Calculations and Measurements - Road Lighting: Fundamentals, Technology and Application - page 205

Fig. 14.3 Observer moving
along with the calculation
grid

obs

obs

obs

obs

experienced by a driver scanning the road area in front of him from one typical,
momentary position. As a consequence, the angle of observation with the horizontal
road surface varies for the different calculation points. For the area between 60 and
160 m in front of the observer, the variation in this angle lies between 0.5 ⓦ and 1.5 ⓦ

for which the reflection tables are applicable as discussed in Chap. 12.
ANSI/IES use the second possibility. They position the longitudinally-moving
observer at a longitudinal distance from each calculation point so that he always
views the road surface under an angle of 1 ⓦ , which is the same angle for which road
surface reflection tables are measured. However, this method does not give a picture
of what one observer momentarily sees when scanning the road area in front of
him. As ANSI/IES works with a standardized eye height of 1.45 m, the longitudinal
distance of their observer is 83 m from each calculation point.
For calculating the veiling luminance needed for glare evaluations (threshold
increment TI for CIE and CEN or veiling-luminance ratio for ANSI/IES), the
longitudinally-moving observer positions are used, as in the second method described
above, so as to obtain the maximum glare value experienced during the drive. The
maximum value obtained from these positions represents the glare value that has to
fulfil the relevant specifications. CIE and CEN take a standardized screening angle
of 20 ⓦ into account in the calculation of the veiling luminance because the car's roof
screens some of the light reaching the driver's eyes direct from the luminaires.

14.1.2.2 Transversal Position

CIE and CEN recommend that in the transverse direction an observer should be
positioned in the middle of each traffic lane. For each of these observer positions the
average luminance, L av , the overall uniformity, U o , and the threshold increment, TI,
is calculated for the entire carriageway (Fig. 14.4 top). The longitudinal uniformity,
U l , is calculated for each centre line (Fig. 14.4 bottom). For L av ,U o and U l the lowest
value obtained for these observer positions is the determining value that should satisfy
the relevant specifications. For TI it is the highest value.