Optimizing Daylight in Different Buildings

Not all buildings are the same, and neither are their daylighting solutions

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Advanced Computer Simulations for Daylighting Design

With an understanding of the significance of daylighting coupled with a grasp on the ways to optimize it in building design and construction, the question now becomes: How do we know if it will give us the desired results? The best way to answer that is to analyze options and determine outcomes using advanced computer modeling specifically for daylighting. Many architects are familiar with some architectural design software (CAD, BIM, etc.) that will do some rudimentary daylighting modeling showing shadow determination and radiance illumination inside buildings. Typically, these programs provide some basic information and have been used by many to get an overall sense of lighting in a building. Other stand-alone or add-on software programs are also used that look specifically at daylighting in a building.

All lighting design and analysis is fundamentally based on the ability to measure quantities of light. In this regard, the international standard is a lumen, which is a measure of luminous flux: the total quantity of visible light (energy) emitted from a source (the sun, artificial light, etc.) for a given unit of time. Since light that is visible to the human eye is actually made up of different wavelengths (hence different amounts of energy), the lumen is actually a weighted average of the makeup of the visible light. Lumens are great for a single point of light, but for architectural purposes, it is more important to know how much light is spread over a specific area (i.e., a square foot, square meter, etc.). Therefore, the more useful standard unit of measure is a lux, which measures the luminous flux (lumens) over a given area. It is standardized so that 1 lux is equal to 1 lumen of light per square meter.

Using lux as the basic unit of measurement, direct sunlight has been measured to provide between 32,000 and 100,000 lux depending on time of day and local conditions. We all know that direct sunlight is very bright to the eye, so levels well below that are considered much more useful inside buildings. Generally, the range of ambient light should be between 300–3,000 lux to be considered desirable. Anything below this level generally requires supplemental electrical lighting, while above 3,000 lux is considered too bright for comfort. There are other standard terms and units of measure that are important to know as well when conducting daylighting simulations.

DAYLIGHTING SIMULATION DEFINITIONS AND STANDARDS

The calculations performed in different daylighting simulations are focused on somewhat different aspects, all intended to quantify the degree of desirable daylight. A list of the common terms and the ways they are used in LEED and WELL projects follows.

Spatial daylight autonomy (sDA) – annual simulation: sDA is a measure of the annual daylight sufficient for a given area, reporting a percentage of floor area that exceeds a specified illuminance level (e.g., 300 lux) for a defined analysis period (10 hours/day, 365 days/year). For LEED projects, scoring varies by building type; higher scores equals more credits. Credit is based on sDA 300/50 percent (300 lux for 50 percent of annual hours). If 55 percent of the floor area meets this threshold, then 2 points are earned. If 75 percent of the floor area is so illuminated, then 3 points are earned. Health-care projects are calculated differently based on perimeter and earn 1 or 2 points respectively for 75 and 90 percent of the defined perimeter floor area.

Annual sunlight exposure (ASE) – annual simulation: ASE is a means to quantify potential visual discomfort within interior spaces. It is defined as the percent of a space in which direct sun exposure exceeds a predefined threshold (e.g., 1000 lux) for some quantity of hours (e.g., 250 hours) in a year. ASE values can be used as a guide to identify and reduce potential sources of glare from sunlight. For LEED projects, the ASE must be reported along with sDA. The threshold for LEED is ASE 1000, 250 h (1000 lux for 250 hours) for 10 percent of the floor space. If the ASE is exceeded in more than 10 percent of the floor area, glare conditions must be addressed, or the space will earn a ‘fail’ score for all LEED daylighting credits. Note that ASE is not an issue when using translucent materials since direct sun exposure is mitigated by the translucent glazing. However, transparent materials, like glass, allow direct sunlight and are problematic for glare. Therefore, glass may require automatic blinds to control glare as part of a successful daylighting design, but the space loses daylight autonomy once the blinds are closed.

Useful daylight illuminance (UDI) – annual simulation: The UDI is an annual illuminance metric that describes multiple categories of ‘usable’ lux levels in a space, reporting a percentage of floor area that falls within a specified illuminance range for 50 percent of the time. Daylight illuminance levels less than 100 lux are generally insufficient for most tasks. Lux levels between 100 and 300 are considered sufficient but may require artificial lighting for some tasks. The range of 300–3000 lux is considered desirable lighting levels in most cases. Illuminance levels of more than 3000 lux are generally not usable, as this is likely to produce visual and thermal discomfort.

Visual daylight glare probability (DGP) – annual simulation: The DGP is a measure that helps to identify potential problem areas where glare is a concern and visual acuity is critical. It is generally used for analysis in work or study areas where people need appropriately balanced lighting to function well. Measurable levels on a scale of 0.0 to 1.0 include: perceptible (DGP above 0.34), disturbing (DGP above 0.38), and intolerable (DGP above 0.40).

Point-in-time – (radiance) simulation: This is a standard metric that looks at specific light levels, contrast ratios, and illuminance patterns at a set day and time using localized weather conditions. This is the common technique used to review daylighting design and glare.

Circadian analysis: equivalent melanopic lux (EML) – point-in-time simulation: The EML is a metric for measuring biological effects of light on humans. It is used in the WELL v2 Standard as means to identify the appropriate light coloration for supporting natural circadian rhythms. WELL awards 1 point for results of 150 EML or 120 EML plus corrected lighting. For conditions of 240 EML or 180 EML plus lighting, 3 points are awarded.

Daylight Simulations

Using the above as a basis, there are three basic types of simulations that have been commonly performed.

Radiance illumination simulation: This analysis can show the pattern of daylight on a floor plan with spot indicators and colorized shading to indicate the range of lux levels. It is common to look at specific points in time, such as the fall and spring equinoxes at early morning, noon, and late afternoon. Noon is most helpful for top lighting (most illuminance), while morning and afternoon are most telling for side lighting when the east/west exposures create a worst case for glare. This type of simulation can be done by selecting either sunny or overcast conditions, but overall, it provides limited information to make design decisions. Typically, sunny skies are chosen to see the most light available in a scenario.
Glare pattern analysis: It is possible to simulate the ‘hot spots’ where glare may become a problem if the lux levels get too high in any areas. A glare pattern analysis helps identify potential problem areas where visual acuity is critical. This is particularly useful for designers when a mix of translucent and transparent fenestration is used.
Daylight autonomy: This simulation is based on determining the average daylight values and the amount of time that the simulated space can be operational with daylight alone at a targeted light level. It is expressed as a percentage of the time that autonomy is achieved. Spatial daylight autonomy (sDA) coupled with annual sunlight exposure (ASE) are the more specific aspects of this type of simulation.

Images courtesy of Kalwall® Corporation

Daylight simulation using the latest computer software allows for accurate prediction of illumination from daylight and can determine the spatial daylight autonomy (sDA) of simulated spaces.

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Originally published in May 2020

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