1. The brightness of a transparent luminous body such as a flame. (An example: a screen of 0.233 square inch in area gives a standard of 2 candle-power. Therefore the brightness of this part of the flame was 8.6 candle-power per square inch.)
2. The brightness of a solid luminous body such a piece of incandescent metal or carbon.
3. The brightness of a matt or rough body which reflects light in all directions.

(All these versions are unfortunate, because unlike other compound units, the quantity is not the product of candle-power into length, but it is the quotient and the divisor is the square of the length).

The illumination of one candle at one foot is equivalent to four candles at two feet and nine candles at three feet.

The foot-candle is a very convenient and comfortable illumination. It is for most people a sufficient illumination for reading, and is to be found on most well-lighted dining-room tables and billiard tables. [Pelham 1911, p16]

Pelham jokes that the correct unit should be candle-toof-toof.

The diagram may be considered similar to the contours on a map and therefore the diagram represents a solid. A section taken through an iso-lux diagram gives the illumination curve for that section. (See Illumination Curve for the curve constructed for the section AB).

"It is not suggested that the construction of these contour lines should often be carried out, and no great importance is attached to them. The study of a few typical cases is interesting." - [Pelham 1911, p52]

The following formulae give the illumination at 'C' from a lamp mounted at 'A'. It combines both this law and the cosine law:

(The first formula requires the length 'AC' to be known whilst the second is derived from the first and simply requires the height ('AB') and angle to be known and has better real-world application).

The curve of cos³θ is given below (assuming height of unity):

Illumination on a horizontal plane decreases so rapidly with the distance that at an angle of incidence of about 37°, or at a horizontal distance of three-quarters the height of the lamp from the ground, it falls off to one-half; at about 45° to one-third; and at 62° or at a horizontal distance or nearly twice the height of the lamp above the ground, one-tenth of the maximum. [Pelham 1911, p29]

This all assumes uniform illumination from the lamp source. Notable differences:

• An arc lamp gives little illumination between 0°-55° due to the obstruction of the lower carbon. The projected area of the tip of the upper carbon varies as the cosine of the angle; therefore cos⁴θ is used. [Pelham 1911, p33]
• A metal filament electric glow lamp has one or more vertical filaments. This creates a distribution related to the sine of the angle of incidence; therefore cos³θsinθ is used. [Pelham 1911, p34]

Varying the height of the lamp results in the following curve (which follows cos²θsinθ):

The maximum illumination occurs at height D, and it may be shown that BA = BD x √ 2  and the angle of incidence BDA is 54°44'. It will be observed that a small change in the height BD (at this height) may be made wihtout appreciably altering the illumination at A.

Raising the lamp √ 2  dimishes the maximum illumination to one-half, and lowering it to 1/√ 2  doubles the original maximum. [Pelham 1911, p37]

The lamp can also be moved through a locus to provide the same minimum illumination at various distances and mounting heights (with variations of the maximum illumination):

The formula gives the curve where c is the candle-power and E is the illumination. For example, if a lamp at A on a post having a height AB gives a certain illumination at the point O, the same lamp moved to C on the post CD will give the same illumination at O.

Near the point H the height may be altered considerably without altering the illumination at O materially.

OG is √ 2  times GH.

This shows that with a given candle-power and given minimum illumination on the ground, the lamps may be spaced at a distance apart not greater than OG x 2. It is for the engineer to consider in any particular case whether the expense of so tall a post as H is warrented, or whether a larger number of posts of height DC would be less expensive. [Pelham 1911, p43]

"Colour Modified" 400W mercury discharge lamps are introduced in 1934. These have mercury, cadmium and zinc added to the discharge tube; the cadmium and zinc introduce blue and red light into the visible spectrum. It has an initial efficiency of 37 lumens per watt. - [Aldington, 1936]

The metrics as given in 1936 are:

The lamp is refered to as the High Pressure Mercury Lamp where the mercury vapour develops a pressure greater than one-half atmosphere when the lamp is in operation. It is realised by 1936 that this is limited, as lamps employing mercury vapour pressures of 100 atmospheres or more have been developed. In general, where the mercury vapour pressure exceeds about 30 atmospheres, the lamps are artificially cooled and are referred to as Super High Pressure Lamps. - [Aldington, 1936]

