US2545896A

US2545896A - Electric lamp, light diffusing coating therefor and method of manufacture

Info

Publication number: US2545896A
Application number: US878A
Authority: US
Inventors: Pipkin Marvin
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1947-02-15
Filing date: 1948-01-07
Publication date: 1951-03-20
Anticipated expiration: 1968-03-20

Description

March 20, 1951 M. PlPKlN 2,545,895

ELECTRIC LAMP, LIGHT DIFFUSING COATING THEREFOR AND METHOD OF MANUFACTURE. Flled Jan '7, 1948 4 Sheets-Sheet 1 i MM H InvenTor: Marvin Pipkin,

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PERCENT OFJNTENS/TV March 20, 1951 M PlPKlN 2,545,896

ELECTRIC LAMP, LIGHT DIFFUSING COATING THEREFOR AND METHOD OF MANUFACTURE Filed Jan. 7, 1948 4 Sheets-Sheet 2 50. CM. Q, Q Q

CANDLES PER C VERTICAL DISTANCE FROM T/P OF BULB //V CM.

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March 20, 1951 M. PIPKIN 2,545,896

ELECTRIC LAMP, LIGHT DIFFUSING COATING THEREFOR AND METHOD OF MANUFACTURE Flled Jan 7, 1948 4 Sheets-Sheet 3 lnven bor: Marvin Pipkin,

His A t torneg.

March 20, 1951 M. PIPKIN 2,545,896

ELECTRIC LAMP, LIGHT DIFFUSING COATING THEREFOR AND METHOD OF MANUFACTURE Filed Jan. '7, 1948 4 Sheets-Sheet 4 lnvn tovz Marvin Pipkin,

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Patented Mar. 20, 1951 ELECTRIC LAMP, LIGHT DIFFUSING COAT- ING THEREFOR AND METHOD OF MANU- FACTURE Marvin Pipkin, Cleveland Heights, Ohio, assignor to General Electric Company, a corporation of New York Application January 7, 1948, Serial No. 878

32 Claims. 1

My invention relates to incandescent electric lamps and diffusing electric lamp bulbs or enclosures, and methods of producing the same.

This application is a continuation-in-part of my copending patent application Serial No. 728,769, filed February 15, 1947, now abandoned and which is assigned to the assignee of this application.

Light-diffusing bulbs have been produced heretofore by using opal glass, that is, glass which is rendered opalescent by the addition of fluorides or phosphates thereto during the manufacture thereof. However, such a glass is expensive, and the light absorption thereof very high, amounting to something like 25 or 30% in bulbs having sufficiently high diffusion to effectively mask the filament or other light source. Moreover, the light absorption is not uniform due to the uneven thickness of the bulb, and this results in the light emitted from the lamp being of quite low and unequal intensity. Such bulbs have therefore had very limited use.

Light-diffusion has also been obtained by enamel coatings on the exterior surfaces of the bulbs. However, such coatings are rather expensive and they also absorb a considerable amount of the light emitted from the lamp, amounting to more than even for white coatings. Moreover, the exterior coatin tends to collect dirt easily. On the other hand, the

application of such coatings to the interior surface of the bulbs has also failed to be accepted generally because of high absorption as well as the tendency to thereby introduce impurities which are injurious to the lamp. Such bulbs have therefore had only a limited'use.

It has also been proposed heretofore to coat the inside surface of electric lamp bulbs by burning metals, and silicon, in powdered form outside the bulbs and then forcing the particles into the bulb where the particles were dusted on the inside surface thereof. However, these oxides did not produce efiicient or satisfactory coatings, and have not been put to any practical use in lamp manufacture due to the fact that the kind or state of the material used, and the method of application or burning, were not such as to make possible control of the particle size of the coating, and would not permit uniformity or adherence of the coating to the extent where such coatings could be employed in a practical electric lamp. Moreover, when it was attempted to burn silicon, it was found to burn very violently, producing sputtering and even explosive flashing, which characteristics make it extremely diflicult, if not practically unusable, because of the incomplete combustion of the silicon. The collected particles were not of proper form, size, configuration, and the incompletely burned flocculent particles of crystaline silicon were not sufficiently adherent and were black, grey or brown in color, which factors prevented use of such a coating as a light diffusing medium in an electric lamp.

The most widely used diffusing bulb has been that in which the inside surface was frosted by etching. That process is quite inexpensive, and the bulb has relatively high diffusion with very low absorption of the light fromthe lamp. Notwithstanding the wide use and general acceptability of this type of diffusing medium there has been evidenced a need for improved and even more efficient light-diffusing moans.

One of the objects of the present invention is to produce lamp bulbs and other devices or enclosures having very high difiusion with minimum light absorption. Actually, I am able to obtain lamp bulbs with very high diffusion and, apparently very small or no measurable absorption, which is indeed surprising and contrary to what has been experienced heretofore with diffusing lamp bulbs.

Another object is to provide a lamp bulb having on its interior surface a superficial lightdiifusing coating. Another object is to provide a simple and inexpensive method of applying such a coating.

Another object is to provide an internal lightdiffusing coating which is not injurious to the lamp but, as a matter of fact, is actually beneficial.

A further object is to provide a previously etched bulb or other surface with a coating which will make the bulb very highly diffusing with virtually no absorption of light due to the added coating.

A still further object is to provide a highly efficient and highly diffusing coating for illuminating ware generally, such as lighting fixtures.

