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Notes re Selection of Correct Angular Settings

  1. All angles are read in a clockwise direction looking down upon the road junction.

  2. Except in a few special cases (such as centrally suspended reflectors) the angle of the reflector is never the same as the angle of the road or kerb line. When a reflector is situated on the kerb line, under normal circumstances it is always desirable to ensure that the maximum of controlled light flux is projected over the road-way. For example, in the case of a straight road with reflectors situtated on the footway, if a 180o fitting were installed, half the redirected light would be projected over the fields or gardens adjacent to the road. The correct reflector must, therefore, have an angular setting over the carriage-way.

  3. The optical design of the reflectors is such that they are theoretically intended for installing at a spacing of 10 times their mounting height, i.e. the major portion of the redirected flux is projected to a point at a distance for the reflector of approximately 5 times the mounting height; this is to ensure maximum compensation for the square law and cosine law factors at the greater distances. Whilst the above is the theoretical layout, the reflectors are so designed with sufficient latitude that they are very favourably applicable to considerably greater or shorter spacings.

  4. The width of the distribution in a horizontal plane has been designed to suit all normal road characteristics. When laying out the angles it is helpful to remember that the projected candle power falls to half the maximum at approximately 8o horizontally from the centre line of the cone of light. In order to illustrate this it will be seen that in the case of a straight road having the reflectors mounted 16 feet high and 160 feet apart, the fittings being of the 165o setting, the illumination across the road at midpoint will be approximately even over the traffic lines of a width between post lines of 40 feet (i.e. 40 feet wide carriage-way when reflectors are over kerb.) The illumination on the kerb will be roughly half that in the centre of the road under these conditions.

  5. As a general guide the following usual angular settings may be of assistance. These are based on a normal straight road:

    (a) Lighting units staggered. 165
    (b) Lighting units staggered. Narrow carriage-way. Important or dangerous footways. 170
    (c) Lighting units one side only with pathway each or opposite side. 155
    (d) Lighting units one side only, path on same side as units and carriage-way up to approximately 30 feet 160
    (f) Right angle road junction with fitting on path opposite centre of opening of third road (remaining units in major road staggered). 195 82½ 82½
    (g) As above, but with remaining units in major road all on same side as three-way fitting. 205 77½ 77½
    (h) Right angle road junction with fitting on corner of branch road (remaining units in major road staggered). 165 112½ 82½
    or 165 82½ 112½
    (i) Central suspension. 180 90 90

  6. Other three-way and also four way junctions may be decided upon on the same basis, but with regard to a four-way all-right-angle crossing with fitting on one corner, it is usually found that the 110o 90o 70o 90o reflector produces the most satisfactory results.

Special Note Regarding Very Acute Angular Junctions (eg Acute Fork Roads)

It has been found from experience that where one of the angles required is less than 35o or 40o it is far more satisfactory to utilise two separate two-way reflectors instead of one three-way unit, the wattage of the respective lamps being adjusted accordingly.

Method Of Measuring Angles

  1. By means of optical surveying instruments on site.
  2. By means of special ESLA protractor on plans (preferably of 1/500 scale).
  3. A very reliable convenient method is by means of an ordinary two-foot carpenter's rule with protractor at hinge (e.g., as manufactured by J.Rabone & Sons, Birmingham, type No. 1165). This rule can be laid on the ground and the angles between the limbs adjusted until, upon sighting along the limb, the desired direction is obtained. For three and four way junctions two such rules may be conveniently used.

Care must be taken when specifying three-way angles to carefully differentiate between those having a major angle of 195o and 165o.

ESLA, 17 Canterbury
Canterbury 17