Popular Science Monthly/Volume 44/January 1894/Window Lights and their Value



MOST subjects of analysis can be studied both in quantity and quality, and light is no exception to the rule. Color as a quality of light has always been a popular study. A vast deal of experiment and attention has been given to the harmonics of color, and many who make no claim to scientific attainment are well versed in it. Every child knows the colors, but the expression "quantity of daylight" has a strangeness and a vagueness about it that are not felt by children only. This is largely because there is such a wealth of daylight about us. "Silver was nothing accounted of in the days of Solomon." Light is not measured, because it is lavished upon us by an unstinting hand. But light can be measured. Although intangible, it has quantity—quantity of effect, if it may be so expressed. If this seems fanciful, it may be remarked that there is nothing fanciful about the measurement of heat, and yet the case is quite analogous. The thermometer is simply a contrivance for measuring variation of intensity in heat. Quantities of heat effect are continually being estimated, for economy in its use is of prime importance. Light, on the contrary, is ready at hand. Ordinarily objects are flooded with a brilliancy of daylight which is as free as air. There can scarcely be need of economy with light when the world is floating in it. But there is nothing impossible in its measurement. Possibly, if the sun were less prodigal in pouring out his rays upon the earth the measurement of daylight would be a more common operation than at present. Every means would be taken to utilize it without waste. We would see the owners of buildings making careful estimates of the light belonging to their properties—even dividing it into lots and renting them separately. At the least, we would see them more jealously than now defending their light from obstructions built around them.

Windows are the natural and all-important resource of the architect. All the light which enters the building must pass through them. It is a very patent fact that the larger they are, and the more numerous, the greater the total amount of light which enters. It is not as widely appreciated, however, that there are other conditions affecting the amount and quality of the entering light that are sometimes ruling in their effect.

Looking from a window, one sees a variety of surfaces—sees them because of their reflecting different intensities and colors of light. If asked to classify them according to relative reflecting power, he will probably speak of the sky first as the great source of light to the window. Then, if he does not ignore them altogether, the objects on the earth are grouped in one class of comparatively little importance. This is not Nature's method. There is no sharp division between the strong reflecting surfaces and the weak, between the sky and the earth. We are prone to such a distinction, because the sky seems prominent as a reflecting surface, but it can be shown that circumstances arise when this is by no means true.

Imagine the window to look out upon a landscape. Earth, foliage, and sky all combine to reflect light into the room. The water may be very dark, from the shadow of overhanging trees, reflecting less than ten per cent as much light as comes from the sky, but there is a path across it where the sunlight is cast back far more brilliant than the sky itself. The beaten highway gleams in the sun so that it is a relief to look away from it into the blue above. Even the foliage, delightfully dark and cool in the shadows, may have a brightness where the sunlight strikes it which is fifty or eighty per cent of the intensity of sky light.

On turning from the country to the city view we find the lessened importance of the sky as a source of light especially emphasized. There the great value of space causes one building to encroach upon the sky light of another until frequently the patch of blue visible from the windows is limited to a mere streak, or may be cut off entirely. If the sky were the only means of lighting, the windows would be useless in such cases; but the fronts of buildings, the paved streets, and other surfaces combine to throw much light into them, and give a reason for their existence.

The reflecting power of the sky dome is due almost entirely to the particles of vapor contained in the atmosphere, and hence must be considerably affected by changes in its aqueous condition. Contrary to what might be imagined, the clearer the sky the less valuable is it as a reflector. The more haze that it contains within limits the more intense the light obtained from it. An observer will recall dreamy summer days when the sun has seemed to shine softly as through a gauze cast over its face and the shadows were mellowed and diffused, yet the sky was white with a radiance painful to the eye. There was little suggestion of ethereal blue in the white light sent down from this atmosphere charged with particles of moisture. Again, a clear day comes; the air fairly dances with brilliancy, and distant objects stand out in the sunlight as clear-cut as a silhouette. The sky is a beautiful Italian blue, but does it occur to one how really dark it is except in the immediate vicinity of the sun? Try to match it with a sheet of blue paper, and it is almost startling to discover what a somber surface the sky dome is. Its value as a source of light is greatly less than on days when it is a duller blue, or when gray better describes its color. Some experiments which are at hand show a difference of over seventy per cent in the reflecting power of the northwest sky on two sunshiny days, and they were by no means extremes of the two conditions of atmosphere discussed. No experiments upon extreme states of the atmosphere are available, but it is safe to say that the reflecting power of the sky dome in this climate is one and a half times greater on some days than on others. Between the extremes are all possible variations.