It is. a still further object of my invention to provide a new and improved incandescent electric lamp which not only provides a highly efficient light diffusing coatin which substantially completely masks the incandescent filament but which also makes it possible to operate the filament at a higher temperature, by virtue of the use of a gaseous atmosphere consisting essentially or exclusively of argon, krypton or xenon, or mixtures thereof, thereby obtaining a higher lumen output and reducing or completely compensating for the slight loss due to the diffusing coating.

Further objects and advantages of my invention will appear from the following description of species thereof and from the drawing.

In the drawing, Fig. 1 is an elevation of one species of apparatus which may be used to coat lamp bulbs in accordance with my invention; Fig. 2 is an elevation in section, of a modified burner; Fig. 3 is an elevation of a further modification of coating apparatus; Fig. 4 is an elevation, in section, of the burner portion of the apparatus shown in Fig. 3; Fig. 5 is an elevation, partly in section, of an incandescent lamp having a bulb coated in accordance with my invention; Figs. 6 and 7 are graphs showing certain characteristics of lamps comprising this invention; Fig. 8 is an elevation of another form of lamp embodying my invention; Fig. 9 is a somewhat diagrammatic elevation, partly in section, of a lamp fixture embodying my invention; and Fig. 10 is a "size-frequency of occurrence" curve which is approximately representative of some coatings made in accordance with my invention. Figs. 11 and 12 are reproductions of actual electron micrographs of silica coatings made in accordance with my invention and the specimens for which were prepared from inside frosted surfaces of electric lamp bulbs. Fig. 13 is a reproduction of an actual electron photo-micrograph showing the particle arrangement of a silica coating produced in accordance with my invention and which was collected directly on a resin film in order to facilitate observation under an electron microscope and without necessitating removal from a glass surface forobservation under an electron microscope.

Generally speaking in accordance with my invention, I provide a highly efiicient radiation or light diffusing coating. For example, this coating may be placed on alight transmitting body or light diifusing body, and comprises finely divided particles of silica formed by the combustion of an inflammable silicon compound. In the combustion of such compounds, there may result gaseous products of organic materials, or a smoke or fume, such as a suspension in a gas of particles of silica which are ultimately collected by or deposited .on the body to be coated and are adherent thereto. The above objectives are attained by the combustion of an inflammable silicon compound which will leave a deposit of extremely fine particles of substantially pure silica adherent or fused to the inner surface of the bulb, or other object to be coated.

The coating density, or the weight of the coating in milligrams per square inch, will of course vary according to the nature and condition of the surface or material to be coated, and according to the light-diffusing characteristics desired. Furthermore, the density of the coating to obtain specified light-diifusing characteristics may be related to the size of the silica particles to obtain benefit-of both maximum diffusion and minimum absorption. I have found that there is an optimum range of coating densities (which of course establishes coating thicknesses) and that there is an optimum range of particle size in order to obtain maximum diffusion with minimum or negligible absorption.

In addition, I have found in the coating of glass or vitreous articles such as clear glass and inside-frosted electric lamp bulbs that very high or maximum light diffusion with minimum absorption is obtained by using coating densities (that is, milligrams of silica per square inch) preferably ranging from a fraction of a milligram, such as milligram, to several milligrams per square inch, although coating densities ranging to about 20 milligrams per square inch may be beneficially employed depending upon the nature of the material to be coated and the lightdiifusing characteristics desired.

Moreover, in connection with particle size of the silica, it has been found that optimum lightdifiusion with minimum or negligible absorption may be obtained by using silica particles the dimension or diameter of which is a fraction of a micron and preferably wherein the average diameter of the particles, as hereinafter defined, is in the range of about to micron, the individual particles ranging in size from about 0.9 micron to 30 angstrom units and less. It will be understood that one micron is equal to 10,000 angstrom units. For example, I have made electric lamps provided with silica coatings in which satisfactory light-diffusion and minimum absorption are obtained and wherein the individual particle sizes range from less than 30 angstrom units to about 7150 angstrom units.

The average diameter referred to herein is the average particle size with respect to surface which, for a given sample, is the diameter of the uniformly sized particles which would give the same total surface per unit volume of sample. This average diameter is calculated as where n is the number of particles in any given size class and d is the diameter representing that class. Some lamps made in accordance with my invention show a size-frequency of occurrence curve of the type approximating that shown in Fig. 10 and which is derived by plotting the number of particles in any given size class against the diametcr representing that class. For those desir-' ing further detailed information relative to the above method of analyzing and studying the frequency of occurrence-particle size characteristics of microscopic materials, reference may be had to the Handbook of Chemical Microscopy. by Chamot and Mason, vol. I, second edition, published 1938 by John Wiley & Sons, Inc., particularly pages 416-419 thereof. The numerical average diameter or arithmetical mean diameter of the particles is, of course, less than the above defined average diameter (113) with respect to surface. In coatings made in accordance with my invention from investigations and studies the coatings are indicated as comprising particles the numerical preponderance of which have a dimension or size less than the shortest wavelength of light or visible radiation, that is less than about 4000 angstrom units.

Electron-microscope investigations have been made to establish and determine the particle sizes of coatings which afford maximum lightdifi'usion with minimum or negligible absorption. These and further studies have furnished additional information as to the configuration and state of the silica particles. Apparently the silica particles are rounded-or in the form of substantially perfect spheres, and the silica is in the non-crystalline or amorphous state.

