Patent Application
20070195257
Non-applicable.
The present invention relates to liquid crystal (LC) or digital micromirror device (DMD) projectors, and more particularly to LC or DMD projectors incorporating an LC or DMD imager panel sub-assembly having an enclosed dust-sealed airspace in front of the imager panel.
Optical imaging systems typically include a transmissive or reflective imager, also referred to as a spatial light modulator, a light valve or light valve array, which imposes an image on a light beam. Transmissive light valves are typically translucent and allow light to pass through. Reflective light valves, on the other hand, reflect only selected portions of the input beam to form an image. Reflective light valves provide important advantages, as controlling circuitry may be placed behind the reflective surface and more advanced integrated circuit technology becomes available when the substrate materials are not limited by their opaqueness. Potentially inexpensive and compact liquid crystal display (LCD) projector configurations may become possible by the use of reflective liquid crystal microdisplays, e.g. employing liquid crystal on silicon (LCoS) chips, as the imager.
In a microdisplay-based optical projector, a microdisplay imager panel forms a pixilated image by spatially modulating light incident thereon, which is then projected onto an image screen by a projection lens with a magnification. Many reflective LCD imagers rotate the incident light polarization on a pixel-by-pixel basis by an angle depending on a relative brightness of a particular pixel in the image. In other words, polarized light is either reflected by the imager with its polarization state substantially unmodified for the darkest state, or with a degree of polarization rotation imparted to provide a desired grey scale. Typically, a 90° rotation provides the brightest state in these systems; a corresponding state of the imager is sometimes referred to as an “ON” state. Accordingly, a polarized light beam is generally used as the input beam for reflective LCD imagers.
Typically, the projected image of the microdisplay is magnified by a factor of 100 or larger. For front or rear projection, the pixels are typically on the order of 10 μm on the microdisplay and 1 mm on the projected screen image. Small defects on the imager surface can become magnified by the projection optics and thus diminish the image quality of the projected image. For example, a 10 μm defect could block the light from a single pixel. For this reason, a class 100 clean room environment must be maintained during the manufacturing of these microdisplay based projection systems.
Further, the imaged surfaces of the microdisplay must remain particle and dust free throughout the life of the projector. Dust that circulates through the projector, e.g. by an air cooling system, can settle on the imaged microdisplay surface and also be imaged onto the image screen, diminishing the image quality on the screen.
A variety of techniques have been proposed for sealing dust out of a projector. U.S. Pat. No. 6,280,036 describes a method of mechanically sealing a microdisplay imager to a field lens in order to eliminate dust. This method, however, may not be suitable for projectors based on color separation, wherein a single field lens is shared between several, typically three, microdisplay imager chips. U.S. Pat. No. 6,394,608 and US Patent Application 2002/0033992 A1 describe sealing the microdisplay imager and optical system inside a dust-proof box. A similar technique is proposed in U.S. Pat. No. 6,350,033. This approach, however, may require manufacturing of the whole optical system inside the box in a type 100 clean room environment, thereby substantially increasing production cost. Also, three-chip projectors often require adjustment on each of the red, green, and blue channels. Sealing the microdisplay chips in a dust sealed chamber prevents such adjustment.
US patent application 2005/0122482 discloses a projector incorporating a color-combining prism and a plurality of micro-displays for modulating light that passes through the prism, wherein the micro-displays are sealed against the prism with a dust-sealed air channels between the prism and respective micro-displays for cooling thereof. This solution is geared towards high-lumen applications, where forced air cooling of the imaged microdisplay surface may be required. It is also restricted to projection systems wherein microdisplay chips are positioned immediately next to the color-combining prism, and is not applicable to projection systems wherein the microdisplay imagers are positioned relatively away and/or at an angle to their respective prism surfaces. An example of such a projector is disclosed in U.S. Pat. No. 6,857,747, which is assigned to JDS Uniphase Corp., the assignee of the instant invention, and is incorporated herein by reference.
High-contrast LC optical projectors typically incorporate trim retarders next to the imager panels. A trim retarder compensates for residual retardation of the micro-display imager panel in a dark (OFF) state. Unlike typical birefringent waveplates providing ¼λ or ½λ retardation, a typical A-plate trim retarder provides between 1 nm and 50 nm of in-plane retardance. The primary benefit of introducing a trim retarder into a display system is to enhance image contrast, while not significantly degrading the ON-state brightness. Trim retarders incorporating both positive and negative birefringence to compensate for retardances resulting from LCoS display panels is disclosed in US Patent Application 2005/0128391, which is assigned to JDS Uniphase Corp., the assignee of the instant invention, and is incorporated herein by reference.
