Know the site - Climatic conditions

The climatic characteristics of a site can dictate the significance of the use of overcast illumination as a design influence and the use of the mirror box artificial sky as a prediction tool. Projects with sites that experience predominantly overcast conditions can be well served by mirror box artificial sky testing. Primarily sunny climates require different daylighting strategies and might only use the mirror box to check performance for the occasional overcast day. Knowing a project's regional climate and lighting characteristics and understanding the percent of daylight hours each month that the sky is clear versus overcast gives important weight to the hierarchy of decisions about the kinds of daylighting techniques to be used in a design for a given locale.

Know your fractions - Daylight Factor

A Daylight Factor (DF) is a measurement representing a ratio of interior illumination relative to what is available outdoors and is expressed as a percentage. Daylight factor is an expression of the impact of architectural geometry. The principle is that building apertures admit the light available from overcast skies as a percentage that is constant regardless of overcast exterior illumination level.

Under overcast sky conditions this percentage relationship holds constant regardless of the overall level of illumination available. This allows the testing results to be interpreted back into predicted interior illumination levels under a range of overcast sky illumination levels that might exist at different times of year or at various locales. Daylight factors can range from small values of 1-2% to what is considered a high value of 8% or more. For example, a 2% DF would represent 30 fc. on a 1500 fc. overcast day and it would represent 12 fc. on a 600 fc. overcast day. Due to the variability of sky conditions and sun position under clear and partly cloudy conditions, daylight factor should be used as a metric only for overcast sky condition testing and measurement.