Designing Daylight Fixtures  

The demand to have access to daylight in a commercial space has never been greater. New building codes and green building initiatives continue to incentivize, and are starting to require, that a certain level of glare-free daylight be available to building occupants and that the building be equipped to realize returns in energy savings when daylight is present. Design research and the resulting new technologies now enable windows to be specified so that they will put the right amount of daylight in the right spaces, be energy efficient, aesthetically pleasing, and within budget. Windows are not just windows anymore. Windows can become daylight fixtures.

This article will provide best practices on how to specify a window into a more sophisticated, better-performing daylight fixture and explore the impact that environmental design decisions can have in determining the type of daylight available at the project site in the first place.

Introducing Daylight Fixtures

It could be argued that the sun is the premier light source currently available. It is more energy efficient than an LED with a longer lamp life too. Add unmatched reliability, unparalleled color rendering, and documented health benefits and it is easy to see why a little daylight is such a desirable solution for the built environment. The most common way to get this coveted daylight into the interior is through a window. In architecture, a window is a source of daylight and, as such, a window could be considered a daylight fixture.

But a lighting fixture does more than simply transmit the light emitted from the light source into the space. Fixtures are designed to manage and distribute light, placing certain amounts of light into specific locations. A spotlight focuses the light emitted by the lamp onto a specific target. High-quality recessed troffer fixtures found in commercial environments are designed to keep higher-intensity light out of certain viewing areas to minimize the likelihood that the fixture will create a distracting glare on a computer screen.

A standard commercial window lacks the ability to provide much management to the daylight streaming through it. In that sense, it may be more accurate to say that a bare window performs more similarly to a bare bulb or naked lamp than a lighting fixture. Available daylight streams through the bare window and into the interior space in whatever intensity and angle it exists. On a sunny day, the intensity of daylight may reach up to, or over, 8,000 foot-candles (fc), making bare windows potentially very bright bulbs.

Adding a solar screen fabric and an automated control system to a bare window gives the window the best ability to manage available daylight. The solar screen fabric reduces the total number of foot-candles allowed into the interior and diffuses direct sunlight. This helps to mitigate glare, preserve views to the outdoors, and improve the overall aesthetic of the window wall. The automated system actively controls the position of the fabric in relation to the position of the sun and sky conditions: lowering the fabric to protect the interior from direct sunlight or overly bright skies and raising the fabric whenever more agreeable conditions exist to maximize the amount of usable daylight allowed into the building. It is the ability to better manage the daylight entering the interior that turns a bare window into a daylight fixture.

Just as bare bulbs and naked lamps are never specified, bare windows, too, may soon become a design anomaly. Designers can specify a solar screen fabric and automation system to satisfy certain performance criteria and turn a regular window into an effective, efficient, and aesthetically pleasing daylight fixture.

Design Goals for Daylight Fixtures

Design objectives for incorporating daylight into a space can range from basic daylight management to saving energy with daylighting and achieving daylight autonomy.

Basic Daylight Management

Preventing glare and excessive brightness from destroying the visual environment and preserving views to the outdoors are two of the key considerations in achieving basic daylight management.

Preventing glare and excessive brightness. In an office setting, the range of useful daylight levels is considered to be between 10 fc and 200 fc at the work plane. As previously mentioned, on a sunny day, daylight levels can reach or exceed 8,000 fc. The solar screen fabric must be capable of significantly reducing the number of foot-candles transmitted into the interior, so that employees can continue productively without distraction or discomfort, even when the sun is at its most intense. The automation system ensures that the fabric is, in fact, deployed to protect the interior when these excessively bright, glare-causing conditions exist.

 

Preserving views. When it is too bright to look comfortably at a window, it is also too bright to enjoy the view beyond the window. A barrier must be placed between the window and the interior to block or filter the intense daylight before it reaches the interior. A vertical or horizontal blind can disrupt the view of the outdoors or eliminate it entirely, while deployed to block the sun. A solar screen fabric has been designed to filter and diffuse intense sunlight and enhance the view of the outdoors, simultaneously. Even when the fabric is lowered, building occupants can see the undistorted landscape or cityscape on the other side.