A 150W version was made commerically available in 1936. - [Aldington, 1936]

Lamps designed for a given mains voltage have the same arc voltage while the current is controlled to give the designated watts. Therefore the lamp current of a 150-watt lamp will only be 3/8 of the current of a 400-watt lamp - this has an influence on the initial efficiency. - [Aldington, 1936]

Account must be given of the temperature at which it is allowable to operate the hard glass tube in which the arc takes place. Too low a temperature on the surface of this tube tends to produce an unstable characteristic; too high a temperature may seriously affect the performance of the lamp during life. - [Aldington, 1936]

The relation of the wattage of a discharge tube to its volume determines the temperature at which the walls of the discharge tube operate. The loading of discharge lamps may, teherefore, be described in terms of watts dissipation per unit volumne of the tube. It has been found that with the types of glass [in use] a loading of some 3.0 watts per cubic centimetre gives high value of efficiency maintenance through life. - [Aldington, 1936]

Quartz allows higher temperatures to be otained without the risk of fracture, it is more resistant to attack of metal vapours at high temperatures and is less liable to electrolytic decomposition under higher pressures. - [Aldington, 1936]

Therefore the dimensions of the discharge tubes for a given wattage are reduced to the lowest possible value consistent with the quartz tube operating at a temperature not greater than 1000°C. The effect is to increase the loading to 60 watts per cubic centimetre which increases the initial efficiency to 40-50 lumens per watt for 100W - 150W tubes. - [Aldington, 1936]

The increased pressure of a quartz lamp uses an incrased in the volt drop per unit length of the arc column, a decrease in the width of the arc column, spectral broadening, and an increase in the emission of radiation at ultra-violet wavelengths. A significant amount of the ultra-violet is transmitted through the quartz. - [Aldington, 1936]

Rhodamine has been used to improve the colour spectrum of quartz lamps. However it absorbs yellow-green light and emits as orange-red; this reduces the overall efficiency of the lamp. Rhodamine is also slowly decomposed under the action of ultra-violet radiation so its efficient life is relatively short. - [Aldington, 1936]

Experiments are ongoing with Barium Sulphide, Cadmium Zinc Sulphide with quartz lamps which will convert the near ultra-violet radiation intro orange or red light. - [Aldington, 1936]

The "Dual Lamp", developed at the Siemens Lamp Works, Preston, combines a mercury discharge lamp and incandescent lamp to reduce colour distortion. It does not require a choke or auxiliary gear. When the lamp is first switched into circuit, the filament dissipates most of the energy, but after about 10 minutes when the discharge tube has reached its designed voltage the thermal switch inside the lamp closes and shorts out a portion of the tungsten filament equivalent to the arc tube voltage. - [Aldington, 1936]

Experiments are also ongoing with combinations of quartz lamps and incandescent lamps. This is proving fruitful with the short run-up time of the quartz lamp (so the filament is only overloaded for a short time), and the mutual temperature gain of the incandescent filament and the mercury discharge tube. - [Aldington, 1936]

The metrics as given in 1936 are:

(This is when the distance is more than ten times the height of the lamp , and it is useless to consider the minimum illumination on the ground either by calculation or by measurement). [Pelham 1911, p41]

The term illumination in this specification denotes the illumination in foot-candles or in lumens per square foot of a surface situated at the ground level and parallel with the plane of the highway. Only light received directly from the installation is considered,light received by reflection from buildings and the like or that received from light sources not forming part of the installation being excluded. [BS 307, 1931]

The term mean test-point illumination in this specification denotes the mean of the illuminations at the test-points in one class of installation or agreed section thereof, but in every case it must be specified whether it refers to the carriageway or the footway. [BS 307, 1931]

The rated illumination is the illumination which would be produced if all the factors which govern the illumination were at their rated value. [BS 307, 1931]

The difference between the values of the rated and the service mean test-point illuminations is necessary in order to allow for the unavoidable variations in the spacing and to provide for decrease of the candlepower of the light sources, variation due to replacements or incorrect adjustment of the light source in the fittings, soiling of glass and reflecting surfaces of the fittings, changes in voltage or gas pressure and other factors which may cause the service illumination to fall below the rated illumination.. [BS 307, 1927]

The service illumination is the illumination at any time during the operation of an installation. [BS 307, 1931]

