The Art of Illumination

UNIVERSITY OF CALIFORNIA ANDREW SMITH HALLIDIE: * By the Same Author. ELECTRIC POWER TRANSMISSION Third Edition, Revised and Enlarged. 632 pages. 285 illustrations. 21 plates. Price $}.oo. POWER DISTRIBUTION FOR ELECTRIC RAILWAYS Second Edition, Extensively Revised and Enlarged. es ~ ~ 1 illustrations 33'P^S t 4$ Price $2.50. McGEAW PUBLISHING COMPANY 114 Liberty St., .NEW YORK. THE ART OF ILLUMINATION BY LOUIS BELL, PH. D. OF THE UNIVERSITY NEW YORK McGRAW PUBLISHING CO. 114 LIBERTY STREET 1902 HALUDIE COPYRIGHTED, 1902, BY THE MCGRAW PUBLISHING COMPANY, NEW YORK. PREFACE. THIS volume is a study of the utilization of artificial light. It is intended to deal, not with the problem of dis- tributing illuminants, but with their application, and treats of the illuminants themselves only in so far as a knowledge of their peculiarities is necessary to their intelligent use. To compress the subject within reasonable bounds, it has been necessary to discuss general principles rather than concrete examples of artificial lighting. The science of producing light changes rapidly and the apparatus of yesterday may be discarded to-morrow, but the art of employing the materials at hand to produce the required results follows lines which are to a very considerable extent subject to fairly well-defined laws. Sins against these laws are all too common, the more so since artificial light has become relatively cheap and easy of application. If this outline of a complex art shall tend to avert even some of the commoner errors and failures in illumination, it will have served its purpose. The author here desires to express his obligations to the beautiful treatise of M. Allemagne for illustrations of early fixtures and to numerous friends, notably Mr. Luther Stieringer, for valuable material and suggestions. November, 1902. 1 16753 CONTENTS. CHAPTER PAGE I. LIGHT AND THE EYE, . i II. PRINCIPLES OF COLOR, 23 V III. REFLECTION AND DIFFUSION, 38 -IV. THE MATERIALS OF ILLUMINATION ILLUMIMANTS OF COMBUSTION, . 56 V. THE MATERIALS OF ILLUMINATION INCANDESCENT BURN- ERS, 83 VI. THE ELECTRIC INCANDESCENT LAMP, .... 95 VII. THE ELECTRIC ARC LAMP, 140 VIII. SHADES AND REFLECTORS, 163 IX. DOMESTIC ILLUMINATION, ...... 183 X. LIGHTING LARGE INTERIORS 211 . XI. STREET AND EXTERIOR ILLUMINATION, .... 244 XII. DECORATIVE AND SCENIC ILLUMINATION, . 275 XIII. THE ILLUMINATION OF THE FUTURE, . .301 XIV. STANDARDS OF LIGHT AND PHOTOMETRY, . 313 IX OF THE ( UNIVERSITY;) I THE ART OF ILLUMINATION. CHAPTER I. LIGHT AND THE EYE. WHILE even the Esquimaux and the Patagonian use artificial light and all civilized peoples count it a necessity, it is seldom used skillfully and with proper knowledge of the principles that should govern its employment. Since the introduction of electric lights that very facility of application which gives them unique value has encouraged more zeal than discretion in their use. It is the purpose of the present volume to set forth some of the fundamental doctrines, optical, physiological, and aesthetic, which underlie the proper use of artificial illuminants, and to point out how they may be advantageously adapted to existing conditions. To begin with, there are two general purposes which characterize two quite distinct branches of the art of illumination. First comes the broad question of supplying artificial light for carrying on such avocations or amuse- ments as are extended into the hours of darkness. Quite apart from this is the case of scenic illumination directed at special objects and designed to produce particular effects or illusions. Lighting a shop or a house exempli- fies the one, lighting a picture gallery or the stage of a theater the other. Each has a distinct purpose, and re- 2 THE ART OF ILLUMINATION. quires special means for its accomplishment. Confusing the purposes or mixing the methods often leads to serious mistakes. Sometimes both general and scenic illumination have to be used coincidently, but the distinction be- tween them should be fully realized even when it cannot fully be preserved. General illumination, whether intended to serve the ends of work or play, must fulfill the following conditions: it must be amply adequate in amount, suitable in kind, and must be so applied as not to react injuriously upon the eye. i/ It must be remembered that the human eye is not merely a rather indifferent optical instrument, but a physical organ which has through unfathom- able ages accumulated the characters wrought upon it by evolution, until it bears the impress and incurs the limita- tions of its environment. It works ^>es* over a ratner li mited retinal area Fi i Indian Goggles. and through a range in intensity of light which, although great, is yet immensely smaller than the range available to nocturnal creatures. It has, moreover, become habituated to, and adapted to, light coming obliquely