Daylighting

Daylighting, also referred to as daylight harvesting, is the practice of reducing electric light levels when daylight is present. Not only does daylighting reduce electrical lighting use, but it can offer significant savings as it is able to reduce the electrical lighting use during peak demand of a building. Daylighting is becoming much more commonplace and is now required by ANSI/ASHRAE/IESNA Standard 90.1-2010 and the California Building Efficiency Standards. Solar screen fabrics and automated systems create opportunities to light more of the floor plate with daylight, without exposing the interior to excessive brightness or glare.

Daylight Autonomy

In some instances, the goal is not to provide partial illumination of a space with daylight, but for daylight to be the exclusive light source for the space. This lofty design objective is referred to as daylight autonomy (DA). The 10th edition of The Lighting Handbook, published by the IES, defines daylight autonomy as “the percentage of the operating period (or number of hours) that a particular daylight level is exceeded throughout the year.” The DA value represents the percentage of the workday that a space could be exclusively illuminated with daylight.

 

Solar screen fabric and an automation system help designs achieve daylight autonomy by ensuring that whenever there are agreeable daylight conditions, the fabric is retracted, allowing the maximum amount of usable daylight into the interior, with minimal probable glare.¹

Daylighting Metrics

There are a few important metrics used when attempting to compare how successfully a design incorporates daylight. Spatial Daylight Autonomy (sDA) quantifies the percentage of area where an illumination level of 30 fc is achieved exclusively from daylight for at least 50 percent of the workday. Higher sDA values indicate that more interior space can rely exclusively on daylight as a light source for at least half of the workday. Daylight Glare Probability (DGP) predicts the occurrence of discomfort and glare in daylit spaces. Lower DGP values are preferred.

Design Decisions that Affect Available Daylight

While specifying a solar screen fabric and automation system gives the window the ability to manage the available daylight, there are many other decisions, made well before the shade and automated system are added, that play a large role in defining what type of daylight is available outside the window and how the daylight is managed once it moves into the interior. Some of these daylight-impacting environmental factors include: siting, façade orientation, window-to-wall ratio, window glass, and even aspects of interior design planning.

Siting

Siting refers to where the building is positioned on the property. The siting of a building can affect the building's access to daylight. If a building is positioned next to another building or obstruction on an adjacent lot, sunlight may be harshly reflected off one building through the windows of the other or blocked out entirely. In the case of a potential reflection, modeling may be necessary to best estimate how the reflection will act throughout the year, as the sun's position changes. If the façade sits in the shadow of another building, access to daylight may be significantly limited.

Façade Orientation

If the passageway between the sun and a window is unobstructed, then the type of daylight available throughout the year is due, in large part, to the angle at which this unobstructed sunlight strikes the window. This angle changes by time of day and by season as a result of the Earth rotating about its axis and revolving around the sun. Although the actual angle at which sunlight strikes a window is unique to its specific location (latitude, longitude), season, and time of day, general rules describing the relationship between the orientation of a building's façade and the type of daylight it receives throughout the day can be applied to help designers work through the concept of their latest projects and identify where opportunities to bring usable daylight into the space may exist.

In the northern hemisphere, windows with northern exposures offer the best access to glare-free, ambient light throughout the day. Eastern exposures are subjected to direct sunlight as the sun rises, but receive softer, ambient light as the day progresses. This is exactly the opposite for western exposures which access soft morning light and then receive more intense exposure as the sun sets. Of all the exposures, windows with a southern exposure require the most continuous daylight management, because they typically receive some level of direct sunlight exposure from sunrise to sunset.

Climate

A great daylight fixture must be able to respond to more than the position of the sun and the resulting angles of the sun's rays, because sky conditions affect the type of daylight available to a space as well. There is a material difference in the daylight available on a clear, sunny day and the soft, diffuse daylight produced on a cloudy day. On an overcast day, the intensity of daylight may reach a few hundred foot-candles. Daylight on a bright, sunny day can exceed 8,000 foot-candles and poses a serious threat of causing glare and visual discomfort for building occupants. While bright, sunny days may require that the most rigorous daylight filters remain in place all day, cloudy days may offer opportunities for the solar screen fabrics to remain at least partly open.

