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# 3 Daylight Factor

Date post: 03-Apr-2018
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Sky models

Overcast sky

7000 (winter) to 20000 (summer) lux on ground

uniform L()=Lz

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Sky models

Overcast sky

7000 (winter) to 20000 (summer) lux on ground

CIE overcast L()=Lz (1+2sin)/3

1 3

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Sky models

Clear sky

30000 (winter) to 100000 (summer) lux on ground

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Sky models

Clear sky

CIE clear sky model

L = fction of zenith luminance and sun position

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Sky models

Clear sky

CIE clear sky model

L = fction of zenith luminance and sun position

Perez All Weather sky model (generalization of CIE clear sky)

L = fction of zenith luminance, sun position, dew point and 5 coefficients L fully defined if diffuse and direct irradiance are known

Ex. for overcast: Perez vs. CIE

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W = Area of each window (m), = Transmittance of each glazing material

= Vertical angle of sky as seen from centre of each windowm

= Maintenance factor based on angle of glazing and cleanliness (0.5 0.9),A = Total internal surface area of space, including walls, floors, ceilings & windows (m)R 2= Area weighted average reflectance of all surfaces making up A(use 0.1 as reflectance for glass).

Average Daylight Factor calculation

Empirical formula

where

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LEED Green Building Rating System

Daylighting credits

8.1 = Daylight 75% of spaces with DF > 2% (1 credit)

8.2 = View for 90% of occupied spaces (2 credits)

Chart for-

Geometry Factor- Min Tvis

- Height factor

FactorHeightWindowMinT

TActualGeometryWindow]sf[AreaFloor

]sf[AreaWindow[%]DFvis

vis =

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Split-flux method for Daylight Factor

UK Building Research Establishment (BRE)D [%] = EP / Eh = sum of: Direct (sky) component: SC

Daylighting protractor lines of sight and correction factors

Internally

Reflected

Component

Direct

Component

Externally

Reflected Component

Eh

Ep

Figure by MIT OCW.

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Split-flux method for Daylight Factor

D [%] = EP / Eh = sum of: Direct (sky) component: SC

Externally reflected component: ERC

Consider as sky component with

different luminance

Internally

Reflected

Component

Direct

Component

Externally

Reflected Component

Eh

Ep

Figure by MIT OCW.

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Split-flux method for Daylight Factor

D [%] = EP / Eh = sum of: Direct (sky) component: SC

Externally reflected component: ERC Internally reflected component: IRC

Use formula

Internally

Reflected

Component

Direct

Component

Externally

Reflected Component

Eh

Ep

Figure by MIT OCW.

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