Patent Application
20070023617
This invention relates to a low cost planar image intensifier tube for use in night vision systems.
Night vision systems are used in a wide variety of military, industrial and residential applications to enable sight in a dark environment. For example, night vision systems are utilized by military aviators during nighttime flights. Security cameras use night vision systems to monitor dark areas and medical instruments use night vision systems to alleviate conditions such as retinitis pigmentosis (night blindness).
Image intensifier devices are employed in night visions systems to convert a dark environment to an environment perceivable by a viewer. More specifically, the image intensifier device within the night vision system collects tiny amounts of light in a dark environment, including the lower portion of the infrared light spectrum, that are present in the environment but may be imperceptible to the human eye. The device amplifies the light so that the human eye can perceive the image. The light output from the image intensifier device can either be supplied to a camera, external monitor or directly to the eyes of a viewer. The image intensifier devices are commonly employed in vision goggles that are worn on a user's head for transmission of the light output directly to the viewer. Accordingly, since the goggles are worn on the head, they are desirably compact and light weight for purposes of comfort and usability.
Image intensifier devices include three basic components mounted within a housing, i.e. a photocathode (commonly called a cathode), a microchannel plate (MCP), and a phosphor screen (commonly called a screen, fiber-optic or anode). The photocathode detects a light image and converts the light image into a corresponding electron pattern. The MCP amplifies the electron pattern and the phosphor screen transforms the amplified electron pattern back to an enhanced light image.
The photocathode is a photosensitive plate capable of releasing electrons when it is illuminated by light. The number of electrons released by the photocathode is proportional to the intensity of the light impinging on it. The photocathode operates by the principles of the photoelectric effect. More specifically, when a light photon enters the photocathode material and the energy of the photon exceeds the binding energy of an electron to an atom on the surface of the photocathode, the electron is excited from the valence band to the conduction band of the photocathode. The electron is then emitted from the photocathode unto the micro-channel plate.
The MCP is a thin glass plate having an array of channels extending between one side (input) and another side (output) of the glass plate. The MCP is positioned between the photocathode and the phosphor screen. An incoming electron from the photocathode enters the input side of the MCP and strikes a channel wall. When voltage is applied across the MCP, these incoming or primary electrons are amplified, generating secondary electrons. The secondary electrons exit the channel at the output side of the MCP and are accelerated towards the phosphor screen.
The secondary electrons exiting the MCP channel are negatively charged and are therefore, attracted to the positively charged phosphor screen, which is coated with phosphor. It should be understood that phosphor is any material that emits light when exposed to electron radiation. The energy of the secondary electrons colliding with the phosphor screen causes the phosphor on the screen to reach an excited state and release photons proportional to the quantity of the secondary electrons. The phosphor on the screen glows when photons are released. An eyepiece lens typically magnifies and collimates the glowing phosphor image. A fiber optic inverter element positioned adjacent to the objective lens may invert the phosphor image for viewing through a goggle eyepiece.
The three basic components of the image intensifier device are positioned within an evacuated housing or vacuum envelope. The vacuum facilitates the flow of electrons from the photocathode through the MCP and to the phosphor screen. The photocathode MCP and phosphor screen are electrically biased so that the phosphor screen is maintained at a higher positive potential than the photocathode. Furthermore, the photocathode, MCP and phosphor screen are each maintained at different electrical potentials. All three components are electrically isolated from one another, while being retained within the vacuum housing.
Referring to
The photocathode is attached to faceplate 15′ having a sloped portion 15A′ and a flat portion 24′ which rests upon a conductive support ring 22 at one end of vacuum housing 12. A metalized layer 25, generally composed of chrome, is deposited upon flat portion 24′ to conductively engage support ring 22. Metalized layer 25 extends continuously along sloped portion 15A′ to conductively engage both photocathode 14′ and faceplate 15′ at interface 19. The abutment of the photocathode faceplate against support ring 22 creates a seal to close one end of vacuum housing 12. The support ring 22 contacts metalized surface 24′ on the faceplate of photocathode 14′. The metalized surface 24′, in turn, is coupled to a photoresponsive layer 26 by means of the chrome deposited layer 25 on photocathode 14′ contained within the evacuated environment of vacuum housing 12. As such, an electrical bias may be applied to photoresponsive layer 26 of photocathode 14′ within the evacuated environment by applying an electrical bias to support ring 22 on the exterior of vacuum housing 12.