Window-to-Wall Ratio

The window-to-wall ratio (WWR) refers specifically to the ratio of the total area of a building façade which is occupied by windows. Although it may seem counterintuitive, creating a presence of usable daylight in the interior and realizing the associated energy savings through daylight harvesting does not require that the building façade be made entirely of glass. A WWR of 30 percent is generally regarded in the industry as the minimum WWR necessary to create any measurable energy savings through daylight harvesting.

In terms of a maximum suggested WWR, this is location specific, as variability in location, climate, orientation, type of glass, interior design, and the type of shading system can dramatically impact both the useful daylight available to a space and how efficiently a space is able to use greater quantities of daylight.

Although the actual upper limit may vary from one project to another, studies on the subject have demonstrated that there is a point in the WWR where the further addition of windows returns only nominal energy savings in lighting and can even negatively impact the overall efficiency of the building, because the added windows increase the heating and cooling demands on the HVAC system. Daylight management technologies capable of reflecting solar heat, such as dynamic shading systems, can enable the WWR of a facade to be larger, without exposing the space to unacceptable levels of heat gain.

Window Shape and Position

The size, shape, and configuration of the windows in the building also play a large role in determining how the daylight will fill up the space.

How deeply daylight will penetrate an area is affected by many variables. The height of the window is one important variable. As a general rule the higher the window head height, the deeper into the space daylight can penetrate.

 

The configuration of the windows influences how the daylight is distributed throughout the space. Continuous bands of windows provide more even or uniform illumination along the interior when compared with punch windows, which create pockets of more concentrated daylight exposure at the window surrounded by areas that are more dimly daylit. Even and uniform illumination is often a critical goal in creating a balanced visual environment and a functional workspace.

Glass Properties

Visual transmittance (Tv) is a metric often used in discussing windows and window treatments. It is the amount of visible light that passes through a window material and into the interior. The visible transmittance of a window is influenced by the number of panes and any glass coatings or tinting that has been applied to it. The typical visible transmittance for a single pane of uncoated clear glass is above 90 percent, which means that more than 90 percent of the daylight that strikes the glass is transmitted into the interior.

The visual transmittance of a window can be lowered by adding additional panes of glass, glass coatings, or tinting. For example, the visual transmittance of the single pane of uncoated clear glass can drop from 90 percent to below 10 percent when a highly reflective coating is added to tinted glass. Adding additional panes of glass is another way to decrease visual transmittance. A typical double-pane of insulated glass with no coating or tinting has a visual transmittance of roughly 78 percent and adding a third pane will decrease transmittance even further.

Tinted glass is designed to absorb a portion of the solar heat and daylight that strikes it, which is how it reduces its visual transmittance. One of the limitations of tinted glass is that there is no way to remove the filter. It always reduces the amount of light coming into the interior. Regardless of whether the light is direct and very intense or soft and diffused, traditional tinted glass treats both types of daylight as if they had the same propensity for disruption, which, of course, they do not.

Interior Space Planning

This increasing demand to bring more daylight deeper into the interior is causing designers to rethink traditional office space layouts, interior furnishings, and the color of the finishes.

Lower partitions. Tall partitions can stop daylight in its tracks, allowing only the light high enough to pass over the top of the partition deeper into the space. Consider an open office space with a 10-foot ceiling height and floor-to-ceiling window wall that is filled with partition walls 8 feet tall placed 10 feet from the window to create rows of private offices along the perimeter. Unobstructed, a 10-foot-tall window could allow daylight to penetrate the interior space up to 25 feet. However, placing the tall partition 10 feet away from the window will effectively block the majority of the daylight from traveling any deeper into the space.

Designers are selecting lower partition walls knowing that every extra inch allows a few more foot-candles further into the space. They are also being more creative in where the partitions are located, choosing to move panels deeper into the floor plate to decrease the divider's ability to obstruct the movement of daylight.

