The outcome of a long investigation into the needs of modern street lighting, this lantern fully meets the requirements of congested streets in heavily built up areas. Excellent visibility for drivers and pedestrians, absence of glare and good light colour are provided by a lantern of most advanced design. It has been accepted by the Council of Industrial Design for Design Index.

Specification

Special Features. The lantern consists of two parts comprising:-
(a) canopy (incorporating light control system) and support assembly.
(b) plastic bowl.

The former embodies fixed lampholder and mains terminal block. The plastic bowl is clamped to the canopy by means of hinge and toggle catches, on release of which the bowl can be hinged to gain access to the lamp and terminal block.

A hollow "D" section neoprene gasket acts as a seal between canopy and bowl. The lantern has been designed for use with the fluorescent mercury lamp.

Optical System. The effectiveness of an optical system is primarily determined by its relative size compared with the source, thus the light from a fluorescent mercury lamp cannot be efficiently controlled by a system designed for the much smaller plain mercury lamp although the converse does not apply.

Moreover, since the fluorescent lamp will tend to obscure any specular reflector at the high angles and any other control system does not normally give a sharp enough run back from the peak, it is difficult to achieve a peak intensity at a reasonably high angle and yet provide the good cut-off so necessary with these high brightness lamps. These difficulties are completely overcome in the Alpha Three by using a reflector system in which a large proportion of the light sent into the peak is directed around the end of the lamp instead of underneath. The distribution is then built up by the contribution from a series of facets and reflector surfaces as shown in the diagram, and excellent performance is achieved. This is indicated by the distribution diagram and optical data.

Materials. The reflector is made from a special high purity aluminium alloy, and the bowl from a clear plastic. Other parts are cast in corrosion resisting aluminium alloy or fabricated from stainless steel. Great care has been taken to avoid any possibility of electrolytic corrosion.

Finish. The reflector is anodised, otehr aluminium parts are subjected to a "Bonderite" process and stoved. The plastic bowl is treated by an antistatic compound.

Installation. The end support has an entry for a 5" length of 1¼" B.S.P. unthreaded barrel. The lantern may be levelled by the adjustment of the three 3/8" B.S.W. hardened socket set screws that grip the barrel. Wiring is brought to three insulated and protected terminals in the canopy. A cable clamp is provided for the incoming leads.

Lamps. ATLAS 250 watt or 400 watt MBF/U mercury fluorescent lamp.

Catalogue Numbers.

PNE.1400/S Semi cut-off lantern for 250 or 400W MBF/U lamps.
PNF.1400/S Aero screened lantern for 250 or 400W MBF/U lamps.

This diagram shows how the side panels of the reflector many be regarded as segments of a large paraboloid. This sends light from the lamp at the focus in a direction parallel to the axis to form the main beams.

By directing the light round the end of the lamp, cut-off characteristics are combined with a relatively high beam angle.

The light control exercised by the side panels is supplemented by facets in the top section which send the light into all directions from directly downwards up to the angle of the main beam.

Half polar diagram parallel to road axis and half polar diagram through major axis at 8° of azimuth for 400 watt lantern with a light output of 19,200 lumens.