A first annular ceramic spacer 28 is positioned below support ring 22. The first ceramic spacer is joined to support ring 22 by a first copper brazing ring 31, which is joined to both the first ceramic spacer and support ring 22 during a brazing operation. The brazing operation creates an air impervious seal between support ring 22 and first ceramic spacer 28. An upper MCP terminal 32 is joined to the first ceramic spacer, opposite support ring 22. The upper MCP terminal is also joined to the first ceramic spacer in a brazing operation. Consequently, a second brazing ring 34 is interposed between the upper MCP terminal and the first ceramic spacer. The upper MCP terminal extends into vacuum housing 12 where it conductively engages a metal hold down ring 36 and a metal contact ring 38. The metal contact ring engages the conductive upper surface 42 of MCP 16, while the hold down ring retains the MCP within the housing. Consequently, an electrical bias may be applied to upper surface 42 of MCP 16 by applying the electrical bias to the upper MCP terminal on the exterior of the vacuum housing 12.
A second ceramic spacer 46 is positioned below upper MCP terminal 32, isolating upper MCP terminal 32 from lower MCP terminal 48. The second ceramic spacer 46 is brazed to both upper MCP terminal 32 and lower MCP terminal 48, as such a third brazing ring 50 is interposed between the upper MCP terminal 32 and second ceramic spacer 46 and a fourth brazing ring 52 is interposed between second ceramic spacer 46 and lower MCP terminal 48. The lower MCP terminal extends into vacuum housing 12 and engages the lower conductive surface 44 of MCP 16. As such, the lower conductive surface of MCP 16 may be coupled to ground by connecting lower MCP terminal 48 to a ground potential external to vacuum housing 12.
A third ceramic spacer 56 separates lower MCP terminal 48 from getter shield 58. The third ceramic spacer is brazed to both lower MCP terminal 48 and getter shield 58. As such, a fifth brazing ring 60 is interposed between lower MCP terminal 48 and third ceramic spacer 56. Similarly, a sixth brazing ring 62 is interposed between third ceramic spacer 56 and getter shield 58.
A fourth ceramic spacer 64 is positioned below getter shield 58, separating the getter shield from output screen support 66. The fourth ceramic spacer is brazed to both getter shield 58 and output screen support 66. As such, seventh and eighth brazing rings 68 and 70 are positioned above and below fourth ceramic spacer 64, respectively.
The lower end of vacuum housing 12 is sealed by the presence of an output screen flange 72. The output screen flange is joined to both the output screen support 66 and fiber optic element 18. A first seal 74 occurs at a point where output flange 72 is joined to screen support 66. A second first seal 76 occurs at a location where flange 72 joins fiber optic element 18. The combination of the three seals (74, 76, and 22), thus, forms an air tight envelope defined by vacuum housing 12 in between photocathode 14 and fiber optic element 18, whereby vacuum housing 12 is constructed by numerous stacked components joined together to form an air impervious chamber.
Still referring to
The quantity, physical form and position of the ceramic spacers integrated within tube 10 present various challenges. In particular, since the image intensifier tubes are susceptible to electrical breakdown across the ceramic spacers, the size of the spacers need to be large enough to prevent voltage potential breakdown. Conversely, the spacers need to be small enough to accommodate a lightweight and compact tube. Furthermore, the variety of spacers, terminals, and support posts are expensive to manufacture and individually inventory. As a result of the numerous components, the assembly process of the image intensifier tube is laborious, complex and costly. Finally, the accumulated tolerances of the individual pacers, rings and support posts impede consistent assembly of the tube. Therefore, it would be desirable to reduce the number of components, as well as the complexity of the components that are integrated into the image intensifier tube.
Two different image intensifier tubes are presently utilized to accommodate either an inverting fiber optic element or a non-inverting fiber optic element. The fiber optic element depends upon the end-use of the image intensifier tube (i.e. night vision goggle or camera). It would be advantageous to provide a single image intensifier tube that may be configured to accommodate either an inverting fiber optic element or a non-inverting fiber optic element.
The present invention advantageously enhances the overall design of the image intensifier tube by reducing component and inventory costs and improving manufacturability and overall assembly of the image intensifier tube.
