight apertures. Additionally, some codes and standards recognize a difference between primary and secondary sidelighted daylight zones, with control in primary zones typically being mandatory, and control in secondary zones being encouraged through power credits.
Below is a short reference to how these codes and standards establish daylight zones in sidelighted spaces (vertical glazing below the ceiling), which, depending on the code or standard, may be called daylight areas. Note that window height (WH, also called window head height) is formally defined in ASHRAE/IES 90.1-2010 and ASHRAE 189.1-2009 as the distance from the floor to the top of the glazing. Also note that the main limitation to the daylight zone is the presence of some type of permanent partition, which may be defined slightly differently across the four codes and standards.
Here we see these rules presented visually in an example space. In each case, the width of the daylight zone is limited by the wall located 1 ft. to the north of the window. In each case, the depth is limited by the wall located southeast of the window, which is located at a distance that is less than one window head height deep into the space.
Some codes and standards, notably the ASHRAE/IES 90.1-2010 energy standard and California’s Title 24-2008 energy code, also establish secondary daylight zones. In the case of sidelighting applications with vertical fenestration such as windows, these are daylight zones extending deeper into the space, controlled separately from primary daylight zones and other general lighting in the space. This level of control is not mandatory but instead encouraged through the use of power credits—that is, a multiplier increasing available watts for the controlled lighting load. If you save energy, the code/standard says, you can have a more power.
As the below graphic illustrates, in each case, the secondary sidelighted daylight zone should be the same width as the primary zone, and another window head height in depth, with the same limitations in regards to 60-inch or taller permanent partitions.
Once the daylight zones are established in a space, we can then decide whether daylight harvesting control is warranted, how many control zones we will need (including what loads will be covered by each controller), and what control method or methods—switching, dimming, etc.—we will use. Each of these areas may be covered by separate code/standard requirements.
Next month, we will examine how to set up daylight zones in toplighted spaces.
By Lighting Controls Association, on October 10, 2011
 The Lighting Controls Association is pleased to announce that it has updated EE201: Introduction to Lighting Control, a popular offering in the Association’s Education Express series of online distance education courses about lighting controls.
The course, authored by Craig DiLouie, principal of ZING Communications, Inc. and LCA’s Education Director, provides an introduction to daylight harvesting and in-depth discussion for each major decision during the design of a daylight harvesting control system. It consists of three learning modules covering these topics:
• Purpose of daylight harvesting
• Typical energy savings
• Typical system
• Importance of transparency
• Ideal applications
• Daylight harvesting and LEED
• Daylight harvesting and energy codes
• Switching versus dimming
• Open versus closed loop
• Control zoning
• Control zoning: daylight availability
• Control zoning: windowed spaces
• Control zoning: skylighted spaces
• Control zoning: energy codes
• Control zoning: granular zoning
• Control zoning tool
• Photosensors
• Photosensors: range of response
• Photosensors: spatial response
• Deadband
• Wireless sensors
• Centralized versus distributed controls
• Analog versus digital controls
At the conclusion of each learning module, an optional online comprehension test is available, with automatic grading; a passing grade (70+%) enables the student to claim education credit.
EE201: Daylight Harvesting is registered with the National Council on Quality in the Lighting Professions (NCQLP), which recognizes a total of 4 LEUs towards maintenance of Lighting Certified (LC) certification. This course is also registered with the California Advanced Lighting Control Training Program (CALCTP) for credit to qualify to receive live training (30 points).
By Lighting Controls Association, on August 9, 2011
 As sunlight scatters through the atmosphere, it turns the entire sky dome into a daylight source. The proper use of daylight coupled with thoughtful electric lighting design and control applications can significantly reduce energy used by lighting.
ALG Online (Advanced Lighting Guidelines) devotes an entire chapter to understanding the impacts of daylight on commercial spaces and effective ways to integrate daylighting strategies into low-energy building designs.
To see more, log in to ALG Online and visit the daylighting chapter. The content covers technical background, daylight sources, design strategies, fenestration technologies, electric light and controls integration and simulation tools.
By Craig DiLouie, on June 13, 2011
 Daylight harvesting is an advanced lighting control strategy used to minimize ongoing owner energy costs. It occurs when a sensor measures daylight levels and signals a control to adjust electric lighting system output to maintain a desired task light level. Variable daylight levels are automatically harvested as energy savings through electric lighting reductions.
Because energy savings will be dependent on factors such as type of available daylight, control response and space and task characteristics, actual savings can be difficult to predict, although studies suggest strong potential. A 2003 study conducted by the National Research Council of Canada discovered 40% energy savings in an open office environment and 50% (with manual blinds) to 70% (manual blinds used optimally, or automatic shading) in private offices. Another 2003 study, conducted by Heschong Mahone Group, found that daylight harvesting strategies can produce 50% energy savings in school classrooms.