In a conventional LCoS display system, illustrated in
Conventional thermotropic liquid crystals in reflective LCoS imager panels are either twisted nematic, e.g. 45° twist (45TN), or vertically-aligned nematic (VAN-mode), which can be electrically switched, or relaxed, to a near homeotropic orientation. Other LC-modes also require trim retarders, if the LC-technology employs a dark-state director orientation near the homeotropic alignment. In the homeotropic alignment, the LC uniaxial positive molecules are oriented normal to the device plane. The dark, or OFF, state may be a switched state or a relaxed state, depending on LC modes. In most applications, a true homeotropic orientation in the dark state is not suitable, i.e. a pre-tilt is required to provide consistent and faster switching behavior.
As a consequence, the display panels exhibit both an in-plane and an out-of-plane residual retardation component, i.e. A-plate and C-plate components, respectively. The C-plate component is typically positive, thereby adding to the net panel retardance at off-axis illumination. Accounting for the imager panel and the light engine characteristics, the residual retardation compensation can be broadly divided into two steps: first, the in-plane retardation component of the imager panel is negated by aligning an A-plate component with an optic axis of the imager panel, which is also the c-axis, at 90° relative azimuth, and then improving the field of view by removing the out-of-plane retardance of the imager with a negative C-plate retarder component.
Importantly, a fine-tuning of the mutual alignment of polarization axes of the trim retarder and the corresponding imager panel is typically required after the projector assembly for obtaining optimum image contrast.
An object of the present invention is to provide a sealed imager panel subassembly having a dust-free space in front of an imaging surface of the imager, that can be assembled independently on the rest of the projection system.
It is another object of the present invention to provide a dust-sealed imager panel subassembly incorporating a rotatable trim retarder for adjustable residual polarization compensation.
In accordance with the invention, an imager panel subassembly for use in an optical projection system is provided, comprising: a reflective imager panel having a front surface opposite to a back surface thereof, for spatially modulating an optical beam incident upon said front surface for producing an image; an optically transparent trim retarder for enhancing the image contrast, the optically transparent trim retarder disposed in front of the front surface of the imager panel; a spacer separating the imager panel and the trim retarder for forming an enclosed airspace therebetween; and, coupling means for securely coupling the trim retarder and the imager panel together in a pre-determined relative position independent of the optical projection system, so that the enclosed airspace is sealed for protecting said enclosed airspace from outside dust.
In accordance with one aspect of this invention, the coupling means of the imager panel subassembly for securely coupling the trim retarder and the imager panel is adjustable, so that to allow rotation of the trim retarder relative to the imager panel while maintaining dust-tight sealing of the enclosed airspace.
In accordance with another aspect of the present invention, the spacer comprises a front plate having an aperture opening for exposing the front surface to a polarized light via the trim retarder through said aperture, and the coupling means comprises means for holding the trim retarder against the front plate for covering the aperture opening and for allowing rotation thereof relative to the imager panel while maintaining a dust-tight seal around the aperture opening.
The invention will be described in greater detail with reference to the accompanying drawings representing preferred embodiments thereof, in which like numerals represent like elements, and wherein:
Three exemplary embodiments of the imager panel subassembly will now be described. It should be noted that while various elements of these embodiments may be suitably coupled or connected to other elements within the exemplary embodiments, such connections and couplings can be realized by direct connection between elements, or by connection through other elements and devices located thereinbetween.
The imager panel 230 is embodied as a LCoS microdisplay; it has a front, or imaging surface 233, and a back surface 238 opposite thereto. The flex cable 235 is connected to the LCoS imager panel 230 for providing electrical video signals to the LCoS chip for controlling LC pixels of the imager panel 230, as it is conventionally done in LCoS-based microdisplays.
In operation, a polarized light beam, not shown in the figures, impinges upon the front surface 233 of the LCoS imager panel 230, passes through a pixilated LC layer beneath the front surface 233, and is reflected back from reflective electrodes imbedded in the imager 230 under the LC layer, so that the reflected beam carries image information imparted on it by the pixilated LC layer via spatial modulation of the beam polarization. The front surface 233 of the imager panel 230 is also referred to herein as an imaging surface. The LCoS imager panels described in this specification, such as the LCoS imager panel 230, are also referred to hereinafter as imager panels, or simply imagers.
