ABSTRACTS OF PAPERS 431 SOME FACTORS AFFECTING THE DESIGN OF ELECTRIC LIGHTING INSTALLATIONS FOR BUILDING INTERIORS By R. O. ACKERLEY, Associate Member.* (ABSTRACT of an Installations Section paper which was published in August, 1945, in Part II of the Journal.) Illumination design is not an exact science: there are three partners in vision—the light, the eye and the object lit—and only the first of these can be scientifically controlled. The science of lighting is that of making the scene appear "right" to the observer, with the essential details easy to see. The final judge of the lighting is the eye. As a preliminary to lighting design the engineer must not only know the physical laws of light control, but also the physiological laws affecting the rela- tionship between light and vision. With this knowledge at his disposal he must answer six major questions: (1) What exactly is the lighting problem? (2) What methods of lighting will best suit the requirements? (3) What illumination values are required? (4) What type of fitting, and what lamps, shall be used? (5) How many fittings will be required, and where shall they be placed? (6) How shall they be installed and maintained? The relation between different parts of the whole problem is shown in Fig. 1. THE LIGHTING PROBLEM The question, What is the Lighting Problem? can be divided into two parts, general and special. The factors affecting the general problem relate to the whole area to be lit, and cover such matters as the function of the room, its structure and decoration, the personnel involved, and special conditions of operation and environment. Analysis of the special problem involves an ap- preciation of the actual job which the eyes of the worker have to do, and a proper analysis of the visual task can be made only by doing the job oneself or by careful study and cross-examination of the operative, eliciting what it is he wants to see, and how he sees it. The answer to these questions will decide from what direction or directions the majority of the light should come, and will determine the choice of lighting system and the posi- tioning of the lighting, fittings. METHOD OF LIGHTING The main methods of lighting are:— General lighting, the illumination of an area in such a way that light reaches every part of it from a large number of different directions, and that the illumination does not vary greatly from point to point. It may be planned in relation either to the area as a whole or to fixed working positions. Angle Lighting, which reaches the work predominantly at a preferred angle, whether in a concentrated beam or over a wide angle. Such lighting is used principally to give good illumination on inclined surfaces, to avoid or create shadows, or to avoid or control specular reflections. Localized Lighting, which provides high illumination over the work and an area around it, and at the same time gives a fair general illumination between work-places. Local Lighting, the illumination of a very small area by a lighting fitting, usually placed very close to the area and giving practically no light outside the immediate confines of the area. Local lighting should never be the sole means of illumination; the average general illumination should not be less than one-tenth of the local illumination. • General Electric Co., Ltd. ILLUMINATION REQUIREMENTS Values of illumination are beneficial far higher than those found even in well-lit premises, but the technical problems in- volved tend to make some compromise desirable. In this country the I.E.S. Code has received general acceptance. In the Code, basic recommendations are made as to the illumination requirements, and illumination values for typical tasks are given in a schedule. Factors that influence illumination requirements are: The size of the detail to be seen.—The smaller the detail, the greater the illumination required. The effective size of detail is a function of its distance from the eye as well as its physical dimensions. The contrast between the object of vision and its immediate background.—The less the contrast, the greater the illumination required. The reflection factor of the object.—Dark objects will require higher illumination than light ones. Motion.—More light is required to see a moving object than a stationary one. The duration of the task.—A task involving concentration re* quires more light than a similar one done intermittently. Welfare.—If persons are to work for long periods in any area, a minimum illumination of 6 f.c. should be provided. The two most important factors are size of detail and contrast, and H. C. Weston has suggested a method of setting out the basic recommendations (Fig. 2).t CHOICE OF FITTINGS AND LAMPS In the choice of fittings the factors to be considered are light distribution, brightness, light output ratio, and mechanical features. Direct-lighting fittings will, in general, give the highest illu- mination on the working plane for the energy expended, but they tend to produce harsh shadows and reflected glare. General- diffusing and indirect-lighting fittings rely progressively on light reflected from walls and ceilings; shadows are softer and risks of reflected glare are reduced. For methods of lighting other thin general, direct-lighting fittings are almost always necessary. The problem of permissible brightness of fittings is one of the most difficult to solve. The reason for the limitation of brightness is the avoidance of discomfort or disability due to glare. The Table below is suggested as a guide for permissible brightness. Recommended Values for Surface Brightness of Fittings Illumination on horizontal plane foot-candles Up to 10 Above 10 For fittings mounted below 8 ft from floor candles/in» For fittings mounted between 8 ft and 16 ft from floor candles/in* 5 10 Maximum within an angle at the eye (5 ft above the floor) of 110° from the downward vertical •f Since the presentation of the paper, Weston's chart has been adopted by th| I.E.S. to replace the basic recommendations in the I.E.S. Code.