According to an aspect of this invention an image intensifier tube is provided. The image intensifier tube has a microchannel plate (MCP), a photocathode and phosphor screen deposited on a fiber optic substrate. A first spacer is positioned between the microchannel plate and the fiber optic substrate. A second spacer is positioned between the fiber optic substrate and the photocathode. The first and second spacers cooperate to provide a spatial relationship among the MCP, phosphor screen and photocathode for effective operation of the image intensifier tube.
According to another aspect of this invention an image intensifier tube is provided. The image intensifier tube comprises a phosphor screen deposited on top of a fiber optic substrate, a microchannel plate (MCP), disposed above the phosphor screen, having electrical input and output contacts, and conductive vias provided through the fiber optic substrate. The conductive vias provide electrical potential to the input and output contacts of the MCP.
According to yet another aspect of this invention an image intensifier tube having a microchannel plate and a fiber optic is provided. The image intensifier tube includes a single spacer positioned between the microchannel plate and the fiber optic.
According to still another aspect of this invention an image intensifier tube having a microchannel plate and a photocathode is provided. The image intensifier tube includes a single spacer positioned between the microchannel plate and the photocathode.
According to another aspect of this invention a method of assembling an image intensifier tube is provided. The method comprises the steps of positioning a spacer above and in direct contact with a fiber optic screen assembly, wherein the fiber optic screen assembly comprises a phosphor screen deposited on a fiber optic substrate. A microchannel plate is positioned above and in direct contact with the spacer.
According to still another aspect of this invention an image intensifier tube having an electron sensing electronic readout anode positioned adjacent to an MCP is provided. The readout anode includes a silicon imager mounted on a ceramic header, and conductive vias are provided through the ceramic header. The conductive vias provide electrical potential to the input and output contacts of the MCP.
The invention is best understood from the following detailed description when read in connection with the accompanying drawing. Included in the drawing are the following figures:
a is a cross-sectional side view of an embodiment of an image intensifier tube according to an aspect of this invention;
b is a detailed view of the embodiment of the image intensifier tube illustrated in
a is a perspective view of the fiber optic screen assembly, microchannel plate spacer and microchannel plate illustrated in
b is a detailed view of the embodiment of the sub-assembly illustrated in
a is a top-side view of the fiber optic screen assembly and microchannel plate spacer illustrated in
b is a detailed view of the fiber optic screen assembly and microchannel plate spacer illustrated in
c is another detailed view of the fiber optic screen assembly and microchannel plate spacer illustrated in
a is a bottom-side view of an embodiment of the microchannel plate spacer illustrated in
b is a top-side view of the microchannel plate spacer illustrated in
c is a cross-sectional side view of the microchannel plate spacer illustrated in
d is a side view of the microchannel plate spacer illustrated in
a is a top-side view of an embodiment of the microchannel plate illustrated in
b is a bottom-side view of the microchannel plate illustrated in
c is a side view of the microchannel plate illustrated in
Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.
The invention is best understood from the following detailed description when read in connection with the accompanying drawing figures, which shows exemplary embodiments of the invention selected for illustrative purposes. The invention will be illustrated with reference to the figures. Such figures are intended to be illustrative rather than limiting and are included herewith to facilitate the explanation of the present invention.
Referring now to the exemplary embodiments illustrated in
As shown, image intensifier tube assembly 100 is of cylindrical form and generally includes three major components, i.e. fiber optic screen assembly 110, microchannel plate (MCP) 106 and photocathode face plate assembly 102. A microchannel plate (MCP) spacer 114 is positioned above fiber optic screen assembly 110. The microchannel plate (MCP) 106 is positioned above MCP spacer 114. A cathode spacer 118 is positioned between fiber optic screen assembly 110 and photocathode face plate assembly 102. A centering ring 117 and getter 119 are coupled to opposing sides of cathode spacer 118. A getter shield 120 is positioned between fiber optic screen assembly 110 and photocathode face plate assembly 102, being radially interior to getter 119. A vacuum condition is sustained within interior chamber 113, disposed between face plate assembly 102 and screen assembly 110.