Thus far the thought has been only of an unbroken expanse of sky, but if clouds float across the field they greatly change the conditions. A cumulus cloud piled high in great masses is carried past the window by the wind. It gleams beneath the sun's rays like a ball of cotton, and pours down a flood of light that may have as much as four times the intensity of the light from the sky directly beside it. At another time heavy thunder clouds will roll up from the horizon—a dark gray, unillumined by the sun. They obscure the sky and replace it by possibly ten per cent of its intensity of light.

Indeed, does it not seem as if there were no stability about the sky light? And yet, brushing the clouds aside, it will be found that the changes in any one day are not usually great. There is enough permanency in its reflecting power to make it serve as a practical standard of comparison—a standard not for the direct sunlight, which so far transcends any other light on the earth as to be unique, but for the vast variety of lights which crowd into the windows—the reflections from brick and stone, from wood and paint, from earth, water, and foliage.

Turning from the sky to the earth, a vast variety of reflecting surfaces is encountered. Each has its peculiar power of altering the light it reflects, both in intensity and quality. The amount of their influence upon window light is apt to be underestimated. Many rooms through the entire day and nearly all rooms for a portion of it have no direct sunlight, and all the light they do receive is entirely by reflection. Of this the portion coming from surfaces on the earth is a very considerable part.[1]

It is true that most surfaces reflect but a small percentage of the light which strikes them, but when that light is the great flood from the sun itself the pittance which comes from them is by no means contemptible. An illustration of this may be given. While reading on the shady side of a railway car one afternoon, a sudden flush of warm-tinted light seemed to cover the page and as quickly it was replaced by a cold gray light. The change was very noticeable and the cause easily revealed. At intervals along the track were stretches of rough-faced masonry, perhaps seven feet high. The stones looked in shadow a dull gray and a buff color, but when the sun struck them they glowed with a light that flashed into the car window. In spite of the fact that the area of sky light was far the greater of the two, that rough retaining wall determined in a great measure the character and intensity of the illumination on the paper.

Light-colored surfaces are the most valuable reflectors, and among them white paint and whitewash stand pre-eminent.[2] Even in shade, when illuminated only by other objects or by the sky, they will give sixty per cent or more of an average sky light. Sometimes they will run up to the full value of sky light, if a reflecting surface near by shines brightly upon them, or they may fall to twenty per cent or even less in deep shadow. But when the direct rays of the sun fall upon a newly whitewashed surface, the volume of light it reflects is almost blinding. Three hundred to four hundred per cent is not too large an estimate to place upon it in comparison with the sky light. On this account the well known expedient is used of whitewashing or painting in some bright tint the walls of a light-shaft or surfaces facing a window which is much shut in. A case is known of the rear of a house so treated being in summer time a source of great annoyance to dwellers on the next street, because of the blaze of light reflected into their rear windows. White marble is quite similar in its powers of reflection. A striking example can be seen in the spires of St. Patrick's Cathedral in New York. Rising far above the dust of the city, they are not yet covered with its grime, and their pure white glistens in beautiful contrast to the dark blue of the sky.

From the white of paint or marble there are many variations in building surfaces, all the way down to black. A large number are to be found among the granites. Some varieties are very dark in tone, reflecting little light unless polished, but the gray granites give considerable light. A freshly tooled gray granite will certainly yield one hundred per cent of sky light when in sunshine, and some varieties give far more. The writer has known the gray granite of an old building to give one hundred and forty per cent, as compared with the sky light of a clear day. When it was in shadow the intensity fell to only ten per cent.

The brownstone, so familiar in building construction, is justly looked upon as a gloomy material for the purpose. Connecticut stone is the darkest, and, especially when oiled, is a somber thing with which to enliven architecture. But even when oiled it does not absorb quite all the light which strikes it. The searching light of the sun will find many little particles among the grains of the exterior that give surface reflection, and this white light mingles with the dark interior hue of the stone to brighten it considerably. The Connecticut stone under ordinary conditions gives some sixty to seventy per cent of the intensity of sky light, when the sun shines brightly upon it. Belleville stone is much lighter, and has been shown to exceed the sky light by twenty or thirty per cent when illuminated by the sun.

Among the bricks there is a large opportunity for choice in their capacity for reflecting light. The Philadelphia pressed brick is popular for its richness of color, but deserves no prominence for its reflecting power. In that respect the coarser Hudson River brick is an improvement. There is, of course, considerable variety, but it may in general be said that walls of Philadelphia brick, and those painted red to imitate that shade, will reflect sunlight to an extent varying between fifty and eighty per cent of ordinary sky light. When illumined by only indirect light from the sky or other sources they have an intensity of fifteen to twenty per cent of the same sky light. With rough brick walls there is always a considerable show of light-colored mortar, and this, with the lighter surface of the bricks themselves, causes a greater reflecting power. It is easy to find surfaces of this character with a reflecting power when illuminated of ninety to one hundred per cent.