The limit of resolution of the electron microscope employed in making these studies was 30 angstrom units, indicating that there are many particles of silica having a size less than this dimension. Moreover, a study of the electron microscope pictures taken of the silica coating indicate that there is a very thin layer of very small particles directly adherent and fused to the glass bulb, and that this under-layer cannot be easily rubbed off or removed, although the superimposed or upper layers can be removed by rubbing or abrasive action. In this manner it will be appreciated that so far as the silica coating is concerned it may be considered to be partially fused or fritted to a supporting body such as an incandescent lamp bulb being coated. The

underlayer, in itself, affords appreciable diffusion properties.

I have found that I can obtain degrees of diffusion such that a clear-glass bulb lamp coated with silica in accordance with my invention'may have a maximum brightness of the order of a fraction of a per cent to a few per cent of that of an otherwise similar lamp having no coating, while an inside-frosted bulb lamp coated in accordance with my invention may have a maximum brightness of the order of a small fraction of a per cent of that of an otherwise similar clear-bulb lamp.

I have also found that, in coating light transmitting bodies and light-diffusing bodies, I may control the general light diffusing characteristics of a silica coating by controlling the particle size. Among the factors which may be considered effective in establishing the particle size to obtain desired light-diffusing characteristics of the coating, I have found that the following are of effect: concentration of the silicon compound, temperature of the flame or the region immediately surrounding the flame, the smoke or fume density, the rate of cooling of the gaseous combustion products or the smoke, and the concentration of oxygen or the oxygenous concentration if a combustion supporting gas is employed.

It has been appreciated for many years that it is desirable to operate a filament of an incandescent lamp in a gas to retard evaporation of the filament and to increase the filament and lamp life. However, the gas so used, by virtue of its thermal conductivity, and energy loss due to the thermal convection losses, results in a reduced filament operating temperature.

Of course, it has also been appreciated heretofore that the inert gases are highly desirable as fillings for incandescent lamps. Moreover, nitrogen has been employed as a filling gas. Furthermore, it has been the practice for many years to use as a filling gas various mixtures of inert gases, such as argon, with nitrogen. However, it has been usual practice in the manufacture of electric lamps to use in all such mixtures a relatively large percentage of nitrogen in order that the mixture have a sufficiently high breakdown voltage characteristic to prevent the establishment of an electric discharge or breakdown across the filament terminals inside the bulb, or across any portion of the filament. While nitrogen does have a breakdown voltage greater than the inert gases, it has a greater thermal conductivity thereby resulting in substantial conduction and convection losses with the consequent reduction in luminous efilciency.

The silica coating which I provide is not only a very efficient light diffusing medium but is also a means instrumental for recovering any. loss in lamp efliciency occasioned by the light diffusing function of the coating, by making it possible to operate the incandescent filament at a higher temperature without increasing the probability of voltage breakdown across the filament terminals. A very high percentage of the silica particles of the above described silica coating are directly and self-adherent to the interior surface of the lamp bulb. By virtue of its adherence and its finely divided nature, the silica coating serves as a contamination prevention means by covering the glass bulb interior surface thereby holding any impurities, such as free caustic soda or carbonates which are always present in lime glasses used in lamp manufacture, and which may otherwise be present on the inside of the glass bulb and which upon dislodgment tend to cause a voltage breakdown across the filament terminals by constituting an undesirable impurity in the gas filling. An inert gas having an atomic weight greater than 39, such as that of the group consisting of argon, krypton and xenon, or mixtures thereof, may be used substantially exclusively as, or as a very high percentage of, the gas filling thereby taking advantage of the lower thermal conductivities of these gases as well as their greater retarding effect on thermal evaporation, and thereby obtaining higher operating filament temperature and lamp efficiency,

without causing arc discharges within the lamp bulb due to voltage breakdown even under rough usage or lamp handling. If an attempt is made to lower the nitrogen content of present day lamps, it is found that such lamps under rough usage tend to form are discharges due to dislodgment of impurities from the bulb wall. Where it is desired to raise the breakdown voltage of a gas filling to a voltage greater than that of the inert gas or mixtures alone, a small percentage of nitrogen, not exceeding 5 per cent by volume, may be used. For example, I may use a gas filling consisting of about 98 per cent argon and 2 per cent nitrogen, by volume, in an incandescent lamp having a glass envelope provided with a silica coating on the interior surface thereof, thereby providing improved light diffusion without any loss .in lamp efficiency.

In coating a bulb or other illuminating ware, I oxidize any of certain volatile or gaseous silicon compounds by burning, thereby breaking down the molecule and forming a fume of silica. These fumes are collected on the inner surface of the bulb, for example, where they form a selfadherent coating of very pure finely divided silica which contains nothing of a harmful nature, not even water of hydration, that might adversely affect the performance of the lamp. Although the coating may be rubbed off with the fingers it is more than sufficiently adherent to Withstand the efiect of jarring or other rough handling, thereby dispensing with the need for a binder of any kind which might introduce harmful impurities into the lamp. However, the adherence of the coating may be increased, if desired, by steaming it, i. e., exposing it to a flow of steam for a brief time. 4

I prefer to use silicon compounds Whose molecules contain no atoms other than silicon, carbon, hydrogen and oxygen. The presently preferred material is ethyl orthosilicate (C2H5)4Si04, which is an oily liquid. Other compounds include silicomethane, silicoethane, silicopropane, silicobutane, silicopentane, disiloxane, methyl silicate, methyl silicane, ethyl silicane, dimethyl silicane, diethyl silicane, tetramethyl silicane, ethoxytriethyl silicane and the like. These are given merely as examples and not by way of limitation, since many other materials might be used. However, in general it is advisable to avoid the use of materials which contain too much 7 organic matter since these might cause a deposit of carbon to be formed.