The LC layer of the LCoS imager panel 230 is preferably protected by a thin cover glass 231, which is typically about 0.5 mm to 1.1 mm thick. The imaging surface 233 of the imager panel 230 is understood herein to be an outer surface of the cover glass 231. Note that the imaging surface 233, as defined hereinabove, should not be confused with an imaging plane of the imager 230, which is commonly understood in the art to be a plane of the beam reflection directly beneath the LC layer of the imager panel.
The front plate 240 is disposed in front of the imaging surface 233 of the imager panel 230, and is held juxtaposed thereto in a fixed position. It has an opening 245 forming an aperture for exposing at least a portion of the imaging surface 233 to the polarized light beam. The optically transparent window 250 is held against the front plate 240 fully covering the aperture opening 245, so that an enclosed airspace is formed within the aperture opening 245 between the window 250 and the imaging surface 233 of the imager panel 230. The front plate 240 functions as a spacer between the front surface 233 of the imager 230 and the window 250, so as to form the enclosed airspace therebetween and, simultaneously, as a circumferential gasket protecting the enclosed airspace from the dust.
The enclosed airspace between the imager 230 and the transparent window 250, also referred to hereinafter as the airgap, is an important feature of the present invention. It will be discussed more in detail hereinafter in the specification with reference to
In a preferred embodiment, the optically transparent window 250 is a birefringent trim retarder plate, which improves image contrast by compensating a residual retardance of the LCoS chip in a dark state, as described hereinabove in the Background section of this specification. Hereinafter, the trim retarder 250 will also be referred to as a birefringent contrast enhancer, or BCE 250. The trim retarder 250 preferably includes at least one C-plate and at least one A-plate for compensating an in-plane and an out-of-plane components of the residual retardance of the imager panel 230, respectively, said A plate and C plate preferably sequentially coupled together to form a monolithic plate. Unlike a conventional ½λ waveplate, the trim retarder 250 provides only about 1 nm to 50 nm of retardance. US Patent applications 2005/0128380 in the names of Zieba et al, and 2005/0128391 in the names of Tan et al, both assigned to JDS Uniphase, the assignee of the current application, which are incorporated herein by reference, provide examples of polarization controlling elements and birefringent trim retarders that can be used as the trim retarder 250.
For optimized performance, an in-plane retardation component of the trim retarder has to be aligned in a pre-determined azimuthal orientation, e.g. 90° relative to the in-plane birefringent optical axis of the imager panel 230, see e.g. the US patent application 2005/0128391 incorporated herein by reference. Advantageously, the present invention provides means for performing this alignment outside a clean-room environment after fully assembling therein the imager subassembly, by rotating the trim retarder 250 in a plane parallel to the imaging surface 233. The angular adjustment of the trim retarder 250 can be performed in a dust-sealed configuration of the subassembly, wherein the enclosed airspace in front of the imager panel 230, and the front surface 233 thereof are protected from outside dust, as described more in detail hereinafter in this specification.
The transparent trim retarder 250 is held in the window frame 260 in a fixed position relative thereto; the trim retarder 250-frame 260 sub-assembly is referred hereinafter as a framed trim retarder 250/260. The window frame 260 has outer “shoulders” 265 about the transparent trim retarder 250. The holding plate 270, embodied herein as a clip frame with an opening 275 for the trim retarder 250, holds the window frame 260 by pressing against the frame shoulders 265, so that to tightly couple together the trim retarder 250, the front plate 240, and the imager 230 by holding the framed trim retarder 250 pressed against the front plate 240 in a dust-tight contact thereto, and the front plate 240 against the imager 230, thereby forming a circumferential seal between the framed trim retarder 250 and the imager 230. This circumferential seal prevents outside dust from getting into the sealed space between the imaging surface 233 of the imager 230 and the transparent window 250.
The front plate 240 can be either fixedly attached to the imager 230, e.g. by bonding directly thereto with a low-modulus adhesive, or can be simply held in its position by resilient fingers 277 and a bent bottom portion of the holding plate 270 protruding below the front plate 240 and limiting movements of the front plate 240 in a vertical direction, and/or by friction due to the pressure applied thereupon by the plate holder 270 via the framed trim retarder 250/260.
