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.

A notable feature of the design lies in the provision made to reduce glare conditions, the lamp being located in the body of the lantern, thus shielding the intrinsic brightness of the light-source at those angles of view which are more liable to impair visibility. By a suitable design of a refractor bowl and top reflector approximately 90% of the light is directed in a 2-way non-axial distribution in the lower hemisphere and the stray upper emission is virtually eliminated. The smooth, well-shaped lines of the lantern are not only pleasing to the eye, but have definite practical advantages in cleaning and maintenance.

Lanterns were installed along Bath Road, Heathrow and had to be modified to a cut-off distribution. (See 1948 Paper for description.) Many street lighting lanterns give their main beams at about 78° from the downward vertical which is a gradient of 1 in 5 and therefore a screen should project from the lantern a distance of at least five times the vertical depth of the optical system. For Bath Road the lanterns with a prismatic glass bowl some 4½ inches deep were screened by two hoods each projecting 14 inches from the lantern and giving an overall width of 3 feet. By using such large screens, the distribution of intensity in the main beam was not seriously affected. (Change in polar diagram indicated by dotted lines). The modified lanterns were mounted on the same posts as the original lanterns at 120 feet spacing in a staggered arrangement. There was a degree of patchiness to be expected and it would've been desirable to employ opposite arrangement in order to obtain a really good brightness distribution from the road.