Design Sky values are derived from a statistical analysis of dynamic outdoor sky illuminance levels. They represent the horizontal illuminance value that is exceeded 85% of the time between the hours of 9am and 5pm throughout the working year. Thus they also represent a worst-case scenario that you can design to and be sure your building will meet the desired light levels at least 85% of the time. Design sky values vary from around 12-15,000 lux at the equator down to around 3-4000 lux at a latitude of ±60°, as shown in Figure 1 below.

Figure 1 - Design Sky Illuminance values as a function of latitude.

Obtaining Design Sky Values

Obviously as different calculation methods give slightly different results, the best way to obtain the Design Sky value for any location is from a published source. However, if this is not readily available, then you can calculate it from the average diffuse sky illuminance formula given by Tregenza, as shown below. The value Y° is the current altitude of the Sun in the sky, given in degrees.

Illuminance values from this equation need to be calculated hourly throughout the year to determine a distribution pattern, from which the 15th percentile is derived. As discussed above, this is because the design sky illuminance is taken to be the illuminance level exceeded 85% of the time. Below is a tool that performs this calculation and displays a cumulative frequency graph of average lux levels. Note that the values obtained from the Tregenza formula vary from around 4,000 Lux up to 10,000 Lux at the Equator.

Useful Links

BRE ETSU Daylight Algorithms

http://eande.lbl.gov/Task21/C2/contents.html