Visual Performance for Motorists - Road Lighting: Fundamentals, Technology and Application - page 27

Fig. 3.16 Calculated
visibility level, VL, for flat
and multi-facetted
hemispherical objects as a
function of the visibility
assessment for flat and
hemispherical objects
positioned 83 m from static
observers on a test road.
Object 20

4

Visibility
assessment

3

lat

hemisphere

2

20 cm with 0.20
reflectance factor. (Bacelar
et al. 2000 )

×

1

0

0 5 10 15 20 25

VL

Fig. 3.17 Visibility level
values, VL, for objects with
the same reflectance factor.
Negative values: objects seen
in negative contrast
(silhouette). Black dots
indicate the lengthwise
observer positions for the
different rows of grid points

- 6 -8

-5

-4

-2

2

-8

-11

-6

From the figure we learn that for the assessment “passable visibility” a visibility level
of 7 to 8 is required.
For a defined object size and shape and for a fixed reflectance factor, visibility
level values can be calculated for the various positions of a defined road lighting
installation (Fig. 3.17 ).
Contrary to the concept discussed earlier of total revealing power, the visibility
level value at each point is only valid for one single reflectance factor. If a different
reflectance factor is chosen, the distribution of the visibility-level value over the vari-
ous positions may be completely different. This makes it difficult to make statements
about the quality of a road lighting installation on the basis of one set of visibility
levels only. Visibility level is therefore less suitable for providing an insight into how
the different aspects of road lighting influence visibility. Different proposals have
been made for defining one critical reflectance value to overcome this problem (CIE
1995 ; ANSI/IESNA 2000 ). This has led to a concept named “small-target visibility”.