of the American Institute of Electrical Engineers, Copyright, 1912 By A. I. E. E. ARC VS. TUNGSTEN STREET-LIGHTING IN SMALL TOWNS BY C. E. STEPHENS PURPOSES Early in the 15th century street lights were ordered in Paris, France, to prevent crimes of violence. This was at a time when the only occupants of the streets after night were belated pedestrians and a few vehicles. In the present day the use of our streets by night is growing and ever increasing. In fact the traffic in certain sections is heavier, in some cities, during the earlier part of the evening than it is at any time during the day. It is necessary to provide street illumination to mark the course, assist the police, also to avoid obstacles and to recognize persons. Police require to observe suspicious characters, and to see numbers on automobiles. It is difficult to estimate the value of good street illumination, nevertheless it is positive. The best lighted streets attract the largest crowds. An increase of intensity of illumination increases the traffic, and property values fluctuate with the density. of the crowd. It is a noteworthy fact that, in many of our cities. and boroughs, one street or section thereof, or perhaps one side of a particular street, is congested with traffic, while other sections in the immediate locality are practically deserted. In some cases this can be attributed to the character of the business houses, but in a large proportion of the cases, there is no doubt that the illumination of the several sections is responsible for the condition. There is, therefore, at the present time a far closer relation existing between public safety and adequate street illumination than existed two or three hundred years since. The appearance of our streets gives the visitor a favorable or unfavorable impression of the city's thrift, depending very largely upon whether or not the illumination is adequate. Well lighted streets have a wholesome appearance which is an incentive to cleanliness. Very little need be said to convince the average citizen of the positive value of well lighted streets. It remains, therefore to profit by the experience with the various systems in use and to select the one which more nearly fulfills the requirements of local conditions. In this connection we may mention some of the defects in the present practice. There is no doubt but that as a whole, stree illumination in American cities is inferior to that of European cities. This is generally understood to be for the reason that European cities have a far greater number of inhabitants per square mile than do American cities, and a lower cost for labor and material in properly maintaining a lighting system, both of which combine to reduce the proportional cost per inhabitant of lighting the street. We believe the chief cause to be that the American public has not been educated to the value of an improved standard of street illumination. This standard has increased wonderfully within the last five years, but there is room for great improvement. A greater number of lamps in some cases, and in others units of greater candle power, have been installed as a result of a spirit of cooperation among the lighting companies. The principal defects in the past have been the absence of a properly diffused light and the use of high brilliancy units so placed as to violate the physiological requirements for satisfactory street illumination. This applies particularly in sections where the cost prohibits any class of illumination other than that of a lamp used for a marker. Until within the last few years, very little attention has been given to scientific illumination, either in commercial, industrial, residential or street lighting. The first improvements were made in the industrial and commercial field. The progressive and successful industrial and commercial concerns soon realized the necessity for better artificial illumination, in order to improve the quality, and increase the output and efficiency of their organization. This resulted in an application of scientific principles of illumination in their establishments which has been followed to a limited degree in residential lighting. The long period of insufficient and generally unsatisfactory illumination of the streets is now being succeeded by a period in which central stations, civic organizations and merchants are making a gigantic effort to improve conditions. This effort has resulted in the installation of a large variety of lighting systems. Great progress has been made in educating the general public to a higher standard of street illumination. To a limited degree citizens have begun to realize the inestimable value of a higher standard of illumination. We have installations worthy of considerable praise, but as yet the improvements of which we boast do not compare favorably with the standards set in other lines of civic betterment. GENERAL CONDITIONS The fundamental problem to consider in the illumination of any street is the intensity of the illumination required and its production at a minimum cost. The cost includes the expenditure of energy, cost of maintenance, and interest and depreciation of the lamps, plant and all auxiliary equipment. The area to be lighted is a long and comparatively narrow strip. The intensity of illumination is proportional to the light intensity of the unit and inversely proportional to the square of the distance from the light source. Assuming that for a given minimum illumination a certain distance between lamps is required, if this distance be doubled, each unit must have four times the light flux and each mile of street will require two times the energy. Inversely, if the distance between units be reduced one-half, to get the same minimum