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Highway Reference Book 1951
Dating from 1951, The Highway Reference Book included a page describing gas lanterns.
STREET LIGHTING BY GAS
The advantage of this lies in the fact that road reflectivity is considerably higher when the light strikes the road at a very
acute angle and in consequence, under ideal conditions, a higher background brightness can be obtained if the maximum light is emitted
at this angle. Furthermore, the light emitted at an acute angle tends to brighten the road surface at some of the points where disturbing
dark patches would otherwise occur i.e. at the base of the next lamp column.
A disadvantage of this type of distribution is that it is almost impossible to avoid some glare, and while the tests made by the
Departmental COmmittee did not reveal any serious loss of visual acuity as a result, there is no doubt that there are imponderable fatigue
effects assoicated with this type of lighting. Nevertheless, it has been widely adopted and the use of high mountings (25 feet) greatly
reduces the glare tendency.
There are at least three types of gas lantern designed to give this type of distribution and these are shown in Figs. 2, 3 and 4.
In the first case the redistribution of light is obtained by the use of strips of curved mirror glass which are rigidly
mounted in a stout brass framework and in the second case light distribution is obtaiend by a faceted steel mirror behind
each group of mantles.
In the case of the "Southport" lamp (Fig. 4), the desired polar curve is produced by the use of special refractors on plates which
focus the light into a beam at about 80°. Examples of the effect obtained by these lamps when installed are given in Figs. 5 and 6.
A second type of light distribution is characterized by a maximum candle power in the region of 75° and a sharp reduction to a
relatively low candle power above 80°. This distribution, often known as "Type 2", greatly reduces the glare and in many cases produces
excellent lighting. A typical lamp of this kind is shown in Fig. 7. Silvered mirror facets, mounted on a strip on either side of the
light source cut off some of the light which would otherwise have been sent out horizontally and reflect this back through the
globe at an angle of about 75°.
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