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Daylighting Primer

We at LightLouver LLC are committed to promoting good daylighting design. We have established the LightLouver Daylighting Optimization Program to collaborate with other manufacturers of architectural elements essential to an integrated, effective daylighting design solution. The following Daylighting Primer presents basic daylighting design principles to assist your efforts to develop an effective daylighting design solution.

Integrated Whole-Building Design

The use of daylight to meet the daytime illumination needs of commercial building occupants is a central tenet to designing low-energy or zero-energy buildings. Consequently, in deciding how to use / integrate daylighting into commercial building projects, architects are advised to consider the whole building, encompassing all its various subsystems, as a single, interdependent functioning system, rather than an assembly of disparate, isolated parts. This integrated whole-building design approach looks at the design and performance ramifications of each individual subsystem design decision on the entire building.

Fenestration design and glazing selection will have a significant influence on lighting, mechanical and interior design. The entire design team should participate in design decisions relative to fenestration and glazing which historically have resided with the architect alone. The benefit of an integrated, whole-building design approach is a greater chance of success towards long-term comfort and sustained energy savings in the building at the lowest possible installation and life-cycle cost.

Windows as Daylighting Elements

Architectural decisions can influence the building’s lifetime energy use more than mechanical and electric lighting decisions. Moreover, the best electric lighting and mechanical systems cannot make up for architectural errors with respect to perimeter zone comfort. Guidelines regarding the use of windows in daylighting new commercial buildings are as follows:

An effective daylighting solution must address each and every one of these design issues:

Glare – Visual discomfort due to large amount of daylight in the occupants’ field of view, typically in combination with a high contrast ratio between the daylight entering the space and the surrounding window frame or wall surfaces.
Poor daylight distribution – Excessive daylight levels near the windows and inadequate daylight levels deeper in the space result in non-uniform illumination levels.
Ineffective electric lighting system “daylight harvesting” controls – Improper design or selection of the electric lighting system controls can reduce the amount of “daylight harvesting” and thus energy savings derived from dimming or turning off electric lights.

Side daylighting relies on windows to allow daylight to enter the proposed daylit space. The size, proportion, location and optical properties of the window and means of daylight distribution will have a significant influence on the quantity and quality of the daylight provided to the space.

The higher the window, the deeper the daylighting zone. The practical depth of a daylighting zone without the use of an optical distribution device is typically limited to 1.2 times the window head height. However with the LightLouver Daylighting System, this zone may be extended up to 4 to 5 times the head height.
If a corridor is beyond this zone and separated with a partially glazed wall, it may be adequately lit with the spill light from the daylit room. With LightLouver units in the daylight window and a standard ceiling height (±10”-0”), plan on adequate daylight within 30 feet from the window.
Horizontal ( strip ) windows provide more uniform daylight.The easiest way to provide adequate, uniform daylighting is with a nearly continuous horizontal band of windows. Punched windows are acceptable, but the breaks between windows can create contrasts of light and darkness areas. This is not a problem if work areas are paired with windows or if other glare control measures are taken.
Large windows require more control. The larger the window, the more important glazing selection and shading effectiveness are to control glare and solar heat gain. Limit large windows close to task areas, and use high-performance windows and window shading treatments ( i.e. operable shade cloth blinds ) to control winter heat loss, reduce glare, and improve thermal comfort.
Use separate fenestration apertures for view and daylight.A good approach for excellent daylighting and glare control is to separate the fenestration glazing into “view” and “daylight” windows. Use high visible light transmission glazing in the upper “daylight” windows, and lower visible light transmission glazing in the lower “view” windows to control glare and increase daylight availability.
Use an optical daylighting system to direct sunlight onto the ceiling. An optical daylighting system, like the LightLouver Daylighting System, will redirect daylight deep into the space onto the ceiling surface. For best performance, the ceiling surface must be smooth and light colored. A sloped ceiling (high near the windows) is an effective way to increase the “daylight” window area within normal floor-to-floor heights, and to distribute daylight deep into the daylit space.
Use horizontal window shapes.Horizontal window shapes provide more even, uniform daylight distribution while vertical windows are more likely to create light/dark contrasts throughout the daylit space. Long horizontal windows are generally perceived as less glaring than tall and narrow ones in the same area. Occupants generally prefer wider openings when the views of interest are of nearby objects or activities.
Use shade cloth on “view” windows to preserve view and obtain filtered daylight.Shade cloth window treatments with a 3 to 5% openness factor will preserve a view out of the window in direct sunlight while allowing filtered sunlight in to daylight the area adjacent to the window. Mini-blinds are not an effective window treatment, because to control direct sunlight they must be fully closed eliminating both a view and filtered daylight.

