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Some 15 years ago, when we first introduced efficient scientific reflectors for Street Lighting[1],
the general conditions under which the equipment was used were very varied and in many cases far different from
those encountered today. For example spacings varied from 100 feet (e.g. conversion of certain arc lighting
installations) to 500 feet. Similarly mounting heights ranged from 10 to 25 feet. It was therefore necessary for
the reflectors to possess extreme flexibility of application. This compelled us to base our optical designs on
a definite principle, namely, comparatively narrow distributions accurately controlled and capable of
modification by the facet arrangement according to the prevailing conditions.
This graphic illustrates this method in diagramamatic form:-
With the advent of the recommendations of the Ministry Of Transport Departmental Committee limiting the
spacing to 100-150 feet, and standardising the mounting height to 25 feet or 15 feet, it has been found possible
to change our basic distribution with several resulting advantages.
By means of a new optical constuction (patent applied for) we have been able to reduce the peak candle power
by approximately 50% and to considerably widen the distribution in order to compensate for the reduction.
Whilst the possible glare effect has been reduced to half, the general illumination has not been affected, and due
to the wider distribution, the road brightness distribution has been greatly improved. The reduction in peak
candle power has also brought the light distribution ratio within the recommended values.
new installations
This new wide angle construction has been applied to the
AL reflectors (for 40 to 200 watt Tungsten Filament lamps and 80, 125, 150, 250 and
400 watt Mercury lamps). The original range of reflectors are being retained for use when compliance with
M.O.T Recommendations is not required and conditions call for a higher intensity and narrower distribution.
existing installations
As explained previously, the light distribution ratios of the original range were higher than those recommended
by the M.O.T. committee, and if it is desired to modify existing installations accordingly, it is therefore
necessary to reduce the peak candle power and broaden the distribution. This can be done by the use of Pearl
or Opal lamps and in certain cases slightly altering the focussing of the lamp as under:
Group 'A'
(25 feet high) |
Use Pearl lamp, and raise 3/4" above normal focus position. |
Use Pearl lamp. |
Group 'B'
(15 feet high) |
Use Pearl lamp and raise 1" above normal focus position, or use Opal lamp. |
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lumen output per 100 feet linear of road
The average lumen efficiency of normal Bi-Multi reflectors is from 80 to 85%. With
this figure and a knowledge of the average lumen output of the lamp throughout life, the above value can be
easily obtained.
[1] The catalogue is not dated, hence this date cannot be calculated. It is assumed that this is refering
to the A Bi-Multi lanterns, but the
AL range was also modified.
[2] This is the only mention in the entire catalogue of 'Clear Mirror' - ESLA also used dimpled mirror segments to try to reduce
glare.
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