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Hitherto Street Lighting Lanterns have been designed to distribute the light downwards so that each unit illuminated a
circular area. In the Directive System, nearly all the luminous energy of the light source is used for illuminating the
long narrow area represented by a street, with the results :-
(1) The minimum illumination is 10 to 20 times that obtained with the existing types of street fittings, depending on the width
of the street.
(2) The small but efficient light source of high intrinsic brilliancy presented by the modern incandescent electric lamp becomes a large and pleasingly
illuminated area.
(3) The cost, both for electrical energy and for maintenance, including lamp renewals, is reduced.
(4) For a small outlay, existing installations can be brought up-to-date, to provide the standard of street lighting
which is now necessary to prevent accidents under modern traffic conditions.
The light distribution of these lanterns comprised two distinct parts. The lower hemispherical flux of the lamp is not modified
but is allowed to fulfil its normal function of illuminating the street or road adjacent to the lamp.
The upper hemispherical distribution, which is generally used to very small advantage, is, in this system, collected by
a specially designed reflector, and is directed upon two plane mirrors arranged radially from the light source at such
angles as will produce powerful illumination up and down the street.
These mirrors are so adjusted that they begin to illuminate the street at the points where the direct light fails to bring the
illumination up to the required standard. Adjustments of the angles of the plane mirrors can be made to allow for different
distances between lamps and to suit various widths of street.
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