Electrical Lighting Coordination

Electric lighting strategy, fixture selection and lighting control method are all influenced by the goal of daylight integration. For buildings primarily occupied during the day, electric lighting serves to augment daylight that is providing the required ambient lighting for most operating hours. In general, ambient illumination levels can be designed significantly less than task levels (but generally not less than one-third of task levels).

A number of suggestions for integrating electric lighting with daylighting in new commercial buildings are presented below.

Use direct/indirect lighting to avoid glare and match daylight distribution. Direct/indirect lighting keeps the brightest light sources out of the occupants’ field of view and is a good fit with daylight distribution, especially with an optical daylighting device like the LightLouver Daylighting System that redirects all daylight unto the ceiling surface.
Pendant-style direct/indirect fixtures are not typically recommended if ceiling height is less than nine feet. For best light distribution, pendant lighting fixtures should be hung at least a foot and a half from the ceiling. A direct/indirect system will generally be more efficient at providing task illumination than an indirect only system.
Balance the light in a deep room. In daylit spaces greater than 30 feet in depth, provide vertical illumination on the back wall (using ceiling fixtures within two feet of the wall, or with wall-washers) with a cool color lamp temperature ( 4000° Kelvin or greater ) to balance luminance differences between the front and back of the room, and to prevent a gloomy feeling. Use walls or partitions with high-reflectance, light-colored surfaces.
Organize fixture layout to match daylighting distribution. To ensure adequate illumination, lighting fixtures should be grouped by areas of similar daylight availability (e.g. in rows parallel to window wall). Arrange lighting circuits in zones parallel to window wall for daylighting, even if controls are not specified, to allow the possibility for controls to be added later as a retrofit.
Re-circuiting is generally difficult and costly in a retrofit project. However, retrofits for daylighting control are possible even with non-optimal circuiting, due to newer dimming and ballast control technology.
Use LED lamps and lighting fixtures. LED lighting is the source of choice for both dimming and switching applications, because it can be efficiently dimmed over a wide range without changes in color and can be turned on and off virtually instantaneously. Most LED lighting fixtures dim to 5 -10% light output, but “architectural” dimmers can dim to 1%.
Match the cool color temperature of daylight. For best color temperature pairing with daylight, specify LED lamps with a minimum temperature color of 4000° Kelvin.

Glare from Electric Lights

A major concern affecting visual comfort, especially where computers are used, is the problem of glare from electric lighting. Several tips for avoiding glare are presented below:

The quality of perimeter spaces depends on the blending and balance between daylight (a strongly directional light from the side, providing high illumination and cool color) and the very different nature of electric lighting. Incorporating a daylighting strategy does not have a negative side effect on electric lighting design. In fact, lighting quality is typically higher in a carefully daylit space.

Follow recommended practice guidelines for down-lights.To minimize direct glare, electric lighting should generally have a minimum Visual Comfort Probability (VCP) of 80% for computer-based tasks, and 70% for other office tasks. Note that VCP is not defined for indirect lighting or any fixture with an upward component. (VCP is defined as the percentage of people who find the lighting free of discomfort glare.)
Keep lamp reflectance out of computer screens.Limit the potential for reflected glare from ceiling lights in computer screens. If ceiling lights are used, limit high-angle brightness to no more than 850 candelas per square meter at 55 degrees altitude (preferably) and at 65 degrees (definitely).
Avoid brightness glare from exposed lamps in the field of view.Obstruct direct views of light sources to avoid glare. Direct/indirect lighting is one method. Careful space planning is another.