The fiber optic screen assembly 110 includes fiber optic substrate 111, phosphor screen 112, deposited on the top side of fiber optic substrate 111, and frit seal assemblies (126, 129 shown) extending through the thickness dimension of fiber optic substrate 111. Two frit seal assemblies (126, 129) are illustrated in
The photocathode face plate assembly 102 includes input face plate 103 and photocathode 104 deposited onto face plate 103. The photocathode 104 may be thermally bonded onto face plate 103. The central axes of photocathode 104, MCP 106 and phosphor screen 112 may be substantially aligned with respect to one another.
Still referring to
Several vacuum-compatible frit seal assemblies are extended through the thickness dimension of fiber optic substrate 111. Five frit seal assemblies are positioned about the circumference of fiber optic substrate 111, as illustrated in
The present invention advantageously uses each frit seal assembly to provide a way to apply discrete voltages to the interior components of tube assembly 100 from the exterior of tube assembly 100, without compromising the vacuum within interior chamber 113. The frit seal assemblies are isolated from one another as they are distributed about fiber optic substrate 111, as best shown in
In assembly, frit beads 122 and contact sleeves 121 (
The input faceplate 103 provides a mounting surface for photocathode 104. The input faceplate may be formed of a non-conductive material, such as, glass or ceramic. Similar to fiber optic substrate 111, the general shape of input faceplate 103 may be cylindrical.
As best seen in
The MCP spacer 114 may be fixed to both MCP 106 and screen assembly 110 with either an epoxy or solder. A variety of epoxies or solders may be used to fix the components. It is contemplated that the epoxy may be a high temperature vacuum compatible epoxy. If a solder is utilized, the solder may be a low temperature solder capable of bonding at low pressure and reflow in a vacuum bake. Non-limiting examples of solder materials are indium thin film or gold/tin thin film. The solder may be in the form of a decal (0.0005″ thick, for example). A decal is advantageous from an assembly perspective, because the thickness tolerance of a decal is minimal (for example, about 0.0001″), thereby providing a relatively consistent assembly process. Furthermore, the flat shape of spacer 114 together with a flat decal further enhances the assembly process.
The cathode spacer 118 is sandwiched between and completely separates screen assembly 110 and face plate assembly 102. Together with MCP spacer 114, the thickness tolerance of cathode spacer 118 effectively controls the distance between MCP 106 and photocathode 104. Optionally formed from a single flat cylindrical ring, cathode spacer 118 may be formed from any conductive or non-conductive material, such as, copper, glass or aluminum. Moreover, since fiber optic substrate 111 may be formed from a non-conductive material, cathode spacer 118 may be formed from a conductive material without concern of shorting out the electrical potentials of phosphor screen 112 and photocathode 104. The cathode spacer may (or may not) extend continuously around the circumference of tube assembly 100. The cathode spacer is optionally coupled to both fiber optic substrate 111 and input faceplate 103 using an epoxy, solder, weld or any other attachment known in the art.
The centering ring 117 illustrated in
Alternatively, the interface between centering ring 117 and cathode spacer 118 does not have to be vacuum tight. Accordingly, centering ring 117 may be joined to cathode spacer 118 via any mechanical fastening means, such as press-fitting. The centering ring may also be coupled to cathode spacer 118 via epoxy, braze or indium film. Although cathode spacer 118 and centering ring 117 are illustrated as separate components in
As described previously, a vacuum condition exists within interior chamber 113 of tube assembly 100. The vacuum facilitates the migration of electrons from photocathode 104 to MCP 106 and then to phosphor screen 112. Although tube assembly 100 is sealed, gas molecules may form within interior chamber 113 over the lifetime of the tube assembly. A getter 119 maintains the vacuum condition by collecting gas molecules within interior chamber 113. The use of getter materials is based on the ability of certain solids to collect free gases by adsorption, absorption or occlusion, as is well known in the art.
The getter optionally takes the form of a cylindrical ring extending around a circumference, as illustrated in
It will be appreciated that an image intensifier tube assembly may include either an evaporable or non-evaporable type getter. In this exemplary embodiment, evaporable getter 119 and corresponding getter shield 120 are selected for illustration. Over the course of operation, the evaporable getter material evaporates and collects on the surface of getter shield 120. If a non-evaporable getter is selected, a getter shield may not be required.
