THE SCIENCE OF LIGHTING

by R. S. YATE Dip. M.I.E.S.

The Arthur Hallett Lectures No.12 in the series March 1966

Artificial lighting is today becoming a subject of extreme importance when allied to industrial application sand efficiency of production. Lighting plays an important part in practically every sphere of human activities and the object of this paper is to analyse lighting as a science and illustrate the trends in modern developments of light sources and fittings. A review of the growth and development of lighting societies devoted to the improvement of lighting is also included.

It was in 1879 that Edison and Swan, working independently, developed the first electrical incandescent light sources. Although this can be considered as the birth of practical artificial lighting, the science of illuminating engineering cannot really have been said to have been born until the last decade of that century. It was then that the fir t step was taken to control and re-direct the luminous flux from the source when Blondel and Psaradouki developed prismatic glassware and an indirect arc lamp was introduced to light a drawing office.

From such a beginning came what is to day one of the most complex and interesting of our modern sciences. It is a science which is often taken for granted and many people still do not acknowledge it as a science. The reason behind this is not easy to find. Perhaps it is because engineers are at times conservative in accepting new principles; perhaps it is that with the tremendous growth of electrical engineering in all its sub-sections the practicing general engineer is sloth to let go of another branch of his trade, particularly one he has always taken for granted and did so easily, and often so wrongly, apply; perhaps with so many other subjects to study there is no time to investigate the vast ramifications of this science. Whatever the reason, lighting is developing rapidly and, as a science, both in the spheres of research and application, must take its rightful place in our modern world. 

Mention has been made that the science of lighting has many ramifications. These cover the field from art to engineering, in which the true lighting engineer must be wholly proficient. A few of these aspects are:

1. Physiology

As there is a close connection between illumination and the eye, it is obvious that the illuminating engineer must have a knowledge of the working of the visual optic and the process of seeing. It has been said that seeing is a partnership of light and vision. Correct lighting can aid vision just as incorrect equipment or layout can seriously impair the vision and cause accidents, fatigue or even deterioration of the sight itself. One of the most serious side effects of uncontrolled lighting is glare, which reduces the pupillary size and so seriously impairs vision. 

2. Psychology

Whenever a lighting installation is planned, it is necessary to consider the psychological factor. Colour plays a large part in our everyday life. In artificial lighting a predominance of red in the spectrum of the source will give an impression of warmth, whereas blue gives a sensation of coolness. Thus, the mood of an interior, be it factory, home, or hospital, can be controlled by varying the colour of the stimulating source. The method of lighting too can have a profound effect on the appearance of an installation. A room lit entirely by indirect lighting or by large, diffused sources gives a feeling of 'deadness' whereas direct lighting correctly employed can created sparkle as well as interesting areas of light and shadow. Such consideration to detail is obvious in many types of installations, but even in industrial areas the effect on the general welfare of workers has undoubtedly reduced absenteeism and staff turnover and has increased production. 

3. Art and architecture

The aesthetics of the lighting in relation to the overall appearance of an installation plays a big part in the total concept. Objects are only visible when light falls on them and their appearance can be drastically altered by changes in the direction of the incident light, the colour of the light, the relationship or of the dominant light to the background illumination and many other factors. Apart from the actual shape and appearance of the fitting containing the light source, light itself is a tool in the hands of the artist. Incorrect use of this tool or lack of liaison between the architect and the lighting engineer at the design stage can only result in an installation which is poor and unsatisfactory. There are many activities where, without the correct light in the correct place, the result can only be classed as a failure. In the sphere of art this is never more important than in the theatre, where an intimate knowledge of drama, music and the dance is a pre-requisite of a successful lighting installation. 

4. Electronics

In the field of light control, be it in a theatre, a television or film studio, a ballroom, a floodlighting installation or anywhere else where the intensity of the light must be adjusted to suit different conditions, electronics is playing an ever-increasing role. The development of the controlled rectifier has provided Control facilities which have never been available in anyone system before. Low voltage discharge lighting units also rely on electronic components and circuitry to operate the source. In the television field, electronics is, of course, in a highly developed state. Without a good working knowledge of electronics, no lighting engineer could design, operate, or maintain any modern control circuit. 

5. Chemistry

Development of new light sources, phosphors, incandescent metals, and discharge gases is dependent on the knowledge of all branches of chemistry. Pure research requires the services of a fully qualified chemist, but even the practicing engineer should have some knowledge of the subject to enable him to utilize the equipment to its best advantage. 

6. Physics

Without a knowledge of the principles of light itself and the-subject of magnetism and electricity, no lighting engineer would be able, to carry out his work.

Apart from a knowledge of circuitry it is important that the engineer be well versed in control gear for discharge lighting as well as all ancillary equipment. 

