Abstract:
A shape measurement apparatus capable of saving time and effort of an examiner and improving measurement accuracy. The apparatus for measuring a spherical shape of an object to be examined comprising a measurement unit where a first projection optical system projects first targets for measurement onto a spherical surface to be measured, a first detection optical system detects images of the projected first targets and a calculation unit obtains a shape of the surface based on the detected images, an output unit which outputs the obtained shape, a mode setting unit which sets any one of a first measurement mode for a convex shape and a second measurement mode for a concave shape, and a condition setting unit which sets at least one of a measurement condition and an output condition based on a mode setting result, the conditions being different between the first and second measurement modes.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a shape measurement apparatus for measuring a spherical shape of an object to be examined, particularly, a shape measurement apparatus suitable for measuring a spherical shape of a cornea of an eye to be examined, a spherical shape of a contact lens and the like. 
     2. Description of Related Art 
     A corneal shape measurement apparatus for measuring a spherical shape of a cornea of an eye to be examined (meridional radius of curvatures of a steepest meridian and a flattest meridian, and an axial angle in the direction of the steepest or flattest meridian) is used for measuring a spherical shape (a base curve) of a contact lens (radius of curvatures on a concave surface side) in addition to the corneal shape measurement. In this case, a convex surface is measured at the time of the measurement of the corneal shape, while a concave surface is measured at the time of that of the base curve of the contact lens. In order to comply with both of the measurements, a measurement target symmetrical to the measurement optical axis (a target which has an annular shape having the measurement optical axis at its center or four targets which are vertically and horizontally symmetrical to the measurement optical axis) is generally used. 
     However, the conventional apparatus does not determine whether the measured spherical surface of the object is convex or concave. Therefore, an examiner need manage measurement data and it is bothersome for the examiner. For example, as is different from the measurement of the corneal shape having a convex surface, in the case of that of the base curve of the contact lens having a concave surface, the examiner makes a memo in printed data that the data is as to the contact lens (the concave surface measurement data). In addition, in the case of the measurement of a base curve of a contact lens having an astigmatic axis depending on a corneal shape of an examinee, the direction of its principal meridian (AXIS: an axial angle) need be reversed laterally, and the examiner performs reverse by manual calculation. 
     Further, in the case of the measurement of the base curve of the contact lens, as is different from that of the corneal shape, a back-surface reflection (a reflection from a convex surface side at the time of the concave surface shape measurement) exists, and it results in a problem of influencing on measurement accuracy. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to overcome the problems described above and to provide a shape measurement apparatus which is capable of saving time and effort of an examiner and improving measurement accuracy. 
     To achieve the objects and in accordance with the purpose of the present invention, a shape measurement apparatus for measuring a spherical shape of an object to be examined comprises a measurement unit where a first projection optical system projects first targets for measurement onto a spherical surface to be measured of an object to be examined, a first detection optical system detects images of the projected first targets and a calculation unit obtains a shape of the spherical surface based on the detected images of the first targets, an output unit which outputs the obtained shape of the spherical surface, a mode setting unit which sets any one of a first measurement mode for measuring a convex shape and a second measurement mode for measuring a concave shape and a condition setting unit which sets at least one of a measurement condition and an output condition based on a mode setting result, the measurement condition and the output condition in the first measurement mode being different from the measurement condition and the output condition in the second measurement mode. 
     In another embodiment of the present invention, a shape measurement apparatus for measuring a spherical shape of an object to be examined comprises a measurement unit where a first projection optical system projects first targets for measurement onto a spherical surface to be measured of an object to be examined, a first detection optical system detects images of the projected first targets and a calculation unit obtains a shape of the spherical surface based on the detected images of the first targets, a second projection optical system which projects second targets of a configuration or shape pattern not symmetric about a point with respect to a measurement reference axis onto the spherical surface, a second detection optical system which detects images of the projected second targets, and a judging unit which judges whether the spherical surface is a convex shape or a concave shape based on a pattern of the detected images of the second targets. 