The holding plate 270 can be coupled either directly to the imager panel 230, e.g. using two or more clamps or clips, or indirectly via a carrier or a holder holding the imager 230. In the embodiment shown in
In a preferred embodiment, the back surface 238 of the imager 230 is in a thermal contact with the back plate 220, for enabling heat dissipation under exposure to a high-power beam, when light absorption by the imager 230 can lead to overheating thereof. In some embodiments, the back plate 220 is attached to a heat sink 210, which may be cooled by a forced air flow within the projector. The back plate 220 preferably includes means for holding the imager panel 230 in a fixed position relative thereto. This means would be obvious to those skilled in the art, and may include a plurality of clamps or clips, such as e.g. four clip fingers 221 shown in
As shown in
In one embodiment of the imager subassembly, an allowable rotation range is ±10° about a vertical position of the trim retarder 250; this range is typically sufficient for fine-tuning of the relative angular position of the trim retarder 250 and the imager 230 during assembly of the optical projector.
In the aforedescribed first exemplary embodiment, the movable window frame 260 holds the trim retarder 250 with two clip holders, which can be spring-loaded. In one embodiment, as shown in
As stated hereinbefore with reference to
In the hereinbefore described embodiments of the imager sub-assembly 200, the thickness of the front plate 240 determines the width of the airgap 300 between the BCE 250 and the imager panel 230. The airgap 300 is an important feature of the instant invention, and will now be addressed with reference to
As shown in these figures, the airgap 300 separates the front surface 233, also labeled as “FRONT OF THE COVER GLASS”, of the LCoS imager panel 230 from the BCE 250. This separation serves several important purposes. Firstly, the airgap 300 partially isolates the LCoS imager 230 from over-heating and stress, that could have otherwise be induced in the LCoS imager 230 by a contact with the BCE 250 during operation. Secondly, the airgap 300, when sealed from the outside dust, serves to reduce detrimental effects of the dust on the image quality.
Indeed, moving a dust particle away from the front surface 233 of the LCoS imager 230 reduces an image contrast between the dust particle image and an image created by the LCoS imager 230 on the projection screen, due to a “blurring” of the particle image. As the particle moves away from the imaging plane of the imager 230, it becomes “out of focus” at the projection screen due to imaging properties of the projection lens. This image blurring, and hence the particle image contrast, can be computed using conventional methods from known optical properties of the projection lens system used in the projector. By way of example,
C=Iparticle/ILCoS,
where Iparticle is an average irradiance of the screen within the particle image area due to light radiated from the LCoS panel 230, and ILCoS is a screen irradiance outside of the particle image area due to the light from the LCoS panel 230. The LCoS imager 230 is assumed to be in an ‘ON’ state and the particle is obstructing/shadowing light from reaching the projection lens due to its transmission and/or polarization properties. Curves 10 and 100 show the estimated image contrast for screen images of particles having a diameter of 10 μm and 100 μm, respectively, in dependence on the particle distance d from the imaging plane of the LCoS imager 230; the particles are assumed to be fully absorbing, non-scattering, and centered in the field of view of the projection lens.
As the curves 10 and 100 demonstrate, the particles image contrast, and therefore—their visibility to an observer, rapidly decreases as the particle-imaging plane distance d increases beyond a few millimeters. To estimate when the particle image becomes invisible to an average human observer, the particle image contrast curves 10, 100 should be compared to a contrast sensitivity function (CSF) of a human eye, which has been extensively studied in the art; this function is represented in
Turning back to
The curves 12 and 120 were computed using the experimental human CSF data of Ramamurthy et al, as follows. First, image dimensions s10(d) and s100(d) for the 10 μm and 100 μm particles, respectively, in dependence on the distance d from the LCoS imaging plane, were calculated using the aforegiven exemplary parameter M of the imaging system and the blurring properties of the projection lens; second, spatial frequencies f10(d) and f100(d), in cycles per degree, were computed from the corresponding image sizes using the equations f10(d)=1/(2·tan−1(s10/l)) and f100(d)=1/(2·tan−1(s100/l)); next, the experimental CSF values given by Ramamurthy et al were extrapolated to obtain contrast values corresponding to the visibility threshold for the computed spatial frequencies; and, lastly, the so obtained contrast values for each of the 10 μm and 100 μm particles were plotted versus the distance d to obtain the curves 12 and 120, respectively.
