Climate based daylight modelling (CBDM) also known as ''dynamic daylight metrics'' is a calculation methodology first developed in the late 1990s to assess daylight quality and quantity. It is used by Building Design
engineer
Engineers, as practitioners of engineering, are professionals who Invention, invent, design, build, maintain and test machines, complex systems, structures, gadgets and materials. They aim to fulfill functional objectives and requirements while ...
s and
architect
An architect is a person who plans, designs, and oversees the construction of buildings. To practice architecture means to provide services in connection with the design of buildings and the space within the site surrounding the buildings that h ...
s to predict
luminance
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls wit ...
and/or
illuminance
In photometry (optics), photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate ...
within buildings using standardised sun and sky condition climate data for a given geographical location. It is a different design metric to
Daylight factor
In architecture, a daylight factor (DF) is the ratio of the light level inside a structure to the light level outside the structure. It is defined as:
:''DF = (Ei / Eo) x 100%''
where,
''Ei'' = illuminance due to daylight at a point on the indoor ...
s which only considers the ratio of the light level inside a structure to the light level outside the structure from an overcast sky.
With CBDM, if used considerately, the facade design of a building can be optimised to maximise useful daylight whilst excluding excessive daylight, which otherwise might cause issues with
glare, visual discomfort, and/or solar gains which can cause
thermal comfort
Thermal comfort is the condition of mind that expresses subjective satisfaction with the thermal environment.ANSI/ASHRAE Standard 55-2017, Thermal Environmental Conditions for Human Occupancy The human body can be viewed as a heat engine where ...
issues. At the same time reducing reliance and operation of artificial lighting.
CBDM calculations are calculated within Building simulation modelling software tools for each and every hour of the year, or sometimes for smaller increments, which allows for daily and seasonal profiles to be tested and optimised
The key metrics reported on within CBDM software are as follows:
* 'Daylight Autonomy' (DA), the amount of time that a point in a room can be expected to achieve a target level of illuminance from daylight. Normally expressed as a percentage for a useful level of illuminance to be met or exceeded, for example 300 lux
* 'Spatial Daylight Autonomy' (sDA); the amount of time that a point on the working plane in a room can be expected to achieve a target level of illuminance from daylight. The working plane is established to represent a useful working height within the space, such as at desk level. The sDA is normally expressed as a percentage target for a useful level of illuminance for a given target, for example; 50% of a 300 lux DA target on the working plane (300/50%)
* 'Useful Daylight Illuminance' (UDI-a); the summed annual occurrence of illuminance on the working plane and during the occupied hours for the space. Typically, the target may be between 100 and 3000 lux. Any hours of illuminance below 100 lux is defined as UDI-s, and typically would require artificial lighting to be switched on. Any hours of illuminance above 3000 lux is defined as UDI-e, and indicates excessive daylight which can cause visual discomfort and glare/contrast issues and typically would require blinds or curtains to be closed.
See also
*
Daylighting
*
Right to light
Right to light is a form of easement in English law that gives a long-standing owner of a building with windows a right to maintain an adequate level of illumination. The right was traditionally known as the doctrine of "ancient lights". A right ...
*
Daylight factor
In architecture, a daylight factor (DF) is the ratio of the light level inside a structure to the light level outside the structure. It is defined as:
:''DF = (Ei / Eo) x 100%''
where,
''Ei'' = illuminance due to daylight at a point on the indoor ...
Notes
{{Reflist
External links
''International Commission on Illumination''
Light
Visibility
Energy-saving lighting
Lighting