Civil Engineering Reference
In-Depth Information
Fig. 16.1
The variables used
in the upward light ratio ULR
ˆ
lum up
ˆ
lum tot
16.2.1.2
Upper Flux Ratio UFR
In contrast to ULR, the upper flux ratio UFR does also take into account light reflected
upwards from the installation. The importance of taking into account reflected light
may be evident by considering an installation with luminaires that have in their
installed position no direct upward light component viz. ULR
0. Such a zero-ULR
installation will nevertheless result in some degree of sky glow because of light
reflected upwards from the horizontal and other surfaces. Soardo et al. (
2008
)infact
showed that an installation carefully designed to have strictly-limited direct upward
light, may nevertheless result in a higher degree of sky glow than an installation with
some less restricted direct upward light. This is because the luminaires of such an
installation may be less efficient as far as the lighting of the road surface is concerned.
Consequently, more light has to be radiated in the downward direction to fulfill the
road-lighting specifications, hence more light is reflected upward from the horizontal
and other surfaces. Such an installation is also less energy-friendly.
The upper flux ratio UFR, an installation related parameter that does take into
account both direct upward light and indirect upward-reflected light, is defined as
(see Fig.
16.2
): the ratio between the luminous flux at and above the horizontal
resulting from the actual lighting installation:
=
•
directly from all the luminaires in their installed position,
•
reflected from the surface area intended to be lighted,
•
reflected from the surrounding surface area lighted because of spill light
and the flux above the horizontal in the hypothetical ideal situation where the
luminaires have:
•
no direct light above the horizontal,
•
all their light is concentrated into the zone towards the surface area to be lighted,
•
that area has exactly the required lighting level.
This definition means that the hypothetical ideal situation has an UFR value of 1.
The larger the UFR value of a real installation, the further away it is from the ideal
situation, and indeed the higher the sky glow will be. UFR can be calculated from
the data of the lighting installation together with the geometric and reflectance data
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