Active Core Sunlighting: Getting Daylight Beyond Windows to a Building Interior

Daylighting Windows Image Credit: iStock / Borisrabtsevich
Image Credit: iStock / Borisrabtsevich
Published On
Jun 14, 2019

Active core sunlighting is the subject of a new design guide (DG-31-18) published by the Illuminating Engineering Society. With this approach, sunlight is captured using solar-tracking optics, delivered deep within the building core, and distributed as general, task and/or accent lighting. By replacing electric lighting at least a few hours of the day, active core sunlighting can save energy and improve lighting quality.

Daylight is a desirable resource for nonresidential buildings. Traditional approaches include toplighting (skylights and nontracking tubular daylighting devices) and sidelighting (windows and clerestories). Light shelves and other elements can deepen daylight penetration. These stationary daylight delivery systems are considered passive.

In contrast, active core sunlighting systems feature moving components that track the sun and use light guides to distribute sunlight within the building. These systems work well with passive daylighting systems. Some active core sunlighting systems use luminaires that deliver sunlight but integrate electric lighting that may supplement or take over illumination as sunlight wanes. Others treat electric lighting as a separate system, though the photocontrols are integrated.

As you might expect, active core sunlighting provides its greatest value in regions with high annual average sunshine probability. It is simpler to implement in new construction, as long as it is given early consideration in design planning. Installation in an existing building may require rerouting HVAC ductwork and plumbing lines to enable a continuous path for light guides.

Active core sunlighting systems may be mounted on a roof or facade. Roof-mounted sunlight collection modules capture sunlight using vertical or horizontal light distribution systems. Facade-mounted systems collect light at each floor and distribute it in the plenum spaces.

A collection module tracks the sun using motors or other motion device, captures and stabilizes sunlight, and concentrates the light for delivery into the building. The sun tracking may be open loop (based on an algorithm; this may be simpler), closed loop (measuring and maximizing delivered sunlight; this may be more accurate), or a combination of the two.

When choosing sunlight collectors, key attributes include ruggedness, sunlight transmission efficiency and degree of light concentration. Typically, the module filters out ultraviolet and infrared radiation, though any filtered ultraviolet and infrared radiation should not be directed so it could damage surface materials or harm people servicing the building envelope. Because sunlight collectors are exposed to the elements, they should be periodically maintained and calibrated to ensure good ongoing performance. Additionally, the filtering element may degrade with prolonged sunlight exposure, so it should be included in maintenance plans.

After the collector, the concentrated sunlight is delivered to the point of use typically by one of two methods. Hollow specular or prismatic ducts can effectively deliver highly collimated sunlight over considerable distances, though longer distances typically require high-quality collection and concentration optics. Solid-optical rods and liquid-filled tubes, often called optical fibers, convey light along a given path using total internal reflection. They are more compact and often flexible but experience higher degradation with distance, generally limiting how far light can be transported while maintaining quality.

Because light absorption can degrade performance and result in failure, special attention needs to be given to selection of light guide materials. Overall, for any light guide, it’s important to evaluate its distance, path through the building, efficiency, spectral selectivity (absorption of certain wavelengths, which can change the light’s color quality), ability to handle concentrated sunlight without overheating, durability, and flexibility (whether it can curve or bend).

Finally, when the sunlight reaches the delivery module or luminaire, its job is to accept the light emitted by the light guide and direct it in a desirable pattern within the space. The luminaire may feature direct or indirect light distribution or a combination. It may provide general lighting, task lighting or accent lighting. Again, the luminaire may be sunlight-only or take a hybrid approach, integrating electric lighting for when sunlight is lacking.

Active core sunlighting should work with electric lighting to provide consistent desired lighting quality. Strategies may involve electric lighting producing similar light pattern and brightness, color temperature, and color rendering as sunlight. The sunlighting system may vary in quality, but quality variation should be mitigated so it is not distracting to occupants. Both systems should deliver the target light levels. For the active core sunlighting system, this relates to a key consideration: how many electrically produced lumens will sunlighting displace.

Overall, active core sunlighting offers a muscular variation on daylighting, allowing deeper penetration of sunlight so it serves as a primary light source throughout a building, not just near glazing. Properly realized, these systems promise benefits including energy savings and high-quality light.

To learn more, get DG-31-18, Design Guide on Active Sunlighting for Buildings , at the IES bookstore at

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