7. Mathematics

In common with all other sciences, it is essential to have a knowledge of this subject to study principles and formulate laws. 

8. Economics

By no means, the least. of the subjects requiring attention by the lighting engineer, economic considerations of each installation must be carefully considered. Apart from the obvious aspect of efficiency of the source, such factors as the quantity and quality of the total illumination can have a great effect on the productivity of the work being carried out under this illumination. Such factors as ease of installation and maintenance of an installation, ease cleaning the equipment, resistance to corrosion of the fitting and components and efficiency of the fitting itself can often contribute to the overall efficiency of a complete factory. The careful weighing up of initial expenditure, costs of cleaning and maintenance, capital depreciation, cost of current used and costs of lamp replacements and other consumable components is within the responsibility of the illuminating engineer. It is up to him to make the correct selection on behalf of his client.

With such a diversity of subjects to cover, unlike practically every other profession, is it surprising that illuminating engineering has such an appeal? Furthermore, the engineer will find himself in many widely opposed situations, from domestic interiors to specialized lighting in factories and mines, from dockyard, to hospitals, from theatres to advertising, from streets to airports. Without light all activity would cease during the hours of darkness and even during daylight hours many activities would be seriously hampered.

In promoting the science of lighting, men and women have come together to pool their knowledge and resources. Lighting societies have started and grown in all the civilized countries throughout the world. The first society was founded in America at the beginning of this century. Quantity rather than quality was the keynote at this stage, but, of course, it was soon realized that both these aspects were inseparable if good lighting practice was to be promoted. Britain was not far behind and the Illuminating Engineering Society there was founded in February 1909. Other countries in Europe also realized the need for this type of society and groups rapidly developed. It soon became apparent that to achieve faster progress and uniformity in research and equipment an international body to co-ordinate the work of these individual societies was needed. In Berlin, the International Commission on Illumination (the C.LE.) was started in 1913. This body is still operating today and has, and still is contributing extremely valuable information and services to the science of lighting.

With the growth of industry in South Africa, a local body or organization was becoming necessary. At first a branch of the British Illuminating Engineering Society was formed in 1951. Three years later the South African National Committee on Illumination was formed and became affiliated to the C.LE. By means of an annual congress held in the various major centres of this country and the work of the numerous technical sub-committees, stalwart service has been rendered to the growth of commerce and industry and the safety of the people in the Republic. Through the Bureau of Standards, a Code of Practice on Street Lighting has been evolved (due for publication soon) and work is progressing steadily on specifications for traffic signals, discharge lighting ballasts and chokes, and lighting fittings. Codes of Practice on industrial lighting and other special applications are also being prepared. Through the C.LE. selected members are appointed as correspondents on various technical committees to keep in touch with all world developments. Their knowledge and experience are made available to all interested bodies and individuals through S.A.N.C.I. 

The future

As would be expected, much of the future development of illuminating engineering will depend on the development of light sources. Since the invention of the first incandescent lamp, this light source has been advanced. Many new light sources have been added. The most significant of the new types is the range of gas discharge lamps which has given to the lighting industry several high light output sources of high efficiency. The development of the electric lamp over the past 90 years is a remarkable and interesting story. From low efficiency sources of short life, we now have lamps having an efficiency of greater than 100 lumens per watt and with lives of well over 8 000 hours. The lighting equivalent of the 'Four-minute­mile' occurred about 8 years ago when the first sodium lamp achieved the goal of 100 lumens per watt efficiency. Like the Four-minute-mile, illuminating engineers have now gone further in their developments and have far exceeded this barrier which, at one time, was thought to be unconquerable.

In recent times poor colour rendering, one of the greatest drawbacks of the higher efficiency discharge lamps, has been overcome. Firstly, fluorescent powders were applied to the outer envelope of mercury discharge lamps to convert the emitted ultraviolet irradiation to useful light in the portions of the spectrum where the source itself gave no light. Later high-pressure mercury iodide lamps were introduced. The construction of this lamp is like the conventional mercury discharge lamp except for minor dimensional differences. The arc tube is fused silica (quartz). In addition to the normal amount of mercury, iodides of sodium, thallium and indium are added to the gas filling. The arc tube is mounted and sealed in a protective hard glass outer envelope. The electrical characteristics are, in general, like the normal mercury discharge lamp but the iodide lamp requires a higher starting voltage so, in addition to the series choke, an impulse generator of some form is required to give reliable starting. The lamp has an efficiency of 70 to 75 lumens per watt and is, now, available in a 400-watt rating. The colour appearance of the light is 'bluish-white’, and colour rendering is like the conventional colour corrected mercury vapour lamps. The initial application of the lamp was inevitably for street lighting, but its use is now being extended to high bay industrial lighting, indoor and outdoor sports arenas, service stations, open industrial and other large areas.

