Understanding the results

This section provides a brief overview of the meaning of the common metrics associated with CBDM calculations.

Useful Daylight Index

CBDM results are expressed in terms of various Useful Daylight Index (UDI) metrics. Each metric describes the amount of time a space is either:

• Too Dim (UDIs)

• Well lit (UDIa, UDIt)

• Too bright (UDIe)

UDIs (Supplementary)

The supplementary useful daylight index is the amount of time (%) a point in a space is insufficiently lit, and the illuminance within the space needs to be supplemented with artificial lighting. This occurs if the illuminance is below the acceptable lux level set on the ‘calculation inputs’ page of the wizard.

UDIa (acceptable)

The acceptable useful daylight index is the amount of time (%) a point in a space is acceptably lit. This generally means the space can be used without needing to switch lights on, and occurs when the lux level is greater than acceptable but less than excessive.

UDIt (target)

The target useful daylight index is the amount of time (%) a point in a space is lit to a desired level. An acceptably lit space may be used comfortably without artificial lighting, but sometimes a brighter environment is desired. This occurs when a point has an in illuminance above the target lux level, but less than the excessive lux level.

UDIe (Excessive)

The excessive useful daylight index is the amount of time (%) a point in a space is lit well above the desired light level. When this occurs, there is a risk of excessive glare, and blinds may be required to reduce the amount of natural light entering the space. This occurs when the point has an illuminance higher than the excessive lux level.

Other Metrics

Minimum UDIa

For a CBDM calculation a space is split up into a series of grid points. Each grid point will have average results representing the entire year. The minimum UDIa result represents the grid point with the lowest average UDIa result, and is useful for describing the distribution of light within a space. For example, a deep room with a large window at one end may have a large amount of the room that is very bright and a large amount of the room that is very dim. The average UDIa value for that room may be very similar to a smaller room where the light is more evenly distributed. The minimum UDIa for the deep room would be much smaller than the minimum UDIa for the room where the light is more evenly distributed.

DFa (Daylight Factor)

The daylight factor for a CBDM grid point is the average ratio of the illuminance inside the building at that point to an unobstructed horizontal plane outside the building. Unlike a regular daylight factor, the CBDM daylight factor predicts the average daylight factor for the entire year taking into consideration the effects of weather.

DA (Daylight Autonomy)

The daylight autonomy figure represents the percentage of points in a space that are above a specific illuminance for at least X-% of the time. 100%-DA represents the amount of time electric lighting will be required annually, between 9am and 4pm.