In order for an image of the 10 μm particle located at a distance d from the imaging plane of the LCoS 230 to be visible on the screen by the viewer, the image contrast has to exceed the contrast detection threshold curve 12 for the particular distance d. Accordingly, an intersection point 15 of the curves 10 and 12 gives an estimate of a minimum separation value d10 between the LCoS imaging plane and the 10 μm particle, at which the particle image becomes substantially invisible to the viewer looking at the image from a distance of 0.5 m. For the exemplary optical system parameters given hereinabove, this minimum separation d10 is approximately 1.8 mm.
Similarly, in order for the 100 μm particle image to be visible, its contrast has to exceed the contrast detection threshold 120, and an x coordinate of the point 150 of intersection of curves 100 and 120 gives an estimate of a minimum separation value d100 between the LCoS imaging plane and the 100 μm particle, at which the 100 μm particle image becomes substantially invisible to the viewer. For the exemplary optical system parameters given hereinabove, this minimum separation for a 100 μm particle d100 is approximately 4.35 mm.
Advantageously, the insensitivity of a perceived quality of the image created on the screen by the LCoS 230 to the presence of 10 μm dust particles between the LCoS sub-assembly 200 having d>d10 and the projection lens, enables assembling of the rest of the projection system, excluding the assembly of the imager sub-assembly 200, within a class 1000, or ISO class 6, clean room rather than within a class 100 (ISO class 2) clean room. The projection system insensitivity to the presence of 100 μm-size particles can further advantageously relax the assembly clean room requirements, and/or make the image quality less sensitive to other optical obstructions of this size located outside the imager subassembly 200. For example, micro-scratches of the front surface of the BCE 250, which can appear e.g. during the system assembly or repairs, are known to have a typical width on the order of 100 μm; therefore, having, in the imager subassembly 200, the front surface of the BCE 250 separated from the imaging plane of the imager 230 by a distance about or exceeding d100˜4.35 mm, is advantageous for making the projection system's image quality insensitive to such scratches.
By way of example, in one embodiment of the sub-assembly 200, the cover glass 331 protecting the LC layer of the LCoS imager has a thickness d0=0.5 mm and the BCE 250 has a thickness dBCE=0.8 mm. In this case, the airgap 300 is preferably at least 0.5 mm thick to ensure that a 10 μm particle on an outer surface of the BCE 250, said surface labeled as “FRONT OF THE BCE” in
In another example, the cover glass thickness d0=1.1 mm, the BCE 250 thickness dBCE=1.4 mm. In this embodiment, 10 μm particles located anywhere outside the imager subassembly 200 will be virtually invisible on the screen for any airgap 300; the minimum airgap 300 thickness corresponding to the 100 μm particle visibility threshold (d100−d0−dBCE) is about 1.85 mm.
Generally, increasing the airgap 300 thickness will advantageously reduce the visibility of the dust defects on the projector screen, with an optimum distance depending on the size distribution of the dust particles within the projector and on optical properties of the imaging system, e.g. of the projection lens. A particular value of the airgap 300 thickness may be limited, however, by other constraints, such as those related to the overall design of the projector, and which would be known to a skilled in the art system designer; for example, it can be limited by a constraint on the overall thickness of the imager subassembly 200 in a particular projection system design; an optimal thickness of the airgap 300 would be clear to those skilled in the art for each particular application.
However, according to the present invention, the enclosed airspace between the BCE 250 and the imager 230 is preferably preserved in any projector design wherein the present invention is practiced, so that the airgap thickness 300 is at least 0.1 mm. The presence of the airgap 300 is advantageous for protecting the imager panel 230 from a direct contact with the BCE 250; such a contact is undesirable since it can lead to over-heating of the imager panel due to light absorption in the BCE 250, and can also result in mechanical deformations and appearance of stress in the imager 230, leading to an undesired stress-induces birefringence therein, and a loss of the image contrast. For these same reasons, the airgap should not be substituted with a transparent, i.e. glass, spacer, as it could adversely affect the imager's performance due to the imager's sensitivity to temperature and stress effects therein. A preferred value on the airgap 300 thickness is between about 1 and 5 mm in the embodiments described herein.
