Luxfer Prisms are a commercial product made of glass of a standard dimension of 4 inches square, having a smooth outer surface and an inner surface divided into a series of prisms. They are formed into plates by the process of electro-glazing, the edges of the prism lenses being welded together, so to speak, by a narrow line of copper which gives the desired stiffness and strength for use in large frames and also an attractive appearance far superior to the ordinary leaded work. These prism plates can be made in any desired size, but for very large surfaces two or more plates, divided by means of iron sash bars, are generally used.
The commercial value of these prisms depends on the quality of glass known as refraction. Prism plates receive the light from the sky (not necessarily from the sun) and refract or turn it back into the room to be lighted. With ordinary windows the light from the sky, passing through the glass, strikes the floor at a point not very far distant from the window. As the floor is usually of a dark color, reflecting perhaps only one-tenth part of the light falling thereon, it will be seen that the rear of room receives only a small portion of the light entering the window. For this reason it has been necessary to make very high stories for deep rooms, in order to even moderately light portions at a distance from the window. When prisms are substituted for the common window or plate glass, the rays of light as they enter the glass are changed in direction, or refracted, and by employing a prism of the proper angle, may be given almost any desired direction. Moreover, by utilizing different prisms in the same plate, part of the rays may be directed to the rear of the room while others are thrown so as to strike near the front. The prism plates do not increase the quantity of light entering the window, but simply re-distribute it, directing it into that portion of the room where it is most needed. By thus changing the direction of light rays a better lighted room can be secured, with less height of story, than when sheet or plate glass is used.
To insure success in the lighting of interiors by means of prisms requires, however, a superior quality of glass, careful scientific calculations and experiments, besides a practical and attractive means of glazing and method of installation. The author believes that these qualifications have been attained by the American Luxfer Prism Company, whose product may be considered as a new building material, and one which has been very successfully applied to the lighting of dark rooms by daylight.
The application of Luxfer Prisms to any particular building depends upon the surrounding conditions and requirements, each case requiring some special treatment, but in a general way the various methods of installation may be divided into classes as follows:
Vertical plates, which are set directly in the sash in place of the ordinary window glass. These are commonly used for the transom lights of store windows and for the upper sash of double-hung windows, or may fill the entire window.
Canopies, which are external prism plates in independent frames placed over a window opening and set at an angle to the vertical, a position similar to that of an ordinary awning.
Pavement prisms, which are set in iron frames in the pavement or sidewalk, in place of the ordinary bull's-eye light. In connection with the pavement prisms, when a well-lighted basement is desired, there is used a vertical plate of prisms, called the Lucidux, which is hung below and opposite the pavement light. This Lucidux receives the light from the pavement prisms, and again changing its direction, projects it horizontally into the basement. (This feature is illustrated by Fig- 525)
The canopies may be made either stationary or adjustable and may be employed in a variety of ways, combining the useful with the ornamental. The Foriluxes are susceptible of many highly decorative effects. In both the vertical plates and Foriluxes, the prisms may be arranged to produce ornamental effects, and by the use of the "Iridian" product a design may be inwrought upon the face of the prism plate to correspond with the design worked into the surfaces of the building and with the style of the entire front. The prism plates weigh no more and often less than plate glass of the same size, while they are much stronger to resist wind pressure, the action of hail and the impact of flying fragments. Although approaching perfection in the transmission of light, the prism plates are not transparent in the ordinary sense, and thus may be used as a screen to hide an unattractive view, or to prevent persons looking either in or out of the window, while at the same time the maximum quantity of light is admitted, The prism plates, owing to the stiff, durable manner in which they are united by the electro-glazing process, are also used for fire protection as a substitute for the ordinary iron fire shutter. The copper glazing forms, as it were, a continuous rivet, which holds the individual prism lights together even after they have become badly cracked by the action of fire and water.
The details of Luxfer Prisms are too complicated to be set forth in a few pages, but they are well described in a handbook published by the company. From a commercial point of view the special advantages of this system of interior lighting are manifold. It transforms rooms, particularly basements, otherwise too dark for occupancy, into income-producing space; in many buildings avoids the use of light shafts, thus saving a large amount of valuable floor space; and in all large or deep rooms effects a great saving in artificial lighting. Once installed, there is no cost for maintenance. The extent to which these prisms have been used by architects, on both new and old buildings, shows that they are to have a decided influence upon commercial architecture.