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ilp archive : journals

public lighting no. 49 vol. 13
January-March 1948

Editorial p13
Hubert Offen Davies p13
Obituary of the APLE's Secretary Hubert Offen Davies by president Thomas Wilkie. He'd held the role for 12 years.
Lighting: Personnel

Lord Mayor of Leeds "Switches On" New Street Lamps p14
Details of a new installation in Leeds.
Lighting: Installations

A Special Type Electric Tower-Wagon p14
The gas department of the City of Coventry have taken delivery of a special tower-wagon for the maintenance of the 500 gas suspension lamps within the city. It is a 50 cwt. Crompton-Parkinson Morrison Electricar and is fitted with a three tier elevator - the maximum height being 25 feet.
Lighting: Equipment, Lighting: Installations

Contributions p14
Mr. Cecil Hughes, A.M.I.E.E. has been elected President of the Elecrtical Trades Commercial Travellers' Association
Lighting: Personnel

Street Lighting Photometry by S. English, D.Sc., M.I.E.E., F.Inst.P. (Technical Director, Holophane Limited) p15
Reproduction of the paper Street Lighting Photometry.

Dr. N. A. Halbertsma (Adviser to Ministry of Works, Holland): Was it desired to measure the horizontal illumination or the illumination normal to the ray of light? He understood that Dr. English measured at right angles to the beam and that was the only way to get a measurement of any value. He did not agree with trying to measure candle-power in certain directions in this way in the streets because it was very difficult to keep out extraneous light. Moreover, there was a greater difficulty in keeping voltages constant when measuring in the street. He would regret very much if this paper distracted attention from the importance of brightness and brightness measurement. Dr. English had spoken of the time when the foot candle was condemned without anything else being put in its place, but he did not think that was a reason for giving it up. In 1908 an Italian named Peri at one of the first Conferences of the International Commission on Illumination presented a paper on the measurement of brightness - then Preston Millar showed the silhouette effect in 1927. It was the angle of emergence that mattered most. As soon as the angle of emergence was 1 degree then there was a uniform factor of reflection and with simplifications of this nature we should make further progress in dealing with the factor of brightness in connection with street lighting.
Mr. E Stroud: If the suggestions made in the draft British Standard were deleted then there would be nothing to measure. Dr. English took the principle laid down for measurement from the Draft but made it more complicated and difficult to measure. There were two serious objections to this modification: (1) Lamps beyond the one under test would have to be put out whilst making the test and (2) the probable inaccurancy of the photometric readings at the low values and long distances. Such measurements would also ignore the horizontal width of the beam and also the distribution closer to the lantern. For a road measurement of achievement, the test suggested in the Draft Specification, which took into account the lantern beyond the one nearest the test point, was surely the most practical. He agreed it would probably be better if this measuremetn were taken a couple of feet into the carriageway and not on the kerb, for a practical reason. The reason why the kerb was chosen, as well as for safety when testing, was to ensure that an adequate spread of light in the horizontal and avoid peaky beams.. If one could be assured of good visibility with the high angle type at 80°, why limited the controlled cut-off type to 75°. He did not sugest that the best visibility was not obtained with the high angle types, as this really depended mainly on the breadth or area of the beam. A wide beam lantern such as was given by using a sodium lamp, or a long source, gave far better visibility than a peaky beam or equivalent intensity than a gas-filled lamp. The high angle distribution was developed to fit in with the wide spacing and low mounting heights of the past. The MOT Report recommended a spacing of 150 ft. - preferably 120 ft. - and with this comparatively close spacing he did not think it was at all necessary to have very high angles of maximum candle-power, which could only increase the glare. A fetish has been made of high road brightness, which should be given its rightful place as one factor in the general scheme and not the major consideration. The brightness of the road surface with a given installation depended on a major degree on the texture of the road surface itslef which varied with time and repairs, and also with atmospheric conditions, and was not under the jurisdiction of the lighting engineer. So long as there was sufficient light, and the obejct was in contrast with the background, it was not of great moment that the background was of a definite high brightness. Cut-off installations could give a high and uniform road brightness and also look very impressive. So much depended on the type of light source and design of fitting, whcih should have a large reflective surface and give a wide distribution.
Mr. P. Hartill: Commented on the question of glare. As Dr. English pointed out, this was not the whole story and did not take into account the general brightness level to which the eyes were adjusted, but this factor must be assumed constant for Holiday's equation of veiling brightness to be valid. The adjustment of the eyes must inevitably be very largely governed by the brightness of the light sources and the author's expression for visibility might be improved if the right hand side were divided by some function of the source brightness. Would it not be desirable in any future specification or Code of Practice, to include some limitation on lantern brightness in certin directions?
Mr. H. S. Alpress: He supported Dr. English in emphasising that this question of visibility and surface brightness taken per se, was a very dangerous aspect from which to regard street lighting.
Mr. L. T. Minchin: We were now in a serious position in street lighting specifications, because having pioneered in 1927, we had no specification at all. The B.S.I. Committe has apparently turned back in mid-stream and decided to go for a Code of Practice. No Code or Specification would be satisfactory unless some such test were provided. Meanwhile we only had the Ministry of Transport report to go upon, and that by specifying lumens per 100 feet - regardless of where the lumens went - had a bad influence on our lighting. The author said that the 1927 Specification contained criteria which bore no relation to visibilty - he doubted whether such a sweeping statement was fully justified: the Specification was used for a number of years by practical engineers which could hardly have happened if it had "no relationship whatever" to the usefulness of the installation. On one point it definitely scored: it paid attention to the brightness of the pavement, which is one of the most important backgrounds in the motorist's field of vision.
Mr. W. H. Burman: The effect of gradients on the illumination of the road was lost sight of. When one got out to the District and used a photometer, one found something quite different. That was very often due to the gradients. For this reason he suggested that mounting height, glare and other factors should not be tied up by hard and fast rules as regards measurement.
Dr. English: The difficulty of measuring candle-power on the road because of the effect of extraneous light had been mentioned in the paper. Laboratory tests, though essential in development work, could not either ensure or check the correctness of installation or the standard of maintenance. We could be sure that if there was satisfactory light distribution at the proper angles, then, with normal road surfaces, we could ensure a reasonable brightness. He was not in favour of single point measurement because it was always easy to get around a single measurement, whereas it was not so easy to get round three separate measurements. Extraneous light had to be excluded from the test surface but this was possible by suitable shielding or switching off offending lamps. He did not know how to take useful measurements on gradients - perhaps the best thing to do would be to rely on visual observation of the suitability or otherwise of the installation for the particular conditions.
Lighting: Distribution, Lighting: History, Lighting: Levels, Lighting: Maintenance, Lighting: Specifications, Lighting: Theory

