Calculations Daylight Factor Concepts

The Illuminating Engineering Society of North America (IESNA) defines Daylight Factor  as the "ratio of Illuminance at a point on a plane, generally the horizontal work plane,  produced by the luminous flux received directly or indirectly at that point from a sky whose luminance distribution is known,  to the Illuminance on a horizontal plane produced by an unobstructed hemisphere of the same sky". The method takes only luminous flux from the sky into consideration (not sunlight) and is generally used with uniform or CIE overcast skies (so that sunlight and solar position is not a factor). Daylight Factor is not calculated when electric lighting is switched on in AGi32 or when sky types other than CIE General Sky Types 1-5 are selected.

The Daylight Factor method is calculated in accordance with the IESNA Lighting Handbook, Ninth Edition.

Daylight Factor ( DF) =

Ei / Eo x 100(%) Ei = Illuminance produced by daylight (direct and inter-reflected) on interior work plane Eo = Simultaneous exterior Illuminance on a horizontal work plane from an unobstructed view of sky

Daylight Factor is used to determine what impact the geometry of the space and location and amount of fenestration have on daylight penetration. For Overcast or Uniform skies, the DF ratio will not change over the year, unless the geometry or fenestration in the room changes. Although the available luminous flux may change (the Daylight Factor Basis), the percentage of light penetrating the space will remain the same). Some uses of Daylight Factor include:

• Comparing different geometric designs side by side.
• Meet certain requirements. For example, the United States Green Buildings Council (USGBC) promotes a Leadership in Energy and Environmental Design (LEED) certification for buildings. Part of the LEED criteria (prior to LEED 2009) specified that a Daylight Factor of 2% was required in 75% of the space occupied for critical visual tasks.
• Understanding the amount of light entering the interior space. This ratio describes that relationship, regardless of the absolute Illuminance values calculated. DF is useful for ascertaining what any changes to the geometric design will have on improving Illuminance and luminance values.

Implementation of Daylight Factor in AGi32

• Daylight Factor calculation points  can be specified in two ways using AGi32: the Calculation Point DF command (work plane only), or, on any surface using the Calculations Points Automatic Placement command.
• AGi32 does not restrict the location of the DF grid to interiors.
• By definition, Daylight Factor is only calculated with Overcast Skies or skies with uniform luminance (CIE Sky Types 1-5).
• Electric lighting is not considered in DF calculations. Electric Lighting should be switched off before Daylighting is applied in order to calculate values for DF.
• Results are given as low-precision values (whole percentage points) by default.