LEED 2009 encourages providing daylight and views to users. IEQ, Credit 8.1 awards 1 LEED point for providing a minimum 25 footcandles of daylight in at least 75% of regularly occupied building areas. IEQ, Credit 8.2 awards 1 LEED point for introducing views in at least 90% of regularly occupied building areas—that is, a direct line of sight to the outdoor environment via vision glazing 90 in. above the floor. Because strong daylight availability is essential to daylight harvesting, these buildings are well suited to this control strategy. Daylight harvesting, in fact, is favored in LEED projects, not only because of daylight availability, but because energy points are based on exceeding ASHRAE/IES 90.1-2007, and because daylight harvesting is not required by this standard, its energy savings can be captured as LEED energy points. Further, the Green Interior Design & Construction version of LEED further awards 2 points for introducing daylight harvesting controls in all daylighted areas (1 point) and/or on 50% of the lighting load (1 point).
Because of the strong energy savings potential offered by daylight harvesting, coupled with advancing technology, codes and standards are now beginning to address daylight harvesting—specifically, International Energy Conservation Code (IECC) 2009, ASHRAE/IES 90.1-2010, ASHRAE 189.1 and Title 24-2008. In review, IECC 2009 and ASHRAE/IES 90.1-2010 are energy standards offered as model energy codes for states and other jurisdictions. ASHRAE 189.1 is a green building standard. And Title 24-2008 is California’s unique energy code.
All of these codes and standards are different and yet have similar major themes. First, they define daylight availability as zones around sidelighting (e.g., windows) and toplighting (e.g., skylights and roof monitors) daylight apertures. Second, they require separate control for general lighting in these daylight zones. The standard may also specify whether the control must be manual or automatic, switching or dimming, stepped switching or simple ON/OFF. And the standard may reward aggressive daylight harvesting with power adjustment credits that can be used to acquire greater design flexibility with the controlled load.
Let’s look more closely at the daylight harvesting requirements published in IECC 2009 and ASHRAE/IES 90.1-2010. First, what is the daylight zone? After all, daylight harvesting is entirely dependent on daylight availability in the space. Both IECC 2009 and ASHRAE/IES 90.1-2010 define daylight zones using formulas based on size of aperture and whether there are any obstructions blocking the light, with ASHRAE’s approach being similar to ASHRAE 189.1 and California’s Title 24-2008 code. Sidelighted daylight zones are defined as depth x width adjacent to the aperture, and toplighted daylight zones are defined as length x width under the aperture. ASHRAE/IES 90.1-2010 includes helpful drawings detailing daylight zones.
IECC 2009 offers a basic approach to daylight harvesting control by simply stating that general lighting in these zones must be separately circuited and controlled from other general lighting in the space. The method of control is not specified, so the designer has a choice of switching or dimming. ASHRAE/IES 90.1-2010 goes much farther with an approach that is similar to California’s Title 24-2008 energy code:
Sidelighted spaces: If the sidelighted daylight zone is larger than 250 sq.ft., then the control method must be automatic and multilevel (or continuous dimming), providing one step between 50% and 70% of the design lighting power, and another between OFF and 35%. ASHRAE/IES 90.1-2010 encourages more aggressive daylight harvesting strategies in sidelighted office, meeting, classroom, retail sales and public space types with credits that can be used to increase the power allowance for the controlled lighting load. Recognized strategies include continuous dimming control and automatic control of general lighting in secondary (deeper) daylight zones in sidelighted spaces.
 Energy standards and sidelighting. Toplighted spaces: In toplighted spaces, if the total daylight area under skylights plus the total daylight area under rooftop monitors is larger than 900 sq.ft., the general lighting must be separately controlled using either a stepped switching or continuous dimming controller, with some exceptions. As with sidelighted spaces, more aggressive daylight harvesting control in toplighted areas is rewarded with power adjustment credits.
Additionally, perimeter lighting in parking garages is required to be automatically reduced in response to daylight, with some exceptions.
Demand for daylight harvesting controls has grown dramatically in recent years, driven largely by sustainability initiatives such as LEED. Since 2005, California’s energy code required daylight harvesting in certain spaces. Now the major energy standards—IECC and ASHRAE/IES 90.1—contain significant requirements for daylight harvesting control, signaling widespread acceptance and adoption of this control strategy in the future.
By Lighting Controls Association, on March 21, 2011
 Lighting Controls Association members will present “Design of Electric Controls for Daylighting,” a three-hour workshop, during the Daylighting Institute at LIGHTFAIR 2011.
The workshop, presented by David Weigand, LC, LEED-AP of Leviton, Gary Meshberg, LC, LEED-AP of Encelium and A. J. Glaser of HUNT Dimming, will occur Sunday, May 15, 2:00-5:00PM.
Energy efficiency through daylighting can only be realized when electric lights are dimmed or switched. This workshop provides information about daylight harvesting control strategies and technologies in a case study format for real-world context, focusing on current approaches, main issues and emerging technologies (e.g., automatic calibration/commissioning, use of multiple sensors), including use of open and closed loop sensing, photosensor characteristics, control algorithms and commissioning.
Learning Objective 1: Learn about various technologies and equipment types used to harvest daylight into energy savings.
