Light-Pollution Parameters - Road Lighting: Fundamentals, Technology and Application - page 240

Fig. 16.4 Luminous
intensities critical for sky
glow at great distances

I

110

I

90

Fig. 16.5 Total permitted
luminaire luminous flux as a
measure to limit light
pollution

ˆ lum, tot

m 2

initial luminous flux of all luminaires installed in an area or installed per area of
surface (Fig. 16.5 ):

∅ lumtot or ∅ lumtot
m 2

16.2.2.2 Virtual Flux Ratio

A straightforward measure for the quantity of lumens leaving the area, as used for
some situations by IDA-IESNA, is based on a virtual enclosure around the perimeter
of the area to be lighted, with the top of the enclosure at least 10 m above the highest
installed luminaire. The limit is then expressed as a maximum percentage of lumens
on the inside surfaces of the enclosure compared with the total initial lumens provided
by the installation. The lighting parameter, virtual flux ratio, is thus the ratio of the
luminous flux on the inside surfaces to the total flux (Fig. 16.6 ):

∅ surf virt
∅ lumtot

16.2.2.3 Virtual Vertical Illuminance

The lighting parameter “virtual flux ratio”, which is the flux on the inside surfaces
divided by the total flux, defined above, is combined by IDA-IES with the parameter