The spacing-height ratio is the ratio of the distance between two adjacent light sources, measured along the centre-line of the roadway, to the height of the light source from the ground immediately beneath it. [BS 307, 1931]

The term test-point in this specification denotes that point on the ground to be illuminated which is equidistant and as far as possible from the light sources which form one complete unit of the system. The test-points will be either on the carriageway or at the edge of the carriageway, and on the road margin or on the footway. The illumination at the test-point in the carriageway is generally the minimum for the carriageway and that at the test-point on the road-margin or footway is generally the minimum for the whole highway. [BS 307, 1931]

The term unit of the system in this specification denotes a single unit of the repeat pattern. A highway lighting system is regarded as a series of sources forming a repeat pattern. [BS 307, 1931]

The organisation is now called the ILP (Institution Of Lighting Professionals) and they publish the Lighting Journal six times a year.

Open-Arc Lamp (also Standard Arc or 2,000 Candlepower Lamp)

Earliest practical form of arc lamp. For use on direct current with carbon electrodes exposed to the atmosphere. The carbons required renewing once a day. The positive carbon forms a crater and is consumed at twice the rate of the negative carbon which becomes pointed. 80%-85% of the total light emitted from the lamp comes from the vapourisation of the carbon at the positive electrode. Feeding of the carbons achieved by an arrangement of solenoids acting against gravity or springs.

A 500W, 9.6A carbon lamp furnished a maximum intensity of about 1200 candlepower. However the light distribution was unsuitable for street lighting: maximum candlepower was downward at an angle of 45° creating a bright ring of light on the street surface near the lamp.

(Note: the polar curve is unsymmetrical. It is caused by the arc wandering from one point of the periphery of the carbon to another. Therefore the values for the polar curve were taken when the arc happened to be on the right side of the electrodes).

The recognized illumination rating for the open-arc carbon lamp has always been 2,000 candlepower. This is incorrect, and probably due to early photometers being unable to measure such quantities. The courts of law have decided that the 2,000 candlepower lamp describes a particular type of lamp and has nothing to do with the actual intensity obtainable from it.

Obsoleted by the Enclosed-Arc Lamp.

Half-Arc Lamp

An Open-Arc Lamp but operated at 6.6A on 50V.

Enclosed-Arc Lamp

The arc is surrounded by an inner or enclosing globe which renders it airtight. Under these circumstances, the carbons are not consumed as rapidly and will burn for 80 to 100 hours without trimming.

Distribution curves for enclosed carbon-arc lamps with reflectors
A - 6.6-amp, A.C. arc
B - 7.5-amp, A.C. arc
C - 6.6-amp, D.C. arc

D.C. enclosed arc gives a natural downward distribution of light due to the existence of a crater in the upper, positive electrode. A.C. enclosed arc gives approximately the same amount of light in the upper and lower hemispheres, and it is usually the practice to equip it with a flat reflector. The greater steadiness of the enclosed lamp and that a relatively greater proportion of flux emitted at angles near the horizontal gave it an advantage for street lighting.

Flaming-Arc Lamp (Open)

Uses carbon electrodes, either impregnated with certain salts which give luminosity to the arc, or with cores which contain the required material. The additives were called the "flaming materials". Carbon life length was typically 12 hours. Commonly rated at 3,000 mean horizontal candlepower.

The Bremer flaming-arc lamp was introduced in 1899. The electrodes are mounted at an angle, their tips projecting into an "economizer" which limits the air supply to the arc and increases the life of the electrodes, and acts as a reflector. An electromagnet is used to blow and spread the arc downwards to prevent damage to the economizer by the arc.

Electrodes mounted at a downward angle to prevent slag build-up of additives; and also ensures most light is directed downwards.

Typical early additives included compounds of calcium (yellow), strontium (red-pink), magnesium and boric acid (white).

Because of the frequent trimming required, the flaming arc in original form was considered unsuited to American street lighting practice, so the enclosed flaming arc was developed.

Flaming-Arc Lamp (Closed)

As above and developed to increase the life of the carbons. Electrodes are again mounted one above the other. Designed for 10A A.C. at the electrodes; many contain autotransformers so can be supplied from 6.6A circuits.