Learning Objective 2: Achieve an understanding of what constitutes good and bad daylighting and how to design a daylight harvesting control system. Students will be engaged in an interactive format to solve real-world problems via a case study approach supplemented by handouts.
Learning Objective 3: Learn about how to properly commission and set up a daylight harvesting control system, with handouts including a generic specification, specification punch list, and detailed commissioning procedures.
Learning Objective 4: Student will engage in problem-solving using real-world examples.
This workshop has been presented by these very knowledgeable industry veterans since 2009 and consistently scores high marks from attendees.
If you’re attending LIGHTFAIR this year in Philadelphia and interested in learning about daylight harvesting controls, be sure to register and attend this workshop.
By Craig DiLouie, on December 23, 2009
Daylighting is going mainstream and daylight harvesting, the energy-saving lighting control strategy that actually makes daylighting “sustainable,” is beginning to catch up. Of particular interest is the fact that daylight harvesting is transitioning from being something encouraged by energy programs to something required by energy codes and standards—not just California’s Title 24, but IECC 2009 and, likely, ASHRAE 90-1.2010 as well. It is likely, in fact, that in the future, most commercial buildings that have windows and skylights will be required to have some type of daylight harvesting control in the adjacent area.
 Green construction is emphasizing daylighting and bringing fresh attention to daylight harvesting. Energy codes are now beginning to require daylight harvesting, which will make it a future staple in construction. Photo courtesy of Leviton. Daylight harvesting’s value proposition is fairly simple: As daylight levels increase in a space, electric light levels can be automatically reduced to maintain a target task light level and save energy. All automatic daylight harvesting control systems need a device that can measure light levels and signal a controller to dim or switch the lights in response to daylight contribution. This device is called a photosensor.
The photosensor is a small device that can include a light-sensitive photocell, input optics and an electronic circuit used to convert the photocell signal into an output control signal, all within a housing and with mounting hardware.
 Photosensors can be mounted on walls, ceilings and as part of light fixtures. Photo courtesy of WattStopper. Photosensors may be mounted on walls, ceilings and even as a part of light fixtures. Fixture-integrated sensors may be installed as part of the original fixture or installed later in the field by attaching to a lamp via a clip and being wired directly to the ballast. The visible size of a photosensor ranges from a golf ball to a standard wall switch.
As daylight harvesting grows in importance, photosensors are becoming more sophisticated. But buyer beware: There is no standard defining how photosensors should operate. When selecting a photosensor, important questions to ask about a given product include: Is it compatible with the given controller? What control method does the system use? What is the sensor’s spatial response? What range of light levels can it “see”? How accurate is its signal? Is it photopic-corrected? How far is it to be installed from its controller? How is it commissioned? How many zones can it support? What are the configuration options? Is it capable of operating reliably within the given environment—heat, cold, moisture? Are there listing or compliance requirements such as UL or ROHS?
“Studies have shown the importance of using daylight harvesting strategies in commercial spaces, particularly with the growing emphasis on architectural daylighting design, but have also illustrated the importance of choosing the right product for the application,” says Daniel Trevino, LEED-AP, Daylighting Product Manager for WattStopper. “This helps maximize energy savings while avoiding user complaints.”
“Using the correct photosensor for a particular application is one of the most critical design aspects,” says Bob Freshman, Marketing Manager for Leviton Lighting Management Systems. “The sensor that is used should be appropriate for the size of the space and the environment in which the sensor is located.”
Control algorithm
Daylight harvesting controls may be “open loop” or “closed loop” systems. Each measures the daylight contribution on the task surface differently. Dual loop is now emerging as a potentially significant technology.
Closed-loop systems measure the combined contribution to light level from both daylight and the electric lighting system. Because the photosensor measures the electric lighting system’s light output, it “sees” the results of the controller’s adjustment and may make signal further adjustments based on this feedback—creating a closed loop.
With closed loop, the photosensor measures actual light levels, so it is sometimes considered more accurate than open loop, Closed loop is considered preferable by some for applications where a specific target light level must be maintained, such as small offices. But control is limited to a single zone and the system must be properly set so that transient light level changes (e.g., white sheets of paper shuffled on and off a dark desk). do not cause overly frequent dimming or switching.
 Closed-loop sensors "see" the results of their adjustment, creating a closed loop. As such, they are typically mounted on the ceiling or as part of a light fixture with a direct view of the task area, and no direct view of sunlight or light sources being controlled. Graphics courtesy of California Lighting Technology Center. 
Open-loop systems measure only the incoming daylight, not the contribution from the electric lighting. The photosensor should not see any electric light and therefore it is mounted outside the building or inside near a daylight aperture facing away from the controlled lighting. Because there is no feedback, it is an open loop.
With open loop, the sensor is not affected by transient light level changes but it does not measure actual light levels. This means that a sensor placed outside a window would not know that the blinds were closed, and dim the lights inside anyway. As a result, open loop is often preferable for applications where accuracy is less important, such as hallways and atria.
 Open-loop photosensors measure only incoming daylight and therefore should be mounted inside the building near a daylight aperture facing away from the controlled lighting, or outside the building. Graphics courtesy of California Lighting Technology Center. 