 

Perimeter walkway. Attaining a private office with a window used to be a symbol of professional accomplishment; a corner office with two or more windows was the crowning jewel of corporate success. Research now reveals that these ever-coveted window offices may have subjected their occupants to unruly glare and heat gain throughout the day and unfairly hoarded all of the available daylight for the upwardly mobile. All of this information has left many designers thinking that there may be a better solution for distributing and managing daylight in the commercial space.

A growing trend in office space design is to consider a perimeter walkway, instead of row of private offices along the perimeter. The walkway solves two problems. As a more common area than a private office, the daylit walkway provides more building occupants with access to daylight and outdoor views. The walkway also provides a distance buffer from the light and heat that can stream in through the window, allowing daylight access, but less daylight-caused discomfort when a person is at their desk.

Lighter interior colors. Specifiers are selecting lighter interior colors and finishes to extend the life of the daylight inside the building. Interior colors and finishes are important, because as daylight enters the interior space, it bounces off of walls, ceilings, floors, and furniture. With every interaction, some of the light is absorbed and the rest is reflected back into the space. As a general rule, lighter colors reflect more of the light back out into the environment.

The reflection index of a surface color describes the relationship between the quantity of light received and then reflected. The higher the reflection coefficient, the better the surface is at reflecting light energy. For example, a white surface typically has a reflection factor of 80 percent, which means it reflects 80 percent of the light that strikes it. Beige can reflect 60 percent of the light energy that strikes it. A black surface typically has the lowest reflectance, with reflection factors commonly falling below 10 percent. Of course, reflective performance can be affected by color, surface, and sheen and the actual reflective performance of any material should be evaluated before being specified. But as a general rule, lighter colors enable daylight to live longer in a space.

Dramatic Comparison

The elements of interior design work together to create either a daylight-friendly or daylight-damping space. Simulations conducted by Purdue University found the overall impact of interior design on daylighting performance to be significant. These simulations compared two open office spaces, identical in every way, except interior design. Space A has an aisle 6 feet wide around the perimeter and light-colored room surfaces. The perimeter of Space B is lined with office areas and the interior is filled with darker materials, finishes, and colors. While Space A and Space B had identical threats of glare with a DGP of 35 percent, the amount of space that could be illuminated by daylight for at least 50 percent of the day was dramatically different. The light-colored space is able to daylight nearly 40 percent more interior area than the dark-surfaced area.

 

The Performance and Efficiency of a Daylight Fixture

Siting, façade orientation, and climate help to determine the type of daylight readily available at the window. Window height, window configuration, and interior layouts all impact how daylight performs once inside the interior space. Just as lighting fixtures take the light from the source and distribute it purposefully, a good daylight fixture takes the available daylight, manipulates it, and delivers the right type of daylight into the interior. The performance of the daylight fixture refers to the effective managing of the available daylight, keeping the glare-causing light out and preserving the views to the outdoors. The efficiency of the daylight fixture refers to maximizing the amount of useful daylight allowed onto the floor plate. The performance and efficiency of the daylight fixture are determined by two items: the solar screen fabric and the automation of the shading system.

Solar Screen Fabric Characteristics

The solar screen fabric is the part of the daylight fixture that physically manages the available daylight by reflecting it, absorbing it, or allowing it to pass through the fabric into the interior. The openness factor and the color of the solar screen fabric are the characteristics that determine the amount of daylight that the shade reflects, absorbs, or transmits. Understanding the role that openness factors and fabric color plays in the overall performance of the fabric is helpful in specifying a solar screen fabric that will perform as desired in a space: combating the specific glare conditions at the window, preserving outdoor views, and maximizing the amount of usable light that transmits into the interior.

Openness factor. The openness factor describes the ratio of open space to fabric material in the woven shade. For example, a shade with an openness factor of 10 percent means that 10 percent of the woven shade is air. The greater the open space between the threads, the greater the openness factor of the shade.