The getter shield is a cylindrical ring, optionally, extending around a circumference and positioned radially interior to getter 119. Although getter shield 120 is illustrated as a cylindrical ring, getter shield 120 is not limited to such shape or form. The getter shield may be formed of any vacuum compatible and structurally stable material. A weld, epoxy or thin film may be applied at the interface of getter shield 120 and fiber optic substrate 111 to temporarily or permanently couple the two components.
Halo is a factor limiting the performance of image intensification tubes and is dependent upon the distance separating the photocathode and MCP. The tolerance of a desirable distance between the photocathode and MCP may be on the order of several microns. In order to limit or minimize halo, the distance between the MCP and the photocathode must be precise. The prior art example, illustrated in
In the exemplary embodiment illustrated in
The photocathode 104, MCP 106, phosphor screen 112 and getter 119 are each separately connected to an electrical power source (not shown). Each of these components operates at a different voltage potential. In particular, phosphor screen 112 is maintained at a higher positive potential than photocathode 104. MCP 106 is maintained at a higher positive potential than photocathode 104 and a lower positive potential than phosphor screen 112. Accordingly, the conductive paths directing electrical voltage to each component are isolated from one another to inhibit electrical shorting of the conductive paths.
Referring now to
Referring now to
Referring back to
Tube assembly 100, shown in
The MCP input path 132 includes multiple conductive regions, i.e. conductive regions 132a through 132e. Conductive regions 132a through 132c are deposited on the exterior surfaces of MCP spacer 114 and conductive regions 132d and 132e are deposited on the exterior surfaces of MCP 106. More specifically, conductive region 132a is deposited onto the bottom side 115 of MCP spacer 114 and is in contact with conductive strip 158 deposited on fiber optic substrate 111. Conductive region 132b extends vertically along a portion of the outer surface of MCP spacer 114 and is connected to conductive region 132a on the bottom side 115 of MCP spacer 114. Conductive region 132c is deposited in an annular shape onto the top side 116 of MCP spacer 114 and is connected to conductive region 132b. The conductive region 132c reaches the outer diameter of spacer 114 and optionally extends around the total circumference of spacer 114. The size of the region 132c may be any dimension sufficient to distribute current.
The top side 116 of MCP spacer 114 is in contact with bottom side 108 of MCP 106. Moreover, a conductive region 132d deposited onto the bottom side 108 of MCP 106 is substantially aligned with and connected to conductive region 132c deposited on the topside 116 of MCP spacer 114. The conductive region 132d optionally extends around the total circumference of MCP 106. Conductive region 132d is substantially the same size as the annular conductive region 132c deposited on MCP spacer 114. Conductive region 132e extends vertically along a portion of the outer surface of MCP 106 and is connected to conductive region 132d. Although not illustrated, conductive region 132e may extend along the entire circumference of MCP 106. The conductive region 107′ on top input side 107 of MCP 106 is connected to conductive region 132e. Conductive region 107′ is a metallic surface that maintains top input side 107 of MCP 106 at a predetermined voltage.
A portion of MCP input path 132 is illustrated in
Similar to MCP input path 132, the MCP output path 130 includes multiple conductive regions, i.e. conductive regions 130a through 130c, which are deposited on the exterior surface of MCP spacer 114. More specifically, conductive region 130a is deposited onto the bottom side 115 of MCP spacer 114 and is in contact with conductive strip 159. Conductive region 130a is sufficiently separated from conductive region 132a, to avoid electrically shorting the two regions. The size, shape and location of conductive region 130a, as well as conductive region 132a, are not limited to the embodiment selected for illustration.
Conductive region 130b extends vertically along the interior cylindrical surface of MCP spacer 114 and is connected to conductive region 130a. Although conductive region 130b extends along a portion of the circumference, as shown in
The conductive region 108′ patterned on bottom output side 108 of MCP 106 is connected to conductive region 130c which is positioned on top side 116 of MCP spacer 114. The centers of both conductive region 108′ and conductive region 130c are substantially aligned to ensure conductive contact between the regions. Conductive region 108′ is a metallic surface that maintains bottom output side 108 of MCP 106 at a predetermined voltage. The conductive region 108′ and 132d are separated by an annular gap “B”, which may be any dimension sufficient to inhibit electrically shorting conductive regions 108′ and 132d.