     Additional objects and advantages of the invention are set forth in the description which follows, are obvious from the description, or may be learned by practicing the invention. The objects and advantages of the invention may be realized and attained by the apparatus for determining corneal ablation data and the corneal surgery apparatus in the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present invention and, together with the description, serve to explain the objects, advantages and principles of the invention. In the drawings, 
     FIG. 1 is an external view schematically showing an ophthalmic apparatus consistent with the present invention; 
     FIG. 2 is a view schematically showing a structure of an optical system included in the ophthalmic apparatus; 
     FIG. 3 is a view schematically showing a configuration (arrangement) of each light source when viewed from an examinee&#39;s side; 
     FIG. 4 is a block diagram schematically showing a control system of the ophthalmic apparatus; 
     FIGS. 5A and 5B are views schematically showing a positional relationship of each detected reflex (target image); 
     FIG. 6 is a view schematically showing a modification of a pattern of a target. 
     FIG. 7 is a flowchart illustrating a flow of a measurement operation at the time of measurements of a convex surface shape and a concave surface shape; and 
     FIG. 8 is a view schematically showing a fixed jig holding a contact lens. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A detailed description of one preferred embodiment of a shape measurement apparatus embodied by the present invention is provided below with reference to the accompanying drawings. The description will be given with an example of a handheld-type ophthalmic apparatus capable of measuring eye refractive power and a corneal shape (corneal radius of curvatures). FIG. 1 is an external view schematically showing an ophthalmic apparatus used in the preferred embodiment, and FIG. 2 is a view schematically showing an optical system included in the apparatus. 
     A measurement window  4  is placed on an examinee&#39;s side (a side of an object to be examined) of an apparatus  1 , and measurement light from an eye refractive power measurement optical system described later is irradiated (projected) onto an eye E to be examined along a measurement optical axis L 1  which goes through the center of the window  4 . Besides, an image of an anterior segment of the eye E is picked up via the window  4 . Two illumination windows  7   a  and  7   b  are provided below the window  4 , and illumination light from anterior segment illumination light sources described later illuminates the eye E through each,of the windows  7   a  and  7   b.    
     In addition, four irradiation (projection) windows  8   a  to  8   d  are provided vertically and horizontally symmetrical about the window  4  being their center. Target light from a target projection optical system described later is irradiated (projected) onto the eye E through each of the windows  8   a  to  8   d . Right below the windows  8   a  and  8   b  are two irradiation (projection) windows  9   a  and  9   b  which target light used for detecting alignment condition in a Z-direction (a direction of a working distance) goes through. 
     A LCD monitor  5  and a switch part  6  are placed on an examiner&#39;s side of the apparatus  1 . The image of the anterior segment of the eye E, alignment information and measurement information are displayed on the monitor  5 . The lower part of the apparatus  1  is a grasping part  2  for the examiner. 
     In FIG. 2, a half mirror  10  is placed on the optical axis L 1  which is a central axis of the apparatus  1  opposed to the eye E, and the eye refractive power measurement optical system  20  is placed on the rear side of the half mirror  10 . On the side of an optical axis L 2  made coaxial with the optical axis L 1  by a half mirror  21 , a light source  22  shared for measurement of eye refractive power and detection of alignment condition in X and Y directions (horizontal and vertical directions), a rotation sector  23  having a slit aperture, a projection lens  25 , and a limiting diaphragm  26  are placed. The light source  22  emits infrared light. In addition, on the rear side of the half mirror  21  on the optical axis L 1 , a photo-receiving lens  31 , a diaphragm  32 , and a photo-receiving part  33  including three pairs of photodetectors are placed. Eye refractive power is measured by obtaining signals indicating the phase difference in accordance with the scanning direction of slit light using the three pairs of photodetectors on the photo-receiving part  33 . The measurement of the eye refractive power has little relationship with the present invention, and the details are omitted (see Japanese Patent Application Unexamined Publication Hei10-108836 corresponding to U.S. Pat. No. 5,907,388 for the details). 
