Architectural daylighting brings together architecture, engineering, environmental science and human experience. Understanding the language behind these disciplines makes research easier to understand, standards easier to interpret and design decisions easier to communicate.
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Why a glossary?
Clear language supports clear thinking.
Understanding architectural terminology is not about memorizing definitions. It is about building a shared language that helps architects, engineers, researchers and students communicate ideas, interpret standards and make better-informed design decisions with confidence.
Daylighting
Daylighting is the deliberate use of natural light to illuminate interior spaces and support their architectural and environmental quality.
Daylighting is more than bringing sunlight through windows. It involves designing building form, orientation, openings, glazing and interior surfaces to make effective use of available daylight. Good daylighting considers its distribution, variation over time, visual comfort and relationship with views, solar gains and electric lighting.
Daylighting is an architectural design task, not simply a matter of adding windows.
Daylight Provision
Daylight provision describes the availability of natural light in an interior and, in assessment methods, whether defined daylight requirements are met.
Daylight provision depends on factors such as building orientation and form, window position and size, glazing properties, external obstructions and interior surface reflectance. Its assessment depends on the method used: some approaches evaluate daylight under defined sky conditions, while others consider performance over time. In EN 17037, daylight provision is assessed using specified criteria rather than a general estimate of how much electric lighting daylight can replace.
Daylight provision depends on both the building's design and the method used to assess it.
Daylight Factor
Daylight Factor (DF, %) is the ratio of indoor illuminance to simultaneous unobstructed outdoor horizontal illuminance under a defined overcast sky.
Daylight Factor expresses this ratio as a percentage. It is calculated for a particular point or assessment position under a standard overcast sky and does not directly describe how daylight changes with the weather, season or time of day. It remains useful for understanding the influence of building geometry, windows and interior surfaces, but it should not be interpreted as an annual daylight performance metric.
Daylight Factor describes a defined sky condition, not daylight availability throughout the year.
Daylight Autonomy
Daylight Autonomy (DA, %) is the percentage of defined occupied hours during which daylight alone meets a specified illuminance target at an assessment point.
Daylight Autonomy uses time-based daylight calculations to evaluate performance across a defined period, taking account of factors such as location, weather, season and occupancy schedules. Unlike Daylight Factor, it does not represent a single sky condition. Its result depends on the illuminance target, the hours assessed and the calculation method, so these assumptions must be stated when comparing results.
Daylight Autonomy measures how often a defined daylight target is met over time.
Spatial Daylight Autonomy
Spatial Daylight Autonomy (sDA, %) expresses the proportion of assessed floor area that meets a specified daylight illuminance target for a defined fraction of occupied hours.
Spatial Daylight Autonomy combines spatial and time-based information. Instead of reporting performance at one point, it indicates how much of the assessed floor area meets a specified illuminance target for a required proportion of occupied hours. The result depends on the target, the time threshold, the occupancy schedule and the assessment method. The exact criteria should therefore accompany any reported sDA value.
Spatial Daylight Autonomy describes how much of a floor area meets defined daylight criteria over time.
Illuminance
Illuminance (E, lux) is the luminous flux incident on a surface per unit area.
Illuminance is measured in lux (lx), equivalent to one lumen per square metre. It describes the amount of light arriving at a surface, not how bright that surface appears to an observer. The illuminance required depends on the activity, visual task and surrounding conditions. In daylighting design, illuminance measurements and calculations help assess whether a space receives an appropriate level of light.
Illuminance measures the light arriving at a surface, not its apparent brightness.
Luminance
Luminance (L, cd/m²) describes the light emitted, transmitted or reflected by a surface in a given direction per unit projected area.
Luminance is measured in candelas per square metre (cd/m²). While illuminance describes the light arriving at a surface, luminance describes the light leaving it in a particular direction toward an observer. It is closely related to perceived brightness and is important when evaluating contrast and glare. Strong luminance differences within the field of view can contribute to visual discomfort, depending on their size, position and surrounding conditions.
Luminance helps explain perceived brightness, contrast and the potential for glare.
Visual Comfort
Visual comfort describes how well a lighting environment supports seeing and performing visual tasks without undue discomfort.
Visual comfort depends on factors such as illuminance, luminance distribution, contrast, glare, the direction of light and the nature of the visual task. Daylight and electric lighting must be considered together, along with surface reflectance, viewing directions and the changing conditions throughout the day. A space with abundant daylight is not necessarily visually comfortable if it produces excessive contrast or glare.