I.P.A. Haldane Essay Competition p24
Details of the Institute of Public Administration's essay competition. Competitors may choose any subject in the field of public administration.

Perception On The Road When Visibitly Is Low by Dr. P. J. Bouma p25
Reproduction of the paper from the Philips Technical Review.

The higher the brightness, the better we can see. Therefore it would be desirable to employ very high intensities of illumination on the highways, but this is limited by considerations of economy. Whatever light is available must be used very economically.
  • (1) We must try to get as much energy as possible (usually electrical energy) for our money.
  • (2) We must get as many lumens as possible per kilowatt input i.e. use lamps with high efficiency.
  • (3) The lumens obtained must be projected on the highway with little loss and in the best possible manner.
  • (4) When designing the system allowance must always be given for the properties of the eye: (a) Under what conditions does the eye function best? (b) How far can these conditions be realised?
The last problem is often difficult to solve because there are a number of factors over which the illuminating engineer has no control (reflection of the road surface and the objects upon it, surroundings of the road, size of the sources of light etc.)

For the study of the performance of the eye there are widely divergent methods of approach. This is best illustrated by mentioning two extremes. One consists in observing the traffic along the lighted roads and from the accident statistics attempting to determine what system of illumination offers the best possibilities of vision. The other extreme consists of a theoretical and laboratory analysis of the process of vision into all its components, and then subjecting each component to an experimental investigation and building up from the components a theory of perception on the road. The best compromise is to investigate the components of vision in the laboratory and to test the results by repeated practical measurements on the road.