Dual-loop photosensing is an interesting new technology that combines open-loop and closed-loop photodiodes looking in different directions. The intended result is greater accuracy than using open loop alone and greater resistance to transient light level changes than using closed loop alone.
 Dual-loop photosensing combines open- and closed-loop technology to increase accuracy and mitigate the impact of transient light level changes. Graphic courtesy of California Lighting Technology Center. Spatial response
The photosensor’s spatial response, also called its angular sensitivity, describes its sensitivity to light from different directions and defines its field of view—what it “sees,” in effect. This is determined by the design of the optical system that gathers and delivers light to it.
“Spatial response is composed of two factors: the angle at which the detected light is striking the photodiode element inside the sensor, and the optics and restrictions of the photosensor housing,” says Norm Dittmann, President of PLC-Multipoint.
The sensitivity of the photodiode changes with its angle—greatest when facing a light source directly and decreasing as the light moves across the sensor’s detection area. Additionally, the rings of a Fresnel lens will not allow light to enter the lens outside of its acceptance angle. A domed sensor will blend different light sources and be able to diffuse low-angle daylight, increasing the angle at which the light can be measured by the sensor.
 The sensitivity of a photosensor changes with its angle. Graphic courtesy of California Lighting Technology Center. If the field of view is too broad, the photosensor may detect light where it shouldn’t, such as detecting direct sunlight near or outside a window, and thereby possibly dim the lights below what is intended. If the field of view is too narrow, the photosensor may become too sensitive to changes in brightness within a localized area, and raise or lower the lights incorrectly.
“The narrower the spatial response, the more closely the photosensor responds to the luminance, or brightness, of the surface at which it is aimed,” says Trevino.
According to the New Buildings Institute, a 60-degree cone of vision is common, but Trevino points to research that suggests a 100-degree field of view for closed-loop photosensors and a 45-degree field of view for open loop. Some sensors provide an adjustable feature to block direct sunlight from the field of view.
Light level response
The photosensor may be limited in the range of light levels it is able to detect. Dusk and dawn lighting control is performed at less than 10 footcandles (fc), daylighted offices are controlled at less than 100 fc, atrium spaces are controlled at less than 1,000 fc, and skylight sensors see up to 10,000 fc of sunlight.
Below are several typical light level ranges that a photosensor “sees” for some applications:
- 1-500 FC for a photosensor mounted on a ceiling in a windowed space;
- 10-5,000 FC for a photosensor mounted in an atrium or skylight; and
- 100-10,000 FC for a photosensor mounted outside a building.
“Lighting designers should ensure that the range of response of the photosensor matches the light levels likely to be found in the area of installation,” says Solayappan Alagappan, Design Engineer for Philips.
 Typical light level ranges that a photosensor "sees" for some applications. Graphic courtesy of Leviton. Type of signal
The photosensor output signal to the controller may be a voltage or current form. As with most commercial building lighting controls, a sensor with voltage output works well when it is within 500 ft. of a controller. A sensor with current output can drive signal thousands of feet and provide better immunity to electrical noise in more rugged power environments. Signal resolution is also critical. This is the relationship between what the photosensor sees and the resulting signal output. A primary question here is whether the sensor uses a linear photodiode or nonlinear photoconductive element.
“Photoconductive sensors use nonlinear sensing elements, which have much more error associated with them,” says Dittmann. “The low light level response has a steep voltage vs. footcandle slope. The response becomes less sensitive in the mid range as the voltage/FC slope becomes lower. Finally, past a point, the sensitivity becomes almost flat, where the sensor is not very responsive. This response curve is not the same value for each sensor, which makes it difficult for the same settings to be applied to different sensors.”
Dittmann says the most important trend in photosensors in the past 3-5 years has been development of photodiode sensors that provide a linear response to light.
“For example,” he points out, “For example, an indoor sensor sees 0 FC and sends 0V as an output, and at 100 FC, it sends 10V as an output, and it’s a straight line in between. This linearity means the sensors can be reliably calibrated, which is very important for setting the correct setpoints on lighting controllers.”
 Photoconductive sensors use nonlinear sensing elements, which present a greater risk of error. Photodiodide sensors provide a linear response to light and hence greater accuracy. Graphic courtesy of PLC-Multipoint, Inc. “Thus photosensor products are available with different range of response—illuminance ranges—to optimize signal resolution, minimize noise levels and avoid saturation,” says Alagappan.
 Table 1. Photosensor characteristics at a glance. Courtesy of PLC-Multipoint. Photopic correction
The photosensor’s spectral response describes its sensitivity to optical radiation of different wavelengths. Photosensors can respond to a greater range of the electromagnetic spectrum than the visible light portion, which the human eye can see. For example, it can respond to ultraviolet and infrared radiation and thereby dim the lights unnecessarily, which can lead to occupant complaints. For this reason, filters are used to as closely as possible match the photosensor’s “eye” to the human eye.
