Daylight Factor [DF%] is defined as the ratio between the internal and external horizontal illuminance, under CIE overcast sky conditions [CIE Sky types 1 or 16].
Considering that daylight factor calculations use an overcast sky, which is defined as completely cloud covered, means that orientation does not influence the results, which in itself means this method is flawed unless you are only considering north-facing windows. Even then, method 1 is likely to under-estimate the median illuminance.
Importantly, this method does not take into account the spatial distribution of light under varying sky conditions, especially clear skies where direct sunlight would have a huge impact on the distribution and quantity of light within an interior.
Our own in-house research and analysis show that following the DF approach, which is essentially a hybrid solution combining daylight factor [overcast sky] with the median external diffuse illuminance [all skies], results in the under-estimation of the illuminance received within interiors.
As an example, a simple test room has been analysed using Method 1 & 2, in 8 principle orientations. The results showed the following:
The Median Daylight Factor was measured to be 1.84%, for all 8 orientations! The London MEDI is 14100lx, giving a Median Illuminance of 259lx.
The same room analysed using CBDM gave a median illuminance in the range of 444lx (north) – 897lx (south), with each orientation having different results, as one would expect. This highlights how important orientation is to building daylight performance and can have a huge impact on the internal illuminance levels.
When comparing the two methods, the Method 1 DF approach [259lx] is 42% lower than the north-facing room [444lx] and a massive 72% lower than the south-facing room [897lx], when analysed using Method 2 CBDM.
This example also highlights that there is an approx. 100% increase in the internal median illuminance between the north and south orientated rooms i.e twice as much light during the year!
Method 1 seems to be a leftover of the old BS8206, albeit a hybrid trapped somewhere being old and new thinking - a halfway house for those not wanting to fully embrace CBDM. There have always been barriers to the adoption of CBDM and many companies prefer to stick with the easier daylight factor approach - maybe CBDM rocks the boat too much! If the focus is on designing energy-efficient, visually and thermally comfortable buildings, then ignoring the local climate in favour of making the analysis easier is in direct conflict with the main objective. Its time to move on from the daylight factor!