Certain gaseous compounds such as silicon hydride or silicanes or silanes containing only silicon and hydrogen ignite when exposed to air or oxygen. Thus, in coating a bulb, a suitable burner can be placed inside the bulb and when such compound is released with oxygen flowing it will be ignited. A definite density of coating can be obtained by regulating the amount of such compound admitted.

One manner of carrying out the process on a lamp bulb is by impregnating any suitable form of wick with a volatilizable liquid compound such as ethyl silicate, and burning the compound inside the bulb in the presence of a stream of air or oxygen to insure continuing combustion and to carry the products of combustion other than silica out of thefbulb.

As applied to lamp bulbs, the coating may be sults, and even better ILSllltS are obtained when the coating is applied to a bulb which previously has had its inside surface frosted, preferably by etching as described in my Patent No. 1,687,510.

As illustrated in Fig. 1 of the drawing, the burner or wick may consist of a ball or wad I of glass thread wound on the end of a wire rod 2. The wad is dipped into the ethylsilicate, and the rod is then inserted in a tube 3 through which a stream of oxygenous gas is blowing. I prefer to use pure oxygen, rather than air, for example, in order to insure complete combustion and subdivision of the particles of smoke and thereby avoid deposits of flocculent silica. The wad is then lighted and the bulb 4 is placed over the burning wad and rotated on a hollow tubular chuck 5 which surrounds the tube 3 and terminates at its upper end in a thin, resiliently compressible collet portion which grips the bulb. A dense white smoke is emitted consisting of silicon dioxide (1. e., silica), water vapor and carbon dioxide, the silicon dioxide being deposited on the inner surface of the bulb in a smooth adherent film, and the water vapor and carbon dioxide being blown out of the bulb and downward through the interior of the chuck 5. The density of the coating may be controlled either by removing the bulb after a predetermined time or by initially applying to the wad l a predetermined amount of the ethyl silicate.

For a 100-watt incandescent lamp bulb having a volume of some 11 cubic inches and a surface area of about 28 square inches, a coating of satisfactory density may be obtained by applying about 11/ or 2 grams of ethyl silicate to the wad I and permitting it to burn until the flame has burned itself out before removing the bulb. A stream of oxygen flowing at the rate of about 100 cc. per second is satisfactory, with a speed of rotation of the bulb of about 70 R. P. M., for example.

In the apparatus illustrated in Fig. 1, the hollow chuck 5 is rotatably mounted on a bracket 6 and is driven from any suitable source of power through a belt I encircling a pulley 8 attached to the chuck. The tube 3 is adjustably supported by a clamp 9 extending from a suitable base II! which also carries the bracket 6. As illustrated, the tube 3 is preferably adjusted so that the wad is approximtaely at the center of the spherical portion of the bulb.

In Fig. 2, I have illustrated a modified set-up wherein the wad is replaced by a receptacle or cup II, of metal for example, having a nippled bottom. I2 set in a metal sleeve or cup I3 at the applied to a clear glass bulb with very good re- 8 upper end of a rod H which is supported in the tube 3 from any suitable stop or projection at the inside of the tube 3 or by having a portion thereof provided with an offset bend to frictionally engage the tube. The cup II contains any suitable absorbent or porous material, which may be a fine powder, for instance the residue of silica such as that deposited on the exterior of the tube 3 in carrying out this process. The ethyl silicate i poured into the cup and ignited as described above. With this arrangement it is possible to use less ethyl silicate, for example about /2 gram as compared with 2 grams in the case of the wad I.

In Figs. 3 and 4.1 have illustrated a further modification of the apparatus shown in Fig. 1. In this case the burner comprises a plurality of nested sheet metal cups I5 similar to that shown in Fig. 2 except that they are open at the bottom. The nippled bottom portions I6 thereby constitute a series of baffles. The lower-most cup I5 is fitted tightly into the upper end of a comparatively fine metal tube I! which extends through the interior of the oxygen supply tube 3. The tube or conduit II is connected at the bottom thereof to a tube I8 which communicates with an outlet I9 in the bottom of a constantlevel reservoir 20. The level of the inflammable liquid silicon compound 2| in the reservoir 20 is preferably above the top of the burner I5, and the fiow of liquid 2| to the burner I5 is regulated by any suitable means such as a valve 22 in the outlet I9 to provide a constant flow of the'liquid.

The oxygen is supplied to the tube 3 through a T fitting 23 which surrounds the tube I1 and which is connected to the lower end of tube 3 by a rubber nipple 24. The lower end of the T is closed by a rubber nipple 25 which surrounds the lower end of the T and a portion of the tube I1. The oxygen therefore flows around the tube I1 and within the inlet tube 3.

In the device shown in Fig. 3, the burner I5 operates continuously. The flow of oxygen around the burner I5 causes the liquid therein to burn with a comparatively short flame issuing from the upper cup I5, the size of the flame being controlled by the rate of flow of oxygen and ethyl silicate. A deposit of silica tends to build up on the top of the burner I5 in the form of a cone or cylinder. This deposit may be periodically removed in any convenient manner, as by slicing or cutting it oil". In coating bulbs of the 100- watt size the ethyl silicate 2| may be fed to the burner I5 at the rate, for example, of about 72' drops, or 3 00., per minute, with an oxygen flow of about cc. per second. Each of the bulbs 4 may be exposed to the silica fumes for a period of approximately 15 seconds.

