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Natural/artificial lighting, daylight factor numerical

Short answer

Natural lighting uses sun/sky through openings; artificial uses electric lamps. Daylight factor DF = (Ei/Eo)×100%. Internal illumination Ei = DF×Eo/100 = 2.0×7000/100 = 140 lux (below typical 300 lux classroom requirement).

What is natural and artificial lighting in the building? Define day light factor with necessary figure and formulae. Calculate the internal illumination of your class room having day light factor 2.0% and outdoor illumination 7000 lux.

Natural lighting

Illumination of interior spaces by the light of the sun and sky through windows, skylights and ventilators. It is free, varies with time, and gives good colour rendering. Design uses window area, orientation and light shelves.

Artificial lighting

Illumination from man-made sources (incandescent, fluorescent, CFL, LED lamps), used when daylight is insufficient or at night. It is controllable and constant, but it uses energy and costs money.

Daylight factor

Daylight factor (DF) is the ratio of the indoor illuminance at a point to the simultaneous outdoor illuminance on an unobstructed horizontal surface, expressed as a percentage.

DF=EiEo×100%DF = \frac{E_i}{E_o} \times 100\% 
 Sky (outdoor Eo)
      \  \  \
   ====[window]====
       \  \
        . point P  (indoor Ei)

Calculation

Given DF=2.0%DF = 2.0\% and Eo=7000E_o = 7000 lux.

Ei=DF×Eo100=2.0×7000100=140 luxE_i = \frac{DF \times E_o}{100} = \frac{2.0 \times 7000}{100} = 140\ \text{lux}

Answer: Internal illumination = 140 lux. (This is lower than the 300 lux usually needed in a classroom, so supplementary artificial lighting or larger windows would be needed.)

✓ Answer checked by Claude Opus 5.5 (high effort)

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