Buff-colored bricks make admirable reflecting surfaces. With the sun upon such a surface it will often reflect one hundred and sixty per cent or more of sky-light intensity. Many opportunities will offer in modern construction for the observer to bring such an illuminated surface against a sky background, and see how much brighter it will ordinarily be. Its practical value as a reflector is greatly lowered in his estimation, however, when he finds that, if the sun is not shining upon it, twenty-five per cent is a liberal estimate of its powers of reflection.

The climax of reflecting power in brickwork is reached in the cream and white enamel brick that are in the market. They are chiefly used for interior work, and their polished surfaces place them in a different class of reflectors from the ordinary building materials.

Slate roofs belong to the same class in a certain degree. The ordinary slate when held in the hand appears a very dark blue; little light is reflected from the interior, and that little is mainly blue, while the very small amount of white light reflected from the surface is not enough to pale the interior hue to any great extent. But turn the slate so as to reflect the light differently, and then there is such an excess of surface reflection as to give even a silvery tone to what was a moment before a dark material, A twist of the wrist will easily make a difference of two hundred per cent in the reflecting power. The extremes and all the intermediate conditions can be seen in slate roofs, and these are in a position where comparison is easily made with the sky. One roof will look almost black, as it stands out against the sky; another that reflects the sunlight will gleam like a white sail on the horizon. At another time it may melt away with a hue and shade that are not to be distinguished from those of the blue sky back of it. The same effect is obtained with shingles, but in a less degree.

The pavement of city streets affords some interesting observations. It is ordinarily looked upon as only fit to be trodden under foot of man, and very little credit is given its reflecting power. Paving stones appear of a rather somber color when held in the hand, and ordinary blue flagstone is similar. Hence it is surprising to learn that dry flagging, when illuminated by sunlight, is about the equal of the highly prized sky as a reflector of light. A considerable number of experiments, taken at various times and places, go to show this. Under ordinary conditions of dirt the pavement may be slightly darker than the bluestone flagging, but they will both give approximately one hundred per cent of sky light. Moreover, their hue is not unlike that of many skies. The idea is perhaps difficult to grasp, in view of their condition; but if a section of New York street could be purified and translated, it would quickly be lost to sight, ascending in a sky of its own color. When in shadow, fifteen to twenty per cent represents the amount of light that the street will give as compared with the sky. If the stones are wet, not more than ten per cent of light comes from them, except where there may be a powerful reflection of white light from the water upon the surface.

It has perhaps been noticed that thus far, in discussing reflecting surfaces, attention has been confined to the changes they produced in the intensity of light. These are not, however, the only alterations that occur. Quality as well as intensity is affected by the surfaces which throw light into the window. Leaving now the intensity of reflecting power, let us endeavor to realize what is taking place in every city street. It may be a thoroughfare filled with the noise of travel, but the air is crowded with silent lines of light. Back and forth they fly in all directions, every surface receiving light, every little grain and fiber acting as a reflector to send it out again. Reflection and re-reflection are taking place with enormous rush and intricacy. But the extent of complication is not yet reached. These various lights are not all white. Each substance has its effect upon the light that it reflects. Some of the light undulations are absorbed, and those reflected give the effect of color characteristics of the object. The colored lights are then flashing back and forth, continually changing as they leave the different surfaces. Neither is this a complete statement of the situation. Each surface is illuminated by a variety of colored lights, depending upon the surfaces opposite it. The color of any one material depends upon the color of the light striking it, as well as upon its own nature. Hence possibly none of the surfaces in the street are seen in their true tones, the hues belonging to them when illuminated by pure white sunlight. They are slightly off color, modified by the colored lights that strike them. All this wonderful play of lights darting across the street is put in action by the steady, powerful, pure white flood of light coming from the sun.

It may be questioned why this condition of affairs is not more noticeable. Why are not the bright hues of the rainbow seen in all directions? The absence of brilliant coloring is briefly accounted for by the large amount of surface reflection taking place, through which we see objects as if a gray glazing had been washed over them.

However, conditions will occur when there is a very noticeable flush of color cast over objects which is not their normal hue, and it can be traced to the predominating influence of some one of the reflecting surfaces. Occasionally most striking effects of this character are seen during the sunset hour. Sun-illumined clouds are in the west holding their bright color when all other sources of light in sky and earth are waning. The predominating power of their colored light is enough to make a modest little sunset in the east, and the sky there will glow with a soft tint sympathetic with the display in the opposite part of the heavens. The western sky may even give a glamour of colored light to the landscape, casting a weird, strange effect over meadow and hillside, ere they disappear in the darkness.

"Where the quiet-colored end of evening smiles miles and miles on the solitary pastures. . . ."

Those who use colored parasols understand perfectly the result, if not the theory, when the lining of the parasol is selected so as to add to natural charms by casting a soft flush of color over the complexion.

