The CBDM report produces results in the form of Useful Daylight Illuminance (UDI) metrics which are as follows:
- UDIs – ‘Supplementary’ – Annual occurrence of illuminances less than the acceptable lux level.
- UDIa – ‘Acceptable’ – Annual occurrence of illuminances between the acceptable lux level and the excessive lux level.
- UDIt – ‘Target’ – Annual occurrence of illuminances between the target lux level and the excessive lux level.
- UDIe – ‘Excessive’ – Annual occurrence of illuminances greater than the excessive lux level.
UDIs corresponds to time where electric lighting will be required in the space. UDIa corresponds to time where the daylight is acceptable and electric lighting isn’t required. UDIt corresponds to time where the desired target illuminance is met by daylight. UDIe corresponds to time where the daylight is excessive and may be too bright for occupants.
For this project the acceptable lux level was set to 100 lux, the target lux level was set to 300 lux and the excessive lux level was set to 3000 lux. These values may well change between projects and are set by the user in the CBDM tool before the calculation is carried out. The weather file used in this project was the 2005 CIBSE London TRY.
The CBDM report also produces results for the Spatial Daylight Autonomy (sDA); this is the percentage of area that is above a specified lux level for a specified percentage of the time or more (t/50% corresponds to the target illuminance for 50% of the occupied time) and the UDIa Min; this is the minimum result for the grid point with the lowest average UDIa result and so is useful in investigating the uniformity of the average UDIa result.