Transcript
ABSTRACTS OF PAPERS 431
SOME FACTORS AFFECTING THE DESIGN OF ELECTRIC LIGHTING INSTALLATIONSFOR BUILDING INTERIORS
By R. O. ACKERLEY, Associate Member.*
(ABSTRACT of an Installations Section paper which was published in August, 1945, in Part II of the Journal.)
Illumination design is not an exact science: there are threepartners in vision—the light, the eye and the object lit—andonly the first of these can be scientifically controlled. Thescience of lighting is that of making the scene appear "right" tothe observer, with the essential details easy to see. The finaljudge of the lighting is the eye. As a preliminary to lightingdesign the engineer must not only know the physical laws oflight control, but also the physiological laws affecting the rela-tionship between light and vision. With this knowledge at hisdisposal he must answer six major questions:
(1) What exactly is the lighting problem?(2) What methods of lighting will best suit the requirements?(3) What illumination values are required?(4) What type of fitting, and what lamps, shall be used?(5) How many fittings will be required, and where shall they be
placed?(6) How shall they be installed and maintained?
The relation between different parts of the whole problem isshown in Fig. 1.
THE LIGHTING PROBLEM
The question, What is the Lighting Problem? can be dividedinto two parts, general and special. The factors affecting thegeneral problem relate to the whole area to be lit, and cover suchmatters as the function of the room, its structure and decoration,the personnel involved, and special conditions of operation andenvironment. Analysis of the special problem involves an ap-preciation of the actual job which the eyes of the worker have todo, and a proper analysis of the visual task can be made only bydoing the job oneself or by careful study and cross-examinationof the operative, eliciting what it is he wants to see, and how hesees it. The answer to these questions will decide from whatdirection or directions the majority of the light should come,and will determine the choice of lighting system and the posi-tioning of the lighting, fittings.
METHOD OF LIGHTING
The main methods of lighting are:—General lighting, the illumination of an area in such a way that
light reaches every part of it from a large number of differentdirections, and that the illumination does not vary greatly frompoint to point. It may be planned in relation either to the areaas a whole or to fixed working positions.
Angle Lighting, which reaches the work predominantly at apreferred angle, whether in a concentrated beam or over a wideangle. Such lighting is used principally to give good illuminationon inclined surfaces, to avoid or create shadows, or to avoid orcontrol specular reflections.
Localized Lighting, which provides high illumination over thework and an area around it, and at the same time gives a fairgeneral illumination between work-places.
Local Lighting, the illumination of a very small area by alighting fitting, usually placed very close to the area and givingpractically no light outside the immediate confines of the area.Local lighting should never be the sole means of illumination;the average general illumination should not be less than one-tenthof the local illumination.
• General Electric Co., Ltd.
ILLUMINATION REQUIREMENTS
Values of illumination are beneficial far higher than thosefound even in well-lit premises, but the technical problems in-volved tend to make some compromise desirable. In thiscountry the I.E.S. Code has received general acceptance. In theCode, basic recommendations are made as to the illuminationrequirements, and illumination values for typical tasks are givenin a schedule. Factors that influence illumination requirementsare:
The size of the detail to be seen.—The smaller the detail, thegreater the illumination required. The effective size of detail is afunction of its distance from the eye as well as its physicaldimensions.
The contrast between the object of vision and its immediatebackground.—The less the contrast, the greater the illuminationrequired.
The reflection factor of the object.—Dark objects will requirehigher illumination than light ones.
Motion.—More light is required to see a moving object than astationary one.
The duration of the task.—A task involving concentration re*quires more light than a similar one done intermittently.
Welfare.—If persons are to work for long periods in any area,a minimum illumination of 6 f.c. should be provided.