Good visual comfort depends on the quality and balance of the visual environment, not simply on light quantity.
Glare Control
Glare control uses design and lighting strategies to reduce visual discomfort or impaired visibility caused by excessive brightness or luminance contrasts.
Glare can arise from bright windows, direct sunlight, reflections or other high-luminance sources within the field of view. The effect depends not only on the brightness of the source but also on its size, position, contrast and the surrounding visual conditions. Strategies for controlling daylight glare include building orientation, window placement, external shading, glazing selection and interior layout. The aim is to provide useful daylight while limiting discomfort and loss of visibility.
Effective glare control balances access to daylight with visual comfort and visibility.
Sunlight Exposure
Sunlight exposure describes whether, and for how long, direct sunlight reaches a defined space.
Sunlight exposure concerns direct sunlight entering a building, whereas diffuse daylight is also available under cloudy skies. EN 17037 provides recommendations for sunlight exposure in spaces where access to direct sunlight is desirable. Assessment depends on the specified date, the space being evaluated and the standard's method. Sunlight can contribute to the experience of a space, but its effects depend on context, and excessive direct sun may cause overheating or visual discomfort.
Good design provides useful sunlight while managing heat and visual discomfort.
View Out
View out describes the quality of the visual connection between an interior space and the outdoors.
A view out can provide information about the surrounding environment, weather and time of day, as well as a visual connection with the wider world. Its quality depends on factors such as the size and position of the opening, the viewing angle and distance, and the range of visible elements and landscape layers. A well-designed view supports orientation and can contribute to the experience and usability of an interior. Daylight provision and view out are related, but they are distinct aspects of building design.
A well-designed view is a valuable quality in its own right, alongside good daylight.
Circadian Clock
The circadian clock is the body's internal timing system, which follows a cycle of approximately 24 hours.
The circadian system helps coordinate daily patterns such as sleep and wakefulness. Light is an important environmental signal for this system, and its effects depend on factors including timing, intensity, duration and spectral composition. Daylight exposure during the day, together with appropriate darkness at night, can help align the body's internal rhythm with the natural day–night cycle. Building design can support access to daylight, while recognising that lighting is only one influence on sleep and wellbeing.
Daylight and appropriate light–dark patterns can help support the body's daily rhythm.
Daylight Glare Probability (DGP)
Daylight Glare Probability (DGP) estimates the likelihood of discomfort glare under a given daylight condition.
DGP is a metric used to estimate the likelihood of discomfort glare from daylight. It considers vertical illuminance at the eye and characteristics of bright sources in the field of view, including their luminance, size and position. DGP can help designers compare window, façade and shading options under defined conditions. As with any metric, results depend on the assessment method and conditions and should be interpreted in context.
DGP helps designers assess daylight glare under defined conditions.
Sky Conditions
Sky conditions describe the distribution of brightness across the sky and the resulting daylight conditions.
Clear, partly cloudy and overcast skies create different daylight conditions indoors. The amount and distribution of daylight depend on the sky, the sun's position, the building's orientation and surrounding obstructions. Standardized sky models represent selected sky conditions for daylight calculations and simulations, helping designers assess performance and compare design alternatives consistently.
Sky conditions influence both the amount and distribution of daylight indoors.
Indoor Environmental Quality (IEQ)
Indoor Environmental Quality (IEQ) describes the combined conditions that shape the indoor environment experienced by occupants.
IEQ brings together several aspects of building performance, including natural and electric lighting, thermal comfort, indoor air quality and acoustics. These factors interact to influence occupants' comfort, experience and ability to use a space effectively. Good architectural design considers them together rather than optimizing one aspect in isolation. Daylight is an important component of IEQ, but it must be balanced with other environmental needs.
Good indoor environmental quality depends on balancing several aspects of comfort.
EN 17037
EN 17037 is a European standard addressing daylight provision, sunlight exposure, view out and protection against glare.
Published in 2018, EN 17037 provides a common framework for assessing important aspects of daylight in buildings. It addresses four areas: daylight provision, sunlight exposure, view out and protection against glare. These aspects are related but distinct, and they require different assessment approaches. The standard helps architects and other building professionals evaluate design options systematically; the applicable edition and national provisions should be checked when applying its recommendations to a project.
EN 17037 provides a shared framework for assessing several distinct aspects of daylight in buildings.