The processes described herein are obviously adaptable to mass production on an automatic machine having a number of heads each corresponding to the arrangement shown in Figs. 1

or 3, for example, and mounted on a rotatable indexing turret or other movable carrier. It is merely necessary for an. operator to periodically replace previously impregnated wads I or cups II as the heads pass the operator's station.

In Fig. 5, I have shown a -watt incandescent lamp in which the bulb 4, either of clear glass or inside frosted, is provided on its inside surface with a coating of extremely finely divided silica indicated by the stippling 26. The filament and other internal parts of the lamp are representative of the ordinary commercial incandescent lamps. The filament 21 of coiled-coil tungsten wire type is mounted on and between a pair of leading-in

wires

28, 28 which extend through the conventional glass stem 29 fused to the neck of the bulb 4. The wires 28 extend to respective contacts on the conventional base 30. The bulb I may be filled with inert gas such as argon.

A remarkable feature of bulbs coated in accordance with my invention is that the thickness or density of the coating can be increased greatly and yet the illumination is extremely uniform, much more so than even in the case of bulbs of opal glass. Moreover, the coating has no adverse efl'ect on the strength of the bulb or the operation of the lamp, and the bulbs blacken to a lesser extent during the life of the lamp than clear or inside frosted bulbs.

The small amount of absorption is shown by tests on 120 volt, 100-watt incandescent lamps. The lamps in the various groups in the table, all were made under similar circumstances and incorporated the same physical features except as noted, and all used a gas filling of 88 per cent argon and 12 per cent nitrogen.

Table [Averages based on large numbers of lamps] 10 lumen per watt, or 4.9% for the silica coated inside frosted bulbs.

The area of the effective bulb surface (that is, above the plane S-S, Fig. 5, which represents the line along which the stem 28 is sealed to the bulb) is approximately 28 square inches, so that the average density of the coating of to 80 mg. total weight is about 1.4 to 2.86 milligrams per square inch. But good results may also be obtained with coatings of about 20 mg. total weight, i. e., 0.7 mg. per square inch.

The high degree of diffusion of bulbs coated in accordance with this invention is illustrated in Fig. 6 by the curves of brightness of 100- watt tungsten filament lamps of the type shown and described in connection with Fig. 5, which are based on measurements of horizontal brightness in candles per square centimeter versus distance in centimeters from the top of the bulb along a vertical line in front of the lamps. These measurements were made in a vertical plane through the lamp axis and perpendicular to the plane of the

leads

28, 28. The curve A is for a standard inside frosted lamp, curve B for a It is striking to note that the efficiencies of the lamps with clear bulbs inside-silica-coated, and inside-frosted-inside-silica-coated bulbs, show about the same efiiciency, that is, the same initial lumens per watt. This result has been shown by tests made on large numbers of lamps in each category, and shows that virtually completely diffusing bulbs for lamps can now be made which show no, or a very small, loss in light output.

A treatment such as those described above provides the bulb with a very highly diffusing coating weighing some 40 to 80 milligrams. Although such a coating provides eminently satisfactory diffusion, particularly when applied to a previously inside frosted (etched) bulb, and show very little light absorption, I have prepared lamps with both clear and inside frosted bulbs which were provided with an extra heavy inside silica coat in order to determine what the absorption would be with much heavier coatings. To this end, I repeated the coating treatment in each of those bulbs seven times, resulting in a coating weighing from 280 to 560 milligrams, that is a coating density from 10 to 20 milligrams per square inch. Test results showed that forsix lamps with inside frosted bulbs the total average lumens per lamp were 1699 and the average lumens per watt were 16.60. For the lamps with clear bulbs inside silica coated seven times, the average for six lamps was 1606 total lumens and 15.73 lumens per watt. For six lamps with inside frosted bulbs, inside silica coated seven times, the average was 1616 total lumens and 15.79 lumens per watt. These results show a loss in average total lumens of 93, or 5.5% for the silica coated clear glass bulbs, and 83 lumens, or 4.9%, for the silica coated inside frosted bulbs. The loss in average lumens per watt was .87, or 5.3%, for the silica coated clear glass bulbs, and .81

' lamps.

has been reduced, respectively, about 3-fold and a l6-fold, and this with no measurable loss in light output. Since the maximum brightness for inside-frosted lamps is below 4.5% of that of clear bulb lamps of the same wattage (Patent 1,687,- 510), this means that the maximum brightness of the silica-coated clear-bulb lamps and the silica-coated inside-frosted lamps is below 2% and 0.3%, respectively, of that of clear-bulb lamps of the same wattage.

Measurements on 100-watt lamps of the same type as those referred to above except that they had clear glass bulbs, showed a maximum brightness averaging 1289 candles per square centimeter. Thus, the maximum brightness of the silica-coated clear-bulb lamps and the silicacoated inside-frosted lamps was about 0.9% and 0.15%, respectively, of that of the clear-bulb lamps.

Fig. '7 shows spectral distribution curves of radiation emitted perpendicular to the plane of the leads from 100-watt tungsten filament lamps with and without silica coatings. Curve D represents a regular inside-frost lamp, curve E an inside frost lamp with silica coating, and curve F a clear-bulb lamp with silica coating. It will be apparent that the distribution in all cases is quite similar.