Solar screen fabrics are typically divided into three categories based on openness factor. A sheer solar screen includes screens with an openness factor ranging from 1 to 18 percent. Translucent solar screens, also called privacy screens, have an openness factor of less than 1 percent, but are not an opaque shade. Blackout fabrics offer 0 percent openness, transmitting no light into the interior. Sheer solar screens are able to provide more balanced performance offering glare control, while preserving outdoor views and providing some degree of daylight into the interior. Translucent and blackout shades provide more rigorous control of the daylight and allow considerably fewer foot-candles onto the floor plate.

The openness factor is a feature that affects both the fabric's ability to control glare and preserve view. Unfortunately, these two performance goals are at odds with one another. As the openness factor increases, more of the fabric material is replaced with open space and there is less fabric present to diffuse the direct daylight that can cause extreme brightness and glare. There is also less fabric obstructing the view to the outdoors. As the openness factor decreases, the fabric becomes a more restraining filter for direct daylight, and, simultaneously, a more present visual barrier to the outdoor view.

 

Fabric color. The color of the solar screen fabric determines whether the fabric will reflect a greater portion of the daylight back out into the space or absorb it. Light colors are more reflective than dark colors, so a lighter fabric will reflect more of the daylight that strikes it. A darker fabric will absorb more of the daylight that strikes it.

The propensity to reflect, absorb, or transmit daylight directly impacts the clarity of the outdoor views that the fabric is able to provide. When daylight strikes a light-colored solar screen fabric, much of the light energy is reflected back toward the window or transmitted into the space. The light diffused through the fabric can create a brightness at the fabric, reducing the contrast between the brightness of the fabric shade and the brightness on the other side of the window. The eye is designed to focus on the area in the field of vision that is the brightest. When multiple items near each other have a similar brightness it is difficult for the eye to focus effectively. When the brightness of the fabric is similar to the brightness of the outdoors, it can be difficult for the eye to see past the bright shade into the equally bright exterior. Darker solar screen fabrics absorb more light than they reflect and so they do not create a competing brightness at the window. The eye is able to focus easily on the outdoor view beyond the fabric.

Although current design trends seem to favor light finishes and materials, when it comes to solar screen fabric selection, light-colored fabrics can increase brightness at the window and decrease views. Darker solar screen fabrics provide clearer views to the outdoors, decrease brightness, and reduce glare.

Solar Screen Fabric Metrics

When determining whether a solar screen fabric can meet the specific needs of a space, it helps to be able to speak quantitatively about the way that fabric manages daylight. There are four solar performance metrics that help to describe how a particular solar screen fabric will perform in a space. The solar performance metrics are: solar transmittance (Ts), solar absorptance (As), solar reflectance (Rs), and visual transmittance (Tv). Solar transmittance (Ts) is the percentage of solar radiation that passes through the fabric. Solar absorptance (As) is the percentage of solar radiation absorbed by the fabric. Solar reflectance (Rs) is the percentage of solar radiation reflected back out by the fabric. The relationship of these performance metrics is:

Ts + As+ Rs = 100 percent present solar radiation.

Visual transmittance (Tv) is the percentage of potentially glare-causing visible light that passes through the fabric. Lower values indicated greater glare reduction. For example, a Tv of 14 percent indicates light reduction of 86 percent.

Ts, As, and Rs. The management of solar radiation as represented by Ts, As, and Rs describes how the solar screen fabric manages the heat component of daylight. If it is important that the solar screen fabric minimize solar heat gain, designers should look for a fabric with a higher Rs value. A dual-sided solar screen fabric is a popular selection when it is important to combat solar heat gain and prevent excessive brightness at the window wall. A dual-sided solar screen fabric has a different color on the interior and exterior sides of the fabric, with a more reflective exterior color to reflect more of the solar radiation back outside, and a darker interior color to protect the interior space against glare conditions.

Tv. Visible light can pass through a solar screen fabric and into the interior in two ways. It can pass directly through the open portion of the fabric weave or it can be diffused by the fabric and still transmitted onto the floor plate. The Tv value, which describes the percentage of visible light transmitted through the fabric, includes both means of transmittance in its calculation and is affected by both the openness factor and color of the solar screen material.

