Light work
By Dick Marlor
February 1999

Flying across the star-filled skies, the only evidence of life below was from the lights of widely scattered farmhouses. The drone of the plane, lateness of night, and lack of any view were enough to put me into dreamland. My senses cleared with what appeared to be the dawning of a new day. As I wiped sleep from my eyes, I realized that the distant yellow glow was not of a new day, but that of my destination, Chicago.

We take electric lighting and the ways it has shaped our civilization virtually for granted. It has the power to change night into day. Good street lighting has even been shown to reduce city crime rates, and this is one of the major applications of the high pressure sodium (HPS) lamp. Most major cities today are illuminated with these. Their trademark yellow glow comes from the electronic transition of atomic sodium within the electric discharge (plasma).

Their efficiency, that is the amount of light a unit of electricity generates, is nearly ten times better than ordinary incandescent lamps, and about twice that of mercury lamps, which they have widely replaced. They produce a lot of light, up to 126,000 lumens for a 1,000 watt lamp, and at low cost. Where color rendition is important, as it is in many industrial and commercial applications, metalarc lamps are preferred. Again, a lot of light at low cost.

The outer bulb for all these 'high intensity discharge' (HID) products demands the superior physical and chemical properties of borosilicate glass. This has been true since the 1930s, when the first mercury lamp was introduced. Pyrex*, which was patented by Corning in 1915 and was widely used, had many of the properties important to a hot operating lamp, except that it would not make an acceptable seal to the wire electrodes. However, a leaded borosilicate glass, Corning 7720 (Nonex), did. It makes excellent seals to tungsten wire, and has all the superior thermal endurance properties of Pyrex. Because of the lead in Nonex, arsenic oxide had to be used to remove the bubbles ('fining') from the viscous melt.

For many years, Osram Sylvania Inc. manufactured a similar composition (SG772) to that of the 1930s Corning glass. But with continued, stringent regulatory issues concerning the use of heavy metals in glass manufacturing, the days of melting a lead/arsenic composition were obviously numbered. In order for any new glass to be an acceptable replacement, it had to fit all the existing glass and lamp manufacturing processes without loss of productivity. This was especially true since this glass had been used throughout the world for so long to make lamps with equipment 'tuned' to the properties of SG772.

Any new glass had to withstand high bulb wall temperatures (up to about 500 degrees C). It had to absorb harmful ultraviolet radiation emitted from the HID arc tube, and be a thermal expansion match with tungsten wire. The wire to glass seal had to stay good during an extended (typically 20,000 hours) lamp life. In short, the new glass had to be 'transparent' to all manufacturers' processes and optically transparent throughout lamp life, as UV will darken most glasses.

Glass melting, forming, and reworking characteristics (viscosity) had to be the same as lead glass to satisfy customer requirements. High boron content controls viscosity effectively, but this can also produce process difficulties. Boron is the most volatile constituent in a borosilicate glass, especially at the high temperatures (1,650 degrees C) required for melting. Typical melting problems, such as 'surface cord' (the appearance of highly siliceous, off-composition streaks of glass with a different refractive index), and melt devitrification (the formation of crystals), can result if volatility is not kept under control.

Recently, a lead and arsenic-free glass, SG773, has been introduced by Osram Sylvania Inc. This new glass is a true 'transparent' replacement for the older formulation, with boron volatility stabilized by the addition of alumina. In fact, the new glass has far less volatility at 1,650 degrees C than the old SG772 lead borosilicate. For certain very high wall loaded bulbs, where there is a high wattage to bulb surface area and temperatures can reach in excess of 500 degrees C, the new glass has also been shown to resist phase separation (opacity). So, even with a very high boron content (15.5 percent by weight B2O3) glass, SG773 can be used in applications where the older glass could not perform.

In 1962, when astronaut John Glenn soared above the Earth in his Mercury capsule, Perth in Australia volunteered to 'turn on the lights' so that Glenn could see Earth at night. Then, most city lighting was predominantly from mercury lamps, which give a bluish-white light. Just recently, when Glenn revisited space 37 years after his first flight, Perth's lights were on again, but this time he saw a yellowish-white light. Even from space, the importance of energy efficient light had become visibly evident. Indeed, achievements in high intensity discharge lighting would be in the dark ages if not for the contributions of boron in glass.

Dick Marlor is team leader of glass technology staff scientists at Osram Sylvania Inc. of Beverley, Massachusetts, U.S.

* Pyrex is a registered trademark of Corning, Inc.

Borosilicate glass benefits

• Resistant to thermal shock
• High aqueous and chemical durability
• High mechanical strength
• Lower glass melting temperature
• Inhibition of devitrification during manufacture