US 3579021 A
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United States Patent Inventor Appl. No. Filed Patented Assignee INCANDESCENT LAMP HAVING LINEAR OUTPUT 5 Claims, 1 Drawing Fig.
U.S.Cl 313/274, 313/315 Int.Cl I-I01j1/96,
' 1-101j 19/52 Field of Search 313/271, 274,315
Primary ExaminerDavid Schonberg Assistant Examiner-Paul A. Sacher Attorneys-Norman J. OMalley and James Theodosopoulos v ABSTRACT: The coiled filament of an elongated tubular incandescent lamp is axially mounted and supported therein by spacers bearing against the inside wall of the lamp envelope. The space between filament turns is less at its ends than at its center in order to yield a substantially uniform output of radiational energy from the entire length of the lamp.
Patented May 18, 1971 3,579,021
STEPHEN F. KIMBALL INVENTOR AGENT 1 INCANDESCENT LAMP HAVING LINEAR OUTPUT BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the field of elongated tubular incandescent lamps and particularly to such lamps having a sufficiently high output as to require a high temperature envelope, such as high silica glass.
2. Description of the Prior Art In the past, elongated tubular-incandescent lamps of the type relating to the field of this invention have generally had a filament which was substantially uniformly coiled throughout its length. Such lamps have been made in lengths of up to 2 or 3 feet and have generally been used for heating. A profile of the radiational output of such lamps taken along the length of the lamps but a short distance therefrom is greater at the center than at the ends. In fact, for a lamp having uniform coiling throughout its length, the output at the center of the lamp is about double that at the ends thereof, when the output is measured on a line parallel to the lamp but only a short distance therefrom. This disparity in the output profile is increased even more by the fact that the ends of the lamp filament operate at a lower temperature, due to end cooling, and emit less radiational energy than does the center.
In many commercial applications where such lamps are used as infrared heaters, the nonlinear output has little practical effect since the heating requirements are not critical. In addition, the lamps in those applications are generally spaced far enough from the materials being heated so as to reduce or substantially eliminate the variation in the output profile.
It is to be remembered that the profile variation is greatest when measured at a distance from the lamp that is relatively short in relation to the lamp length.
Recently, photocopying applications have arisen where such lamps are used at a relatively short distance from the material being heated or illuminated. A typical application is in reprographic or photocopying equipment where an illuminating lamp can be positioned a short distance from a master copy being reproduced. Or a heating lamp can be used to fuse a thermoplastic film on the reproduction. In these applications, the photo copying is conducted at such high speeds and at such proximity to the lamps, that the output profile thereof is critical. A nonuniform output as is obtained from a lamp having a filament that has uniform turn spacing throughout is generally unsatisfactory for such applications.
In some cases, also, there are sufficient edge losses in the photocopying equipment as to require a lamp having higher output at its ends than at its center in order to compensate for the losses.
SUMMARY OF THE INVENTION We have found that the variation in output profile of elongated, tubular, high temperature lamps can be markedly reduced when the ends of the filament thereof are caused to operate at higher temperatures than the center of the filament. More specifically, we coil the ends of the filament at a higher pitch, that is, at a high number of turns per inch (TPI), than the center of the filament. The closer proximity to each other of the turns at the end results in a higher operating temperature thereat than at the center. The radiational energy emitted therefrom is thus greater than at the center, the effect of which is to be substantially improve the linearity of the output profile over that of the prior art lamps.
The improvement in linearity is directly dependent on, among other things, the ratio of the turn spacing at the ends of the filament to that at the center. A ratio only slightly greater than 1.0 will improve the linearity only slightly. However, there may be situations where such a ratio, say, 1.05 or 1.10, is satisfactory, as where the ends of the filament extend beyond the edges of the paper being heated or illuminated and where the distance between the lamp and the paper is less than about one-twentieth of the lamp length. In such a situation, the nonlinear output of a lamp has less effect than in one where the filament extends only to the edges of the paper and where the distance between the lamp and the paper is about one-fifth or one-tenth of the lamp length. In this case, a ratio greater than 1.1 would preferably be necessary to obtain a satisfactory amount of radiational energy on the entire surface of the paper being heated or illuminated. However, the turn spacing at the ends of the filament should be great enough to prevent any substantial shorting out of individual turns.
