Basic Lighting Quantities - Road Lighting: Fundamentals, Technology and Application - page 8

Fig. 2.2 The influence of road-surface reflectance on perceived brightness, with illuminances con-
stant: a smooth dry surface; b smooth wet surface; c rough dry surface; d rough wet surface. (Van
Bommel and De Boer 1980 )

2.2.1 Horizontal and Hemi-spherical Illuminance

It is important for pedestrians and cyclists that the lighting reveals potentially dan-
gerous obstacles lying in their path and any irregularities in this. The horizontal
illuminance is therefore usually used as the basic lighting parameter (Fig. 2.3 ). Since
most objects are not flat but three dimensional, some road lighting standards specify
the strength of the lighting at ground level in terms of the hemispherical illuminance
rather than the horizontal illuminance (see again Fig. 2.3 ). Appendix A gives the
calculation formulas for horizontal and hemi-spherical illuminance.