Title:
APERTURE FLUORESCENT LAMP FOR COPYING MACHINES
United States Patent 3767956


Abstract:
An aperture fluorescent lamp wherein luminous flux is radiated uniformly along the aperture. In a first embodiment, a variable area aperture is provided. In the second embodiment, a variable area reflector is formed on the outside surface of the lamp.



Inventors:
BAUER G
Application Number:
04/887846
Publication Date:
10/23/1973
Filing Date:
12/24/1969
Assignee:
XEROX CORP,US
Primary Class:
Other Classes:
313/113, 313/635
International Classes:
H01J61/35; (IPC1-7): H01J61/35; H01J61/42
Field of Search:
313/109,113,117,220,221,18R 240
View Patent Images:
US Patent References:
3275872Reflector fluorescent lamp1966-09-27Chernin et al.
3225241Aperture fluorescent lamp1965-12-21Spencer et al.
2407379Combination bactericidal and illuminating lamp1946-09-10Morehouse
2135732Device for producing visible light1938-11-08Randall et al.



Primary Examiner:
Demeo, Palmer C.
Claims:
What is claimed is

1. An aperture fluorescent lamp comprising:

2. An aperture flourescent lamp comprising: an elongated tubular glass envelope having electrodes sealed into its opposite ends and containing an ionizable medium therein;

Description:
BACKGROUND OF THE INVENTION

In the xerographic process as described in U.S. Pat. No. 2,297,691, a base plate of relatively low electrical resistance such as metal, etc., having a photoconductive insulating surface coated thereon is electrostatically charged in the dark. The charged coating is then exposed to a light image. The charges leak off rapidly in the base plate in proportion to the intensity of light to which any given area is exposed, the charge being substantially retained in non-exposed areas. After exposure, the coating is contacted with electrostatic materials which adhere to the remaining charges to form a powder image corresponding to the latent electrostatic latent image remaining after exposure. The powder image then can be transferred to a sheet of transfer material resulting in a positive or negative print, as the case may be. Since dissipation of the surface electrostatic charge is proportional to the intensity of the impinging radiation, light sources of uniform and sufficient intensity must be provided so that the photoconductive insulator can be properly exposed.

It is generally known that the luminous intensity from aperture fluorescent lamps decreases sharply from the center towards the ends thereof. This lack of uniformity along the length of the lamp is a disadvantage when the lamps are used in xerography wherein it is desired to have substantial uniform illuminance across the width of the material to be copied. By limiting the aperture, or slot, to a length which is less than that of the positive column, or distance between the thermionic electrodes, the prior art provided an aperture fluorescent lamp which produced a substantially uniform luminous intensity along the axis of the aperture. However, restricting the useful length of the lamps to an aperture of less than the positive column of the lamp has two disadvantages associated therewith. Light produced within the lamp in the areas corresponding to the non-aperture portions of the lamp is not completely utilized, decreasing the efficiency of the lamp. In addition, the shortened length of the aperture effectively limits the width of the material which may be copied.

SUMMARY OF THE INVENTION

The present invention provides an aperture fluorescent lamp having a uniform luminous intensity along its aperture axis, and, in particular, wherein the uniform luminous intensity is obtained along the entire positive column of the lamp. In a first embodiment, the aperture is wider towards the ends of the glass tube envelope. In a second embodiment, the outside of the glass tube envelope, except for the aperture, is covered with a reflecting member whose surface area increases towards the tube ends. Both embodiments utilize the entire positive column of the lamp and uniform luminous intensity can be achieved without a significant loss in lamp efficiency.

It is an object of the present invention to provide an aperture flurorescent lamp which produces a uniform luminous intensity along the aperture axis.

It is a further object of the present invention to provide an aperture fluorescent lamp which produces uniform luminous intensity along the aperture axis wherein the length of the aperture is equal to the positive column of the lamp.

It is still a further object of the present invention to provide an aperture fluorescent lamp wherein the length of the aperture is equal to the positive column and the width of the aperture varies along the aperture length.

It is a further object of the present invention to provide an aperture fluorescent lamp wherein the aperture is equal to the positive column and wherein a reflecting layer of varying surface area is formed on the outside of the glass envelope except for the aperture.

DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention as well as other objects and further features thereof, reference is made to the following detailed description which is to be read in conjunction with the accompanying drawings wherein:

FIG. 1 shows an aperture fluorescent lamp with a shortened aperture as found in the prior art;

FIG. 2 is a cross-sectional view along line 2--2 of the lamp shown in FIG. 1;

FIG. 3 shows an aperature fluorescent lamp with the aperture width increasing towards the ends of the tube envelope in accordance with the teachings of the present invention;

FIG. 4 is a cross-sectional view along line 4--4 of the lamp shown in FIG. 3;

FIG. 5 shows another embodiment of the aperture fluorescent lamp of the present invention which illustrates a variable width aperture;

FIG. 6 is a cross-sectional view along line 5--5 of the lamp shown in FIG. 5;

FIG. 7 shows an additional embodiment of the aperture fluorescent lamp of the present invention which utilizes a reflecting layer whose surface area increases towards the ends of the tube envelope; and