     On an optical axis L 3  made coaxial with the optical axis L 1  by the half mirror  10 , a light source  11  which emits visible light, a fixation target plate  12  on which a fixation target is formed, and lenses  13  and  14  are placed to form a fixation target optical system. The light source  11  and the fixation target plate  12  integrally move in the direction of the optical axis L 3  by a fixation target moving part described later to fog the eye E. In addition, a dichroic mirror  15  is placed between the lens  14  and the half mirror  10 . On an optical axis L 4  on the reflecting side of the dichroic mirror  15  (the optical axis L 4  is mode coaxial with the optical axis L 3  by the dichroic mirror  15 ), an image forming lens  16 , a telecentric diaphragm  17  and a CCD camera  18  having an image pickup element are placed to form an observation optical system. The CCD camera  18  has a sensitivity of near-infrared and infrared regions. The observation optical system serves as a target detection optical system for detecting targets projected onto the eye E, and as a part of a corneal shape measurement optical system. 
     Reference numeral  40  indicates the target projection optical system, which is composed of four groups of first target projection optical systems  40   a  to  40   d  as a part of the corneal shape measurement optical system placed on the circumference of a single circle having the optical axis L 1  at its center, and of second target projection optical systems  50   a  and  50   b  for irradiating (projecting) the target light used for detecting the alignment condition in the Z-direction. 
     The first target projection optical systems  40   a  and  40   b  are placed so that each of their projection optical axes intersects with the optical axis L 1  at a predetermined angle in the horizontal direction of the apparatus  1 . Likewise, the first target projection optical systems  40   c  and  40   d  (illustrations are omitted in FIG. 2) are placed so that each of their projection optical axes intersects with the optical axis L 1  at a predetermined angle in the vertical direction of the apparatus  1 . Reference numeral  41   a  to  41   d  are point light sources which emit infrared light,  42   a  to  42   d  are spot diaphragms, and  43   a  to  43   d  are collimating lenses which project targets at a infinite distance onto the eye E. 
     The second target projection optical systems  50   a  and  50   b  are placed below the first target projection optical systems  40   a  and  40   b  (in FIG. 2,  50   a  and  50   b  are deviated toward the optical axis L 1  for convenience in illustration), and are placed symmetrically with respect to the optical axis L 1 . The second target projection optical systems  50   a  and  50   b  are provided with point light sources  51   a  and  51   b  which emit infrared light, and spot diaphragms  52   a  and  52   b , and project targets at a finite distance onto the eye E. 
     FIG. 3 is a view schematically showing a configuration (arrangement) of the light sources  41   a  to  41   d  included in the first target projection optical systems  40   a  to  40   d  and the light sources  51   a  and  51   b  included in the second target projection optical systems  50   a  and  50   b , when viewed from the examinee&#39;s side. The light sources  51   a  and  51   b  are placed at positions not symmetrical about a point with respect to the optical axis L 1  (an asymmetric pattern). The target projection optical system  40  irradiates light to form totally six reflexes (target images) at the periphery of a cornea Ec of the eye E off the corneal center. 
     Besides, in FIG. 2, the anterior segment illumination light sources  45   a  and  45   b  which emit near-infrared light are placed at the same height as the distance from the optical axis L 1 , and placed to have a predetermined positional relationship with the optical axis L 1 , so as to illuminate the eye E from an oblique-lower direction. The light sources  45   a  and  45   b  irradiate light at a finite distance, and form two reflexes on the cornea Ec. The reflexes are detected by the camera  18  as target images not symmetric about a point with the optical axis L 1  (an asymmetric pattern). 
     FIG. 4 is a block diagram schematically showing, primary parts of a control system of the apparatus  1 . An output image from the camera  18  is provided with a predetermined processing and captured in an image memory  61 . Besides, the image from the camera  18  is displayed on the monitor  5  via an image synthesizing part  62 . A character generating part  63  generates various characters and letters to be displayed on the monitor  5 , and a signal therefrom is electrically synthesized with a picture signal from the camera  18  by the image synthesizing part  62 . An image processing part  65  detects a signal from the image captured in the image memory  61 , and a calculating/controlling part  60  obtains positions of the target images based on the signal detected by the image processing part  65  to measure a spherical shape such as a shape of the cornea Ec, a shape of a contact lens, or the like. In addition, the calculating/controlling part  60  is connected to the light source  22 , the light sources  41   a  to  41   d , and the light sources  51   a  and  51   b , the photo-receiving part  33  for measuring the eye refractive power, a fixation target moving part  57  and the like, and controls measurement of the corneal shape and measurement of the eye refractive power and calculates the eye refractive power. 
