Lighting Quality Parameters - Road Lighting: Fundamentals, Technology and Application - page 90

8.1.2 Uniformity

In the case of a luminance pattern on the road in which there is a large difference
between the luminance value of the darkest part and the value of the average road-
surface luminance, it may become difficult to detect objects against that darkest part
because the eye adapts itself to the higher average luminance. It is for this reason
that the concept of overall luminance uniformity has been introduced. This is defined
as the ratio of the minimum to the average road-surface luminance, L min /L av .Itis
immaterial where on the road the minimum luminance is located, because a motorist
needs to be able to see objects at all possible points all over the road. The symbol
used for overall uniformity is U o . For the determination of U o , the lateral position
of the driver is mostly taken as being one quarter of the carriageway width from the
near-side kerb.
A continuous sequence of bright and dark spots on the road in front of a driver can
be extremely discomforting. This discomfort aspect of a road-lighting installation
can be limited by specifying a minimum value for the longitudinal uniformity. This
is defined as the ratio of the minimum to the maximum road-surface luminance
on a lengthwise line parallel to the road axis (L min /L max ) length . The symbol used is
U l . Apart from the actual luminance difference, it is also the rate of change of this
difference that plays a role. The slower this rate, the less is the problem with the
non-uniformity. With greater luminaire spacings the rate of change is lower, which
means that somewhat lower values of Ul l can be tolerated.

8.1.3 Lighting of the Surrounds

We have seen that concentrating all the light on the road surface alone and keeping
the surrounds in the dark, makes it difficult to see objects at the side of the road or on
the pavement. Moreover, the visibility of the surrounds is important in being able to
judge the course of the road ahead. In order to ensure that the cut-off of light at the
kerbside is not too sharp, the lighting quality parameter of surround ratio (SR) has
been introduced. This is defined as the ratio of the average horizontal illuminance
on 5 m strips adjacent to the left and right-hand sides of the road to the average
illuminance on 5 m strips on the left and right-hand sides of the road itself (Fig. 8.1 ):

E surround (right + left)
E road (right + left)

SR =

In this definition the illuminances on the left and right-hand adjacent strips together
make up the average. In cases where one of these two strips has a high lighting level
and the other one a really-low level, the average can be relatively high. Consequently,
the SR value will be high as well, although visibility against the darker adjacent
strip is not good. Therefore, in the European Standard for road lighting, a different
parameter is used (CEN 2013 ). This is called the edge-illuminance ratio (EIR). It
is defined separately for the left and for the right hand site of the road. Each of the