FIG. 8 is a cross-sectional view along line 8--8 of the lamp shown in FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, there is shown a prior art aperture fluorescent lamp, the construction of which is similar to the lamp of the present invention, the improvements thereover which constitute the present invention being described hereinafter with reference to FIGS. 3 through 8. The lamp comprises an elongated glass tube 10 forming the envelope into the ends of which are sealed a pair of electrodes 12 and 14. The electrodes may be of the thermionic type, each comprising a tungsten filament coated with electron emitting material consisting of alkaline earth oxides and the support and lead-in wires 16 and 18 sealed through the usual stem press and connected to terminal pins 20 and 22 of a base 24. The envelope is filled with inert gas, for instance argon or a mixture of argon with another rare gas as helium, at a pressure of a few millimeters of mercury with sufficient mercury to provide a vapor pressure of a few microns in operation. It should be noted that the present invention, as described hereinbelow, may be utilized in high pressure discharge lamps. A narrow aperture, or slot, 30 is provided by scraping out the reflective and phosphor coatings over a minor portion of the interior periphery, for instance over an arc of 60°. The actinic energy output of aperture lamps is dependent on, among other factors, the geometrical configuration of the aperture. The amount of luminous flux emitted through the aperture is proportional to the area of the aperture. As discussed hereinabove, it is known that the luminous intensity of an aperture fluorescent lamp along the length of the lamp decreases towards the ends. Therefore, the aperture in conventional lamps only extends for a portion of its length as shown in FIG. 1.

Referring now to FIG. 2, there is shown a cross-sectional view along line 2--2 of FIG. 1. Reflective coating 26 is applied to the inside surface of the glass envelope 10 over the major portion of the periphery and a phosphor coating 28 is applied thereover. The reflective coating may comprise powdered materials such as titanum dioxide having a particle size less than one micron, magnesium oxide, zinc oxide, zirconia, or metals such as aluminum or silver.

The aperture shown in FIG. 2 is clear of the reflective and phosphor coating. It should be obvious that the invention as hereinafter described is applicable to a reflector fluorescent lamp with the phosphor coating applied to the aperture. The phosphor coating 28 may be made thick enough to reflect into the envelope a large portion of the light emitted from the phosphor coating, thereby eliminating the necessity of a separate reflective coating. The phosphor coating 28 may comprise calcium halophosphate activated with manganese and antimony or any other suitable fluorescent lamp phosphor. The methods of applying the reflective coating and phosphors to the tube walls and forming a phosphor or clear aperture is well known and will not be described herein.

FIG. 3 shows one possible lamp configuration wherein a variable area aperture is formed. A variable area aperture is provided whereby the total length, or positive column of the aperture lamp is utilized while, at the same time, providing a substantially uniform luminous intensity along the total length of the aperture fluorescent lamp. Portion 40 of the aperture is selected to correspond to the length of aperture 30 as shown in FIG. 1 such that the decrease in lamp brightness from the center of the lamp towards the ends normally would become pronounced if the length of aperture 40 was extended without an attendant increase in aperture area. However, in accordance with the teachings of the present invention, the aperture changes in area towards the ends of the lamp, corresponding to the step-shaped portions 42 of the aperture. Portion 42 may be formed in the same manner as apertures 30 or 40, discussed hereinabove. The increased aperture area towards the ends of the lamp (portions 42)increases the amount of luminous flux radiated thereat, since a greater area of the phosphor coating 28 is seen than along aperture portion 40. In other words, the amount of luminous flux emitted by the phosphor coating 28 through an aperture is proportional to the area of the aperture.

FIG. 4 is a cross-sectional view along line 4--4 of FIG. 3 wherein the dotted lines indicate the step-shaped aperture portions 42.

Referring now to FIG. 5, there is shown another embodiment of the novel aperture lamp of the present invention. The output of aperture portion 50, as defined by dashed lines 54 and 56, is chosen to provide substantially uniform luminous intensity, as discussed with reference to aperture 30 and aperture portion 40 hereinabove. The remaining portions of the aperture shown in FIG. 5 comprise curved portions 52. This aperture configuration is designed to increase the luminous intensity at a rate equal to the rate of decrease in the intensity which would normally occur if the aperture area was constant along the lamp length.

The variable area portions of the apertures as illustrated in FIGS. 3 and 5 are illustrative of the type of configurations which may be utilized. Many other variable area aperture configurations may be provided and still be within the purview of the present invention.

FIG. 6 is a cross-sectional view along line 6--6 of FIG. 5 showing the effect of the changed aperture geometry.

Referring now to FIG. 7, another embodiment of the present invention is shown. In this embodiment, reflecting layer 26, shown in the prior figures, is omitted. It is known that only a portion of the luminous flux generated inside of the lamp is radiated through the aperture. Some other portion of the luminous flux is radiated through the glass envelope. It has been determined that when a reflecting layer covers the outer surface of the envelope the flux radiated through the aperture is increased. Substantially uniform luminous flux is radiated through the aperture by providing a variable area reflecting layer which covers the outside of the lamp and which widens towards the lamp ends. It is assumed that the thickness of phosphor layer 28 is chosen to permit a portion of the luminous flux generated in the lamp to pass therethrough. The reflecting layer may comprise aluminum, silver or any other suitable light reflecting material and is deposited on the lamp surface by known deposition techniques, such as by evaporation. The reflecting layer may also comprise aluminum foil.

The width of reflecting layer 62 formed on the surface of envelope 10 varies in proportion to the normal fall-off in luminous intensity along the length of the aperture. It should be noted that reflecting layer 62 also provides for better starting and cooling of the lamps. Better cooling increases the lifetime and efficiency of the lamps since the chemical processes that cause lamp deterioration slows at lower temperatures.

An alternate embodiment to the embodiment shown in FIG. 7 would include the reflecting layer 26 since reflectors are normally not perfect i.e. a portion of the luminous flux generated in the lamp will pass through reflecting layer 26 and impinge upon reflecting layer 62.

FIG. 8 is a cross-sectional view along line 8--8 of FIG. 7 showing the reflecting layer 62.

The embodiment shown in FIGS. 3, 5, and 7 are illustrative of the various types of variable area aperture and reflecting layer configurations that may be utilized in the present invention and various other configurations may also be utilized.