Princeton Firestone Library

Daylighting Design

Iteration 5: Electrochromic Glazing

29 January, 2016


Currently, the glazing across the atrium is a 14% VLT (Visible Light Transmission) glass with a 50% opaque brise-soleil mounted below the glass, resulting in an approximate overall 7% VLT. The temporal map below shows yearly illuminance at the circulation desk (center of bay 3), based on typical weather data for nearby Trenton, NJ. Below that, the illuminance distribution on July 11, at 2pm, one of a number of worst case times in mid-summer months. Clearly the current installation cannot accommodate the target of 500lux maximum.


Electrochromic glass produced by Sage Glass has the capacity to control not only the worst case conditions (see the illuminance levels below), but also adapt to lighting conditions across the year.

In these simulations, all angled glazing (1-4) are set to 1% VLT, and the vertical glass (5) is set to 60% VLT. End glazing is set at 34% VLT. This temporal map shows that in the worst case, direct sun is cut down to below the 500lux threshold, except for times earlier than 7 AM from light entering through the east wall, when the library is closed.

Illuminance distribution at July 21, 2pm. Striping is due to shading by the mullions.


Given that 1% glass can handle direct sun penetration, we propose programming the electrochromic skylights to respond to the sun path and geometry, such that the patches of darkest glass are no larger than required, and the contrast at their edges is softened by bands of glass of the middle tones (6% and 20% VLT). Pre-programming the system using the algorithm developed for the representations below, the skylight would become a dynamic, slowly shifting work of art throughout the day and year.

Try moving the sliders on the image above – during the winter months, little adjustment is required, but during summer months, much of the skylight must be dropped to 1% VLT. By maintaining dynamic control, the space never feels dark and gloomy.


In this illustration of the glazing control concept (simulating May 21 or July 21), the VLT of each glass pane is adjusted based upon where direct sunlight passing through that pane would strike within the space. Sunlight striking the work-plane 30″ above the floor causes the glass pane VLT to be set at 1% (red rays). Sunlight striking the lower wall area sets the glass VLT at 6% (orange rays), the upper wall 20% (yellow rays), and the high wall 60% (green rays).  If the sunlight strikes none of these surfaces, the glass pane will also be left clear at 60% VLT. That control patterning is based on the control of direct sunlight. In addition, if at certain times the overall light levels including inter-reflected light would exceed the desired illuminance threshold, the algorithm would respond to reduce the glass VLTs accordingly.

Three views of May 21 / July 21st: Upper left shows the rays inside the space, darkness on the walls represents the zones of control; upper right shows the space from the exterior, and at bottom is a rendered set of images (transmissions and color are approximate and representational).

This program would only operate during sunny days in which direct sun hits the space. A photocell on the roof would watch for cloudy skys, and lighten the glass accordingly. Additional programs could also be accommodated, such as a weekend / after hours mode that drops the transmission to 1% across the roof.

This solution provides maximum flexibility across the space throughout the year, and a unique method for controlling daylight, utilizing the capability of the electrochromic glazing to provide a dynamic shading response precisely tailored to every current sun and sky condition.