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953 (F. I.)
ILLUMINATION DATA


ILLUMINATING ENGINEERING DATA


Systems of Lighting.
With the development of the Gasfilled Lamp the Illuminating Engineer is provided with a complete range of incandescent lamps from the 20 watts size to the 1500 watts size. He can thus choose the lamp which with a suitable reflector will give exactly the amount of light he requires.

Broadly speaking, there are three courses to choose from :-

  • General Lighting, in which a more or less even illumination is provided over the whole of the working area;
  • Local Lighting, consisting of small individual units each allotted to some particular bench or machine;
  • A judicious blending of both.
Wherever possible, (i.e., where the elevation of the building and the arrangement of the plant permit), general lighting is preferable on the score of economy, both as regards first cost and upkeep. A small number of relatively large and economical units may be used instead of a large number of small and less efficient units. There is, therefore, a saving in wiring and installation cost, in current consumption, and, what is probably more important, in lamp renewals. Moreover, if the installation is well designed, re-arrangement of the shop can be carried out without re-wiring.

A system of purely local lights is only suitable for the smallest shops, or in certain very special instances.

Where general lighting is possible, there are often special positions or machines where individual lights are required, but these should be so chosen that no violent contrasts in intensity occur between various portions of the shop.


Types of Fittings.
Fittings may be classified according to the method in which they distribute the light :
  • Direct Type Reflectors are placed above the lamp to redirect the horizontal and upward distribution on to the working plane. To this class the Benjamin Steel Reflectors and Reflector fittings belong.
  • 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 ceiling or a top reflector and thence re-directed to the working plane.
  • 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.
For general purposes, Direct Type Fittings have the highest light efficiency, that is to say, the greatest proportion of the light emitted by the lamp is usefully employed. Care must be exercised to ensure that glare from exposed filaments does not occur; they should therefore be suspended well above the normal line of vision. The shadow effects from Direct Type Reflectors are, however, somewhat harsh, thus rendering them unsuitable for certain purposes.

The best results from Semi-Indirect Fittings are obtained when they may be suspended at a minimum height of from 8 to 9 feet. Their efficiency is appreciably less than that of direct type reflectors, but they cast much softer shadows and the opalescent bowls effectively prevent glare. They are not suitable for use in foundaries or situations where much dust is likely to collect in the bowls.

Indirect Fittings provide a light practically devoid of shadows at a still further sacrifice of efficiency. In industrial lighting their use is usally restricted to the drawing office and a few special purposes.


Definitions.
Before giving a few simple rules used in designing lighting lay-outs, it will be well to define the principal terms employed.

The Foot-candle is the unit of intensity of illumination; it is equal to the intensity produced on a surface by a source of 1 candle-power at a distance of 1 foot. Table A gives the intensities which have been found desirable for various positions and classes of work.

Table "A"
Position or Process Intensity in Foot Candles
Roadways and yard thoroughfares 0.05 - 0.25
Storage Spaces 0.5 - 1.0
Stairways, passage ways, aisles 0.75 - 2.0
Rough manufacturing such as rough machining, rough assembling, rough bench work 2.0 - 4.0
Rough manufacturing involving closer discrimination of detail 3.0 - 6.0
Fine manufacturing, such as fine lathe work, pattern and tool making, light coloured textiles 4.0 - 8.0
Special cases of fine work, such as watch making, engraving, drafting, dark coloured textiles 10 - 15
Office work, such as accounting, typewritting, etc. 4.0 - 8.0


The Lumen is the unit of light flux. It is equal to the amount of light required to produce an intensity of 1 foot candle over an area of one square foot. Table B shows the number of Lumens emitted by various sizes of OSRAM Vacuum and Gasfilled lamps.

Table "B"
Type Of Lamp Watts LUMENS
100-130 Volts. 200-260 Volts.
OSRAM Vacuum Lamps 20 163 147
30 255 229
40 341 330
60 535 515
OSRAM Gasfilled Lamps 60 750 -
100 1300 1200
150 2100 1800
200 2800 2500
300 4600 4200
500 8500 7300
1000 18000 16500
1500 28000 26000

The Working Plane is the surface to be illuminated. For bench work and factory operations, it is generally taken as a plane parallel with, and 3 feet above, the floor.

The Mounting Height is the vertical distance between the Working Plane and the centre of the lamp filament.

The Spacing Distance is the horizontal distance between two adjacent lamps.

The Spacing Ratio is the ratio to be observed between the Spacing Distance and the Mounting Height when uniform illumination is desired. Reflectors are made in various designs - Distributive, Concentrating, etc. The appropiate spacing ratio for each type is given on the pages where each type is listed.


Planning an installation.
The first step towards planning a Lighting Scheme is to decide the intensity of Illumination which is to be allowed.

Table "A" shows the desirable figures for various positions and types of work, but the actual value to be taken depends very largely on the fineness of the work carried out and the character of the surroundings, the higher fitures being those for situations where fine or detailed work is done or where the walls and general surroundings are dark.

In settling the intensity to be allowed in any building, the Illuminating Engineer must be guided chiefly by experience, and if any doubts should arise, the G.E.C. Illuminating Engineering Service, particulars of which are given elsewhere, is always available.

The next stage is to determine the amount of current necessary to produce this intensity. For Direct Lighting this figure is obtained with the formula :-


Watts = ((Intensity in Foot Candles) * (Area Of Working Plane In Square Feet))/ a Constant


For average conditions the value of this Constant may be taken as 4.5 for OSRAM Vacuum Lamps and 7.0 for OSRAM Gasfilled Lamps.

An increase in wattage of from 20% to 50% should be made if the surroundings are abnormally dark, or if there is an undue risk of dust accumulating on the reflectors.

If semi-Direct or Indirect Lighting is to be employed the Wattage determined as above should be increased by 33.3% or 100% respectively.

Finally, the number and size of points and the type of Reflector have to be decided upon. The area to be illuminated may be divided into a number of squares or approximate squares and one point may be placed at the centre of each.

The approximate total Wattage divided by the number of points gives the Wattage of each Lamp.

The choice of reflector can be made by comparing the permissible mounting height with the spacing ratio of the standard reflectors.

The rules set forth above give satisfactory results for general lighting. It should be borne in mind, however, that in large installations and in many specialised trades every scheme should be carefully considered on its own merits.


The G.E.C. Illuminating Engineering Service.
The G.E.C. Illuminating Engineering Department has a staff of expert engineers who have had long and varied experience in dealing with lighting problems. Their services are unreservedly, gratuitously and without obligation, at the disposal of clients of the Company. All problems submitted with receive individual attention. If necessary, an engineer will visit the premises under consideration to investigate the actual working conditions, render an expert opinion, and, if required, preapre a complete lay-out for the installation.