The two most important factors are size of detail and contrast,and H. C. Weston has suggested a method of setting out the basicrecommendations (Fig. 2).t
CHOICE OF FITTINGS AND LAMPS
In the choice of fittings the factors to be considered are lightdistribution, brightness, light output ratio, and mechanicalfeatures.
Direct-lighting fittings will, in general, give the highest illu-mination on the working plane for the energy expended, but theytend to produce harsh shadows and reflected glare. General-diffusing and indirect-lighting fittings rely progressively on lightreflected from walls and ceilings; shadows are softer and risks ofreflected glare are reduced. For methods of lighting other thingeneral, direct-lighting fittings are almost always necessary.
The problem of permissible brightness of fittings is one ofthe most difficult to solve. The reason for the limitation ofbrightness is the avoidance of discomfort or disability due toglare. The Table below is suggested as a guide for permissiblebrightness.
Recommended Values for Surface Brightness of Fittings
Illumination onhorizontal plane
foot-candlesUp to 10Above 10
For fittingsmountedbelow 8 ftfrom floor
candles/in»
For fittingsmounted
between 8 ftand 16 ft
from floor
candles/in*5
10Maximum within an angleat the eye (5 ft above thefloor) of 110° from thedownward vertical
•f Since the presentation of the paper, Weston's chart has been adopted by t h |I.E.S. to replace the basic recommendations in the I.E.S. Code.
THE LIGHTING PROBLEM
GENERALCONSIDERATIONS
FUNCTION OF ROOMSTRUCTURAL FEATURESDECORATIONPERSONNEL (1)WELFARE (2)
SHADING (7).OBSTRUCTION SHADOW (8)COLOUR CONTRASTUSE OF HIGHLIGHTSSILHOUETTE (9)
METHOD OF LIGHTING
GENERAL (10) and/or SPECIAL (II)
ROOM PROPORTIONSMOUNTING HEIGHT
(ROOM INDEX)
REFLECTION FACTOR(WALLS AND CEILINGS)
CHOICE OF FITTINGAND LIGHT SOURCE
LIGHT DISTRIBUTIONBRIGHTNESSSPECTRAL CHARACTERLIGHT OUTPUT RATIOMECHANICAL FEATURES
CHOICE OF FITTINGAND LIGHT SOURCE
LIGHT DISTRIBUTIONBRIGHTNESSSPECTRAL CHARACTERLIGHT OUTPUT RATIOMECHANICAL FEATURES
FACTORS AFFECTING ILLUMINATION
SIZE
CONTRAST
REFLECTION FACTOR
MOVEMENT
DURATION (13)
WELFARE
TOTAL ILLUMINATIONREQUIRED
I
ILLUMINATIONCALCULATION (12)
IDEPRECIATION
FACTOR
ABSORPTIONFACTOR (15)
COEFFICIENT OFUTILIZATION
FLOOR AREA
POSITIONSREQUIRING
LOCALILLUMINATION
rLOCAL
ILLUMINATION
MOUNTINGHEIGHT
SPACING/HEIGHTRATIO (16)
GENERALILLUMINATION <l<)
CONVENIENTFIXING
MAINTENANCEFACILITIES
TOTAL LUMENSREQUIRED
MAXIMUM SPACING
EXPLANATORY NOTES .(1) Age, sex and degree of skill of worker. (2) Psychological and aesthetic con-
siderations only. (3) Moisture, dirt, murk, corrosion, inflammability, temperature,etc. (4) Rough, smooth, glossy. (5) Factors related to angle of work: whetherhorizontal or inclined, two- or three-dimensional, etc. (6) Angle of view, obscura-tion and operational movement of object and worker. (7) Shadow due to form asdistinct from (8) Shadow due to obstruction. (9) True or reverse, i.e. light objectagainst dark ground. (10) Planned in relation to area or to working positions.(11) Local, diffused, concentrated or of special spectral composition. Angle lighting,localized lighting. (12) By iso-footcand}e diagram, polar curves or empiricalmethods. (13) Whether visual task requires prolonged concentration or is inter-mittent. (14) General illumination must never be less than minimum required forwelfare or than one-tenth local illumination. (15) Allowance where necessary forlight absorption in the atmosphere. (16) Appropriate to selected fitting.
