DAYLIGHTING GLOSSARY
Learn about daylight in buildings.
Essential terms to help design and evaluation.
DAYLIGHTING IN BUILDINGS - GLOSSARY
This glossary explains essential daylighting terms in plain language, helping readers design or shape buildings and use or furnish interior spaces to improve comfort, enhance energy performance, and create a healthier, more comfortable living environment. For a deeper technical context, please read the articles and view the presentations on Daylighting Research. For 'visual' examples of how daylight transforms buildings and interiors, please explore the Daylighting in Buildings Gallery. If you are designing, refurbishing a building, about to purchase a flat or property, or remodeling an interior space and need help, please get in touch.
Daylighting
Daylighting is the (intentional) use of natural light in buildings.
Natural light, or daylight, is one of the most powerful design assets in (sustainable) architecture. It reduces reliance on artificial lighting, improves visual comfort, supports well-being, and strengthens inhabitants' connection to the exterior environment, weather, and the seasons.
Daylight Factor
Daylight Factor (DF, %) quantifies the effectiveness of natural light entering an interior space.
Daylight Factor is a ratio that expresses the amount of 'light' (Illuminance measured on a grid) available indoors relative to the illumination outdoors, measured at the same time. It is typically used in 'overcast sky' conditions, when direct sunlight is not present. This value incorporates the effect of the opening (window or skylight), accounts for the color and reflectance of interior walls, ceilings, and surfaces, and considers the room's geometry. It is relatively easy to measure and helpful for making quick daylighting comparisons of indoor spaces.
Illuminance
Illuminance (E, lux) is the measure of the amount of light (Luminous Flux: lm) received on the surface (area: m2).
Illuminance requirements for rooms typically range from 50 to 750 lx. For example, 100 to 200 lx is recommended for general lighting in corridors and staircases; 500 lx is necessary for comfortable reading and writing; below 150 lx when working on a computer screen; and 1000 to 2000 lx in a dentist's chair. The illuminance of direct sunlight can reach 100,000 lx, while diffuse skylight may range from 3,000 to 18,000 lx. Illuminance levels can be measured precisely with a luxmeter. Free smartphone apps can also be used to estimate rough illumination levels.
Luminance
Luminance (L, cd/m2) is the amount of light emitted, passing through, or reflected from a surface.
Luminance is a photometric measure of luminous intensity per unit area in a given direction; it describes the amount of light that is emitted, transmitted, or reflected from a surface and determines how 'bright' that surface appears. Luminance is often informally associated with brightness, but whereas luminance is an objectively measurable quantity (cd/m²), brightness is a subjective visual sensation that cannot be measured directly as a physical value.
Visual Comfort
Visual comfort is a subjective state of well-being influenced by the lighting conditions in a building's interior.
Visual comfort describes how pleasant and strain‑free a space feels to look at and work in. It depends on luminance (how bright surfaces appear), contrast between surfaces, and other lighting metrics that help keep brightness within ranges the human eye finds comfortable and prevent glare. Achieving visual comfort involves many design factors, such as typical viewing directions, glare control, light distribution, overall illumination levels, task lighting, and the 'quality of view' to the outside. Good visual comfort supports occupants’ productivity and well‑being and contributes to an aesthetically pleasing interior.
Sunlight Exposure
Sunlight exposure (h) refers to the duration of direct sunlight entering a room or other interior space (e.g., atrium) in a building.
The EN 17037 'Daylighting in Buildings' standard categorizes the duration of sunlight exposure on a specific day of the year into three performance levels: 1.5 h (Minimum), 3 h (Medium), and 4 h (High) of direct sunlight availability in an interior space. Sunlight exposure is essential for maintaining physical and mental health, primarily by supporting vitamin D synthesis and regulating the body's internal (Circadian) clock. This is why sunlight exposure should be provided in specific areas of hospitals, schools, kindergartens, and elderly homes, as well as in the living rooms of residential buildings.
Glare Control
Glare control is about limiting overly bright light sources or strong contrasts that make it uncomfortable or difficult to see.
Effective glare control keeps direct and reflected bright spots out of the main field of view and prevents strong contrast that can cause eye strain. It can involve shading devices (such as blinds, louvers, and overhangs), careful placement and shielding of luminaires (artificial light sources), appropriate window sizing and positioning, and thoughtful selection of surface reflectance (matte or shiny). Good glare control supports visual comfort, helps people see tasks clearly, and allows daylight and artificial light to be used without creating disturbing bright patches or reflections.
