Mercury-vapor lamps of the glass-tube low-pressure type give from 9 to 13 lumens per watt, according to size. The quartz tube mercury vapor lamp gives about 20 lumens per watt.

Multiple flame arc lamps using impregnated carbon electrodes vary greatly in efficiency according to the electrodes used. The longer the life of the electrodes between trimmings the lower the efficiency. Short life electrodes may give as high as 28 lumens per watt and long life electrodes 17 lumens per watt when the lamp is clean, but this value rapidly falls off with the accumulation of globe deposit so that the service value is uncertain.

All of the foregoing information regarding lumens output per unit of input of various illuminants applies to new lamps in the proper adjustment. There is a depreciation or falling off in value. with use from various causes as described more in detail later on.

Candle-power Distribution from Bare Light Sources.-The distribution of light about the more common interior illuminants now in use comes under two general classes or combinations of the two. The gas-mantle upright burner, which is a straight tube and the vacuum tungsten lamp, the filament of which is largely straight vertical wires, give polar candle-power curves approximately circular in form on each side of the center. The gas-filled tungsten lamp with close coiled filament and the open-flame gas burner give polar candlepower curves nearly circular about the center. The inverted gas mantle gives a curve nearly semicircular below the horizontal with some light above.

Effects of Accessories on Distribution and Brightness.-The distribution of light from a source can be considerably modified by the use of reflectors and its brightness can be modified by the use of diffusing glassware or enclosing globes. Such modifications are usually nec-. essary for interior illumination and the accessories for accomplishing it are to be discussed in Mr. W. F. Little's lecture. A careful study of the possible and available modifications is essential to design. A few of the principal things that can be accomplished by reflectors and globes may be reviewed.

With mirrored reflectors approaching the parabolic in form, over an incandescent electric or gas mantle burner, the maximum concentration of light can be obtained. This arrangement represents the extreme in the control of light. With reflectors now available almost any distribution between this and the natural distribution of the bare lamp can be obtained. Where the accurate control necessary to extreme concentration of light is required, mirrored surfaces,

consisting of glass silver plated on the back has proven to be the best for most commercial purposes of interior lighting, because the silvered surface is not exposed to tarnish as in the case of plated metal. On account of the streaked light resulting from the use of smooth mirrored surfaces corrugations are frequently necessary to eliminate these streaks.

Where less concentration than that obtainable with a mirrored surface is required it can be obtained either by selecting a mirrored reflector of different design or by the use of a reflector with more diffusing reflecting surface. The opaque surfaces of this character in most common use are white enameled metal and matte-finished aluminum.

The class of reflectors commonly known as deep bowl and also to some extent known commercially as intensive and extensive types have considerable use in general illumination of rooms because they combine a fairly wide distribution of light with a covering or shading of a part of the sources of light in a large room. They cannot, however, from the nature of the case, hide the source of light completely unless hung very low. Reflectors of the deep bowl type obtain their wide distribution at angles about 45 degrees from the vertical by reflecting the light back past the axis of the lamp and reflector. In this they differ from the shallow bowl or dome type of reflector which is considerably larger in diameter. Since the shallow dome type does not intercept so much light flux it has less internal multiple reflection and does a larger part of its useful lighting by direct light from the source, and the physical efficiency is higher for lighting a large area than with the deep bowl type.

The flat reflector which enables considerable light to escape horizontally without interception by the reflector has rather limited application in the lighting of interiors. Its principal use is in locations where the lamps are so far apart that any reflector of greater depth would interfere with the illumination midway between the lamps, and the escape of light upward is to be avoided as far as possible.

Deep bowl shapes of reflectors made in opal or other diffusing glass have a considerable application in direct lighting of interiors. Reflectors of this type are also sometimes used inverted for semiindirect lighting. Where they are used for semi-indirect lighting, it is hygienically desirable to keep the brightness of the reflector surface as low as possible to avoid contrast glare, and for this reason the more dense types of glass are to be preferred for such cases.

Prismatic reflectors offer a control of light which approaches that of the mirror. Considerable light passes through the reflector at the tops and bottoms of the prisms.

For indirect lighting and semi-indirect with dense reflectors it can be shown theoretically that the best reflector for the purpose would distribute light evenly over the whole ceiling area served from one fixture.