In brief review, a current carrying wire, male pin or female contact extending from a power supply (not shown) is connected to MCP electrical input contact 150. The current is conducted through MCP electrical input contact 150 to conductive strip 158. Conductive strip 158 is in contact with conductive regions 132a through 132c deposited on the exterior surface of MCP spacer 114. The current is thereafter conducted through conductive regions 132d and 132e deposited on the exterior surface of MCP 106. The conductive region 132e is connected to conductive region 107′, which maintains top input side 107 of MCP 106 at a predetermined voltage.
Moreover, a current carrying wire, male pin or female contact extending from a power supply is connected to MCP electrical output contact 152. The current is conducted through MCP electrical output contact 152 to conductive strip 159. Conductive strip 159 is linked to conductive regions 130a through 130c deposited on the exterior surface of MCP spacer 114. The current is thereafter conducted to conductive region 108′, which maintains bottom output side 108 of MCP 106 at a predetermined voltage. The bottom output side 108 of MCP 106 may be maintained at a higher or lower voltage potential than top input side 107 of MCP 106.
Referring now to
Referring now to
With regards to the assembly of tube assembly 100 of this exemplary embodiment, the phosphor screen 112 is evaporated, plated or bonded on the top surface of fiber optic substrate 111. The frit seal assemblies are inserted through apertures 123 positioned in fiber optic substrate 111. The frit seal assemblies are heated until the glass frit melts and forms an optionally hermetic vacuum-tight seal. The MCP spacer 114 is oriented above fiber optic substrate 111 so that conductive regions 132a and 130a align with and conductively engage with conductive strips 158 and 159, respectively. A conductive epoxy, weld, solder or thin film, for example, may be employed to fix MCP spacer 114 to fiber optic substrate 111. The MCP 106 is oriented above MCP spacer 114 so that conductive regions 132d and 108′ physically contact and conductively engage with conductive regions 132c and 130c, respectively, of MCP spacer 114. A conductive epoxy, weld, solder, or thin film for example may be employed to adhere MCP 106 to MCP spacer 114. The cathode spacer 118 may be coupled to fiber optic substrate 111 via a brazing, welding or application of conductive epoxy, for example. Getter 119 is positioned adjacent cathode spacer 118 and optionally coupled to cathode spacer 118 via a brazing, welding or application of conductive epoxy, for example. If getter 119 is of the evaporable type (as illustrated in
The input faceplate 103 is positioned above cathode spacer 118. The centering ring 117 is coupled to fiber optic substrate 111 and input faceplate 103. The centering ring 117 is optionally coupled to cathode spacer 118. An indium seal (optionally an indium decal) may be positioned at the mating interface of centering ring 117 and fiber optic substrate 111 and the mating interface of centering ring 117 and input faceplate 103. A minimal amount of pressure is applied to the indium seals and the entire assembly is thereafter reflowed in a vacuum bake to form a hermetic seal at the aforementioned mating interfaces.
Although assembly steps are described herein, the assembly process is not limited to the steps or step order as described. Rather, the assembly order and assembly components may vary widely from the above description.
Referring now to
Referring now to
The top surface of the ceramic header of silicon imager assembly 320 may be substantially planar. For the purposes of comparison, the top surface of various prior art silicon imager assembly embodiments include a protruding rib segment for electro-optic focusing purposes. Accordingly, the exclusion of a protruding rib segment in this exemplary embodiment and more particularly, the reduction of a manufacturing step to create the protruding rib segment, may represent a cost savings. Alternatively, the ceramic header of the silicon imager assembly may be non-planar to incorporate the cathode spacer and the MCP spacer.
The steps to assemble tube assembly 300 are similar to the assembly process associated with tube assembly 100, with the exception of assembling silicon imager assembly 320. A portion of the exterior circumference of silicon imager assembly 320 may be coupled to centering ring 317 with an indium seal, braze, weld, solder, epoxy, or any other fastening method known in the art. The interface between silicon imager assembly 320 and centering ring 317 may be hermetically sealed to maintain a vacuum within tube assembly 300.
While preferred embodiments of the invention have been shown and described herein, it will be understood that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the spirit of the invention. Accordingly, it is intended that the appended claims cover all such variations as fall within the spirit and scope of the invention. Also, the embodiments selected for illustration in the figures are not shown to scale and are not limited to the proportions shown. Although attachment means have been described herein, it will be understood that any attachment means known in the art may be utilized.