Well-being
Well-being describes how healthy, comfortable, and satisfied people feel in an interior space.
In buildings, well-being is influenced by factors such as access to daylight, visual comfort, air quality, acoustic conditions, temperature, and a sense of connection to the outside, as well as workspace ergonomics. Good daylighting and glare control help support well-being by reducing eye strain and fatigue, improving mood, and making interiors feel more pleasant and inviting. A space that supports occupants’ well-being can enhance productivity, reduce stress, and encourage people to spend more time in it.
Daylight Provision
Daylight provision describes how much of a building’s lighting needs can be met by daylight over the year, helping to reduce artificial lighting use and energy costs.
Well-designed daylighting can supply useful light for many hours each day, even under overcast skies, while offering excellent color rendering. By considering factors such as building location, surrounding obstructions, window size and orientation, glazing type, and interior surface reflectance, architects can use daylight provision and reduce electric lighting energy demand. Strong daylight provision supports visual comfort and well-being, can lower operating costs, and contributes to the overall quality and value of the building.
View Out
View out describes the quality of visual connection from an interior space to the outdoors through windows or other openings.
A good 'view out' provides useful information about the local environment, weather changes, and the time of day, and offers mental refreshment by giving occupants a change of scenery and a shift in focus. The quality of a view depends on factors such as the viewing angle and window size, the distance to outdoor elements, and the number of distinct visible layers (e.g., sky, landscape, and ground). Quality 'View out' can enhance occupants’ sense of security and well-being, reduce fatigue associated with long periods indoors, and significantly increase a property’s perceived and market value.
Circadian Clock
The circadian clock is the body’s internal 24-hour timing system that regulates sleep–wake cycles, hormone release, body temperature, and many other biological processes.
Daylight exposure strongly influences the circadian clock, especially the timing, intensity, and color of light reaching the eyes. Morning light helps synchronize the circadian clock with the natural day–night cycle, while poorly timed or insufficient light can disrupt it, leading to sleep problems, reduced alertness, and lower overall well-being. In buildings, access to appropriate daylight and well-designed electric lighting can support healthy circadian rhythms, improve mood and daytime performance, and help occupants feel more rested and balanced.
Daylight Glare Probability (DGP)
Daylight Glare Probability index (DGP, %) is a metric that predicts the likelihood that occupants will experience discomfort glare from daylight in a given view and lighting condition.
It is based on a combination of vertical eye illuminance and the brightness, size, and position of glare sources in the field of view, and expresses glare as a probability from 0 to 100%. Typical DGP ranges are used to classify scenes from “imperceptible” or “just perceptible” glare through to “disturbing” and “intolerable” glare, helping designers evaluate and compare daylighting and shading strategies.
Correlated Color Temperature (CCT)
Correlated color temperature (CCT, K) describes the appearance of the color of white light from a lamp or light source, expressed in kelvins.
Lower CCT values (around 2700–3000 K) appear “warm” and more yellowish, while higher CCT values (4000–6500 K) appear “cool” and more bluish, similar to overcast daylight or clear sky at midday. CCT influences how a space feels: warmer light is often associated with relaxation and hospitality, while cooler light is typically used for workspaces and tasks that require alertness. In daylighting and lighting design, choosing appropriate CCTs for electric lighting helps create a coherent visual environment that complements natural daylight and supports comfort and well-being.
Sky Conditions
Sky conditions describe the distribution of sky luminance across the hemisphere at a given time, influenced by factors such as cloud cover, overall brightness, and the presence or absence of direct sunlight.
Commonly referenced sky conditions in daylighting design include clear, partly cloudy, and overcast skies, each producing different daylight distributions, contrasts, and variability inside buildings. Designers use standardized sky models (for example, uniform overcast or clear sky with sun) to simulate how daylight will enter a space and to test performance across a range of conditions. Understanding sky conditions helps in sizing and positioning windows, selecting shading strategies, and predicting daylight provision, glare risk, and visual comfort over the year.
Indoor Environmental Quality (IEQ)
Indoor environmental quality (IEQ) refers to the overall quality of the indoor environment as experienced by occupants, encompassing visual, thermal, acoustic, and air-quality factors.
It includes aspects such as daylight access and visual comfort, temperature and air movement, noise levels and sound insulation, and ventilation and pollutant control. Good IEQ supports occupants’ health, comfort, and productivity by reducing strain and stress and making interior spaces more pleasant to spend time in. Daylighting design, glare control, and appropriate electric lighting play a key role in the visual component of IEQ, working with HVAC and acoustic design to create high-quality indoor environments.
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