An object on the road is noticed because there is a contrast. Objects can therefore most quickly be perceived when care is taken to provide strong contrasts everywhere i.e. by trying to make all objects appear very dark against a bright background. Since there are a number of factors over which we have no control, this is not always possible. By constrast just perceptible we mean the ratio dB:B where B is the brightness and dB the smallest perceptible difference in brightness. B:dB is called the constract sensitivity and it must be made as large as possible. For road lighting we are mainly concerned with the interval between 0.1 and 1 candle/m2. In this interval the quotient B:dB is found to increase rapidly with the brightness; it varies from 18 to 33 which means that contrasts of 3 to 5.5% lie on the limit of perceptibility. This was under the most favourable laboratory conditions - under practical conditions the contrast sensitivity is much smaller, by a factor of more than 3. Under real road conditions the contrast-sensitivity is still lower, as is understandable considering that the non-uniformity of the distribution of brightness and a certain amount of glare play a part as distributing factors. [The lower the contrast-sensitivity, the worst it is]. Given an installation that is good in every respect, the contrast sensitivity will not lie more than a factor 1.5 below the dotted line, so that at an average level of brightness it will amount to at least 4.5 (contrasts of about 22%) while at higher levels of brightness (such as occur with light-coloured concrete roads) it may rise to 6 (17% perceptible contrast). Contrast sensitivity is also dependent on age - it decreases with age. So far, we've dealt with "just perceptible" contrasts - in practive, it is also of great importance that a certain contrast, given by two different brightnesses, should be made as striking as possible. Thus the objects are more readily noticed and driving on the road becomes much less of a strain. This can be achieved by eliminating all disturbing influences as far as possible.

Once an object has been noticed, we also want to recognise it as quickly as possible. This recognition is closely related to the visual acuity. As a measure of the visual acuity G we may take the inverse ratio of the minimum value of the angle of vision within which a detail must be seen to be observed, or a quantity proportional thereto. This value G is limited on the one hand by the structure of the retina of the eye, thus by the size of the separate light-sensitive elements, and on the other hand by the focusing errors of the eye. When the level of brightness is lowered then G is reduced. (The cause of the decrease lies partly in the expansion of the pupil, and the associated focusing errors.) At very low levels of brightness visual acuity is affected also by the transition from cone vision to rod vision. Under laboratory conditions and at high levels of brightness it is possible to observe details within an angle of vision of about 1 minute. With road lighting, even under favourable conditions, it will not be possible to reach much lower values than 3 minutes.

Quick Recognition
It is particularly of importance for fast traffic that objects on the road can be recognised quickly. It is a question of the time elapsing between the moment when the obejct comes into the field of vision, under sufficiently favourable conditions for observation, and the moment of reaction to what is seen (braking, swerving, etc). This period of time may be divided into four parts :-

  • (a) The time taken for the image to be built up on the retina. This depends on the intensity of the light. At the levels of road brightness (0.1 - 1 candle/m2), the 'exposure time' lies between 1/4 and 1/20 second. In the case of objects with little contrast, or where there is any intense glare, considerably longer times may occur.
  • (b) The time it takes to become conscious of this image and to recognise the object. This depends upon the clearness and the "striking" power of the image and especially whether it is anything common or uncommon. At very high levels of brightness, (b) plays the main role, while at low levels (road lighting) both (a) and (b) are of importance.
  • (c) The time during which the situation is weighed up. What must I do?
  • (d) The time take to set the reaction in motion. These last two periods of time have nothing to do with the eye and are influenced, among other things, by psychological factors (shock, concentration, distraction, difficulty of the problem etc.)

Disturbing Influences: Glare
In general the disturbing effect of a given source of glare is all the greater if the general level of brightness is lower and the visual task to be performed is more difficult. It follows that at the lower brightnesses of road lighting we must try to lmit this disturbing influence to a minimum. Glare may be manifested in several ways:

  • Glare depreciates the performance of the eye. This can be manifested by a reduction in the contrast sensitivity (major reduction) and visual acuity (minimum reduction). Glare does not so much obscure the shape of the object but rather often causes it to fade entirely in the background.
  • After-effect, after-images. After the source of glare has disappeared from the field of vision, its disturbing influence may still continue for some time (successive glare).
  • Hindrance. This is more psychological: the general feeling of hindrance, annoyance, and distraction of attention. These factors are difficult to measure.

Disturbing Influences: Non-uniformity
The non-uniformity of the distribution of brightness on the road surface may also have a very disturbing effect that manifests itself in a depreciation of the performance of the eye. For a given installation the smallest perceptible contrast was measured with the visibility meter at different states of wetness on the road surface. It was found that with increasing wetness of the road surface the dark spots became darker and the light spots got lighter and smaller. It is thus clear that the unfavourable effect of the increasing non-uniformity of the distribution of brightness exceeds the favourable effect of the increase in the brightness itself. We may say that on an average with an installation with sheilded sources of light and with a dry road surface the disturbing effect of the glare of the fixed light sources on the contrast sensitivity is the same as that due to a lowering of the level of brightness by a factor 1½, while the disturbing effect of non-uniformity is equivalent to a lowering of the level of brightness by a factor of 2.