Enclosing the arc increases the life of the electrodes to approximately 100 hours, but reduces the candlepower of the lamp. Due to poor candlepower maintenance (by deposit of ash on the globes) and because of unsteadiness of light, the enclosed flaming-arc lamp was superceeded for street lighting.

Magnetite-Arc Lamp (also Luminous Arc)

Arc lamp with metallic electrodes: copper for the upper electrode and a magnetite stick for the lower. It has a high efficiency, white colour and good candlepower maintenance. Electrode life from 100 to 350 hours (depending on amperage of lamp and length of arc). Must be used with D.C., so usually supplied from mercury-arc rectifier sets.

Diagram of connections for luminous-arc lamp.

The magnetite arc met with much favour in both utilitarian and ornamental street lighting in the USA, with the ornamental 6.6A magnetite arc being most common.

Typical distribution curves of luminous-arc lamps with different equipment.

The electromagnetic ballast is a simple series inductor or "choke" which limits the lamp current. This inductor has significiant resistive and inductive losses which increase with the age of the ballast. For a 150W ballast these losses might amount to 15%.

The inductance of an electromagnetic ballast results in a low power factor for the combined lamp-choke which can be partically compensated by the use of a capactor across the mains.

The vast majority of brackets have one arm which supports one lantern. Other examples can have two, three or four arms to support multiple lanterns.

Brackets are often made from the same material as their assoicated column (tubular steel, concrete), although some manufacturers offered metal brackets for concrete columns.

Early examples are highly decorative with scrollwork and finials. By the 1950s, the designs became more austere and functional.

With many early lanterns requiring fixing via the top of their canopy, many brackets are bent and angled. The most familar shape is the swan neck (see left).

A bracket isn't normally required for post-top lanterns. These attach directly to the column.

They are often revised and updated. The current standards (2013) are:

BS 5489 Part 1: Lighting of roads and public amenity areas.
BS 5489 Part 2: Tunnels.
BS EN 13201 Part 2: Performance requirements.
BS EN 13201 Part 3: Calculation of performance.
BS EN 13201 Part 4: Methods of measuring lighting performance

BS EN 13201 Part 1 will no longer appear as a standard, and will probably appear as a guideline document.


Previous standards were:

• BS 307/1931: Street Lighting. Published in 1927.

  It specified a test point and minimum illumination values (as foot candle classes):

  Class	Minium Illumination	Minimum Height Of Source	Spacing Recommended	Spacing Height Ratio
  A	2.0 foot candles	30 feet	90 feet	3
  B	1.0 foot candles	25 feet	100 feet	4
  C	0.5 foot candles	21 feet	105 feet	5
  D	0.2 foot candles	18 feet	108 feet	6
  E	0.1 foot candles	15 feet	105 feet	7
  F	0.05 foot candles	13 feet	104 feet	8
  G	0.02 foot candles	13 feet	130 feet	10
  H	0.01 foot candles	13 feet	130 feet	10

  It allowed a simple way to order and check the work of contractors and give general guidance in the planning of new work.
  
  
  

• BS 398/1930: ???. Publication date: 1930

  This documented specified the symmetrical light distribution from light fittings. Street lighting was part of the "Direct" class, and not less than 90% of the flux was in the lower hemisphere. The polar curves were divided into five classes:-

  1. Extra Narrow

  2. Narrow

  3. Intermediate

  4. Wide

  5. Extra Wide

  By the 1930s it was realised that both these reports were unsuitable, and had lead to some very highly directional reflectors and refractors giving narrow beams of high intensity, leading to glare and streakiness on the road.

  They set a fashion for high peak intensity.

  They were quickly succeeded by the MOT report in 1937 and subsequently obsoleted.
  
  
  

• BS 1308: Reinforced Concrete Street Lighting Columns. Published in 1946.
  
  
  
• BS 1249: Cast Iron Street Lighting Columns. Published in 1951.
  
  
  
• BS 1788: Street Lighting Lanterns. Published in 1951.

  This document covered the mechanical and constructional features of lanterns, with regard to safety, durability and easy of maintainance.

  It did not cover photometric requirements.
  
  
  

• BS Code Of Practise (BSCP) 1004 Part One: Street Lighting - Traffic Routes. Published in 1952.

  This refined the MOT Report of 1937 on Group 'A' roads and was supplimented by BS 1788 which was published the previous year.

  The total downward light the now the most important.
  