“Daylight by nature has infrared, ultraviolet and visible light, but the human eye is sensitive to visible light only,” says Trevino. “If the goal is to control electric lighting in response to incoming daylight in a way that satisfies the human eye’s visual needs, then a photosensor must have the same type of sensitivity as the human eye.”
When daylight and an electric light source are mixed in the same space at different levels, the photosensor’s control algorithm should automatically undergo photopic correction. Open-loop and closed-loop proportional control algorithms can accomplish this. However, when daylight and two spectrally different types of electric lighting—such as fluorescent and incandescent—are mingled in the same space, correction is not currently possible with today’s technology.
“The problem with photopic correction is that it becomes very situational,” Dittmann warns. “An installation may have initially a certain type of lamp installed, such as a cool white fluorescent lamp. The control system may have corrected for that color. Years later, the lamps may be replaced with a warmer color lamp. The shift in colors is not insignificant, and could cause the control settings to not be correctly applied.”
 Typical mounting locations for photosensors based on application. Graphics courtesy of PLC-Multipoint, Inc. 
Trends
“We see the top three trends in photosensor development to be in the following areas: wireless, fixture integration and self-calibrating product,” says Freshman. “We look at wireless as being a key growth area. It is an ideal solution for retrofit applications with great cost savings that occur from not having to run new wires and because of faster installation. Fixture mounted sensors have been a strong growth category as well.”
“There is an evolution toward integrating daylighting controls with other types of lighting controls,” says Trevino. “For example, manufacturers are beginning to develop integrated systems that combine occupancy sensors, relay panels, daylighting controls, even integration into lighting fixtures. Secondly, there’s been an evolution towards self-calibrating/commissioning controls. Some manufacturers have introduced devices that automatically calibrate for operation. Some minor adjustment may be necessary to trigger this capability, but the process is far more automated than even five years ago. This greatly simplifies the setup process, which has always been one of the biggest hurdles to successful daylighting control.”
“Daylight regulation in combination with blind controls will be a future technology trend,” says Alagappan. “We may also see low-resolution cameras playing the role of photosensors, intelligently compensating for changes in room interiors (surface reflectances). And individual light control may emerge as a trend, in which fixtures within one zone have different daylight settings and one intelligent photosensor controls all of the fixtures. For example, the window and aisle-side fixture of the room could receive different control level signals from the photosensor.”
Special thanks to the following Lighting Controls Association member representatives for their valuable contributions to this whitepaper (listed alphabetically by company):
Bob Freshman, Marketing Manager for Leviton Lighting Management Systems
Solayappan Alagappan, Design Engineer for Philips
Norm Dittmann, President of PLC-Multipoint
Daniel Trevino, LEED-AP, Daylighting Product Manager for WattStopper
By Craig DiLouie, on July 12, 2008
 Daylighting and daylighting control is recognized as best practice in energy codes and industry standards. Automatic daylighting controls can reduce energy consumption by 35-60%, according to the New Buildings Institute.
California’s Title 24-2005 energy code, for example, requires daylighting control in daylit areas larger than 250 sq.ft., and ASHRAE 90.1-2010 and future versions of IECC will likely contain similar daylighting control provisions.
LEED-NC v.2.2 encourages the introduction of daylight into at least 75% of regularly occupied building areas. The Advanced Buildings Benchmark requires that lighting in daylighted areas be controlled by a daylighting control system. The ASHRAE Advanced Energy Design Guide for Small Office Buildings requires daylight dimming controls for luminaires within 12 ft. of North/South window walls and within 8 ft. of skylight edges. And the Northeast Collaborative for High Performance Schools encourages daylighting throughout school buildings and automatic daylighting controls controlling at least 40% of the connected lighting load.
To be successful, however, daylighting control projects require special attention to placement of photosensors and predicting daylighting impacts, integration of daylight and electric lighting, and potential energy savings.
“Daylighting and electric lighting do not inherently know how to play together in the same space,” says Judie Porter, CEM, LEED-AP, program manager for Architectural Energy Corporation. “Add photosensors to the mix without understanding exactly what the sensors see or how the electric lighting responds to the control settings, and the results may be undesirable.”
One example, she says, is using the software to identify glare issues, which can make a space uncomfortable. “We have seen examples of a Band-Aid, such as paper taped on glazing, used to remedy a high daylight illuminance design in a computer room with a low task illuminance criterion,” Porter adds. “We have also seen photosensors rendered useless by occupants taping over the sensor because they were placed or commissioned improperly and light levels in the room were not adequate.”
With proper planning and design of the daylighting controls, she points out, these problems can be avoided in many projects.
Enter SPOT, or the Sensor Placement + Optimization Tool. Aimed at lighting and daylighting designers, energy consultants, electrical engineers and students, this new free software tool helps designers to quantify electric and daylighting with associated energy use in a given space and across all seasons.
The latest version of SPOT (v.4.0), released in May 2008 and funded by the California Energy Commission’s Public Interest Energy (PIER) Program and Energy Design Resources, recently earned Lightfair International’s highest honor for the Most Innovative Product of the Year. SPOT also won top honors in the Research, Publications, Software and Unique Applications category, and the Attendee’s Choice Award by popular vote.