     Further, a memory part  66  is capable of storing the obtained spherical shape data (the radius of curvatures and the axial angles of the steepest and flattest meridians) such as the obtained corneal shape data or the like, the obtained eye refractive power data and the like. Various data stored by the memory part  66  are sent to a printer  70  via an outward output part  67  so that measurement data are printed out. 
     With the configuration as described above, the operation will be described referring to a flowchart shown in FIG.  7 . Firstly, measurement of a spherical shape of a convex surface of the cornea Ec will be described. Secondly, measurement of a spherical shape of a concave surface of a contact lens will be described. 
     &lt;Measurement of a Spherical (Convex Surface) Shape of a Cornea&gt; 
     The light source  22 , the light sources  45   a  and  45   b , the light sources  41   a  to  41   d , and the light sources  51   a  and  51   b  light up, and the window  4  is positioned opposed to the eye E, corneal reflexes of those light sources and an image of the anterior segment are thereby picked up by the camera  18  to be displayed on the monitor  5 . On the display of the monitor  5  shown in FIG. 4, reference numerals  22 ′,  41   a ′ to  41   d ′, and  51   a ′ and  51   b ′ indicate the corneal reflexes of the light sources  22 ,  40   a  to  40   d , and  51   a  and  51   b , respectively. Reference numeral  45   a ′ and  45   b ′ indicate the corneal reflexes of the light sources  45   a  and  45   b . Light emitted from the light source  22  is irradiated (projected) onto the eye E along the optical axis L 1  to form the reflex  22 ′ on a corneal vertex. 
     Further, at a predetermined position on the monitor  5 , an aiming marker  110  having a square shape generated by the character generating part  63  is displayed being electrically synthesized by the image synthesizing circuit  62 . The center of the aiming marker  110  is considered as an alignment center in X and Y directions, and the examiner performs alignment in the X and Y directions by moving the apparatus  1  with respect to the eye E so that the reflex  22 ′ is positioned at the center of the aiming marker  110 . Furthermore, an alignment condition in the Z-direction is detected by comparing the distance between the reflex  41   a ′ and the reflex  41   b ′, with the distance between the reflex  51   a ′ and the reflex  51   b ′ (see Japanese Patent Application Unexamined Publication Hei6-46999 corresponding to U.S. Pat. No. 5,463,430 for judgment of the alignment condition). 
     From the image of the anterior segment captured in the image memory  61  by the image processing part  65 , central coordinates of each reflex (target image) are detected. The control part  60  judges whether the object is measured on the convex surface shape (the measurement of the corneal shape) or on the concave surface shape (the measurement of the base curve of the contact lens), according to the positional relationship among the detected reflexes (a relation in the configuration (arrangement) of the reflexes). Precisely, a configuration (arrangement) pattern of the reflexes at the time of the measurement of the convex surface shape is stored in advance as a reference pattern, then the positional relationship among the reflexes detected by the image processing part  65  is compared with the reference pattern in order to judge whether the measurement is performed on the convex surface or concave surface. 
     FIG. 5A is a schematic diagram showing a positional relationship among the reflexes detected at the time of the measurement of the convex surface shape. At the completion of the alignment, the reflex  22 ′ being the central reference point, the reflexes  41   a ′ to  41   d ′, the reflexes  51   a ′ and  51   b ′, and the reflexes  45   a ′ and  45   b ′ are detected in a positional relationship as shown in FIG.  5 A. In other words, the reflexes  51   a ′,  51   b ′,  45   a ′ and  45   b ′ are positioned below the reflex  22 ′ being the central reference point. The configuration pattern is stored in the memory part  66  as the reference pattern. When this configuration pattern is detected, the control part  60  judges that the convex surface shape is measured, and controls measurement in the mode for the concave surface shape measurement. 
     Based on the judgment of the alignment condition in the Z-direction and in the X and Y directions, the control part  60  starts the measurement automatically giving a trigger signal when the predetermined alignment condition is completed. The control part  60  calculates the corneal shape data of the eye E such as the corneal radius of curvatures, the axial angle and the like, based on a photo-received position (a detected position) of the reflexes  41   a ′ to  41   d ′ detected by the image processing part  65 , The calculated corneal shape data are stored into the memory part  66  while being displayed on the monitor  5  by the character generating part  63 . At the time of the corneal shape measurement, the corneal radius of curvatures and the axial angle may be calculated if three reflexes (target images) are detected as described in Japanese Patent No. Hei1-19896. 
