Daylight Factor – Comprehensive Aviation and Architectural Illuminance Glossary

Daylight Factor (DF)

The Daylight Factor (DF) is a fundamental measure in both architectural and aviation lighting design, representing the ratio of indoor illuminance at a specific reference point to the simultaneous outdoor illuminance, under a standardized overcast sky. Expressed as a percentage, DF is defined by the formula:

[ DF = \left( \frac{E_i}{E_o} \right) \times 100% ]

where Ei is the indoor illuminance (in lux) at a specified point (usually on a horizontal working plane), and Eo is the outdoor horizontal illuminance under the CIE Standard Overcast Sky. This standardized approach isolates diffuse daylight, intentionally excluding direct sunlight and artificial lighting, to ensure robust assessment under the most challenging daylight conditions.

DF is central to daylighting design in settings like airport terminals, control towers, and large public buildings. It ensures that interiors remain visually comfortable and energy efficient, even in overcast weather. Regulatory frameworks and sustainability certifications (such as LEED, BREEAM, or EN 17037) often mandate minimum daylight factors for various space types. For example, general workspaces typically require a DF of at least 2%.

DF is easy to calculate and conservative, making it ideal for early-stage design, code compliance, and legal assessments regarding rights to light. However, it does not account for annual climate variations or direct sunlight, which are addressed by advanced metrics such as Spatial Daylight Autonomy (sDA).

Illuminance (Ei, Eo)

Illuminance is the luminous flux incident per unit area on a surface, measured in lux (lx). In daylight factor analysis, the two key types are:

• Ei (Interior Illuminance): The measured or simulated illuminance at a specific indoor location, typically on a horizontal work plane (such as a desk or floor). In aviation facilities, this includes check-in counters, waiting zones, and security areas.
• Eo (Exterior Illuminance): The horizontal illuminance measured outdoors, in an unobstructed area, under the CIE Standard Overcast Sky. Eo serves as the reference value for DF calculations.

Illuminance is crucial for ensuring task visibility and safety, especially in complex environments like airports. It is determined by factors including window area, glazing transmittance, sky luminance distribution, room geometry, and surface reflectances. Simulation tools may discretize the sky and window aperture, summing the contributions from multiple sky patches to calculate accurate illuminance values.

CIE Standard Overcast Sky

The CIE Standard Overcast Sky is an internationally recognized sky luminance model, defined by the Commission Internationale de l’Éclairage (CIE). It is used as the reference condition for daylight factor calculations to ensure consistency and a conservative, worst-case assessment of daylighting performance. The luminance profile is:

[ L(\varphi_{sky}) = L_z \frac{1 + 2 \sin \varphi_{sky}}{3} ]

where Lz is the zenith luminance, and φsky is the sky element’s altitude angle. The zenith is three times as bright as the horizon, and luminance is uniform in all compass directions at a given altitude.

This model is essential for regulatory compliance and forms the basis for all daylight factor calculations, especially in aviation architecture and legal rights to light assessments.

Sky Component (SC)

The Sky Component (SC) is the portion of the daylight factor received directly from the visible sky, through windows or other openings, with no intermediation by external or internal reflections. SC depends on window size, location, orientation, and the presence of external obstructions.

The SC is typically calculated using geometric projection or graphical tools (like the Waldram Diagram) to determine which sky patches are visible from the indoor reference point. Each visible patch contributes proportionally to its solid angle and luminance per the CIE model. In large, unobstructed airport terminals, SC can be the dominant daylight source, but it diminishes in urban or shaded settings.

Externally Reflected Component (ERC)

The Externally Reflected Component (ERC) accounts for daylight reflected from external surfaces (such as pavements, aprons, or adjacent buildings) into the interior. This component is particularly relevant in aviation settings, where extensive light-colored pavements or roofs can boost daylight entering through façades.