The aforedescribed imager panel subassembly 200 therefore provides the benefits of the dust protection similar to those provided by the prior-art solutions, e.g. those disclosed in U.S. Patent Application 2005/0122482, and U.S. Pat. No. 6,280,036. Importantly, it provides additional considerable advantages over the prior art solutions disclosed in the aforecited patents and other known prior art. Indeed, the solution of the present invention enables a significant reduction in time and complexity of the assembly that has to be performed in class 100 clean room environment, thereby reducing manufacturing time and cost. Indeed, in the prior art solutions, a significant number of optical elements had to be assembled together before a resulting rather complex assembly can be dust-sealed; typically, these optical elements include all three imager panels, a beam-splitting centerpiece including at least one beam splitter and, possibly, lenses. Contrary to that, the solution of the present invention provides a separate relatively simple dust-tight subassembly, e.g. the subassembly 200, for each individual LCoS panel 230, so that the rest of the projector can be assembled outside of the clean room, or in a clean room of a lower “cleanness” standard, e.g. in a class 1000 clean room.
Next, the imager subassembly of the present invention advantageously includes the trim retarder 250, which, in addition to functioning as a contrast enhancer, serves also as an optically transparent window providing an optical path to the LCD imager 230, while assisting in sealing the imaging surface 233 of the imager 230 from dust.
Furthermore, the present invention provides the imager subassembly wherein the angular orientation of the trim retarder 250; or the BCE, relative to the imager 230 is adjustable without jeopardizing the dust-tight sealing of the imager subassembly. This unique feature of the preferred embodiment of the invention enables fine-tuning of the LCoS imager performance outside of the clean room environment, thereby further simplifying the projector assembly and calibration; it also enables post-production adjustments of the BCE-imager alignment for optimizing image contrast.
Of course, other embodiments of the imager-BCE subassembly of the present invention can be realized, that posses some or all of the aforedescribed important advantages of the invention. Two of them will now be briefly discussed with reference to
Similarly to
Differently from the first exemplary embodiment shown in
In some embodiments, the imager subassembly is sealed air-tight after the assembly thereof by applying a sealing agent, e.g. 3M's epoxy adhesive DP-105, circumferentially around the edges of the sub-assembly 400 to seal the contact areas between the front plate 340 and the back plate 320, and around the cable 235 opening at the top of the subassembly. In these embodiments, a vent hole covered with a dust filter, e.g a HEPA (high efficiency particulate arrestance) filter, is advantageously provided in the subassembly to ensure even air pressure inside and outside the dust sealed airspace and prevent associated mechanical deformations of any portion of the subassembly 400. In the embodiment shown in FIGS. 7A-B, the vent hole 342 is provided in the front plate 340, and is covered with the dust filter 344. This is further illustrated in
The aforedescribed first and second exemplary embodiments of the invention both provide an important ability to tune the relative angular alignment of the LC imager 230 and the BCE 250 while still maintaining the dust seal therebetween. In preferred embodiments at least ±10 degree variation in the mutual alignment of the imager 230 and the BCE 250 is allowed, so that the A-plate of the BCE 250, which is a residual LC panel retardance compensator, can be individually tuned to each LCoS panel used in the manufacturing of the subassembly of the present invention as the imager 230, for compensating of small variations in the LC layer orientation parameters, and for improving thereby the yield of the microdisplay subassembly.
This feature can be omitted in some cases to simplify the subassembly, e.g. as the accuracy of the LCoS microdisplay chips alignment is sufficiently improved, so that alignment deviations from chip to chip are insignificant.
In this embodiment, the coupling means for securely coupling the trim retarder and the imager panel together in a pre-determined relative position independent of the optical projection system, so that the enclosed airspace is sealed for protecting thereof from outside dust, is formed by the adhesive, and any fastening means for attaching the front plate 440 to the back plate 320, such as the bolt 390, and two protruding fingers 349 of the front plate 440 for engaging the back plate 320, as shown in
Of course, numerous other embodiments may be envisioned without departing from the spirit and scope of the invention utilizing all or some of the aforedescribed features of the present invention in various combinations; for example, it should be understood that each of the preceding embodiments of the present invention may utilize a portion of another embodiment.
The many features and advantages of the present invention are apparent from the aforegiven description and, thus, it is intended by the appended claims to cover all such features and advantages of the invention that fall within the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described with reference to the three exemplary embodiments, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.