  
  

• BS 1840: Tubular Steel Columns for Street Lighting. Published in 1952.
  
  
  
• BS Code Of Practise (BSCP) 1004 Part Two: Street Lighting - Roads other than Traffic Routes. Published in 1956.

• BS 1788: Street Lighting Lanterns. Revised in 1964.

• BS 4533: Light Fittings. Published in 1976.

Left: The BTH Urban Enclosed (SM278) - available as both side and top entry.

Ceramic arc tubes were preferred over quartz (as used for high pressure sodium discharge lamps) as ceramic arc tubes were more resistant to attack by the metal halide salts within the tube.

The lamp was not recommended for dimming.

It was superseded by the CDO-TT range (which could be dimmed).

The lamp was manufactured by Philips as the MASTER City White and was available in 70W and 150W sizes. The 'CDM' was a Philips code, presumed to stand for 'Ceramic Discharge Metal halide'. The 'TT' was another Philips code representing the ES lampholder.

Ceramic arc tubes were preferred over quartz (as used for high pressure sodium discharge lamps) as ceramic arc tubes were more resistant to attack by the metal halide salts within the tube.

The lamp could be dimmed with special operating gear.

It superseded the earlier CDM-TT range.

The lamp was manufactured by Philips as the MASTER City White and was available in 70W, 100W, 150W and 250W sizes. The 'CDO' was a Philips code, presumed to stand for 'Ceramic Discharge Outside'. The 'TT' was another Philips code representing the ES lampholder.

Although proposed as a technology in the mid 1960s, practical considerations (such as the corrosion of the metal wires passing through the arc tube) prevented the production of lamps until 1981 when Thorn unveiled their TSH (Tin Sodium Halide) lamp. Unfortunately the bulb required special gear so research continued throughout the 1980s but halted when GE cancelled the project (when they purchased Thorn).

It was left to Philips to continue the project and solve the technical problems. Their CDM range was announced in 1994.

English Parliamentary Candle was set up about 1860 as an official standard to be used for the purpose of testing gas. It is described as a pure spermaceti candle weighing 1/6th of a pound and burning at a rate of 120 grains per hour. It was set up as an official standard for the purpose of gas-testing.

International Candle was adopted as the international unit of candle-power by Great Britain, France and the United States of America. Such a candle is defined as being equal to one Pentane candle, or 1.11 Hefner units (the Hefner being the German unit and denoting a luminous intensity of the Hefner lamp under standard conditions).

It has been superceeded by the Candela.

The candle power as a unit of light intensity originally represented the horizontal intensity of light from the British Standard Candle, but since the value can be more accurately preserved and reproduced in the incandescent lamp. This arbitrary value is now maintained in tested incandescent lamps in the National Physical Laboratory, and in other countries. The unit of light in general use in Great Britain, France, United States and other countries (Germany excepted) is termed the "International Candle."

The relations of the International Candle to other terms are as follows:-

• 1 International Candle - 1 Pentane Candle (Great Britain)
• 1 International Candle - 1 Bougle Decimale or 0.104 Carcel Units (France)
• 1 International Candle - 1.11 Hefner Units (Germany)

Traditionally columns were made from cast iron, but during the 20th century, other materials were used such as steel, concrete, fibre glass, plastic and even wood. Steel became the most popular by the end of the century, but concrete was extremely popular in the decades following World War II due to metal shortages and rationing.

Initially, particularly when cast from iron, columns were extremely decorative. Such embellishments gradually disappeared and columns became simpler over the years.

Cast-iron and early concrete columns were designed to withstand vehicle collisions but such thinking radually disappeared during the 1960s with designs which would crumple and even shear off at the base.

Some columns were designed with a door at their base for the housing of fuses, control gear and timing mechanisms within the column. Earlier columns, such as those originally designed for gas lanterns, had no such compartments.

See: Ballast.

Low pressure sodium has a CRI of zero (which is why it has been excluded from current specifications).

Typically in older specifications, the maximum intensity is at 70° to the downward vertical with a negligible amount of light at 80° and above.

Left: The DAVIS GR100 - originally an ELECO design.

The separate incident rays are all reflected according to the laws of reflection, but owning to the fact of the various parts of the surface being inclined at different angles to those rays, the latter are reflected in all directions, or a beam composed of scattered rays results.