The greatest achievement in the discharge lamp field recently took place with the introduction of a high - pressure 'colour-corrected' sodium discharge lamp. The sodium lamp, always acknowledged as the source of highest efficiency, was never 'socially' accepted because of its mono-chromatic light and extremely poor rendition of colours. The lamp was used only in streets. Many complaints were received from nearby residents, particularly the ladies, who refused to be seen under these lights. Apart from this use and the limited application of the lamp in certain industries and for colour floodlighting, the future of the lamp seemed to be limited. The new lamp has now opened the market for almost any application where a high wattage, high efficiency lamp of good colour appearance is required. The first lamps manufactured are rated at 400 watts with an efficiency of 95 to 105 lumens per watt. The design is essentially the same as the high-pressure mercury vapour lamp except that sodium is used in place of mercury. The high-pressure sodium vapour however is active chemically and rapidly attacks all known glasses. For this reason, special translucent ceramic discharge tube is used in place of the conventional quartz tube. The colour appearance of the lamp is close to that of (1 vacuum tungsten filament lamp and very close to that of an incandescent black body at 2 100°K. There is no change of colour with life. The lamp requires the usual simple series choke and power factor correction condenser as used for mercury lamps. No voltage step-up is required.

As the light sources developed, new fittings had to be designed to accommodate them. With increases in efficiency and the consequent tremendous increase in surface luminance of the lamp, all the aspects of glare have had to receive careful study and attention. Glare can be a serious deterrent to good vision. Even though higher illumination levels are becoming more prevalent in industry and commerce, it has been found that where glare was not controlled, production fell sharply, instead of rising as was hoped. This problem has been realized for many years and many methods have been suggested to assess and regulate the amount of glare in an installation. One of the best methods which is today accepted and applied by lighting engineers in many countries in the BZ method published in England in 1961. Along with the recommended levels of illumination for most industrial and commercial tasks, have been published the limiting glare indices. Careful design is a requirement of every new lighting installation if efficiency and comfort for the people who are to work under it are to be major factors. The choice of fitting with its methods of glare control should be a matter of careful consideration and application.

Many methods of glare control have been devised and one method which indicates the high degree of engineering which has gone into this work is shown in Fig, 1. Here is illustrated the difference in reflection characteristics between various surfaces. The newly developed machined parabolic face surface seen in Fig. 1 shows how the light is directed downwards out of the zones where glare would be a problem. The reflecting surface is manufactured from aluminium which ensures high efficiency of the fitting. Typical fittings using this reflector system are rated under the British system as BZ1 and BZ2, which indicates that they are suitable for use under the most critical conditions of glare control.

For over fifty years the method of design of lighting installations for interiors has been based on the illumination incident on the visual task. Various organizations in several countries have published codes of practice giving their recommendations as to the amount required for practically every conceivable task. These values have differed widely from country to country depending on many factors. A new approach to lighting design has now been made by Prof. R. G. Hopkinson of the University College of London, which may lead to greater uniformity if adopted. His recommendations, while admitting the validity of the old method for lighting on the work, are based on the design of lighting for a whole building rather than for any specific task. His experiments and observations have shown that 'the degree of satisfaction given by lighting is regulated by the brightness pattern in the visual field'. Based on luminance requirements not only of the task, but of the immediate surrounding area, the general environment and the light sources, a code of practice incorporating all these factors promises a new and interesting approach to design problems. Much research will be necessary but as Prof. Hopkinson remarked 'it is a challenge to and would be necessary for the health of the lighting profession'. As far as South Africa is concerned, we do not yet have the research facilities to stimulate the interest of many of our younger people. For this reason, the profession has an extremely limited following and is seldom accepted as a branch of engineering. To overcome this and to introduce new candidates to the profession it is necessary to publicise it and stress its diverse nature and interests as widely as possible. Facilities must also be granted to anyone wishing to pursue the subject to receive expert instruction and laboratory practice. At the present, these facilities are limited only to one university and one technical college. The Witwatersrand Technical College has for some years organized successful courses for the layman on the 'Art and practice of lighting' and has recently introduced Illuminating Engineering as a subject in its advanced technician’s course. Even amongst commercial organizations selling lighting little attempt is being made to train its personnel in the science of lighting.

Th's situation should not be allowed to exist, as with advancement of the country, lighting must go hand in hand with industry and commerce to ensure maximum efficiency and productivity. In South Africa with its shortage of skilled labour, every attempt should be made to cultivate this most interesting and valuable of our contemporary sciences. 

REFERENCES

1. International Lighting Review. Vol. 54, No.5.

2. Transactions of the Illuminating Engineering Society. Vol. 30 No.3.

3. I.E.S. Code of Practice for the Lighting of Interiors.

4. G.E.C. Publication 'BZ and All That'.

5. G.E.C. Journal of Science and Technology Vol. 31 No.2.