Window-to-Wall Ratio (WWR)
Window-to-Wall Ratio (WWR) expresses the glazed area of a façade as a proportion of its total area.
WWR is the ratio of glazed area to the total façade area, calculated according to the convention used for the project or assessment method. It influences daylight availability, solar heat gains, views, glare risk and energy performance. Increasing the glazed area does not automatically improve daylight quality: results also depend on orientation, glazing properties, shading, room depth and the intended use of the space. Good design balances these factors rather than relying on window size alone.
Good daylight design balances glazing area with orientation, shading and room use.
Building Orientation
Building orientation describes how a building is positioned in relation to the sun and the cardinal directions.
Building orientation influences daylight availability, sunlight exposure, glare risk, passive solar heat gains and energy use throughout the year. Its effects depend on the climate, surrounding buildings, façade design and how spaces are used. Orientation is an early design decision that can create opportunities for good daylight and energy performance, while later design choices determine how effectively those opportunities are used.
Good orientation creates opportunities that later design decisions can build upon.
Building Form
Building form describes the overall shape, proportions and spatial organization of a building.
A building's depth, height, geometry and spatial organization influence how daylight reaches its interior. Deep floor plans can limit daylight penetration, while atriums, courtyards, rooflights and carefully arranged spaces can bring light further into a building. These strategies must be considered alongside visual comfort, energy use and the intended function of each space.
Building form shapes how daylight reaches and is experienced throughout a building.
Early Design
Early design is the stage when fundamental decisions about daylight and building form can still be shaped.
Building orientation, form, massing, floor depth and window placement are often established during the early design stages. These decisions strongly influence how much daylight can enter a building and how it is distributed. Evaluating daylight early allows architects to compare alternatives while fundamental aspects of the design can still be adjusted, rather than relying mainly on later measures such as additional glazing or shading.
The best time to shape daylight performance is while the fundamental design decisions are still open.
Building Performance
Building performance describes how well a building meets its functional, environmental and occupant-related objectives.
Building performance covers several interconnected aspects, including energy use, daylight and electric lighting, thermal comfort, indoor air quality, acoustics, usability and occupant experience. These aspects can influence one another: for example, increasing glazing may improve access to daylight and views while also increasing solar heat gains or glare. Successful design balances competing requirements instead of optimizing one measure in isolation.
Good building performance balances environmental goals with the needs of occupants.
Sustainability
Sustainable building design seeks to balance environmental impact, social value and long-term performance.
Daylighting can contribute to sustainability by reducing the need for electric lighting and supporting a comfortable indoor environment. Its overall benefit depends on how daylight is integrated with orientation, glazing, shading, electric lighting controls, heating and cooling. Sustainable architecture also considers energy and resource use, material impacts, durability, adaptability and the needs of building occupants across the building's life cycle.
Daylighting contributes most when it is integrated with the building's wider environmental strategy.
Daylight Simulation
Daylight simulation uses digital building models to estimate daylight conditions and performance.
Daylight simulation uses computational methods to estimate how daylight enters and is distributed within a building under specified sky, weather and time conditions. Depending on the method, it can assess daylight availability, annual performance, sunlight exposure or glare. Simulations help architects compare design alternatives before construction, but their reliability depends on the model, input data, assumptions and assessment method. Results should therefore be interpreted in the context of the question being investigated.
Daylight simulation makes design alternatives comparable before a building is constructed.
Occupant Wellbeing
Occupant wellbeing describes how the built environment can support people's health, comfort and everyday experience.
The built environment can influence wellbeing through daylight, views, thermal comfort, acoustics, indoor air quality and the organization of space. Access to daylight and a meaningful view can contribute to a pleasant indoor experience, while glare, overheating, poor air quality or unwanted noise can undermine it. Designing around occupants means considering these factors together and recognising that individual needs and activities differ.
Good buildings respond to the people who use them and the activities they support.
Life Cycle Assessment (LCA)
Life Cycle Assessment (LCA) is a method for assessing environmental impacts across a product or building's defined life cycle.
Life Cycle Assessment evaluates environmental impacts associated with defined life-cycle stages, which may include material production, construction, building operation, maintenance, replacement and end of life. The stages and impacts covered depend on the scope and boundaries of the assessment. LCA extends the analysis beyond operational energy alone and can help architects compare materials, systems and design options. Considered alongside daylight, energy performance, durability and occupant needs, it supports more informed environmental decisions.
LCA helps designers compare environmental impacts across the defined life cycle.