The effect on particle size of varying the flow of oxygen, and, therefore, the character of the 11 flame, is illustrated by the following results obtained on inside frosted 100-watt bulbs with the apparatus illustrated in Figs. 3 and 4. In one case, a fiow of ethyl silicate at the rate of 120 drops per minute, with an oxygen flow of 468 cc. per second, and an exposure of 30 seconds to the smoking flame produced a coating weighing 41 milligrams in which the particles had an average diameter ((13) of 2400 angstrom units, the maximum diameter being 4500 angstroms and the minimum diameter being less than 40 angstroms. Fig. 11 is a. reproduction of an actual electron micrograph (20,000)!) of the silica coating from the inside surface under the conditions stated and shows the configuration of the particles as mechanically suspended in nitrocellulose for electron microscope study. Fig. 13 shows the type or kind of random distribution of the silica particles and the arrangement thereof. In another case, with afiow of oxygen of 107 cc. per second, a flow of ethyl silicate of 120 drops per minute, and an exposure of 30 seconds, the weight of the coating was 40 milligrams, and the particles had an average diameter (dz) of 4300 angstroms, the maximum diameter being 7150 angstroms and the minimum diameter 100 angstroms. Fig. 12 is a reproduction of an actual electron micrograph (20,000X) of the silica coating deposited on the inside frosted surface of the latter bulb and removed and mechanically suspended in nitrocellulose for analysis under an electron microscope in order to show the shape of the individual particles. The latter bulb was considerably more diffusing than the former; in

fact, it contained more silica than was necessary for good diffusion.

In another bulb made as described in connection with Fig. l, the average particle diameter (dz) was 3400 angstroms, with a range of indi vidual particle sizes from 6000 angstroms to less than 30 angstroms.

The frequency of occurrence of the particles in any given size class for these coatings is approximately as shown by the curve in Fig. 10.

I have found that the diffusion characteristics of the silica coating may also be controlled after deposition by wetting or exposing the coating to a liquid or vapor, such as steam. When a liquid or vapor comes in contact with these particles of silica, the particles are apparently ruptured and they assume a heterogeneous form and arrangement affording less diffusion. In substance, the particles so treated form a thinner layer and the light diffusion is less. Accordingly, the ultimate diffusing characteristics of a silica coating may be controlled by exposing the deposited silica coating to moisture, thereby providing a convenient method of determining the characteristics desired of the entire coating or a part thereof.

The silica which I employ as a light diffusing element in an incandescent lamp also makes it possible to obtain the improved diffusion in a lamp without any loss whatsoever in over-all lamp efficiency (lumens per watt). Even though, as indicated above, the loss in efliciency due to the silica coating is small, I have found that any loss so incurred may be completely recovered by using filling gas compositions which heretofore have been considered unsuitable in lamp manufacturing practice.

As stated generally above, it has been the usual practice in most general purpose incandescent lamps to use a relatively high percentage of nitrogen as a constituent with an inert'gas as the gas filling. This use of nitrogen. in spite of tel its high thermal conductivity, has been deemed necessary and desirable because of the hi h breakdown voltage characteristic of nitrogen. In other words, in order to obtain a resultant or over-all high breakdown voltage of the gas composition, the incident greater heat loss by use of nitrogen was accepted, even though it entailed a reduction in lamp efficiency as compared with the use of an inert gas alone.

A gas filling consisting of per cent argon and 10 percentage nitrogen has a breakdown voltage which is four times that of pure argon. Stated in other words, the breakdown voltage of pure nitrogen is about ten times that of pure argon under the same conditions. Fo this reason, it will be understood that heretofore in order to obtain freedom from internal arcs during rough handling of lamps, it has been the practice to use a relatively high percentage of nitrogen as a componentin the filling gas. For example, in one commercially standardized line of watt lamps, it has been and is the practice to use a filling gas consisting of 88 per cent argon and 12 per cent nitrogen.

Lamps as constructed above. using a relatively high percentage of nitrogen, that is ranging from 10 to 20 per cent, of course have been satisfactory from the standpoint that these lamps are free from internal arcing even under rough usage. However, as stated, by virtue of the relatively high percentage of nitrogen and its higher thermal conductivity as compared with the inert gases such as argon, krypton and xenon, it was necessary to accept the incident greater heat loss thereby necessarily imposing on the lamp 9. loss in efiiciency with respect to that theoretically possible.

By using a gaseous atmosphere consisting essentially of an inert gas, such as argon, krypton or xenon, or mixtures thereof, either with or without a small percentage of nitrogen, or by using a gaseous atmosphere consisting exclusively of an inert gas such as argon, krypton or xenon or mixtures thereof, and by the use of the silica coating I am able to prevent voltage breakdown or are discharges across the filament terminals, whereas in similar lamps using the same gas composition in the absence of silica a large percentage of the lamps arc-over internally. In this manner, I effect operation of the filament at a higher temperature affording a gain in efficiency over that present in an otherwise similar lamp using a relatively high percentage of nitrogen as a constituent in the filling gas.

When-the silica coating which I provide is employed in a lamp it has been found that the amount of nitrogen employed may be substantially reduced to an amount heretofore considered unusable in a practical lamp. Although the percentage of nitrogen employed with the stated inert gases may vary depending upon the type of application for which the lamp is designed, I have found that the amount of nitrogen by volume may be maintained at a relatively minor percentage, not exceeding 5 per cent by volume of the gas composition. For example, in a 100 watt lamp as herein described, I prefer to use a gas composition consisting of an inert gas, such as argon, constituting 98 per cent by volume, and 2 per cent nitrogen.

