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Electrical Applications 2 by David W. Tyler

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Illumination 125
Light entering the room directly from the sky
—Light entering the room by reflection from
an external vertical surface
Light entering the room by reflection from
an external horizontal surface
Figure 7.30
The luminance of the sky depends on the season of the year, the
hour of the day and the distance of the room from the equator, i.e.
on its latitude, since the height of
sun above the horizon depends
on these factors. The average level of illumination on a horizontal
surface in the open, which is a measure of the luminance of the sky,
measured at Greenwich using north ligh