Climate-Based Daylight Modelling (CBDM) uses location-specific weather data to simulate how daylight varies within a building throughout the year. Unlike traditional daylight calculations based on a single sky condition, CBDM considers changing weather, sun position, season and orientation, providing a more realistic representation of annual daylight performance.

Daylight Autonomy (DA) is the percentage of time that a target illuminance level is achieved, or exceeded. The measure is typically taken across a year, during hours of operation. Theoretically, DA could be measured daily, weekly, monthly or any other time period. DA is typically measured across the working plane of a space, with DA values being calculated at each grid point.

Spatial Daylight Autonomy (sDA) defines the percentage of time that a target illuminance (typically 300lux) is achieved across a defined percentage of floor area (typically 50%).

In England, the DfE requires and sDA (300/50%) ≥50% across at least 50% of the working plane, during occupied hours.

Useful Daylight Illuminance (UDI) is a measure of daylight availability, during hours of operation. Rather than a specific target illuminance, UDI analyses the percentage of time that a target range of illuminances can be achieved.

Typically, UDI is divided into three ranges; UDI-s (<100lux), UDI-a (100-3000lux), and UDI-e (>3000lux).

UDI-s (<100lux) is a measure of daylight availability, during hours of operation, below the target range of 100-3000lux.

In England, the DfE GDB specifies that the UDI-a (100-3000lux) should be ≥80% (ave), across the working plane of each space, during occupied hours. This means that the UDI-s & UDI-e cannot exceed 20% (combined).

UDI-a (100-3000lux) is a measure of daylight availability, during hours of operation, within the target range of 100 - 3000lux.

In England, the DfE GDB specify that the UDI-a (100-3000lux) should be ≥80% (ave) across the working plane of each space, during occupied hours.

UDI-e (>3000lux) is a measure of daylight availability, during hours of operation, above the target range of 100-3000lux.

In England, the DfE GDB specify that the UDI-a (100-3000lux) should be ≥80% (ave) across the working plane of each space, during occupied hours. This means that the UDI-s + UDI-e cannot exceed 20% (combined).

ET is the target illuminance used within BS EN 17037 daylight provision assessment.

It represents the daylight illuminance that should be achieved across a specified proportion of the reference plane for a defined proportion of daylight hours.

ETM is the minimum target illuminance used in BS EN 17037.

It is assessed across a greater proportion of the reference plane than ET and is intended to ensure that daylight is distributed adequately across the wider occupied area.

Daylight Factor is the ratio between the daylight illuminance at a point inside a building and the simultaneous unobstructed outdoor horizontal illuminance under a standard overcast sky.

It is expressed as a percentage and represents a static daylight condition rather than annual daylight performance.

Average Daylight Factor is the average Daylight Factor across a defined working plane within a room.

It was historically used extensively within daylight design guidance, particularly for assessing whether rooms were likely to achieve acceptable levels of natural light.

Minimum Daylight Factor is the lowest Daylight Factor measured or calculated across the assessed working plane.

It provides an indication of the least well-lit part of a room and can be useful when considering daylight distribution.

Daylight uniformity describes how evenly daylight is distributed across a space.

It is commonly expressed as the ratio between the minimum and average Daylight Factor across the assessed area.

The Sky Component is the proportion of daylight at an internal point that arrives directly from the sky, excluding externally reflected and internally reflected light.

It is one of the components that contribute to the overall Daylight Factor.

The Externally Reflected Component is the portion of daylight reaching an internal point after reflecting from external surfaces such as neighbouring buildings, ground surfaces or other external obstructions.

The Internally Reflected Component is the portion of daylight reaching a point after reflection from internal room surfaces such as walls, ceilings and floors. The lighter the surfaces finsihes, the greater the IRC.

Vertical Sky Component measures the amount of skylight reaching the centre of a vertical window from an unobstructed overcast sky.

It is expressed as a percentage and is commonly used to assess the potential effect of new development on daylight reaching neighbouring buildings.

The No-Sky Line separates the part of a room that has a direct view of the sky from the part that does not.

Changes in the No-Sky Line can be used to assess whether a proposed development materially affects daylight distribution within an existing room.

Annual Probable Sunlight Hours represents the total number of hours during a typical year that direct sunlight could reach a window, taking account of the movement of the sun and local obstruction.

Winter Probable Sunlight Hours represents the potential hours of direct sunlight available during the winter period.

It is commonly considered alongside APSH when assessing sunlight to neighbouring windows.

The 25° rule-of-thumb test is an initial screening method used to assess whether a proposed obstruction is likely to materially reduce daylight reaching a neighbouring window.

Where the obstruction subtends an angle greater than 25° from the horizontal, at the centre of a window window, more detailed daylight analysis may be required. The BRE states

"If this angle is less than 25° for the whole of the development, then it is unlikely to have a substantial effect on the diffuse skylight enjoyed by the existing building.”