     The corneal shape data are sent to the printer  70  via the outward output part  67  using the print switch provided in the switch part  6  in order to be printed out. At this time, the data may be clearable if provided with a notice informing whether the convex surface shape measurement or the corneal shape measurement. 
     &lt;Measurement of a Spherical (Concave Surface) Shape of a Contact Lens&gt; 
     Next, the measurement of the base curve of the contact lens will be described. After pouring water into a concave part formed on a holder  101  of a fixed jig  100  shown in FIG. 8, the convex surface of the contact lens is mounted on the holder  101 . Then, the same alignment as that for the measurement of the corneal shape is performed on the concave surface of the contact lens, and an automatic measurement is performed. At this point, the control part  60 , as described above, detects the positional relationship among the reflexes (target images) detected by the image processing part  65  from the image of the anterior segment captured in the image memory  61  by the camera  18 , and judges whether the object is measured on the convex surface shape (the measurement of the corneal shape) or the concave surface shape (the measurement of the base curve of the contact lens). 
     FIG. 5B is a schematic diagram showing a positional relationship among the reflexes detected at the time of the measurement of the concave surface shape. In measuring the concave surface shape, a configuration (arrangement) pattern of the reflexes is obtained by vertically and horizontally reversing that obtained at the time of the measurement of the convex surface shape shown in FIG. 5A with respect to the reflex  22 ′. That is, the reflexes  51   a ′,  51   b ′,  45   a ′ and  45   b ′ are positioned above the reflex  22 ′. When the reflexes are detected with such a configuration pattern, that pattern does not coincide with the reference pattern. Therefore, the coordinates of all of the detected reflexes are reversed vertically and horizontally (rotated 180 degrees about the measurement optical axis on the image). Then, the configuration pattern becomes applied to the condition of the reference pattern. If the configuration pattern after the calculation coincides with the condition of the reference pattern, the control part  60  confirms that the analysis common to the measurement of the concave surface shape can be performed, and sets the mode for the concave surface shape measurement. 
     Further, when the concave surface shape of the contact lens is judged to be under measurement, the control part  60  displays letters “CL” on the monitor  5  using the character generating part  63  to inform the examiner of the measurement of the base curve of the contact lens. 
     Furthermore, in order to suppress reflection light (a back-surface reflection) from the convex surface of the contact lens, the control part  60  makes adjustments so that target projection light intensity of each light source is reduced within a range where the radius of curvatures may be calculated while the back-surface reflection is suppressed (the reduction of the light intensity may be applied to light sources used at least for the spherical shape measurement.) Detecting sensitivity to the reflexes may be reduced instead. This reduction may be performed by adjusting photo-receiving sensitivity of the camera  18 , using the circuit of the image processing part  65  or a processing software thereof, and the like. 
     As in the case of the measurement of the corneal shape, when the predetermined alignment condition is completed, the control part  60  starts the measurement automatically giving the trigger signal, calculates the radius of curvatures and the axial angle of the concave surface of the contact lens, and displays the calculated result on the monitor  5 . At this time, if the contact lens to be measured is for astigmatic correction, it is necessary to calculate the axial angle. For this purpose, the control part  60  performs calculation for horizontally reversing a principal meridian axial angle (AXIS) obtained from the four reflexes for the spherical shape measurement (i.e. transform the axial angle of 45 to 135 degrees), and displays a value applied to the axial angle in the case of wearing contact lens (the axial angle when viewed from the convex surface) as the measurement result. In addition, at the same time, the control part  60  makes the memory part  66  store the obtained shape data (the radius of curvatures and the axial angle on the concave surface) as the data from the measurement of the base curve of the contact lens. The shape data stored into the memory  66  is sent to the printer  70  via the outward output part  67  using the print switch provided in the switch part  6 , and printed out with the letters “CL” indicating that the shape data is obtained from the measurement of the base curve of the contact lens. 