 

The Tv value of a fabric has a considerable impact in how the fabric performs with regard to managing glare, maximizing the presence of useful daylight, and preserving the view of the outdoors. Consider three different solar screen fabrics. All three have an openness factor of 3 percent, but they are each a different color, resulting in a unique Tv value for each fabric. Darker fabrics absorb more light energy than they reflect or transmit, so, when openness factors are identical, darker fabrics will have a lower overall Tv value than a light fabric.

In this example, based on daylighting simulations conducted by Purdue University, the three different colored fabrics have Tv values of 5 percent, 9 percent, and 13 percent. The darkest fabric has a Tv value of 5 percent. A mid-range color fabric has a Tv value of 9 percent. The lightest fabric has a Tv value of 13 percent. As detailed in the table, the darker fabrics were able to keep glare probability low, while preserving the view. The lightest fabric offered the worst protection from glare and provided the poorest performance in view preservation. However, the mid-range color offered the most optimal solution, because it was able to provide a greater degree of daylight autonomy than the darkest solar screen fabric.

It should be noted that achieving an optimal daylighting solution in a space is not trivial. It requires careful consideration of the environmental factors addressed in this article, such as siting, orientation, WWR, etc., but it also requires a prioritization of performance goals (glare control, view preservation, daylight autonomy) and a knowledge of the daylighting solutions available.

Best Practices for Selecting a Solar Screen Fabric

Perhaps the most common mistake made when specifying a solar screen material is selecting a fabric based on color with little regard for the openness factor or Tv value. The little known truth in the design community is that the fabric provides more than aesthetic appeal, it dictates the performance of the window as a daylight fixture and, as such, it is critical to get it right. Here are some best practices to consider when selecting the right solar screen fabric for a space.

Determine the priorities. First determine the daylight management priorities for each space. Identify the areas where glare control is critical and the spaces where achieving a greater degree of daylight autonomy or preserving an outdoor view may be more important. Functional spaces such as office spaces and conference rooms may prioritize glare control over view preservation, whereas social and transitional areas, such as hallways and cafeterias, can enjoy greater levels of daylight, without disrupting the purpose of the space. Once the priorities have been established, it is easier for a designer to find a fabric that appropriately balances the need to keep glare out, without unnecessarily obstructing the view and while allowing usable daylight in.

Openness factors and orientation. Solar screen fabrics with different openness factors are often specified throughout a project to best accommodate different daylight conditions and different types of space. Eastern and western facades that experience direct sun exposure during sunrise or sunset are often fitted with solar screen fabrics that have an openness factor of 3 percent or less (often 1 percent). Fabrics with openness factors of 4 percent or less are often placed on windows on southern facades with a direct view of the sun.

Tv value and optimal performance. Once the appropriate openness factor has been determined to manage the presence of direct daylight, the Tv value can help a designer select a fabric that will provide the optimal balance of daylight transmission, daylight absorption, and daylight reflection. There are three basic statements to describe the relationship between the Tv value and fabric performance. Remember that lower Tv values reduce the likelihood that the solar screen fabric will become an overly intense or glaring light source on the window. Lower Tv values can offer better view preservation, because the fabric remains muted and does not create an additional bright spot to draw the focus away from the outdoor panorama. To optimize energy efficiency, it is possible to increase the Tv value, so that the solar screen fabric still provides adequate glare control and view preservation, and also allows enough daylight into the interior to achieve a level of daylight autonomy.

Functionality of an Automated Shading System

A solar screen fabric actively manages daylight by filtering, absorbing, and reflecting it. An automated shading system ensures that the shade fabric is deployed when needed, to the exact position it is needed, and then retracted whenever possible to maximize the amount of ambient light allowed onto the floor plate.

Ensure protection from glare and bright daylight. An automated shading system uses a combination of sunpath diagrams, local sensors, and programmed settings to determine when a window is being threatened by direct sunlight or bright sky conditions and lowers the fabric shade to protect the visual environment from glare and hot spots. The advanced scientific algorithms and automation eliminate the guesswork and man-hours spent to properly position and reposition a manual shade throughout the day, day after day.