NUMBER, WATTAGE AND POSITIONOF FITTINGS FOR GENERAL
LIGHTING
NUMBER, WATTAGE AND POSITIONOF FITTINGS FOR LOCAL
LIGHTING
Fig. 1.
ABSTRACTS OF PAPERS 433
APPARENT SIZEOF DETAIL TO BE
SEEN
ILLUMINATIONRECOMMENDED(FOOT-CANDLES)
CONTRAST!
Fig. 2.—Chart, prepared by Weston, for determining illuminationrequirements according to basic characteristics of the task and inconformity with the I.E.S. Code.
The illumination required for any size is found onScale A if the reflection factors involved differ widelyScale B if the reflection factors involved differ moderatelyScale C if the reflection factors involved differ little
(Scale B represents average conditions and is, therefore, shown in heavy type.)General lighting is recommended for values shown thusGeneral or (general + local) lighting is recommended for values shown thus
1020
Local supplementing general lighting is recommended for values shown thus . . 200
Where reflected glare is likely, brightness should be limitedover the whole area of the fitting below the horizontal.
The main characteristics of lamps that must be considered arespectral composition of the light, source brightness and area, andeconomic factors. Spectral composition is important in relationto the power of light to reveal colour. The appearance of colourin an dbject depends upon the capacity of that object to reflectonly selected wavelengths. If these particular wavelengths aremissing from the source they cannot be reflected. Sourcebrightness and area are important in relation to problems oflight distribution and control. The higher the brightness ofsource, the smaller the reflector that is needed to project anyrequired amount of light. On the other hand, high-brightness
sources are far more likely to cause glare, either directly or byreflection from glossy objects. In general, high-brightnesssmall-area sources are desirable where light has to be providedover a limited angle only; low-brightness large-area sources haveadvantages where general illumination is required over a largearea.
Fluorescent tubular lamps are likely to be important in post-war lighting, owing to their qualities of very low brightness, largearea of source, good colour rendering, low radiant-heat output,high efficiency, and long life.
NUMBER AND POSITION OF FITTINGS
In general-lighting systems, mounting height and illuminationrequirements are primarily responsible for dictating the numberof fittings and their positions.
Mounting height determines the maximum spacing permissiblefor reasonable uniformity, and influences the amount of lightthat can reach the working plane only after reflection from wallsand ceilings. In large rooms of normal proportions it isgenerally wise to use all the height conveniently available. Theillumination requirements determine the minimum total lumenoutput required from the fittings. This is calculated by meansof a utilization factor which takes into account the light-outputratio and light distribution of the fitting, the mounting height inrelation to room length and breadth, and reflection factor ofwalls and ceilings. Some allowance must be made to compensatefor the light losses that inevitably occur under service conditions.Under good conditions of operation and maintenance, an allow-ance of 30% is found by no means excessive for this purpose.The final decision as to the wattage rating and placing of eachpoint is determined by practical considerations of convenientfixing positions and of symmetrical layout within the area.Where it is desired to provide limited areas of high brightness,iso-footcandle diagrams can be usefully employed to estimate theillumination that will be obtained.
The principal points to be considered in relation to installationare the need for flexibility of control, ample sub-fusing, adequatecarrying capacity of conductors, and facilities for easy mainten-ance. Whatever the maintenance routine, periodical light-meterchecks of illumination are essential.
THE FUTURBIn the future, a greater appreciation of the importance of task
analysis is likely to lead to an understanding of the possibilitiesof making work more easy to see. All experience points to asteady demand for higher illumination; it is not sufficient touse the old methods with increased wattage of lamps, becausesuch methods all too frequently result in a feeling of being op-pressed by too much light. One solution is the use of large-arealow-brightness sources, and fluorescent light sources will go along way towards making this kind of light practicable. Anothermethod lies in the art of allowing the required light to fall uponthe object to be illuminated without falling direct on the eyes.The simplest method of achieving this is by the use of reflectorswith louvres.
Possible developments in fluorescent lamps include new coloursand sizes, improved control circuits, high-frequency or higher-voltage operation, and more rapid and positive starting. Theexcellent quality of the light from daylight fluorescent lamps forsupplementing natural daylight may have considerable influenceon future building design, particularly in heavily built-up areas.Finally, among the tools of light control, plastics and anodizedaluminium are likely to play an important part.