     Besides, the measurement mode in the initial state of the apparatus  1  is set for the convex surface shape, which is for the corneal shape measurement. After setting the mode for the concave surface shape measurement, the mode is kept the same till it is changed to the mode for the convex surface shape measurement. Once the mode is changed for the concave surface shape measurement, if the coordinates of all the reflexes are reversed vertically and horizontally, and a judgment is made whether or not they comply with the condition of the reference pattern, the calculation may be performed effectively. And, when the convex surface shape is measured again, since the configuration pattern of the reflexes is not compatible with the condition of the reference pattern, the configuration pattern is reversed vertically and horizontally again. If the reversed configuration pattern coincides with the condition of the reference pattern, it is judged that the convex surface shape measurement has been performed. Then, the vertical and horizontal reverse may be stopped. 
     The vertical and horizontal reverse of the reflexes requires a simple calculation and a short processing time. Therefore, if the configuration pattern of the reflexes is judged not compatible with the condition of the reference pattern, the reflexes may be reversed vertically and horizontally so as to be judged with respect to the condition of the reference pattern again. In addition, in the preferred embodiment, when the concave surface shape is measured, the reflexes are reversed vertically and horizontally, and the same calculation as that for the convex surface shape measurement is performed thereon to obtain measurement data. However, the present invention is not limited thereto. When the reflexes are not arranged intendedly, another program different from that program for the convex surface shape measurement may be run for the concave surface shape measurement to calculate the measurement data. 
     Further, the mode for the convex surface shape measurement and that for the concave surface shape measurement are not set automatically, but may be set by manual operation by the examiner using a mode changing switch  6   a  placed in the switch part  6 . 
     Furthermore, the mode for the concave surface shape measurement may be that for the contact lens as well. Generally, in measuring a contact lens using this kind of ophthalmic apparatus, a concave surface shape is measured to obtain the base curve data. When the measurement mode is selected for the contact lens, the control part  60  displays the letters “CL” on the monitor  5  and on the printed-out data, and sets the measurement condition such as reducing the light intensity of the measurement light for the spherical shape measurement. 
     In the preferred embodiment described above, totally nine reflexes are provided on the eye E, including the illumination light, but the present invention is not limited thereto. For example, the reflexes of a number which is capable of performing the measurement of the corneal shape and the like, may be formed on the object, while the targets projected onto the object may be arranged in an asymmetric pattern with respect to the optical axis L 1 . Since the corneal radius of curvatures may be measured by projecting two targets being symmetrical about a point and one target on the circumference of the same circle as the two targets, the targets for judging whether the convex or concave surface may be used as the measurement targets. Moreover, it is essential only that the targets for judging whether the convex or concave surface have an asymmetric pattern. For example, a triangle-shaped pattern as shown in FIG. 6 may be used. 
     Furthermore, in the preferred embodiment, the reference pattern stored in advance and the configuration pattern of the actual reflexes are consistent with each other, but the present invention is not limited thereto. For example, among the plural reflexes detected by the camera  18 , the detecting condition of part of the reflexes are captured to detect whether the convex or concave surface is measured. When using the nine reflexes presented in the preferred embodiment, as shown in FIG. 5A, if one of the reflexes at the upper edge of the image of the anterior segment and two of the reflexes at the lower edge are detected, it may be judged that the convex surface shape is measured. 
     As above, in the preferred embodiment, the description is given to the handheld-type ophthalmic apparatus provided with functions of the corneal shape measurement and the eye refractive power measurement. However, the present invention is not limited thereto. For example, the present invention may be applied to an ophthalmic apparatus of a stationary type or an apparatus having only the function of the corneal shape measurement. Additionally, the present invention may simply be applied to an apparatus for measuring a spherical shape of the convex and concave surfaces of correctives like the contact lens, the spectacles lens and the like. In this case, it is judged whether the measurement is on the convex or concave surface, and the judgment result is therefore added to the measurement data to be displayed and printed out, thereby eliminating the trouble of management by the examiner. 
     As described above, according to the present invention, the apparatus may be provided, which is capable of avoiding troubles for the examiner and has an excellent operability. In addition, the measurement accuracy is improved. 
     The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in the light of the teachings described above or may be acquired from practice of the invention. The embodiments chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.