An electronic control gear controlling a 150W lamp would typically have losses of 5%.

Reliability and length-of-life are current issues with electronic gear which are far more electronically complex than older electromagnetic ballasts.

See also: Ballast.

Measured in lumens per watt or lm/watt.

Left: The Eleco Lunar (PT 1006) - an example of a striking design.

• M.S.C.P * 4π = Total Lumens.
• Foot Candles * sq. ft. = Lumens.

The Foot-candle is a unit of intensity of illumination; it is defined as the illumination produced by a source of one candle power on a surface everywhere one foot distant from the source.

Frequently, though more generally in American literature, the foot-candle is defined as an illumination of one lumen per square foot, this definition involving a consideration of luminous flux.

Since the candle-power of a souce varies with the direction, there is much to be said for the latter method of measurement.

The first incandescent bulbs used carbon filaments in a vacuum; these were then replaced by tungsten filaments in a vaccum; to be finally superceeded by gas filed tungsten filament bulbs. (An inert gas was pumped into the bulb to slow down the loss of tungsten atoms from the filament).

The original bulbs used a coiled filament, whilst coiled coil tungsten filament lamps started appearing in the 1930s: these became the standard. Old 1930s open GLS lanterns (such as ESLA Bi-Multis or BLEECO open dome refractor lanterns) should be fitted with clear single coil bulbs.

Typical wattages are: 40W, 60W, 75W, 100W and 150W (for Group B roads); 200, 300, 500 and 1000W (for Group A roads).

Variations of the GLS source are:

Clear Lamps: These are clear glass bulbs.

Pearl Lamps: Are the most common types of incandescent lamp. These are internally etched to produce a diffusing finish and usually have a grey unlit appearance.

Opal Lamps: Are less common, being sprayed inside with a diffusing coating. They have a white unlit appearance (similar to a coated elliptical MBF or SON).

In the 1930s, it was believed this was governed by three factors:

1. The brightness of the lamps.
2. Position relative to the observer.
3. A factor which takes account of the relative movement of observer and lamp.

This became the standard.

When bowl became the preferred term, globe was used to describe spherical, decorative glass or plastic bowls for street lights.

There are other categories depending on the shape of the surface:

• Hemispherical: Luminous flux on a small hemisphere with a horizontal base, divided by the surface area of the hemisphere.
• Horizontal: Illuminance on a horizontal plane.
• Semi-Cylindrical: Total luminous flux falling on a curved surace of a very small semi-cylinder divided by the curved surface area of the semi-cylinder.
• Vertical: Illuminance on a vertical plane.

The term "lantern" is now superceeded by "luminaire".

1. Spinning
   1. Steel
   2. Copper
2. Fabricated Box
3. Casting
   1. Iron
   2. Silicon Aluminium
      1. Gravity Die
      2. Pressure Die
   3. Plastic Moulding

• The reflected ray lies in the same plane as the incident ray and the normal, but on the opposite side of the latter.
• The angles of incidence and reflection are equal.

1. Absorption: Part or practically all of the light losses its identity and is converted into heat.
2. Refraction: Bending of a ray of light due to its passing from one transparent medium to another of greater or less density.
3. Reflection: Throwing back or redirection of rays by a surface.
4. Diffusion: The breaking up of a beam and spreading of its rays in all directions by the medium through which it passes or by the surface upon which it falls.

Various technologies, and combinations of those technologies, are used in a lantern:

1. Reflector
   1. Vitreous Enamel
   2. Glass Or Plastics
      1. Silvered
      2. Aluminised
      3. Prismatic
   3. Anodised Aluminium
2. Reflector-Refractor
3. Refractor
   1. Single Piece
   2. Sealed Two Piece
   3. Glass
      1. Soda-Lime
      2. Heat Resisting
   4. Plastic
      1. Processed Sheets
      2. Injection Moulding
4. Diffuser

1. Incandescent Filament (GLS)
2. Discharage Lamp
   1. Low pressure sodium (SO/H)
   2. Mercury
      1. Discharge MA/V, MA/H, MB/U
      2. Blended MBT/V
      3. Bulb Fluorescent MBF/U
      4. Tubular Fluorescent MCF/U
3. Gas
   1. Low pressure
   2. High pressure
4. Arc Lamp

1. Direct Type Reflectors are placed above the lamp to redirect the horizontal and upward distribution onto the working plane. (As typified by Benjamin Steel Reflectors and many Reflector Fittings.)