Available free, SPOT helps designers to quantify electric and daylighting with associated energy use in a given space and across all seasons.
“SPOT enables designers to simulate and understand the impact of their designs and achieve optimal photosensor system selection, placement and settings—helping them to find the best balance between required annual light levels and energy savings,” says Jennifer Scheib, staff engineer for Architectural Energy Corporation and co-developer of SPOT. “Light levels and energy savings can be compared among differing designs for specific geographic locations.”
Version 4.0 includes performance characteristics for specific photosensor manufacturers (based on the 2007 NLPIP Specifier Report on Photosensors by the Lighting Research Center). It can also be used to calculate compliance with daylighting metrics for LEED and the Collaborative for High Performance Schools (CHPS) rating systems, enabling the generation of printable reports.
SPOT uses a Microsoft Excel platform with a RADIANCE engine, handles toplighting and sidelighting daylight sources, and can model any electric light source using existing IES files. The program has two main functions: Design Tool followed by an Analysis Tool.
The Design Tool consists of all geometric and site inputs and calculates electric lighting performance and annual daylighting results and performance. “It’s simple enough for most lighting designers and others to navigate and understand,” says Scheib.
 The Electric Lighting Results page presents nighttime workplane light levels with the electric lighting on at 100% light output.  The Daylighting Results page presents daytime workplane light levels over a year. The Electric Lighting Results page presents nighttime workplane light levels with the electric lighting on at 100% light output.
The Daylighting Results page presents daytime workplane light levels over a year.
The Analysis Tool provides recommended photosensor placements for the analyzed space.
“These recommended placements provide good starting points for analysis and typically require further iteration to fine tune the photosensor system design,” says Zack Rogers, staff engineer for Architectural Energy Corporation and SPOT lead developer. “The iterative process requires more technical knowledge.”
 The Analysis Tool provides photosensor location recommendations and allows the user to define other photosensor scenarios for further analysis. The Analysis Tool provides photosensor location recommendations and allows the user to define other photosensor scenarios for further analysis.
The software produces commissioning reports for the analyzed photosensor system to aid with field implementation. A DOE-2 output function has been added, enabling the annual electric lighting simulation to be integrated into a whole-building energy analysis.
“SPOT reflects the growing importance of quantifying light in a space,” Rogers points out. “Designers can use SPOT to evaluate the daylight and electric lighting in any type of space using a variety of electric lighting control strategies. It plays a unique role in providing the kind of information necessary for design teams to more consistently and successfully integrate daylighting and electric lighting, and it is neutral to the type of technology or manufacturer’s equipment specified.”
SPOT provides annual performance calculations and lighting energy savings.
Click here to learn more about SPOT and download the software free.
 SPOT provides annual performance calculations and lighting energy savings.
By Craig DiLouie, on December 13, 2007
What are the benefits of combining advanced lighting control strategies in the same space? Are the energy-saving benefits of lighting controls persistent over time? Can advanced lighting controls be successfully applied to open offices given concerns about jurisdiction conflicts, lighting uniformity, etc.? Can they enhance worker satisfaction?
A new office lighting field study addresses these questions. Involving about 90 workers in a real-world open-office environment, the one-year study determined that occupancy sensing, daylight harvesting and individual occupant dimming control worked together in the building to produce average energy savings of 47% while correlating with higher occupant environmental and job satisfaction.
The study demonstrates that sophisticated lighting control strategies can be combined successfully to generate persistent, large energy savings in open-plan offices while improving occupant satisfaction with their jobs and workspace.
“The industry has long sought objective evidence that lighting controls not only save energy, but also benefit organizations in other ways such as occupant satisfaction,” says Dr. Guy Newsham, senior research officer for National Research Council Canada – Institute for Research in Construction. “This research provides such evidence.”
 The study
The one-year study occurred within four floors of an open-plan office building in Canada. The building selected for the project was attractive to the research team for several reasons. First, it contained a sophisticated control system operating in an open-plan office setting, an environment often perceived as unfriendly to sophisticated control strategies. Second, this control system combined three control strategies—occupancy sensing, daylight harvesting and individual dimming control. Third, the control system was already installed and in operation. Finally, the site manager was agreeable to the research team not only monitoring energy savings, but also surveying occupants on matters related to environmental and job satisfaction.
Four years earlier, the building had installed 195 direct/indirect lighting fixtures to replace 530 recessed 2×4 T8 deep-cell parabolic fixtures. The new fixtures, centered over the cubicle workstations and containing 3x32W T8 lamps powered by two electronic ballasts, reduced installed lighting wattage by about 40%.
The direct/indirect lighting system features advanced controls, while the parabolic system does not. Workers occupying 86 workstations on three and a half floors participated in the study by using the advanced controls, while workers on half of one floor were still using the old parabolic system, a setup that allowed a comparison between the two groups. Monitoring software was installed to collect detailed data for a period of one year.
