Characteristics of Daylight

Daylight is a dynamic light source that is unmatched in terms of the power, variability, and complexity of the light it provides. Daylight spans an exceptionally broad range of intensities, impacted both by the time of the day and weather conditions, with a color composition that changes regularly throughout a 24-hour cycle.

Dynamic Intensity

Time of day, time of year, and daily weather all affect the intensity of the daylight incident on a window at any given moment, giving the intensity of daylight the characteristic of constant change and a certain degree of unpredictability. Outdoor daylight illuminance can range from 120,000 lux for direct sunlight at noon, which is enough to cause eye pain, to less than 5 lux under thick storm cloud cover. The position of the sun relative to any window changes throughout the day and year as the Earth rotates around its own axis and the sun. Clear or cloudy sky conditions also influence the intensity of available daylight, as can obstructions and reflections in the immediate environment. This variability and unpredictability of solar intensity provides a challenge for the building designer in controlling the amount of the sun’s heat and light entering the building.

The Dynamic Spectrum of Daylight

Visible light has a wavelength range from 400 to 700nm. Violet, indigo, and blue light exist at wavelengths from 400 to 480nm; green light has a wavelength of around 510nm; and red and orange light have wavelengths from 590 to 700nm. White light is a mixture of all of the colors of the visible spectrum. The color of the light perceived by the human eye depends on the relative intensities of each wavelength seen. Although daylight appears essentially neutral like white light, the balance of color (or spectral power distribution) actually changes as a function of the position of the sun in the sky, on the sky conditions (cloudy sky or cloudless sky). For example, daylight has a higher blue component in the morning and a higher red component as the sun sets.

Photo courtesy of SageGlass/Phil Daubman Photography

CASE STUDY: BUTLER COUNTY HEALTH CARE CENTER, DAVID CITY, NEBRASKA

The design of the Butler County Health Care Center in David City, Nebraska, featured a curved, 22-foot-tall glass curtain wall, totaling more than 3,000 square feet, which flares out at a seven-degree angle in curved pie-shaped forms. The design team at Visions in Architecture knew they needed to manage the potentially harsh daylight exposure that the south-facing facade would experience. They also wanted to fully incorporate daylight and views into the wellness and rehabilitation center in order to offer occupants access to the documented benefits in stress relief, decrease in pain medication dependence, and improved patient outcomes that exposure to abundant natural light can provide.

The architects chose EC glazing because of the elegant solution to the heat and light control it provided and the ability to maintain views of nature that patients in the rehabilitation center could enjoy. Given the outward slope and curvature of the facade, shading this facade using conventional means would have been very difficult. The tinting in the dynamic curtain wall is programmed to follow the solar path, more deeply tinting sections of the glass at the time of the day when they receive direct solar radiation.

With EC glass, it is possible for patients and staff at Butler to enjoy unobstructed outdoor views of the nearby park and golf course from a comfortable environment that maximizes the presence and, hopefully, the restorative effects that natural daylight can provide.

As an added bonus, the center was able to reduce HVAC requirements so effectively that it was able to reduce the size of the heating and air-conditioning unit by one size.

Photo courtesy of SageGlass/Phil Daubman Photography

A large facade incorporating electrochromic glazing at Butler County Healthcare Center in Nebraska. This project features a complex curved facade with an outward slope, which provides panoramic views for the center’s patients.