In addition, the increased pitch at the filament ends must not extend excessively along the length of the filament; otherwise, the output of the higher pitch end itself would be excessively nonlinear. Generally, the higher pitched coiling should not extend more than one-third of the coiled filament length from the end thereof.
BRIEF DESCRIPTION OF THE DRAWING The single FIGURE is an elevational view of a lamp in accordance with this invention showing a filament having higher pitch turns at its ends than at its center.
DESCRIPTION OF THE PREFERRED EMBODIMENT Filament 3 has integral terminal portions 4 at each end, the
pitch of terminal portions 4 being greater than the pitch of the mainbody of filament 3. As previously mentioned, each terminal portion 4 should be less than one-third the total length of filament 3. Filament 3, including terminal portions 4, preferably consists of a single length of coiled wire, preferably tungsten, in order to avoid mechanical and electrical connections, for example, welds, between the ends of filament 3. Lamps of the type under consideration herein generally operate at such a high temperature that such connections could be locations of early failure of a filament. Of course, filament 3 can consist of 2 or more wires wound together in parallel on the same mandrel for the purpose of increasing the current carrying capacity of a filament without increase in the diameter of the filament wire. Filament 3 is supported throughout its length by metallic spacers 8 bearing against the inner wall of envelope 2.
The ends of filament 3 are connected to ribbon connectors 5 embedded in press seals 6 at each end of envelope 2. Ceramic bases 7 are attached to the ends of press seals 6 and ribbon.
connectors 5 are electrically connected to external contact terminals in bases 7.
In particular example of a 1,000-watt, T2%, -volt quartz-halogen infrared lamp in accordance with this invention, envelope 2 had a length of 17% inches and a diameter of 8 millimeters. Filament 3 consisted of two 3,660 millimeter lengths of 9.95 mil tungsten wire bifilarly wound on a 35 mil mandrel. The mandrel, of course, was removed prior to the mounting of filament 3 in envelope 2. Filament 3 had an overall length of 15% inches and was supported throughout the length of envelope 2 by wound tungsten spacers 8 spaced about one-half inch apart. The ends of filament 3 were connected to molybdenum ribbons 5 by lead-in wires 9. One end of lead-in wire 9 was welded to ribbon 5 and the other end 10 was coiled, thereby permitting the end of filament 3 to be threaded thereinto to provide a secure mechanical and electrical connection.
Terminal portions 4 of filament 3 were each 3 inches long which was about 20 percent of the total length of filament 3. Terminal portions 4 were coiled at 43 turns per inch while the central portion of filament 3 was coiled at 35 turns per inch, the ratio therebetween being 1.22 to 1. At normal operation the filament temperature at terminal portions 4 was 2,343 K. and the filament temperature at the center was 2,245 K. Thus terminal portions 4 operated at a temperature 98 K. higher than the center of filament 3 and the increased radiation therefrom resulted in a lamp output profile that, when measured at a distance of 1 inch from the lamp, was more linear than a control lamp, the filament of which had uniform turn spacing throughout its length.
1. An incandescent lamp having improved uniformity of radiational output along the length of the lamp comprising: an elongated tubular high silica glass envelope having press seals at each end thereof; a continuously coiled tungsten filament axially disposed within said envelope, said filament having coiled integral terminal portions, said coiled portions having a greater pitch than the central portion of said filament; and
spacers supporting said filament within said envelope.
2. The lamp of claim 1 wherein the length of each of said terminal portions is less than one-third the total coiled length of said filament.
3. The lamp of claim 1 ,wherein the space between turns of said terminal portions is sufficient to prevent any substantial shorting out of individual turns.
4. The lamp of claim 1 wherein the ends of said filament are threaded into coiled lead-in wires, said lead-in wires being connected to ribbons embedded in said press seals.
5. The lamp of claim 1 wherein the ratio of the TH of said terminal portions to the TPI of said central portion of said filament is greater than about 1.05.