The high intrinsic brightness of the light source is shielded by the reflector assembly in the lantern hood, thus avoiding glare conditions, while the combined reflector refractor assembly affords a controlled two-way non-axial distribution, promoting adequate road surface and background brightness when installed in accordance with Group "A" requirements.

The lantern employs a 400-watt MA/V mercury discharge lamp, burning in a horizontal position and controlled by a magnetic arc deflector to ensure satisfactory life and performance. The lamp is located within the body of the lantern, at which position its high intrinsic brightness is shielded at those angles of view approaching the horizontal. Upward light emission from the lamp is controlled by silvered prismatic reflectors which re-direct this flux into the lower prismatic bowl at predetermined angles of incidence. The end sections of the mirror reflectors are provided with a specially designed prism profile to utilise the light flux, which would otherwise be lost by regular reflection into the end castings of the lantern. This feature also promotes uniform flashing of refractor bowl.

The shallow prismatic bowl is of heat-resisting glass construction, embodying a diagonal arrangement of prisms on the inner surface, and a substantially smooth exterior for ease of maintenance. The prism design has been arranged to control both the direct and indirect light emission from the lamp, and to afford a 2-way non-axial distribution with controlled cut-off characteristics in the vertical plane.

A special feature of this distribution lies in the fact that a very steep gradient of candle-power values is effected in the glare zone between 80° - 90°, and this factor materially improves the visibility for road users. The combined optical system of top reflectors and lower prismatic bowl, ensures a high light output ratio, of which 95% is directed into the lower heimsphere. The lantern body is constructed with hgih-grade silicon aluminium alloy to Specification L.33, and other metals employed in the lantern assembly are cadmium plated to ensure resistance to corrosion in service.

The design of the unit, which is arranged for side entry, ensures that the weight of the lantern is evenly distributed on the bracket arm over the entire length of the hood. Top entry can, however, be arranged as an alternative, where the installation conditions are such that this form of suspension must be employed.

The lower reflecting bowl is carried in a hinged frame secured to the main body of the lantern by means of a single captive wing-nut, thus allowing easy access to the lantern interior for maintenance and relamping purposes.