   These have the highest efficiency, but care should be taken that glare from the exposed filaments should not occur. The shadow effects are, however, somewhat harsh, thus rendering them unsuitable for certain purposes.

   In the 1910s, three specific subcases were recognised. These cases were sometimes quoted for very early street lighting reflectors.

   • Extensive Reflectors. These give a wide distribution of light and are suitable for rooms which are low in comparison with their width or length, or for rooms employing a single central fitting. Such reflectors provide uniform illumination over a surface whose radius equals the height of the lamp above the working plane, from which it is clear that if a large space is to be illuminated by several lamps, they must be fixed a distance apart equal to twice their height.

     Usage: General Room illumination.

     

     
     

   • Intensive Reflectors. These provide an intermediate illumination i.e. a uniform illumination over an area whose radius is three-quarters the height of the lamp above the working plane, and consequently should be placed a distance apart equal to one and a half times their height.

     Usage: Desk Illumination.

     

     
     

   • Focussing Reflectors. These throw an intense light over a small area, and are brought into use if special illumination of any particular object is required. Such a reflector will provide a uniform illumination over an area whose radius equals half the height of the lamp above the working plane, and lamps should be placed a distance apart to their light.

     Usage: Object Illumination.

     

     
     

   • Concentrating Reflectors. These are required in cases that require exceptional illumination.

   • There are also Dispersive and Distributing Industrial reflectors - see APLE Conference 1932, Credenda.

     Usage: Object Illumination (such as shop-window illumation.)

     
     

   In 1930 (BS 398/1930), the classication of the polar curve was divided into five classes, which cast 90% of the total flux in the lower hemisphere:
   • Extra Narrow.
   • Narrow.
   • Intermediate.
   • Wide.
   • Extra Wide.

   
   

2. Semi-Indirect Fittings are provided with an opalescent bowl beneath the lamp. Only a portion of the light is transmitted directly by the bowl, the greater part being reflected on to the white celining or a top refelctor and thence re-directed to the working plane.

   Their efficiency is appreciably less than that of direct type reflectors, but they cast much softer shadows and the opalescent bowls effectively prevent glare.

   
   

3. Indirect Fittings consist of an opaque bowl, having an internal reflecting surface, suspended beneath the lamp. Thus the whole of the light is directed to the ceiling, or to a top reflector, and thence to the working area.

   They provide a light practically devoid of shadows at a still further sacrifice of efficency.

   
   

1. Prestige And Display
2. BS Code Of Practise
   1. Group A
      1. High Angle
      2. Medium Angle (including Aeroscreened)
      3. Cut-Off
      4. Uni-Directional
   2. Group B
      1. Narrow B1
      2. Wide B2
      3. Tree Lined

The Lumen is the unit of light flux. It is equal to the amount of light required to produce an intensity of one foot-candle over an area of one square foot.

From the source, unit flux will be given out within each unit solid angle, the total luminous flux being equal to that within each unit solid angle multiplied by the number of such angles about the source.

The unit of luminous flux defined above is known as the lumen, and thus from the source the total flux of light must equal 4π lumens.

If such a source be surrouned by a sphere of radius equal to one foot, the total flux emitted will be intercepted by the surface of the sphere, and the flux incident upon each square foot of the surface will be given by :-

flux (4π) / area (4π) = 1 lumen per square foot.

since the flux is 4π lumens and the area 4π square feet.

According to the definition of illumination it is clear that the illumination on all points on the surface of the sphere is the same and equal to one foot-candle, all points being the same distance - one foot- from the source of one candle power.

400W bulbs were first available, quickly followed by 250W units. A 150W version was launched in 1936, but, due to the medium pressure of the mercury vapour, suffered from efficacy problems. Therefore the race was on to develop a high pressure version of the bulb which could operate well at lower wattages.

The orientation of lamp burning was also important, and several designations were used:

/V Vertical cap up

/D Vertical cap down

/H Horizonal

/U Universal

Mazda (BTH) sold the lanterns under their Mercra name. The GEC could only use the Osram name for tungsten filament lamps, so sold their mercury lanterns as Osira up until the second world war. Philips used Philora, whilst Siemens used Sieray.