As a standard, reference may be had to a commercial line of 100 watt, volt incandescent lamps employing inside frost and using 88 per cent argon and 12 per cent nitrogen. The luminous efliciency of such lamps, made in large quantities for commercial production, in lumens per watt is about 16.3. Lamps built in accordance with my invention employing inside frost and an inside coating of silica as herein explained, and using a gas filling consisting of 98 per cent argon and 2 per cent nitrogen, also have an efficiency of 16.3 lumens per watt, indicating that the same amount of light is emitted and the same efficiency obtained, while obtaining greatly improved diffusion. Stated in other words, any loss in efficiency occasioned b the use of the silica coatirig is completely recovered and compensated for by using the higher percentage of the stated inert gases.

Furthermore, extensive tests conducted on lamps employing the inside silica coating and using a gas filling, consisting of 98 per cent argon and 2 per cent nitrogen, show that these lamps are entirely free of internal arcs even under the most rough usage. More particularly, in one set of tests conducted on 57 regular lamps employing inside frost and 98 per cent argon and 2 per cent nitrogen, one third of the lamps developed internal arcs under the rough usage or fragility test. On the other hand, in similar tests conducted on the same number of lamps having an inside frost and an internal coating of silica and using a gas filling consisting of 98 per cent argon and 2 per cent nitrogen, no lamps showed internal arcing. This test is merely representative of a large number of similar tests so conducted which conclusively show that the silica coating prevents the internal arcing of the lamps even under rough usage.

The pressure of the gas filling is not critical in carrying out my invention so that I may use any practically feasible pressure providing the necessary safety in use. For example, the pressure may be less than one atmosphere, or greater than one atmosphere. In some general purpose lamps the cold gas pressure is about 600 mm., and during operation the gas pressure approaches or reaches about one atmosphere (760 mm.).

In Fig. 8, I have illustrated the invention as applied to the inside surface of the outer bulb 3| of a high pressure mercury arc lamp which may be of the type disclosed in Patent 2,094,694 to C. Bol et a1. and which is assigned to the assignee of this application. The bulb 3| encloses a quartz mercury arc tube 32. In lamps of this nature which may be employed to produce either or both visible radiation and ultraviolet radiation, the diffusing coating may be employed to diffuse the visible radiation and to produce beneficial diffusion of the ultraviolet radiation.

In Fig. 9, I have shown the invention as applied t0 the inner surface of a glass globe 33 surround- 2. A glass electric lamp bulb having on the inside surface thereof a light-diffusing coating of particles of amorphous silica of substantially spherical configuration and having an average diameter within the range from about A to 75 micron, said coating having a density ranging 14 from a. fraction of a milligram to several milligrams of silica per square inch.

3. A radiation diffusing body having a coating thereon consisting essentially of fine particles of amorphous silica having substantially spherical configuration and wherein the density of the coating is within the range from about 0.5 to about 20 milligrams per square inch.

4. A glass electric lamp bulb having on the inside surface thereof a light-diffusing coating of particles of amorphous silica of substantially spherical configuration and having an average diameter within the range from about to micron.

5. A glass electric lamp bulb having on the inside surface thereof a light-diffusing coating of particles of silica self-adherent to the inside surface of said bulb and the numerical preponderance of which have a size less than the shortest wave length of light.

6. A glass electric lamp bulb having on the inside surface thereof a light-diffusing coating of particles of silica having an average diameter of a fraction of a micron and a substantial number of which particles are fritted and self-adherent to said surface.

7. A glass electric lamp bulb having on the inside surface thereof a light-diffusing coating of self-adherent particles of silica having an average diameter within the range of about A, to micron.

8. A glass electric lamp bulb having on the inside surface thereof a light-diffusing coating of self -adherent particles of silica having an average diameter of the order of a half micron.

9. A glass electric lamp bulb having the inside surface thereof etched and coated with a light-diffusing layer of particles of silica the numerical preponderance of which have a size less than the shortest wave length of light.

10. A glass electric lamp bulb having the inside surface thereof etched and coated with a. light-diffusing layer of particles of silica having an average diameter of a fraction of a micron.

11. A glass electric lamp bulb having the inside surface thereof etched and coated with a light-diffusing layer of particles of silica having an average diameter within the range of about to /5 micron.

12. A glass electric lamp bulb having the inside surface thereof etched and coated with a light-diffusing layer of particles of silica having an average diameter of the order of a half micron.

13. A glass electric lamp bulb having on its inner surface a coating consisting of extremely finely divided rounded particles of silica so that the maximum brightness of an ordinary incandescent lamp comprising such a coated bulb is of the order of a fraction of a per cent to a few per cent of that of said lamp with a clear bulb.

14. A glass electric lamp bulb having its inner surface etched and provided with a coating consisting of extremely finely divided rounded particles of silica so that the maximum brightness of an ordinary incandescent lamp comprising such a coated bulb is of the order of a fraction (13f a per cent of that of said lamp with a clear ulb.

15. A light-transmitting electric lamp bulb having on the inside surface thereof a light-diffusing coating consisting of a deposit of fumes of an inflammable silicon compound.

16. A glass electric lamp bulb having its inside surface etched and coated with a light-difaccuses fusing layer consisting of a deposit of fumes of an inflammable silicon compound.

17. An electric lamp having on the inside surface of a bulb therefor a thin light-diffusing coating of finely divided particles of amo p silica self-adherent to said surface, a filament mounted inside said bulb, and a gas filling consisting of about 98 per cent of an inert gas and 2 per cent nitrogen by volume.

18. An electric lamp bulb having on the inside surface thereof a thin light-diffusing coating of rounded particles of amorphous silica, a substantial number of said particles being fused and self-adherent to said surface.