“Our research group has had a strong interest in lighting controls over many years, and the opportunity to conduct a field study offered a great complement to the laboratory studies we’ve conducted,” Newsham points out. “There was almost no information available on the long-lasting success of energy-saving lighting control technologies when used in combination in real buildings. In addition, a field study allowed us to explore effects that simply can’t be addressed in anything but a real workplace, but such as those related to organizational productivity. I think everyone agrees that new technologies should demonstrate benefits for occupants and their organizations, as well as energy savings, and such outcomes will promote their adoption.”
 The control system
The direct/indirect fixtures contained an integral occupancy sensor and photosensor. The center lamp, connected to a fixed-output electronic ballast, produced the indirect (uplight) component of the fixture; this ensured uniformity of light on the ceiling. The two outboard lamps, connected to a dimmable electronic ballast, produced the direct (downlight) component of the fixture; light output varied based on signals from four control inputs.
If the occupancy sensor detected vacancy in the workstation below, it signaled the dimming ballast to gradually dim the downlight (outboard) lamps until reaching 0% light output, at which point they were switched off. If the sensor detected occupancy, it signaled the ballast to start the lamps and restore light output to the last set level.
The photosensor monitored light levels on the below task plane, which received variable contributions from daylight available through windows. When light levels exceeded the occupant-set level, the photosensor signaled the dimming ballast to dim the downlight lamps.
Occupants could also dim the lamps forming the direct, or downlight, component of their lighting fixtures via an on-screen slider on their desktop PCs, thereby enabling them to choose their own preferred task light levels.
With this setup, researchers were able to study the overall effect of the combined control system, and estimate the relative contributions of each control type to the overall savings, for a period of one year. The study was conducted in three phases—phase 1 (39 workdays) with just the occupancy sensors and individual dimming controls active, and with a sensor time delay of 8 minutes with 7 minutes of dimming before shutoff; phase 2 (140 workdays) with all controls enabled, and with an occupancy sensor time delay of 12 minutes with 3 minutes of dimming before shutoff; and phase 3 (61 workdays), the same as phase 2 but with email reminders encouraging occupants to use the individual control feature in their workstations.
Just before the study was initiated, the control system was recalibrated. As new employees were hired and entered the study area, or existing employees were re-assigned, the IT department, responding to a request from the energy manager, quickly re-enabled the individual control feature, which would prove critical in sustaining this control strategy.
A fourth control strategy—global automatic on/off switching from a central point of control based on a daily schedule (7:30 AM to 5:00 PM workdays)—was in effect but not included in the study.
The results
By replacing the recessed parabolic fixtures with the direct/indirect fixtures, energy savings of about 40% were realized and lighting power density was reduced from about 0.92W/sq.ft. to about 0.54W/sq.ft.
The combined control system increased lighting energy savings to 67-69% compared to the old parabolic system. Further, the direct/indirect fixtures operating with the control system produced 42-47% energy savings compared to if the fixtures operated at full light output without the controls. All energy savings resulting from the use of the controls were accompanied by concomitant demand reductions. Because the controls ensured that not all lighting power was used at any one time, the average lighting power density in use was about 0.28W/sq.ft. The site manager estimated a simple payback for the advanced system, based on energy cost savings alone, to be 2-4 years in a new installation and 4-12 years in a retrofit installation.
If installed alone, the occupancy sensors would have produced an estimated average 35% savings, daylight harvesting 20% and individual dimming control 11%. Daylight harvesting savings were higher in perimeter workstations, as would be expected (due to closer proximity to windows), and the researchers estimate that savings would have matched the occupancy sensor savings if perimeter fixtures had been allowed to dim below 50% output based on the photosensor signal (deeper dimming based on occupancy sensors or personal control was allowed).
Occupant surveys demonstrated a correlation between the presence of the controls and higher job and environmental satisfaction. While individual dimming’s contribution to overall energy savings was relatively small, researchers credited the improvements in occupant satisfaction to the individual control feature. The researchers are currently looking deeper into the relationship between the controls and worker satisfaction, and hope to publish their results by 2009.
“This study demonstrates that the right package of controls, properly maintained, can produce large, persistent energy and demand savings coupled with benefits to occupants and their organizations, and refutes suggestions that these kinds of control systems cannot work well in open-plan offices,” concludes Newsham. “Although such systems do have a higher initial cost than standard office lighting systems, the overall benefits may justify the investment, especially in the context of other investments organizations make in their employees and their work environments.”
The field study was supported by the Government of Canada, BC Hydro Power Smart and Ledalite Architectural Products. To see the complete study, click here.
By Craig DiLouie, on September 12, 2007
Daylight harvesting is a lighting control strategy that automatically adjusts electric lighting system usage based on available daylight. It relies on a photocontrol system to measure ambient light levels, then switch or dim the affected electric lighting to maintain the desired level of illumination. An effective daylight harvesting system will save energy while being virtually unnoticed by occupants.
Daylight can be captured in a space through sidelighting, such as through vertical windows, or toplighting, such as through skylights. After Heschong Mahone Group, Inc. conducted a study of toplit photocontrol systems and found them to be cost-effective and persistent sources of significant energy savings, which was used to justify new code requirements for toplit spaces in California, the firm next studied sidelit photocontrols.
