DAYLIGHTING Magazine issue 20 January/February 2020

SMART GLAZING

Making smart-tinting glazing even smarter

Shadow Mapping with Photographic Obstruction Maps for Better Daylighting, by Andrew McNeil, Facade Performance Specialist, Kinestral Technologies, Inc.

Human reaction to direct sunlight through glass, which translates to glare or thermal heat, is immediate: shades go down. Those same occupants, however, are slower to raise the shades when discomfort disappears. Consequently, shades stay down longer than necessary, depriving occupants of daylighting and views are blocked.

Automated shading solutions are growing in popularity because they can be programmed to raise shades when

the risk of glare and thermal discomfort is low, enhancing daylighting. They’re increasingly common in large commercial office buildings, but window shading in smaller buildings and residences are left out of automation – primarily because of cost. The cost of centralized control hardware and commissioning sensors on a rooftop might be more easily absorbed in a building with a thousand windows than a building with only 10 windows.

Dense urban environments are often shaded from direct sunshine by neighboring buildings, awnings, fins, trees, mountains, and other contextual obstructions for intermittent periods. When site shadowing isn’t taken into account by the shading control system, shades can be deployed unnecessarily.

Shading Automation Today

People vividly remember when the shading system isn’t doing the right thing – even if it’s only for two hours of the work week or 95% correct automation. From an occupant’s point of view, satisfaction can only be achieved when automatic shading is at least 99.5% accurate. Automation should not activate shading when a window is in a shadow.

To account for shadowing, some automated shading vendors generate annual shadow schedules for windows using ray tracing CAD models on site. Most automated shading control systems can account for exterior geometry. What they do is they build a CAD model of the building and the site and then they ray trace from the windows to the site. The trouble with this is that setting up the CAD model, verifying that as accurate can cost money and add a lot of time to a project.

CAD model availability is also spotty. For major cities like Paris, there’s probably a good CAD model of most of the city. In most small or mediumsized cities, all that’s often available are the footprints of the buildings. Heights have to be estimated. Any errors in the alignment or the height of exterior buildings used can get baked into scheduled automation and can be difficult to diagnose and correct afterthe-fact. While not insurmountable, shadow modeling adds additional costs to projects that small projects can’t absorb. The cost of setting up the shadowing schedules can be justified when amortized over hundreds or thousands of windows.

Photographic Obstruction Mapping: A New Way to Account for Shadows

Photographic obstruction mapping is about achieving more accurate automation and doing it more economically so that it can be used on small buildings. That was our goal at Kinestral Technologies when we developed our photographic obstruction mapping tools. Our goal was to enable our Halio® smarttinting windows to tint or clear to near 99.9% accuracy from the view of the occupants – and make that practical for even smaller projects. Large projects can use our system to spot check the accuracy of the ray-tracing method.

The idea behind photographic obstruction mapping is simple: a camera is used to collect angles of exterior obstructions to a window. The photos are remapped to angular pixel coordinates, and the area of the sky that’s visible from the window is traced. The traced sky is then queried in realtime to determine whether or not a window sees the sun or is in shadow.

To do this, we developed a prototype camera based on the raspberry pi system. It has a 180-degree fisheye lens, allowing a full view left to right and from sky to ground. A black shroud makes it possible to hold the camera against the window so that it’s aligned to the plane of the window and the lens is very close to the glass. The black color reduces reflection for a clearer image. Because it’s nearly impossible to hold the camera perfectly level, we added an accelerometer that can measure the angle, which can then be used to correct image rotation so that they are level.

... we developed a prototype camera based on the raspberry pi system. It has a 180-degree fisheye lens, allowing a full view left to right and from sky to ground.

Fig. 1: The image above is the corrected version of the first image.

No lens is perfect, so we also correct for angular distortions. To account for this, we put the camera on a motor and step the camera every few degrees. Figure 1 shows the uncorrected and corrected image.

We call our shadow mapping image format an orthonormal pseudocylindrical projection. The X-axis has the azimuth angle and the Y-axis has profile angle. The benefit of using this projection is that lines that are Orthonormal to the field of view are straight in the projected image. And it turns out that a lot of the built environment is orthogonal so that can be a helpful feature. Figure 2 demonstrates two renderings of a cube with grids. On the left is the equirectangular projection that is commonly used in virtual reality: the vertical lines are straight, but the horizontal lines are all curved. On the right is the orthonormal projection which shows straight horizontal and vertical lines.

Electrochromic Glass: Perfectly Accurate Modern-Day Automated Shading

Electrochromic technology has advanced dramatically. Continuous tints (versus a fixed number) and transition speeds 10x faster than earlier entrants make it possible to respond quickly to changing conditions.

In a real-life setting, the Halio window control system queries the photographic obstruction mapping in real-time relative to the sun’s path throughout the year to determine when to tint a window or not and by how much. Our system can query the system every minute, every hour, or every 30 seconds.

Photographic obstruction maps are quick to create and easy to setup. Obstruction maps allow smart-tinting facades to admit diffused daylight when a window is in shadow for prolonged periods of time, improving the daylight amenity provided to occupants. Additionally, photographic obstruction maps provide a visual record of the condition at the time of setup, an asset for troubleshooting and customer support when buildings are erected or demolished.

Close-up of the camera based on the raspberry pi system

Fig. 2: Two renderings of a cube with grids. On the top is the equirectangular and below it is the orthonormal projection which shows straight horizontal and vertical lines. record of the condition at the time of setup, an asset for troubleshooting and customer support when buildings are erected or demolished.

Daylighting is about maximizing daylight without sacrificing occupant comfort. Photographic obstruction mapping makes it possible for systems like our Halio smart-tinting glass to improve automation performance for small to medium-sized projects.

For more details about photographic obstruction maps see “A Photographic Method for Mapping Angular Locations of Exterior Solar Obstructions” published in The Journal of Building Engineering.

www.kinestral.com

Andy McNeil is a daylight specialist whose current work focuses on developing fenestration products that increase the quantity of daylight and the quality of daylight, i.e., dramatically reduced glare. Before Kinestral Technologies, Andy was a lighting consultant at Arup.

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