Calculations and Measurements - Road Lighting: Fundamentals, Technology and Application - page 225

Illuminance on the eye or meter caused by i th E eye, i luminaire or luminaire

element (pixel)

Angle between light incidence from i th

ʸ i luminaire or luminaire element

and viewing or measuring direction

Luminance of the i th

L luminaire, i luminaire element in the direction of the meter

The solid angle of the i th ˉ i luminaire element as seen from the meter

For discomfort glare, there is the glare control mark (G). However, due to the
psychological nature of discomfort glare, the value of G cannot be derived from
measurements.

14.2.5.3 External Influences

Stray light reaching the road surface direct from shop windows, traffic signs, vehicle
lights, and even from the moon, as well as light reflected onto the surface from light-
coloured clothing, walls of nearby buildings and so forth, can add considerably to
the amount of light reaching the road surface from the lighting installation alone. The
influence of stray light is, of course, particularly noticeable when seeking to measure
minimum values. Suppose, for example, that the minimum illuminance given by the
road-lighting installation alone at a point on the road surface is 0.5 lx. Stray light
contributing only 0.05 lx will then already account for a 10 % discrepancy. Stray
light that does not appreciably influence the illuminance measurement (light from
an oncoming car, for example) can, due to its direction of incidence, have a marked
effect on the magnitude of the luminance reading.
For measurements to be meaningful, the installation's supply voltage should be
kept to its nominal value while the measurements are being made; only then will
the nominal flux from the lamps be obtained. If control of the supply voltage is
not possible, it should at least be monitored so that in the event of supply variations
occurring, corresponding corrections to the light output can be made. For installations
containing new discharge lamps, 100 h of operation should have elapsed before
measurements are taken, in order to be sure that the lamps are stabilised.
Before luminance measurements are carried out it is important to check that the
normal reflection properties of the road surface have not temporarily changed for one
reason or another. Such changes can be caused by large accumulations of surface dirt
(mud, oil, leaves, salt—employed to melt snow and ice—and so forth) or, in the case
of bitumen-based surfaces during very hot weather, by partial melting or softening of
the surface itself. Where it is the dry-surface luminance that is being measured, care
must, of course, be taken to ensure that the surface is perfectly dry. An additional
source of error or uncertainty will arise where the wet-surface luminance is being
measured, since this surface condition is not accurately reproducible.
Finally, since field measurements are only valid for the conditions prevailing at
the time of the survey, it is important to record a detailed description of the area
surveyed and to make a note of all those factors that might conceivably influence the
results: factors such as lamp type and age, luminaire and ballast type, supply voltage,
state of maintenance (including date last cleaned), weather condition (temperature,