The Part Played By Each Component
The the factors dealt with play an important part. There is a lack of unanimity in literature as to whether the visual acuity and the closely related velocity of observation are also important factors. This depends very much on the speed at whcih one is driving on the road. As the speed increases it is necessary to be able to recognise objects at greater distances and thus within smaler angles of vision, so that the visual acuity begins to play a more and more important part.

Purkinje Effect
Rods and cones differ in the following aspects:

  • By their different location on the retina. Near the centre of the retina there are only cones while closer to the edges there are only rods. The part containing cones is the part with which we see most sharply. It is for that reason we stare directly at objects we wish to see sharply.
  • The cones can observe colour differences. The rods cannot.
  • By the difference in the range of brightness within which they function best:
    • Daylight brightness levels: Exclusively cone vision.
    • Artificial brightness indoors: Usually exclusively cone vision.
    • Road lighting: Mixed mainly with cones.
    • Full moon: Mixed mainly with rods.
    • Stars: Exclusively Rods.
  • By the difference in their eye-sensitivity curve. The cones are apparently most sensitive to yellowish green, the rods to bluish green. If we were to reduce the brightness to such a low level to get purely rod vision then the reddish kinds of light would ultimately become less efficient. In road-lighting practice we work in the upper part of the transitory region; when a reddish light source is employed (i.e. sodium), it will therefore be "inferior in brightness" only for the darkest parts of the field of vision and not for the brighter parts. The result is that the dark objects appear extra black against the much lighter road surface (greater contrast).

Influence Of Colour On Vision
This is confined to the three most important kinds of artificial vision: incandescent light, mercury light and sodium light:

  • (a) Contrast sensitivity: Little difference.
  • (b) The power of greater contrasts: Sodium light offers important advantages (see above)
  • (c) Visual acuity: Mercury and sodium are superior to incandescent.
  • (d) Velocity of observation of stationary objects: mercury and sodium are superior.
  • (e) Velocity of observation of moving objects: sodium light is superior and mercury light inferior to incandescent.
On the road, one is concerned more with case (d) than with case (e) since the movements on the road are for the most part towards or away from us; tranversally there is little displacement of objects in our field of vision.

  • (f) Glare - influence on visual acuity and constast sensitivity: Little difference between the three kinds of light.
  • (g) Glare - after-effect (i.e. the time elapsing after the disappearance of the source of glare until the visual acuity has again attained a certain value): The recovery times for white light are a factor 1.8-2.0 longer than the corresponding times for sodium light.
  • (h) Glare - psychological effects: in general, yellow light is found to be less disturbing than white light.
Lighting: Colour, Lighting: Levels, Lighting: Physiology, Lighting: Theory, Lighting: Weather

Maidenhead Installs Fluorescent Lighting p31
Details about a fluorescent installation in Maidenhead.
Lighting: Installations

Patents And Design Acts: Finall Report of the B.O.T. Committee by S. T. Madeley p32
Details of the reports issued by The Board Of Trade.

"Majestic" Low-Pressure Gas Street Lanterns by Elm Works, Ltd. p33
Description of the Majestic low-pressure gas street lantern as shown at the 1947 Southport Conference.
Lighting: Luminaires

Gas Illuminated Guard Posts in Westminster p34
Detail of the C.C.S. control by Sugg which provides for any number of guard posts with centre island refuge lamps to be simultaneously lighted and extinguished by one clock controller.

REVO Concrete Standard p35
The REVO Phidias concrete standard has been passed by the Royal Fine Art Commision. The column is fitted with the REVO twin 80-watt fluorescent lamp unit and the fitting fits flush up to the standard without the need for bracing or supporting stays and the column is entirely free of embellishment. The column has been named after the Greek Sculptor.
Lighting: Columns, Lighting: Luminaires

APLE Conference - Eastbourne 1948 p35
Arrangements are now going ahead for the Eastbourne Conference. There will be an indoor exhibition of street lighting apparatus and equipment where leading manufacturers will be displaying modern equipment. The Exhibition Hall will be the Winter Garden next to the Conference Hall. The conference will run from Monday, September 13th to Friday, September 17th.
APLE: Conference

Clock Washing p35
Article on clock washing from an Oldham paper. Clock washing was done by the Oldham Lighting Department.

APLE Conference - Eastbourne 1948 p36
List of hotels in Eastbourne.
APLE: Conference

Correspondence p37
Corrrections of discussion points by Mr. Leslie T. Minchin, B.Sc. ("The motorist's decision as to whether they should use their headlights was not influenced by the height of the columns", "Semi-cut-off" distribution should be used for Group B) and R. S. Bradley ("The problem of side street lighting is an old one").
APLE: Journal

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