The 45° approach is commonly used as an initial planning assessment to consider the relationship between a proposed residential extension and neighbouring windows.

It provides a simple geometric indication of whether a development may have a significant effect on daylight received at neighbouring windows.

The process involves measured the 45° line project in both plan and elevation. If the centre of the window falls within the 45° angle on both plan and elevation a significant amout of light is likely to be blocked.

The BRE 3x rule is a simple screening test used to identify whether a proposed development is likely to have a significant effect on daylight to existing windows.

Where the development is at least three times its height above the centre of the existing window away, the loss of light is generally considered small.

Where the proposed development fails this test, the BRE 25° rule-of-thumb test can be applied.

Glare is visual discomfort or reduced visual performance caused by excessive brightness or large differences in luminance within the field of view.

Glare may result from daylight, direct sunlight, reflections or electric lighting.

Discomfort glare causes visual discomfort without necessarily preventing a person from seeing an object or completing a visual task.

It is influenced by source luminance, source size, position within the field of view and background luminance.

Disability glare reduces visual performance by scattering light within the eye, creating a veiling effect that reduces contrast.

Daylight Glare Probability is a metric used to estimate the likelihood of discomfort glare caused by daylight.

It considers vertical illuminance at the eye and the luminance, size and position of bright sources within the field of view.

Unified Glare Rating is a method used to estimate discomfort glare from electric lighting installations.

It considers the luminance, apparent size and position of luminaires relative to the observer, together with the background luminance of the space.

Luminance contrast describes the difference in brightness between adjacent surfaces or objects.

Excessive contrast can contribute to visual discomfort, while insufficient contrast can make visual tasks difficult to distinguish.

Luminous flux is the total quantity of visible light emitted by a source, weighted according to the spectral sensitivity of the human eye.

It is measured in lumens (lm).

Luminous flux describes the total quantity of visible light emitted from a source.

Luminous intensity describes the amount of luminous flux emitted in a particular direction.

It is measured in candelas (cd).

A candela is equivalent to one lumen per steradian. Luminous intensity is therefore a directional quantity and is fundamental to describing the light distribution of a luminaire.

Photometric files such as IES and LDT files contain luminous intensity data measured at multiple angles around a luminaire.

Lux is the SI unit of illuminance.

Illuminance describes the quantity of luminous flux falling onto a surface.

It is measured in lux (lx), where:

1 lux = 1 lumen per square metre

Illuminance is commonly used to specify lighting levels on working planes, floors, roads and other surfaces.

For a point source, illuminance generally reduces with the square of the distance from the source and is also affected by the angle at which the light strikes the surface.

The foot-candle is a non-SI unit of illuminance still widely used in the United States.

One foot-candle represents one lumen of luminous flux distributed over one square foot:

1 fc = 1 lm/ft²

The relationship between foot-candles and lux is:

1 fc = 10.764 lux

or approximately:

1 lux = 0.093 fc

For example, an illuminance of 500 lux is approximately 46.5 foot-candles.

Luminance describes the light leaving a surface in a particular direction, whether that light is emitted directly, reflected or transmitted.

It is measured in candelas per square metre (cd/m²).

Unlike illuminance, luminance is closely related to the apparent brightness of surfaces and light sources as seen by an observer. It is therefore particularly important in glare analysis, visual comfort and the assessment of contrast within the visual field.

Luminous efficacy describes how efficiently a light source converts electrical power into visible light.

It is normally expressed in lumens per watt (lm/W).

A higher luminous efficacy indicates that more visible light is produced for each watt of electrical power consumed.

A solid angle is the three-dimensional equivalent of a planar angle.

It is measured in steradians (sr).

A full sphere contains 4π steradians. Solid angle is fundamental to photometry because luminous intensity is defined as luminous flux per unit solid angle.

Reflectance is the proportion of incident light reflected by a surface.

It is usually expressed as a percentage or decimal value.

Surface reflectance has a significant effect on both electric lighting and daylight performance because light reflected from ceilings, walls and floors contributes to the overall distribution of light within a space.

Transmittance is the proportion of incident light that passes through a material.

For glazing, visible light transmittance describes the proportion of visible light passing through the glass and is a key input in daylight modelling.

Absorptance is the proportion of incident light absorbed by a material rather than reflected or transmitted.

For a given wavelength or spectral range:

Reflectance + Transmittance + Absorptance = 1

For a point source, illuminance on a surface perpendicular to the direction of light reduces in proportion to the square of the distance from the source.

Doubling the distance therefore reduces the illuminance to approximately one quarter.

The relationship is:

E = I / d²

where:

E = illuminance in lux
I = luminous intensity in candelas
d = distance in metres

Where light strikes a surface at an angle, the illuminance is reduced according to the cosine of the angle between the incoming light and the surface normal.

The relationship is:

E = I cos θ / d²

This is an important principle in lighting calculations because the orientation of a surface can significantly affect the illuminance it receives.