Daylight Factor

The concept of Daylight Factor (DF) was developed in the United Kingdom in the early 20th century. Daylight Factor is a ratio that represents the amount of illumination available indoors relative to the illumination present outdoors at the same time under overcast skies. Daylight Factor is typically calculated by dividing the horizontal work plane illumination indoors by the horizontal illumination on the roof of the building being tested and then multiplying by 100. For example, if there were 20,000 lux available outdoors and 400 lux available at any given point indoors, then the DF for that point would be calculated as follows DF = 400/20,000 *100 or DF=2.

Daylight Factor is to be used under overcast sky conditions only. Daylight factor is the most common metric used when studying physical models to test daylighting designs in ‘overcast sky simulators’. It is reasonably easy to calculate in real buildings or physical models with illumination meters. It is possible to calculate DF with digital models but care should be taken to understand the ‘sky model’ that is referenced and interpret the data accordingly.

Daylight Factor outputs are helpful in making quick comparisons of relative daylight penetration under overcast sky conditions and is arguably less useful in climates with a great deal of sun. However, most climates across the United States have substantial periods of overcast skies and DF is a useful metric to inform design decisions for these periods.

Early versions of the USGBC, LEED rating system originally required a DF  2 for at least 75% of the critical visual task zones to achieve indoor environment credit 8.1. British Standard Institution, BS 8206-2 requires DF  2 or 5 depending on electric lighting requirements to support human well-being.

Daylight Factor can be reported with static or dynamic measures, however it is most commonly considered statically (at a single point in time) as shown above. In fact, the stability of DF regardless of the time of day and year (assuming an overcast sky) is one of the benefits of the metric.

• The Natural and Artificial Lighting of Buildings, The Journal of the Royal Institute of British Architects, Vol. XXXII, No. 13, pp. 405-426 and 441-446).