Siemens were advertising the Sieray-Dual Lamp in 1936 as a combination of mercury discharge lamp and tungsten filament. The light quality was considered better from this bulb, and it didn't require any gear (the tungsten filament acting as ballast with its fixed resistance). The designation for this bulb became MAT and was also known as Blended Mercury.

There were a couple of variants (which extended the lamp's code):

T With a tungsten filament. See above.

F Coated with a fluorescent power to improve the light quality.

First known use was in the Bremer flaming-arc lamp; the electromagnet is used to blow and spread the arc downwards to prevent damage to the economizer by the arc.

Later used to allow MA/V lamps to be mounted horizontally. The small magnetic coil is positioned in a lantern above the lamp to correct the upwards bend of the arc caused by heat convection. The magnetic field of the coil pushes the arc back into the centre of the arc tube. Power consumption is typically 1W.

If the magentic arc deflector isn't used for a horizontally mounted MA/V bulb, the arc will rise sufficiently to hit the arc tube and destroy the bulb.

1. Hand operated (both individual and group)
2. Time clock operated (both individual and group)
3. Centralised control using high frequency impulse or DC Bias.
4. Contactors in cascade.

The Mean Spherical Candle Power can be averaged from the MHCP by multiplying by 0.8 to 0.85.

Imagine a luminous body to be surrounded by a sphere and a number of points uniformally distributed over the surface of the latter to be taken.

If the candle-power be now measured in the direction of radii to each of these points, the average of all the intensities so found is called the mean spherical candle-power of the source.

The M.S.C.P. of a metal filament lamp can be found by multiplying the Mean Horizontal Candle Power by a factor which varies between 0.8 and 0.85.

They published their first report in 1937 (with an interim report in 1935 which covered main roads only) which superceeded and obsoleted the British Standards 307 report of 1927. The amount of light from a lantern was just one criterion (it was the principle subject of BS 307). Definite spacial arrangements of lanterns in terms of height and distance were standardized, and there were limits placed on the peak intensity of the beam, leading to improvements of glare. It was also the first document to divide roads into Group 'A' and Group 'B'.

1. Symmetrical
2. Axial Asymmetric
3. Non-Axial Asymmetric
4. Uni-Directional

1. Low labour costs for maintainance.
2. Lanterns more easily cleaned on the ground.
3. Maintenance can be effected without causing a traffic obstruction.
4. Elimination of risk of accidents with tower wagons.
5. However... installation costs more and additional equipment needs maintaining.
6. Jarring and shock can cause filament or bulb failure.

See also: Cut-Off.

At first, there was much laughter about the "queer yellow lamps" but motorists soon realised their benefits (good visibility, less glare, better performance in fog). Therefore the trial installation was kept.

The first installation in the UK was in Purley Way, Croydon, outside the airfield, also in 1932 (see below). This was probably prompted by the successful trial of the GEC's MA discharge lamp in East Lane, Wembley, early in the year.

It appeared that this first installation (100W DC) was installed in bucket-type cut-off lanterns. It lasted four years before being replaced with a larger installation of Wardle Liverpools on a catenary system, which lasted until the 1970s.

The bulb was typified by a two piece construction - the inner arc tube was enclosed in an outer tube which comprised a Dewar flask. When the inner tube was spent, it was removed and replaced - the outer tube, which was expensive to construct, was retained.

The Dewar vacuum chamber provided thermal insulation, allowing the inner tube to reach its optimum temperature of 260^oC.

The original wattages and efficiencies were:

Philips sold the bulb under the Philora name - technical information can be found here.

On the 15th Novemebr, 1938, ELMA reclassified the wattage and efficiencies as follows:

By the early 1960s, the efficiencies of the lamps had improved:

The orientation of lamp burning was also important, and several designations were used:

/V Vertical cap up

/D Vertical cap down

/H Horizonal

/U Universal

The lamp was superceeded by the SOI design.

Such a device would be used for SOX, SOX Plus and SOX-E.

DAVIS also sold this device as a converter unit from MBF to SON. Chokes can be connected in series, so the low loss choke decreased the wattage by a set amount (from 80W for MBF to 70W for SON for instance) and the thyristor acted as an ignitor.

A data sheet for these devices can be found here