ERC is calculated by analyzing the visible portion of external surfaces from each window, their reflectance, and incident sky luminance. The formula sums the contributions from each external patch, adjusted for glazing transmittance and solid angle. ERC is especially significant when direct sky view is limited but reflective surfaces are abundant.

Internally Reflected Component (IRC)

The Internally Reflected Component (IRC) describes daylight that, after entering the space, reaches the reference point via one or more reflections from internal surfaces (walls, ceilings, floors). IRC is vital in deep-plan spaces or those with limited window area, often making up a significant share of the total DF.

IRC depends on:

• Surface reflectances (higher values yield more IRC)
• Room geometry and openness
• Placement and size of windows

High-reflectance finishes (e.g., white ceilings, light-colored walls) maximize IRC, enhancing daylight penetration and uniformity. Simulation tools track multiple bounces to model IRC accurately, but empirical approaches can provide rough estimates for simple spaces.

Glazing Visible Transmittance (τvis)

Visible Transmittance (τvis) is the fraction of visible daylight that passes through glazing, typically ranging from 0.3 (tinted or coated glass) to about 0.8 (clear glass). τvis is determined by glass composition, coatings, and thickness, and is a critical parameter in daylight factor calculations.

A high τvis maximizes daylight ingress but may increase glare and thermal gain. In aviation facilities, selecting an appropriate τvis is essential for balancing daylight provision with comfort and energy efficiency.

Solid Angle (Ω)

The Solid Angle (Ω) quantifies the apparent size of an object or sky patch as seen from a given point, measured in steradians (sr). In daylighting, solid angles determine how much of the sky or external environment is visible through a window from the reference point.

The larger the solid angle subtended by the window, the greater the potential daylight contribution. Maximizing Ω from key interior locations is a central design strategy in terminal and large public building architecture.

Luminance

Luminance is the measurable brightness of a surface (in candelas per square meter, cd/m²) as perceived by an observer. In daylight factor analysis, it refers to the brightness of sky patches (as modeled by the CIE sky) and window elements, as seen from inside.

Luminance influences both the quantity and quality of daylight, affecting visual comfort, glare, and the legibility of signage and displays in aviation environments. Design must balance high luminance for daylight provision with the risk of discomfort glare.

Inter-reflection

Inter-reflection is the process by which daylight, after entering a space, is reflected multiple times by room surfaces before reaching the observer or reference point. This process enhances the IRC and determines how deeply daylight penetrates into the interior.

High-reflectance finishes (white or light surfaces) and open geometries maximize beneficial inter-reflection, promoting even daylight distribution and reducing reliance on electric lighting. Accurate modeling of inter-reflection requires advanced simulation tools.

Spatial Daylight Autonomy (sDA)

Spatial Daylight Autonomy (sDA) is a dynamic, climate-based metric expressing the percentage of a space’s floor area that receives at least a minimum daylight level (commonly 300 lux) for at least 50% of annual occupied hours. sDA accounts for both diffuse and direct sunlight, as well as annual weather variations, providing a holistic assessment of daylight availability.

sDA is increasingly required by modern codes and certifications, complementing the conservative DF approach. High sDA values indicate successful daylighting design; however, excessive direct sun must be managed to avoid glare and overheating.

Annual Sunlight Exposure (ASE)

Annual Sunlight Exposure (ASE) measures the percentage of an area exposed to more than 1000 lux of direct sunlight for over 250 hours per year. ASE identifies zones at risk of glare and thermal discomfort, helping designers balance daylight provision (sDA) and occupant comfort.

Optimal design achieves high sDA while keeping ASE below thresholds (often <10%) to ensure both ample daylight and visual/thermal comfort in highly glazed environments, such as airport terminals.

By understanding and applying these daylighting concepts—especially the Daylight Factor and its components—designers and engineers can deliver visually comfortable, energy-efficient, and code-compliant spaces in both aviation and general architectural contexts. This ensures a better experience for occupants, reduced energy costs, and support for sustainability goals.