19. A glass electric lamp bulb having the inside surface thereof etched and coated with a thin light-diffusing layer of amorphous silica.

20. The method of controlling the light-diffusing characteristics of a light-diffusing article having thereon a coating of finely divided particles of amorphous silica which comprises wetting at least a part of the silica coating to lessen the light-diffusing property of the part which is wetted.

21. In an incandescent lamp, the combination comprising a bulb, a coating of finely divided particles of silica on the interior surface thereof, a filament mounted inside said bulb, and a gas filling consisting essentially of an inert gas of the group consisting of argon, krypton and xenon, or mixtures thereof, and an amount of nitrogen not exceeding 5 per cent by volume.

22. In an incandescent lamp, the combination comprising a glass bulb, a coating of finely divided particles of silica adherent to the interior surface of said bulb, and a filament mounted inside said bulb, said coating serving the dual function of a light-diffusing means and as a means for preventing the dislodgment of any impurities from the inside surface of the glass bulb which would tend to lower the breakdown-voltage across said filament.

23. In an incandescent lamp, the combination comprising a glass bulb, a light diffusing coating of finely divided silica particles adherent to the interior surface of said bulb, a filament mounted in said bulb, and a gas filling in said bulb comprising an inert gas, said coating serving as a means for holding any impurities occurring on the interior surface of the bulb.

24. In an incandescent lamp, the combination comprising a glass bulb, a coating of finely divided particles of silica on the interior surface of said bulb, a filament mounted in said bulb, and a gas filling consisting essentially of an inert gas from the group of such gases having an atomic weight greater than 39, or mixtures thereof.

25. In an incandescent lamp, the combination comprising a bulb, a coating of finely divided rounded particles of amorphous silica on the interior surface thereof, a filament mounted inside said bulb, and a gas filling consisting essentially of argon.

26. In an incandescent lamp, the combination comprising a glass bulb, a light-diffusing coating of finely divided particles of silica on the interior surface of said bulb, a filament mounted in said bulb, and a gas filling in said bulb consisting essentially of an inert gas of the group consisting of argon, krypton and xenon, or mixtures thereof,

said coating serving as a means for preventing dislodgment of impurities from the interior surface of said bulb, and said inert gas permitting operation of the filament at a higher temperature than that obtainable by use of gas fillings 16 other than the stated inert gases whereby increased luminous efllciency is obtained to recover any loss in efficiency due to said coating.

2'1. In an incandescent lamp, the combination comprising a glass bulb, a coating of finely divided rounded particles of amorphous silica adherent and fused to the interior surface of said bulb, a filament mounted inside said bulb, and a gas filling consisting of about 98 per cent argon and 2 per cent nitrogen by volume.

28. An electric incandescent lamp comprising a sealed glass bulb having a filament mounted therein, said bulb having its inside surface etched and coated with a thin light-diffusing layer of finely divided rounded particles of silica self-adherent to said surface.

29. An electric incandescent lamp comprising a sealed glass bulb having its inside surface etched and coated with a thin light-diffusing layer of finely divided rounded particles of silica selfadherent to said surface, gas within said bulb of the group consisting of argon, krypton, xenon and mixtures thereof and a small amount of nitrogen not exceeding 5 per cent by volume, and a filament mounted in said bulb operable at temperature greater than that in lamps of corresponding rating to afford recovery of any loss in luminous efilciency occasioned by the light-diffusing function of said layer.

30. In hollow illuminating glassware a radiation transmitting body having an etched surface and a coating of rounded particles of amorphous silica directly and self-adherent to the etched surface.

31. An electric incandescent lamp comprising a sealed glass bulb having its inside surface coated with a thin light-diffusing layer of finely divided particles self-adherent to said surface and to prevent dislodgement of any impurities from the inside surface of the bulb, gas within said bulb of the group consisting of argon, krypton, xenon and mixtures thereof and a small amount of nitrogen not exceeding 5 per cent by volume, and a filament mounted in said bulb operable at temperature greater than that in lamps of corresponding rating to afford recovery of any loss in luminous efiiciency occasioned by the light-diffusing function of said layer.

32. The method of treating a glass electric lamp bulb having an opening therein to form a layer of light-diffusing material on the internal surface thereof which comprises the steps of producing within the interior of the bulb a combustible mixture of an organo-silicon compound which is reducible by burning to silica, igniting the mixture and supplying through said opening a steady stream of oxygen sufficient for complete combustion of the organo-silicon compound to thereby disassociate it into silica and other volatile products, continuing the combustion and maintaining the fumes in contact with the inner surface of the bulb until a thin diffusing coating of silica particles is deposited thereon while carrying the products of combustion other than silica out through said opening.

MARVIN PlIKIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITE'DSTATES PATENTS Number Name Date 573,206 Chavez and Herman Dec. 15, 1896 580,248 Biemann Apr. 6, 1897 (Other references on following page) 17 UNITED STATES PATENTS Number Name Date Hamburger and Lely Sept. 30, 1919 Gustin Jan. 15, 1929 Fagan Nov. 3,1931 Fuwa. Apr. 19, 1932 Ferguson July 5, 1932 Hageman et a1. Mar. 7, 1933 Biggs et a1. Apr. 23, 1935 Bahlke et 'al. July 7, 1936 Parker Sept. 28, 1937 Weinhart Oct. 19, 1937 Callan- Nov. 22, 1938 Heany Jan. 6, 1942 Hyde Feb. 10, 1942 Number Number Germany Mar. 20, 1934