EP0125062A2

EP0125062A2 - Lamp envelope with improved resistance to solarization and lamps made therefrom

Info

Publication number: EP0125062A2
Application number: EP84302721A
Authority: EP
Inventors: Paulette I.K. Onorato; Michael N. Alexander
Original assignee: GTE Laboratories Inc
Current assignee: Verizon Laboratories Inc
Priority date: 1983-04-22
Filing date: 1984-04-19
Publication date: 1984-11-14
Also published as: EP0125062A3

Abstract

A lamp envelope is fabricated from a soda lime silicate glass containing an effective amount of cerium oxide or a compound thermally decomposable to cerium oxide attemp- eratures below about 1450°Cto inhibit solarization of the glass under the influence of ultraviolet radiation having a wavelength equal to or less than 254 nanometers and lamps made from same.

Description

FIELD OF THE INVENTION

This invention relates to lamp envelopes being resistant to solarization. More particularly, it is concerned with fluorescent lamp envelopes being resistant to solarization.

BACKGROUND OF THE INVENTION

When fluorescent lamps are operated, a portion of the ultraviolet radiation produced by the excitation of the mercury fill is absorbed by the glass making up the envelope of the lamp. As a result of the absorption of ultraviolet radiation, both permanent and transient color centers develop in the glass which tend to decrease the light transmittance of the glass envelope over time. The gradual darkening of the glass as a result of exposure to sunlight or ultraviolet radiation is termed solarization.
Although the loss in light transmittance of the glass envelope of a fluorescent lamp may be only a few percent or even a fraction of one percent, such losses contribute to the loss in lumen output of the lamp over time and are thus highly undesirable.
In current glass formulations employed in fluorescent lamp fabrication, small amounts of antimony oxide or titanium dioxide are frequently employed to inhibit solarization. The disadvantages of using antimony oxide to decrease lamp glass solarization lie in its high cost and toxicity. Titanium dioxide, on the other hand, while less expensive per pound than antimony oxide, tends to change the devitrification and melting properties of glass compositions to which it is added.

OBJECTS AND SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a lamp envelope which exhibits improved resistance to solarization due to ultraviolet irradiation.
It is another object of this invention to provide a fluorescent lamp which exhibits improved brightness and lumen maintenance.
It is yet another object of the present invention to provide a means for enhancing the solarization resistance of soda lime silicate glass fluorescent tube envelopes which eliminates the need for the use of costly and potentially hazardous toxic materials.
These and other and further objects and advantages are achieved in accordance with the present invention wherein there is provided a lamp envelope comprising a soda lime silicate glass composition containing an effective amount of a cerium compound to inhibit solarization of the glass by ultraviolet radiation.

DETAILED DESCRIPTION

The mechanism of solarization effects in soda lime silicate glasses is not completely understood, but absorption of light by solarized glasses is attributed to hole traps and electron traps produced by the ultraviolet radiation. The addition to such glasses of an oxide of a metal capable of exhibiting multiple valence states tends to counter these effects. While it has been the practice in the past to add antimony oxide to soda lime silicate glasses employed in fluorescent tube manufacture, it has been found in accordance with this invention that cerium oxide is more effective for this purpose, is less costly, and eliminates the problem of handling the toxic antimony compound.
While not espousing one theory to the exclusion of others, it is believed that the decrease in uv-induced visible absorption (solarization) of soda lime silicate glasses which is observed when cerium replaces antimony as the dopant is due to the ability of Ce ⁺³ to capture radiation-induced holes and of Ce ⁺⁴ to absorb less visible light than the intrinsic hole traps.
It is preferred that a cerium compound be added to the soda lime silicate glass used in the fabrication of fluorescent lamp envelopes in an amount less than 1.0 weight percent, more preferably in an amount less than 0.2 weight percent and most preferably in an amount of about 0.1 weight percent. The cerium compound is added either in the form of cerium oxide or a compound of cerium thermally decomposable to cerium oxide at the temperatures employed for melting of the glass batch, i.e. at about 1450°C or below.
A typical fluorescent lamp comprises a tubular glass envelope having electrodes, an inert ionizable gas and a charge of mercury therein, and the inside surface of the tubular glass envelope is coated with a phosphor.

Examples

The following examples are provided to enable one skilled in the art to practice the invention. The examples are merely illustrative of the invention, and are not to be construed as limiting the scope of the invention which is defined by the appended claims.
Glass samples having compositions given in Table 1 were prepared from ultra-pure (i.e. 99.999% pure) materials.
Portions of the basic glass batch formulations given in Table 1 were doped with various concentrations of iron (a component found in most commercial soda lime silicate glasses), added to the glass formulations as ferric citrate. To other portions of the laboratory glass batches of Table 1 were added dopant levels of CeO₂, As₂O₃, V₂O₅, PbO₂ and Ti0₂. The dopant ions were added by dissolving their oxides in acid, typically nitric acid, and adding an appropriate amount of the solution to the glass batch.
The resulting batch in each case was mixed for four hours, melted at about 1450°C for four hours in a quartz crucible, transferred to an annealing oven at 560°C, and cooled slowly to room temperature. The crucible was then broken away from the glass which had formed, and prisms were cut and polished from the crude glass sample.
Solarization of each glass composition was measured by passing a light beam, chopped at 338 Hz, from a helium-neon laser through the glass prism and detecting the intensity of the light beam at 633 nm wavelength after passage through the sample. A beam splitter, placed in the path of the light beam prior to its passage through the sample permitted double beam detection to compensate for any variation in the initial beam intensity. The sample was irradiated by means of a mercury lamp that emitted 254 nm and 185 nm wavelengths. The intensity of this radiation impinging on the glass samples was made similar to that which impinges on the inside of the glass envelope of a 40 watt T12 fluorescent lamp.

