REVO ELECTRIC CO., LTD., TIPTON, STAFFS.

There are three distinct methods of light cotnrol which form the basis of the design of street lighting fittings in common use, the cut-off, the semi-cut-off and the non-cut-off. The accompanying illustrations show examples of each of these types, for use with Sodium and Mercury vapour discharge lamps.

C.9777 illustrates a lantern suitable for 140 watt Sodium lamp in which the light source is completely screened at approximately 20° below the horizontal. It is specially designed, however, to let a certain amount of reflected light to be emitted up to 10° below the horizontal so that the space-height ratio can be greater than that usually associated with cut-off fittings without causing alternate light and dark bands to appear across the width of the thoroughfare.

The special feature of the lanterns C.10735 and C.10766 is the independent control of the light by reflectors and refractors, thus directing the light emission with the minimum of loss. In this type the maximum candle-power is emitted at 15° below the horizontal and the intensity is progressively reduced above this angle.

It is recommended in the Final Report (August, 1937) of the Departmental Committee on Street Lighting, set up by the Ministry of Transport, that the distribution of light from lanterns "should be designed primarily to produce as uniform and high a background brightness as possible, subject to the avoidance of undue glare ... The background includes not merely the carriageway and footway surfaces, but also other surfaces, such as those of buildings or fences against which objects may be seen." It follows that to obtain uniformity of brightness across the whole width of the essential background, it is necessary that sufficient light be emitted laterally from the lanterns to illuminate the sides of a thoroughfare and produce on those regions a brightness which may be comparable with the brightness of a carriageway. This has been taken into account in the design of the lanterns illustrated, the lateral spread of the light being sufficiently wide to ensure a high degree of brightness uniformity over the total useful background when the lanterns are correctly spaced and sited.

The normal spacing-to-height ratios on straight roads are 4:1 for the cut-off type and up to 6:1 for the semi-cut-off and non-cut-off types. Where a road bends, however, it is important that lanterns to the non-cut-off, or semi-cut-off type are correctly sited so that successive lanterns do not appear to be too widely separated from the motorist's viewpoint. The limitation of the desirable angular separatin of successive lanterns may involve linear spacings of as little as 60 ft. where pronounced bends occur. In the case of the cut-off type, experience has shown that the angular separation is not so important, so that a fairly uniform linear spacing can be adopted regardless of bends.

The lanterns illustrated are suitable for the lighting of Class A roads to the requirements of the Ministry of Transport when mounted at a height of 25 ft. This company also manufacturers a large range of lanterns suitable for Class B roads, for which the recommended mounting height is 15 ft. Among other products are ornamental lanterns to suit local to suit local architectural features, and also standards and brackets of pleasing design.

The company has its own Technical Research Department which is constantly developing new principles of light control and applying them to various types of apparatus, as well as assisting responsible authorities with their own peculiar street lighting problems.

C.10392 shows an exmple of a lantern of the non-cut-off type for use with 250-watt or 400-watt vertically-burning mercury vapour lamps. In this type a high value of candle-power is emitted at angles near to the horizontal for the purpose of taking full advantage of the specular component of light reflected from the road surface at large angles of incidence.