intensity of illumination will require only one-fourth the light flux and one-half the energy. If the problem could be solved on an energy basis alone, it would therefore be logical to use a maximum number of light units with a corresponding reduction in their flux and energy consumption. It will be noted, however, that increasing the number of units also increases the installation and maintenance cost of the system. There is a point beyond which the cost for any increase in the number of units will exceed the saving in energy consumption. A point cannot be fixed which will apply generally on account of the innumerable variables (such as the intensity requirements in different sections of the city, obstacles which prevent a proper location and distributions of lamps, energy costs, etc.) which are involved. When considered from the standpoint of economy without regard for illumination and decorative requirements, if energy cost is low, large units at great distances apart are better, and if energy cost is high, small light units placed at frequent intervals are more economical. It should be noted that the above applies only to the illumination of long and comparatively narrow areas where the flux of light required varies with the distance between units, that is, when the distance between the units is doubled, the area which a particular lamp must light is only doubled. This does not apply in the illumination of large rectangular areas where the light units are placed on the basis of the square, since under these conditions when the distance between units is doubled the area which each unit must light is increased to four times, the total light flux is increased four times, (to maintain the same minimum intensity of illumination) and the efficiency remains the same. In street lighting, therefore, to keep cost within reasonable limits the intensity of illumination is necessarily very low. Heretofore we have had to be satisfied with a spot light form of illumination in our streets, but recent improvements in efficiency and the satisfactory operating characteristics of our most modern street illuminants make it possible to light our streets economically and comfortably by night. The increased standard of illumination of the street has become necessary on account of the extensive use of our streets at night. The character of this illumination is general as distinguished from local lighting, and on account of the low intensity should be quite uniform or else effective illumination midway between lamps will be further reduced by the points of high intensity immediately under the lamp. Having mentioned some of the general requirements of street illumination, it is in order to discuss conditions, quantity and quality of the light and to note its relation to intensity. The requirements for good street illumination may be considered under the following heads: Uniform intensity; distribution. Intrinsic brilliancy of light source. We see things by virtue of their difference in luminosity. Difference in color at low intensities is barely discernible, and regardless of the quantity of light received by objects, the differ ence in the amount reflected to the eye from black objects and bright objects is comparatively small. That is, a black object will reflect approximately 25 per cent of the light reflected by a bright object. The ability to perceive objects, therefore, depends upon the capacity of the eye to distinguish differences in luminosity. According to Fechner's law the eye can perceive, over a wide range of intensity, a fairly constant fractional difference in luminosity. In bright light it ranges from one-half of one per cent to one and three-fourths per cent. This difference decreases with light intensity until at one lux the eye is hardly able to perceive shadows. The ability of the eye to perceive differences in luminosity depends largely upon the condition of the eye. If the eye is adapted to the dark, shade perceptions are possible at extremely low light intensities. In street lighting if you can see an object fairly well between lamps, i.e., at the point of minimum light intensity, the ability to see is materially affected by glare, and to changing of light intensity due to moving along the street. It is of importance, therefore, to provide a uniform intensity of illumination, particularly in streets where intensity is low, and to select, equip and locate the light source to avoid glare. By uniformity is meant a uniform illumination at all points. throughout the length of the streets. At street intersections and other points where traffic is likely to be congested a higher intensity of illumination is needed. The intensity at these points can be ten times that of the minimum illumination at other points and not be objectionable, since there are no moving vehicles which will approach alternate light and dark spots so fast but that the eye cannot easily accommodate itself to the different intensities of illumination. Assuming the minimum intensity of illumination as unity, the ratio between maximum and minimum illumination for business or other sections of the street where a high intensity is maintained, should not exceed ten to one. For residence sections, parks and outlying districts the ratio should not be greater than five to one. This ratio of maximum to minimum illumination is smaller in the latter case because of the fact that where intensities of illumination are quite low the effect of the comparatively light and dark spots produces objectionable glare. Furthermore, it is in these sections of our streets that are found the fast moving cars, automobiles and carriages. Uniformity is of greater importance, therefore, in the residence or other sections or cities where |