Example 1

The solarization behavior under 254 nm and 185 nm irradiation of soda lime silicate glass samples containing 400 ppm iron as the only dopant, measured at 633 nm wavelength, appear in Table 2.
The data of Table 2 indicate that typical commercial soda lime silicate glass compositions containing about 400 ppm iron as a contaminant, exhibit a monotonically increasing solarization under 254 nm and 185 nm ultraviolet irradiation, as described above, which approaches about 1.3-1.5% absorbance at 633 nm wavelength after about 2 hours. This solarization results in a corresponding decrease in the overall output of a fluorescent lamp fabricated with a glass envelope having such glass compositions.

Example 2

A sample of soda lime silicate glass containing 0.10 weight percent CeO₂ was prepared by the method detailed above. The sample was subjected to 254 nm and 185 nm ultraviolet irradiation, as described above, and the solarization (change in absorbance) was measured at various visible wavelengths. The data appear in Table 3.
The data of Table 3 illustrate that the addition of small amounts of cerium to soda lime silicate glasses greatly increases the resistance of such glasses to solarization effects caused by exposure to the short wavelength ultraviolet light. This improvement has been found to exist across the visible spectrum.
Arsenic (III) oxide was found to be not nearly as effective as Sb₂0₃ as an anti-solarization agent in these glasses. Lead oxide additions inhibited solarization, but not as well as either cerium oxide or antimony oxide additions. Vanadium oxide was found to be quite effective as an anti-solarization agent, but tended to impart an undesirable color to the glass. As mentioned above, titanium dioxide was found to be an effective anti-solarization agent, but in the amounts required, tends to adversely affect the devitrification and melting properties of the glass.

Example 3

In a third example, the behavior of soda lime silicate glasses doped with about 0.1 weight percent cerium oxide in a typical fluorescent lamp was determined. The spectral absorption curve of a glass sample which had been exposed to the 254 nm and 185 nm, as described above, ultraviolet light for two hours was convoluted with the spectrum of a typical Cool White halophosphate phosphor activated with antimony and manganese and with the eye sensitivity curve. The resultant curve was corrected for the path length of a Lambertian source on a 0.032 inch thick glass (typical fluorescent lamp wall thickness) to give the predicted lumen loss for a typical 40 watt T12 fluorescent lamp. According to these results, the replacement of 0.17 weight percent Sb20₃ presently used commercially in soda lime silicate fluorescent lamp envelopes with 0.10 weight percent Ce0₂, produces in a 40 watt T12 lamp a 20 lumen increase in light output. Other fluorescent lamps that have higher uv irradiation fluxes (intensities per unit area) at the envelope surface, such as T8, HO, and VHO types, should be expected to exhibit equal or greater improvement in solarization resistance (compared to alternative glass formulations) than that illustrated in Example 3 for a 40 watt T12 lamp.
Thus, in accordance with this invention, the fabrication of fluorescent lamp envelopes with a soda lime silicate glass formulation, doped with about 0.1 weight percent cerium oxide, results in lamps having higher light output and greater resistance to output losses due to solarization effects. Moreover, this result is achieved with the accompanying elimination of a costly and toxic ingredient presently widely employed commercially in fluorescent lamps.
Other types of lamps which generate uv irradiation fluxes at their envelope surface should also be expected to exhibit improvement in solarization resistance by using the cerium doped glass as discribed above. Such lamps would be electrodeless discharge lamps as described in U.S. Patent No. 4,266,166 and 4,266,167 to Proud et al., electric discharge lamps having positive resistance characteristic, (e.g. as discribed in U.S. Patent No. 1,901,128) and lamps containing an ultraviolet generating medium having no mercury therein.
there have been shown and described what are at present believed to be the preferred embodiments of the present invention, it will be obvious to those skilled in the art to which this invention pertains that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

Claims

1. A lamp envelope comprising a soda lime silicate glass containing an effective amount of a cerium compound to inhibit solarization of said soda lime silicate glass by ultraviolet radiation.

2. A lamp envelope in accordance with Claim 1, wherein said soda lime silicate glass contains an effective amount of a cerium compound to inhibit solarization of said soda lime silicate glass by ultraviolet radiation having a wavelength equal to or less than 254 nanometers.

3. A lamp envelope in accordance with Claim 1, wherein said cerium compound is thermally convertible to cerium oxide at a temperature of below about 1450°C.

4. A lamp envelope in accordance with Claim 1, wherein said cerium compound is cerium oxide.

5. A lamp envelope in accordance with Claim 4, wherein said cerium oxide is present in said soda lime silicate glass in an amount of less than about 1.0 weight percent.

6. An electrodeless discharge lamp having a lamp envelope according to Claim 1.

7. An electric discharge lamp having a lamp envelope according to Claim 1.

8. A lamp having a lamp envelope according to Claim 1, wherein said lamp contains an ultraviolet generating medium having no mercury therein.

9. A fluorescent lamp comprising a tubular glass envelope being a lamp envelope as claimed in any one of Claims 1 - 5, having electrodes, an inert ionizable gas and a charge of mercury therein and a coating of phosphor on the inside surface of said tubular glass envelope.