Maximize access to usable daylight. Perhaps the greatest advantage offered by an automated shading system is not that it lowers the shade to block glare, but that it reliably raises the shades when conditions are agreeable. It is common practice to pull manual shades down to eliminate glare or excessive brightness and to rarely touch the shade again. In fact, studies have documented that once a shade is pulled down, it oftentimes remains down, even after the problematic conditions are no longer present.

Leaving shades deployed day after day causes two issues. Unless the manual shade is made of a transparent, woven solar fabric, the shade may entirely obstruct the outdoor view, eliminating the access to daylight and views originally intended and negating the presence of the window. It also minimizes the presence of usable daylight in the interior. If the shade is always deployed, it essentially works as an additional layer of tinting, further filtering all daylight, even the good stuff.

The ability of the automated shading system to maximize the amount of usable daylight in a space is becoming increasingly more necessary as more projects attempt to achieve daylight autonomy. As a reminder, daylight autonomy occurs when all or a portion of a building's lighting needs can be met exclusively with daylight. One popular metric for measuring daylight autonomy is the sDA value, which quantifies the percentage of area where an illumination level of 30 fc is achieved exclusively from daylight for at least 50 percent of the workday. Only deploying shades when necessary and allowing the maximum amount of usable daylight into the space, whenever it is available, is how automated shading systems help to increase sDA values for projects and help designers achieve daylight autonomy.

 

Automation and sDA

Automated shading systems have been found to more than double the amount of interior space that can be exclusively illuminated with daylight and improve access to views, while maintaining glare protection. Simulations conducted by Purdue University found automated shades maintained the glare protection (at 35 percent DGP) provided by a perpetually closed shade, more than doubled the amount of space that could be exclusively daylit during a large portion of the workday and dramatically improved view preservation.

In fact, automated shades provide a better daylight management solution than reducing the WWR and lowering the visual transmittance of the glass. Purdue University also simulated the performance of two spaces with significantly different WWR and glass types. Using an automated shading system in a space with a 90 percent WWR and typical commercial glass, with a Tv of 65 percent, nearly doubled the amount of space filled with usable daylight during the workday and dramatically improved view preservation, without increasing the probability of glare at all, when compared with the same space with a 30 percent WWR and glass with a more restrictive Tv of 30 percent.

Compare Different Automated Shading Systems

A key differentiator between one automated shading system and another is the amount of information that the system takes into consideration in order to determine the optimal position of the solar screen fabric.

The most basic automated shading systems are programmed to respond to the sun's position. They use the season, sunpath, and time of day to determine whether the solar screens should be deployed and, if so, to what position, half-way down the window, entirely down, etc.

Unfortunately, there are factors that affect the relationship between daylight and a window that are not considered in this basic equation. If parts of a building sit in the shadow of another building, solar screens may be attempting to manage daylight that has already been blocked. Beyond shadows, sky condition is another factor that can dramatically impact the need for protection, but sky conditions are an unpredictable, real-time piece of information outside the grasp of these more rudimentary automated systems.

Some systems include local sensors at the window. These local sensors detect, in real time, conditions that may require an alternative solution to the already programmed fabric position. If excessively bright light streams through on a bright day or soft, more diffuse daylight is available on a cloudy day, even at noon, or the shadow of a nearby structure changes, or if seasonal snow cover has created an exterior shade, these systems use the measurements of the local sensors to override the previous positioning plan. On bright days, the solar screen fabric is lowered until the intensity of the penetrating daylight wanes. On cloudy days, the solar screen will raise and remain retracted as long as ambient light levels remain within a predefined, usable range.

Now windows can be specified to satisfy project-specific daylight performance and efficiency objectives. Selecting the right fabric and the right automation system is a critical piece of maintaining the integrity of the visual environment and successfully incorporating daylight to achieve daylight management, daylight harvesting, or daylight autonomy. With the right equipment, a specifier can turn a window into an efficient daylight fixture.

Lutron Electronics, headquartered in Coopersburg, Pennsylvania, designs and manufactures energy-saving light controls, automated window treatments, and appliance modules for both residential and commercial applications. Its innovative, intuitive products can be used to control everything from a single light, to every light and shade in a home or commercial building. www.lutron.com