Street lighting works by illuminating the road surface so that objects such as pedestrians may be seen in silhouette. There is thus an interest in being able to calculate the luminance pattern on the road so that it may be designed to give adequate visual performance to the driver. To do this one needs to know the intensity distributions and positions of the lanterns used and also the luminance factors of the road surface. The luminance factor Q (luminance/illuminance) varies greatly with angle of incidence so that a table of luminance factors is needed for each surface. Following the calculation through one obtains the simple basic formula for luminance of a point L = (1/H)2(I)(r), where H is the mounting height of the lanterns, I is the intensity given in the relevant direction and r is the reduced luminance coefficient (Q cos3y), y being the vertical angle of incidence.