The light from the horizontal burning sodium lamp is symmetrical at right angles to its long axis and gives approximately 50% of the total flux in the mid 60° zone and 25% each in the upper and lower 60° zones. The mid zone light below the horizontal is already given in the useful directions for street lighting. The upward light requires redirecting along the roadway and the lower light usually gives too high an intensity directly under the lanterns. The middle zone of light is transmitted in its normal direction without control other than a slight diffusion from the body glazing. The upward light is redirected, by means of specially curved diffusing mirrors, to augment the mid zone light and part transmitted to illuminate the area directly under the lantern. The control of light from the lower zone is effected by utilising the optical principle of "First Surface Reflection" from a polished surface. The inverted "V" shape or "W" of the lower part of the lantern body is glazed with clear polished glass and the panes are plaed at such an angle with reference to the light source that the major part of the light rays which impinge upon them is reflected off at an angle from 70° to 80° from the downward vertical.