Light Yesterday and Tomorrow Page 86

to continue indefinitely. But they will change! Evolution is rife in the lamp industry.

Looking ahead into the next half century, lighting scientists can justifiably expect progress to continue unabated because man’s need for more and better light has not yet been satisfied. No one is convinced that present methods of generating light are more than early steps in a long journey!

Incandescent Illuminants

The first question might be — what about advancements with metallic wire or incandescent illuminants? The answer seems fairly definite. A filament material superior to tungsten is unlikely (all the available known elements have been investigated), and there is but little prospect for better gases to surround it excepting krypton or xenon, both of which are inherently expensive. These rare gases exist in the atmosphere in minute percentages and are difficult to segregate. True, they can increase incandescent lamp efficiencies some 10 percent; but so long as a liter of krypton costs about ten dollars, and xenon five times more, the use of these lethargic gases as heat blankets will likely be confined to the smaller bulbs — such as miner’s lamps — and only where highest efficiency is paramount. Mechanical and metallurgical perfections will undoubtedly continue to produce superior filament ruggedness, smaller and stronger bulbs and more pleasing surface finishes, but withal, the present-day efficiency of the tungsten lamp could at most be increased by a small percentage. The three-hour photoflood lamp leads this class, at 36 lumens per watt. The ultimate ceiling is set at the softening temperature of tungsten, just short of melting, and certainly the efficiency of this illuminant will never exceed 50 lumens per watt, because molten tungsten emits but 52.

Nevertheless, there is no evidence of any major decline in the usage of this convenient and versatile source. Curiously enough, the advent of the fluorescent lamp ten years ago did not decrease the consumption of incandescent lamps. For several years to come, the tungsten filament lamp will likely represent more than 75 percent of the illuminants in general lighting service. Today, in numbers, the sales of fluorescent lamps represent about 10 percent those of the incandescent lamps, but the former are exceeding this percentage.

Metallic-Vapor Sources

High-pressure mercury-vapor lamps have a hopeful future because their efficiencies are already well in excess of 50 lumens per watt and may reach 100 — perhaps more — depending on the discovery of improved methods of handling the high temperatures (1000 degrees C) and pressures (10 atmospheres) of the mercury arc. Water cooling seems a cumbersome expedient, so air cooling will likely continue. Present objectives are not primarily to increase efficiencies. Rather they are concerned with improved methods of shortening and condensing the arc to obtain better focusing and superior projection control, since neither lenses nor reflectors can efficiently handle the light from a large area. Also the shorter the arc, the brighter it becomes. Other objectives include improving the color of the mercury arc, obtaining a better balance between useful life and lamp cost, and making the lamp capable of instant restarting after a current interruption. Today’s mercury lamps have to cool for five to ten minutes before they will re-start, but methods are now known of overcoming this annoyance, and will in certain cases be put into practice.

The discovery of a bulb material more refractory than clear quartz would be a long step toward better high-pressure mercury lamps. Another giant advance would be a means of utilizing the large amount of invisible ultraviolet radiation generated by the typical mercury arc. This brings into focus the matter of some type of fluorescent glassware or enclosure, or perhaps a fluorescent paint or other material incorporated into the surfaces illuminated. The object would be to change the useless, short-wave radiations to longer wave quality, thus increasing the usable illumination. For example, in the case of mercury lighting for highways, fluorescent markers and safety painting could be used, or even fluorescent dye material sprayed on the gravel of the roadway edges.

Other types of electric arcs, especially in small wattages, are being carefully studied. One is the pin-point zirconium arc. However, high-pressure vapor lamps are not likely to enter the fields of store or home lighting because these places cannot conveniently use great volumes of light in a single package. Much remains to be learned about photochemical uses of these illuminants and herein is an intriguing, undeveloped field.

Fluorescent Illuminants

Today’s fluorescent lamp has come a long way in ten years, and bids fair to continue its meteoric career, growing in efficiency, variety of color, useful life, and in simplified methods of operation. With the exception of the extremely large and extremely small lamp sizes, it may be able to do most of the world’s lighting jobs better than any other known lamp. Today’s efficiencies of about 60 lumens per watt may be doubled in the next decade, this through improved phosphors to provide a better transformation of ultraviolet to visible light, coupled with gradual betterment of the inert gas mixtures within the tubes, and perfected electrodes. Superimposed upon all physical improvements is the gain that would result from using alternating current at frequencies higher than 60 cycles. For instance, going from the conventional 60 to 400 cycles would increase the luminous output roughly 20 percent, and halve the ballast size. Unfortunately, there is the matter of high-frequency generation and distribution.

Conceivably, other methods of generating short-wave radiant energy may compete with the present method of an electric discharge through low-pressure mercury vapor, but phosphor responses must be geared to the wavelengths generated, so the task is a lengthy one. Tomorrow’s research may clarify the “one-way traffic” movement or down-hill procedure of