That is, in a small room, with one central fixture, the whole ceiling would be evenly illuminated; or in a large room with a fixture in the center of each bay each reflector would evenly illuminate that bay. By confining a considerable portion of the light flux to the center of the ceiling with a fixture hung in the middle of the room, more of the light flux will reach the working plane after one reflection from the ceiling than if the distribution over the ceiling were more uniform. The more even the distribution the greater the amount of light lost by absorption at the walls. However, from the standpoint of the desk worker there is some advantage in having the ceiling evenly illuminated as there is some tendency to specular reflection from the brightest portions of the ceiling causing a slight veiling glare. This glare is not so pronounced if the ceiling is evenly illuminated. An indirect reflector giving uniform ceiling distribution must be of the deep bowl type, but this type has a very sharp "cut-off" or transition from high to low illumination at the edge of the reflector. This causes a shadow on the ceiling which is objectionable and calls for some modification of uniform ceiling distribution. Two principal ways of overcoming this have been worked out in practice which work well with non-concentrated light sources. One is to use a shape similar to the deep bowl distributing type for the lower part of the reflector and a flaring bell-shaped one for the upper part. The other plan is to use a large reflector of the shallow bowl-shape. The former plan is used mainly with mirrored reflectors where it is desirable on account of first cost, to keep down the size while the other plan is used with white enamel reflectors and for semi-direct lighting with large glass bowls. While it may be immaterial for the engineer who plans the lighting installation how the result of eliminating dark shadows from the ceiling is accomplished it must nevertheless always be kept in mind that good design calls for the elimination of these shadows to a large extent by tapering off the brightness from the center to the edges of the illuminated area covered by each reflector.

For semi-indirect lighting a plain bowl somewhat shallower than

Orna

a hemisphere is likely to give the best results in efficiency. mental designs in which the maximum diameter of the bowl is greater than the diameter at the top cause considerable loss of light because of the light which is intercepted by the part of the bowl projecting inward. Therefore when such designs are used this extra loss should be recognized in the calculations and a decision reached whether the ornamental effect attained is sufficient to justify the loss.

While the placing of lamps and shaping of semi-indirect bowls is not as important as in the case of indirect reflectors of the opaque type, it is not by any means a matter of indifference. The lamps should be placed in a position not to cause undue shadows on ceilings or walls or too uneven illumination on the bowls as viewed from below.

Angle reflectors may be obtained giving a number of different types of distribution for special purposes such as show window lighting, bulletin board lighting and other cases where more light is wanted on one side of the plane through the lamp axis than on the other. They cannot be classified into general types as there is such a variety. Makers data should be thoroughly studied as to the forms available.

Shifting the position of a lamp in a reflector by the use of different forms of shade holders may materially change the light distribution.

In the selection of reflectors for any purpose it is always well to remember the fundamental principle that control of the light flux is the end to be desired if the flux is not to be wasted by escaping to places where it is not needed or positively undesirable. The larger the percentage of the total flux of light from the lamp which the reflector intercepts and reflects in desired directions the higher the efficiency; unless, however, the natural undirected flux from the lamp approximates the distribution desired. With reflectors which must confine the light flux of the lamp within rather restricted areas as in show windows and for localized lighting of work benches and the like it is important to use reflectors large enough to intercept a considerable portion of the light flux. There is apt to be a tendency to cut down reflector sizes to save first cost but such reduction usually means a permanent impairment of efficiency. This is also true in the lighting of a large high room of the armory or coliseum type where the lamps must be placed high and all light eminating from reflectors at angles only a little below the horizontal is likely to undergo serious loss by striking dark roof and walls.

Sky Brightness Characteristics useful for design of natural illumina

tion are given in Table III. It will be seen that there is an enormous variation in the brightness of the sky during what are ordinarily

considered daylight hours. Calculations of daylight illumination of interiors should therefore be made on the basis of maximum and minimum values.

The sky is the principal source of daylight illumination of interiors, as the illumination obtained directly from the sun may be considered as purely incidental and frequently avoided by the use of shades.

In connection with daylight we have first to consider the amount of sky exposure through side or ceiling windows; the amount of illumination (excluding reflection from walls and other buildings on any point) varying directly according to the area of the exposure as projected from the point in question. Part of the window area may be obstructed by buildings and in certain cases the reflection of light from these buildings (or in other words their brightness) must also be taken into account as well as that of the sky.

Illumination from Direct Sunlight in the open has been found to reach approximately 9000 foot-candles, in Virginia, during the summer months as measured on a horizontal plane. Extensive measurements made there by Prof. Herbert H. Kimball, of the U. S. Weather Bureau, show that the total illumination from sun and sky during the middle of the day consists of about 20 per cent. skylight and 80 per cent. direct sunlight. Sunlight shining into interiors therefore may have about 80 per cent. of its outdoor value.

With clear glass windows the only sky brightness which is useful for illumination of the room is that directly visible from the interior of the room. If the window is obstructed by buildings the sky brightness is not available. Where a window is exposed to sky