from above, and resents strong illumination, whether natural or artificial, from any other direction. It seems to be well established, for ex- ample, that the distress caused by the reflected glare from sand, or water, or snow, and the grave results which fol- low prolonged exposure to it, are due not so much to the intensity of the light as to the fact that it is directed up- into ward the eye and is quite insufficiently stopped by the rather transparent lower eyelid. Ordinary dark glasses are small protection in this case, but if the lower LIGHT AND THE EYE. 3 part of the eye be thoroughly guarded no difficulty is found. The Alaskan Indians have evolved a very effective protection against snow blindness in the shape of leather goggles with the eye arranged as shown in Fig. i . The eyepiece is merely a round bit of dark leather with a semicircular cut made for the peep hole, the resulting flap being turned outward and downward, so that the eye is fully guarded from brilliant upward beams. Blackening the whole lower eyelid with burnt cork is stated by one distinguished oculist to be completely efficacious for the same reason. It is more than likely that the bad effects ascribed to the habit of reading while lying down are due largely to the unwonted direction of the illumination, as well as to the unusual direction of the eye's axis. All these matters are of fundamental importance in planning any illumination to facilitate hard visual work. Their significance is that we are not at liberty to depart widely from the distribution and character of natural day- light illumination. Of course,, one realizes immediately that the eye is neither fitted nor habituated to working to advantage in anything like the full strength of sunlight, but its more general properties steadiness, absence of pronounced color, downward cblique direction, wide and strong diffusion, freedom from sharp and black shadows these must be followed rather closely in ordinary artificial illumination, or the eye, that has been taking form through a million years of sunlight and skylight, will re- sent the change. The eye is automatically adjustable, it is true, for wonderfully diverse conditions, but persistent and grave changes in environment are more than it can bear. Now from a practical standpoint the key to artificial 4 THE ART OF ILLUMINATION. illumination is found in the thoughtful contemplation of what is known as Fechner's law, relating to the sensitive- ness of the eye to visual impressions. It is stated by " Helmholtz substantially as follows : Within very wide limits of brightness, differences in the strength of light are equally distinct or appear equal in sensation, if they form an equal fraction of the total quantity of light com- pared." That is, provided the parts of the visual picture remain of the same relative brightness the distinctness of detail does not vary materially with great changes of absolute brightness. Now since, barring binocular vision, our whole perception of visible things depends, in the absence of strong color contrasts, upon differences of illumination, the importance of the law just stated needs little comment. It implies what experience proves, that within a rather wide range of absolute brightness of illumination our vision is about equally effective for all ordinary purposes. Fechner's law, to be sure, fails when extremely brilliant lights are concerned. Few persons realize, for instance, that the sun is twice as bright at noon as it is when still 10 to 15 degrees above the horizon, still less that its bril- liancy is reduced more than a hundred fold as its lower limb touches the horizon. Yet while the eye does not de- tect very small changes or properly evaluate large ones in a body so bright as the sun, the mere fact that one can see to work or read about equally well from sunrise to sunset is most convincing as to the general truth of the law. Full sunlight at noon is generally over-bright for the eye if it falls directly upon the work, but with half of it one can get along very comfortably. All this is most important from the standpoint of artificial illumination, since it means that within rather wide LIGHT AND THE EYE. 5 limits of intensity artificial lighting remains about equally effective for most practical purposes. The actual amount of illurriination necessary and desir- able, the terms by which we measure it, and the laws that govern its intensity are matters of primary importance which must now occupy our attention. A simple and definite standard of light is greatly to be desired, but we do not yet possess it. The p/actical and generally legal standard in English-speaking countries is the standard candle. This is defined to be a spermaceti candle of certain definite dimensions, weighing one-sixth of a pound avoirdupois, and burning 120 grains per hour.

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