“There was a general concern that the rumors of poorly functioning systems were commonly associated with sidelit spaces,” says Lisa Heschong, principal of Heschong Mahone. The resulting Sidelighting Photocontrols Field Study, funded by Southern California Edison, Pacific Gas & Electric and the Northwest Energy Efficiency Alliance, examined photocontrol system performance in 123 sidelit spaces in west coast buildings.
“We found that the performance of systems in sidelit spaces was far more variable than in toplit spaces, but that when they were working well, they were capable of a similar level of energy savings, demand reduction and persistence as toplit systems,” says Heschong. “Unfortunately, half the systems in sidelit spaces were found to be not saving any energy, and were not functioning for a wide variety of reasons. Of the systems that were saving energy, only half of those were performing well. Thus there is a lot of room for improvement.”
The Field Study points to significant technical potential for this control strategy, finding the top-performing systems to be saving 1.1 kWh/sq.ft./year, or 51 percent lighting energy savings, while achieving a peak net demand reduction of 0.6W/sq.ft. in controlled daylit areas. Assuming this is the achievable potential for photocontrol systems in sidelit applications, Heschong Mahone estimated that 3,190 GWh/year could be saved nationwide based on the current commercial building stock of 58 billion sq.ft. Assuming an average commercial rate of $0.08/kWh, this equates to $80 million in annual energy cost savings.
 Fisheye view of daylit classroom at Evergreen State College Childcare Center in Olympia, WA. Balanced daylight from two sides of the space increases probability that a photocontrol system will work well. Photo courtesy of Heschong Mahone Group. “I was personally surprised that the best sidelit systems were performing nearly as well as the best toplit systems,” says Heschong. “This is very good news. Some of the best-performing systems were quite old—12 to 16 years old. Thus the technology from a decade ago was fully capable of success.”
The Field Study, however, revealed that only 25 percent of the sidelighting photocontrol systems in the studied buildings were functioning well, whereas almost 100 percent of the toplighting photocontrol systems were found to be functioning well in the previous study. What went wrong?
The researchers discovered that 52 percent of the photocontrol systems were not functioning at all. The most common reason: The system had been intentionally disabled, most often due to occupant complaints. The system also may not have been working because it had never worked (typically due to incorrect or incomplete installation), it had never been commissioned, there was not enough daylight, or the system was incompatible with a present building automation system.
The choice of dimming versus switching did not make a significant impact on the likelihood of success. Dimming systems failed less often than switching, but saved less energy when functional. “Overall, dimming and switching were equally likely to be saving energy, but the two systems have very different challenges,” says Heschong. “Switching systems seem to work best in simple, very well daylit spaces, where the source of electric light is not directly visible to occupants, such as in combination with indirect luminaires. Dimming systems are more likely to be ‘tuned’ down to low levels where they are not saving as much energy as they could, and left continuing to function at these low levels. Dimming also generally requires more knowledgeable designers and installers, and more money. It is important to know that both can be successful, and choose the appropriate application.”
Commissioning may have been a factor. Unfortunately, interviewed facility managers were unclear about the concept, and so it could not be correlated to a greater likelihood of system success or failure. The researchers did determine conclusively, however, that in buildings where the occupants had been trained on the performance of the photocontrols, the systems were more likely to be successful.
“The most common design error was trying too hard—putting photocontrols in spaces that do not have good daylighting,” says Heschong. “Open office spaces with low windows and high partitions are the most common case. Or private offices that are only used infrequently, where occupancy sensors would be far more cost effective. We also saw many cases where the circuited area for photocontrols—i.e., the ‘daylit zone’—was too large, and so the back or a far corner of the zone was not controlled well.”
Just as important: What went right?
The Field Study found that sidelighting photocontrols were most likely to be functioning in spaces with higher daylight levels and more uniform daylight distribution. The controls performed best in owner-occupied buildings, with large open spaces and no partitions, and with daylight entering the room from more than just one wall. Systems that kept it simple, without too aggressively pursuing energy savings, were also more likely to be successful. Systems in classrooms were least likely to fail, but saved the least energy when working.
And the lessons learned? According to Heschong, demand for photocontrol systems has been increasingly dramatically, but the supply of knowledge needs to catch up. She pointed out three rules of thumb to follow:
Rule #1: Make sure the space will receive sufficient daylight. “There is really no point in installing photocontrols in spaces where the daylighting is marginal, or causes visual comfort,” says Heschong.
Rule #2: Keep it simple. “Elaborate integration schemes or highly complex control protocols are more likely to fail sooner or later, when someone down the line does not understand the subtleties,” she says.
Rule #3: Communicate system specifications, design intent and physical location very clearly. “Currently, there is not a deep reserve of common sense in the construction industry about these systems, thus you cannot rely on the contractor filling in the blanks or figuring it out on the fly,” she adds. “Substitutions are usually fatal. If you can’t clearly describe exactly how the system is supposed to work, chances are no one else will be able to figure it out either.”
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