The importance of the concept illuminance rather than illumination in the practice of public lighting is stressed, and the complexity in both calculation and measurement arising from its dependence on several factors is discussed. A practical method is outlined for calculating the average value and the uniformity of road surface luminance, if certain assumptions (which may reasonably be regarded as allowable for the conditions which apply in street lighting) are accepted; additional data on road surface reflection characteristics and lantern light distributions will be required for the design of installations to a given standard. The difficulties of measurement of road surface luminance (as seen by the normal road user) are also discussed, and a practical form of luminance meter is described, together with its use for the determination of the reflection characteristics of a particular road surface. (A)
Abstract