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ESLA Bi-Multi Reflectors and The MOT Requirements

A general guide relating to both existing and new installations together with description of a new type of reflector

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:

Road Classication Type of Existing Reflector
Type "AL"
(Clear mirror)[2]
Type "B"
(Clear mirror)[2]
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. -

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.