But illustration of the influence of the effect of surfaces upon the color of light can be found in more prosaic subjects. Buildings offer examples. Ordinary brick and stone often have more influence than is imagined upon the light that enters windows. A brilliant although exceptional example of this occurred once in the writer's office. It has an eastern exposure and nearly opposite, at a distance of over one hundred feet, is a large red-brick building. The sky area exceeds all others, and ordinarily the amount of gray or light-blue light entering is enough to entirely overcome the effect of the red surface. On this special occasion, however, after a rainy day the sun suddenly burst through the clouds. The face of the building was illuminated by clear white sunlight, and stood out brilliantly against a backing of heavy, dark clouds. The effect in the office was most noticeable. Where had been nothing but a cold gray light in an instant a glare of red was cast over everything. Table and book that had been dull looked warm in color, and the walls appeared, as if by magic, in the most delicate rose tint fit for a fairy's boudoir. True, these were conditions most admirably adapted to illustrate the point that surfaces opposite windows can affect the quality as well as the intensity of the light reflected, but others more common lead to the same conclusion.

An excellent opportunity for alternating contrast is offered by a ride on an elevated city railway. Let one select a time in the afternoon when the eastern sky is not so bright as to obliterate the effect, and seat himself on the right-hand side of a down-town train in New York city. If the buildings are not too far removed from the track, a very decided change is noticed as each block is passed. Where the opening of the street brings a considerable sky area into view an ordinary gray light is cast upon the newspaper. This is succeeded by a sudden flush of rose as a high block of red-brick buildings is passed, and again a street opening allows the western sky to assert itself. Moreover, let it be noted that this may occur not with a bright sunlight pouring upon the buildings, but when they are in shadow, except as the eastern sky illuminates them.

Occasionally the effect of surfaces opposite the windows upon the color of the light can be noticed inside of buildings, even with no exceptional atmospheric conditions existing. On any clear day, by limiting the rays striking a marble slab in a certain room chiefly to those from a brick building opposite, one can change the white marble to a deep rose tint of a most beautiful shade.

These various illustrations have many corroborations in experience. They show that the light entering windows must be considerably influenced in color by reflecting surfaces opposite, even though the effect be not noticeable. Usually this is not a matter of any concern, because a slight difference in hue from their appearance under pure white light does not materially affect our appreciation of most objects. There are, however, special operations which are thus affected to an extent that is of practical importance. A conversation with a cotton broker, for example, will do much to increase respect for the sensitiveness of the trained eye, and convince one of the practical bearing of these matters. Careful observation is needed to judge cotton and grade different samples, and the eye becomes wonderfully skilled in doing so. A steady, clear sky light is desirable. Evidence is at hand of the injurious change upon the light caused by alterations in buildings across the street from a cotton sample room. In the course of building operations in Pearl Street, New York city, a dull, buff-colored wall was taken down and replaced by red brick. The result when the sun shone upon the surface was a noticeable and injurious change in the light that came into the windows. A flush of pink was cast over the cotton samples, perhaps too slight to be noticed ordinarily, and yet giving a tone to them which interfered with the judgment of their quality.

And now a few words in conclusion concerning the service the light performs when it finally reaches the room. That light which passes directly from the window to the eye is of no benefit, except as it enables one to see the outside view. Nearly all the light serves a far more useful purpose. It enters the window and sets in operation on a smaller scale the same phenomena that are taking place in the street. It is bandied back and forth between walls, carpet, furniture, and occupants. The light that these various surfaces reflect gives impressions of form and color by which we appreciate objects. By means of it we see our friends' faces, enjoy the pictures, read the book. It should be noted that light and color are entirely subjective. They are effects produced in the brain by different kinds of light undulations. We perceive the color of the upholstery and carpet because these have the faculty of sorting out undulations of special wave lengths and reflecting them. We perceive the outline of chair or of face partly by change of color, but chiefly by light and shade, the difference in intensity of the undulations coming from them and from objects behind them. To make the objects in the room distinct the light entering the windows must be conserved as much as possible. This will best be accomplished by banishing all dark materials. Heavy hangings absorb light as well as dirt. Oak or enameled furniture reflects the light that black walnut or rosewood absorbs, and a light-colored wall will do almost as much as a sunny disposition to fill a room with sunshine and good cheer.

  1. In a number of cases carefully determined in city locations it has been found that the sky gave only from eight to forty per cent of the total light reaching a point inside a window on the ground floor. The remainder came from opposite building surfaces and from the street—this, with the sun shining upon these reflecting surfaces. At another time of day the sky value would be comparatively greater, yet not so much as might be imagined, because all the surfaces would not lie in shadow at the same time.
  2. The percentages stated in the following pages are based upon a large number of photometric measurements of the light from building surfaces, made by the writer at various times and places.