The subject of daylight, and especially glare, is complex and ambiguous. As a newcomer to the space, I found my basic questions typically elicit replies that begin with every engineer’s favorite prefix: “it depends…”. Questions that may seem straightforward like “what is glare?” or “how much daylight makes a space ‘daylit’?” cannot easily be answered. There are a lot of metrics for the same thing, and these metrics seem to be changing much faster than marketplace understanding. However, it is possible to demystify some of this complexity with a basic conceptual organization that really anyone in the building industry can wrap their head around.
How to Design Buildings with High-Quality Daylighting
Ultimately, what designers, owners and occupants want is a space with quality daylight. We can break daylight quality down into four key parameters:
- Daylight sufficiency
- Glare avoidance
- Proper spectrum
- Subjective preference
1. Daylight sufficiency
To discuss sufficiency, we first need to talk about how daylight quantity is typically expressed. Generally, this is in Lux, which in layman’s terms is light intensity. Light sufficiency has three elements:
Light Intensity + Floor Area + Time
How much daylight, over how much of a space, for how much of the year gives you a visual sense of your daylight sufficiency. These parameters are pretty intuitive and they serve as the basis for one of the most common metrics for daylight sufficiency, Daylight Autonomy (DA). This is the metric LEED uses for its daylighting credit and the approach adopted by future European standard EN17037 on daylight of buildings for the assessment of daylit spaces. While these three parameters serve as the constants of sufficiency, the variable is always application. Needs in a laboratory setting are different than in a typical office setting, for example. Depending on needs, lux levels, percent of floor area and time can all be adjusted. What is good about DA is it provides an adjustable framework that lets you analyze daylight sufficiency, while modifying based on application or customer specific needs. In future blogs we will talk more DA (and its cousin sDA, Spatial Daylight Autonomy) as well as other daylight sufficiency metrics.
2. Glare Avoidance
Glare is generally understood to mean light levels that are distracting or uncomfortable. Perception of glare is influenced by many factors and glare itself has numerous sub-types. We will dig deeper into glare’s nuances at a later date, but we can cover some basics of how glare is measured to understand how to approach glare control.
Lux levels can serve as a decent proxy for other more specific glare measurements. That is to say, at a certain lux level it’s safe to say glare is likely for some number of occupants in a space, at some points in time. LEED again provides guidance with its Annual Sunlight Exposure (ASE) metric. This is primarily a Lux-based metric though. Daylight Glare Probability (DGP) is designed to understand the probability that a person, in a given space, will experience no glare, tolerable glare, or intolerable glare at different points in time. The subject of glare measurement deserves a lot of attention on its own, but here’s the basic takeaway: these two metrics are good (albeit imperfect) indicators of glare, and many modeling tools use them to show building designers glare risk as well as the relative effectiveness of different glare mitigation approaches.
3. Proper Spectrum
In an earlier blog post (Daylight, A Powerful ally for our Health!), we spoke about the growing understanding of the “non-visual” effects associated with light. This is largely about how different spectrums of light have non-visual properties that have been shown to impact peoples’ health and wellbeing. Like glare, this is a complex issue deserving of its own attention. The point here is that light quality needs to consider how different spectrums of light, at different times of day, impact people. So, “proper spectrum” is time dependent, not a single value. Our understanding of these impacts is still maturing, but no discussion of daylight quality is complete without considering it.
4. Subjective Preference
Quality cannot be determined by metrics and models alone. Daylight quality, like quality of other goods and services, is subjective. No amount of data showing a space has quality daylight will convince people who feel it’s too dim, too bright, full of glare etc. It is true that the goal of various daylight models and metrics is to try and serve as accurate predictors of perceived quality, and some are even informed by human research in this area. The reason why this merits listing on its own is to reinforce the point that we can’t forget about people. Assessing the real world impact on human beings needs to be a critical part of any approach to delivering quality daylight.
While it is complicated, understanding daylight is ultimately about recognizing the four parameters of daylight quality in a space. These parameters can serve as the guideposts for a daylighting strategy. As building owners and designers strive to create better buildings for people, it’s important to recognize that daylight is a valuable resource for building occupants, one worth managing well. Having a solid foundation in the basics of daylight is a good place to start.