Abstract:
A test apparatus and method for testing the zoom of a miniature digital camera module is presented. A combination of focus targets are illuminated by different colors of light and simultaneously viewed by the digital camera module. The focus targets range from a far target to a close target and are positioned in the apparatus accordingly. The zoom capability of the camera can be either electrical or manual, and the test apparatus can be adapted to either zoom configurations allowing the zoom of the camera lens to be controlled by a tester. The brightness of the image of the focus targets captured by the digital camera is monitored to determine whether the focus of the camera is maintained over the range of zoom.

Description:
RELATED PATENT APPLICATION 
   This application is related to U.S. patent application Ser. No. 10/930,351, filed on Aug. 31, 2004, and assigned to the same assignee as the present invention. 
   This application is related to U.S. patent application Ser. No. 10/929,651, filed on Aug. 30, 2004, and assigned to the same assignee as the present invention. 
   This application is related to U.S. patent application Ser. No. 10/930,353, filed on Aug. 31, 2004, and assigned to the same assignee as the present invention. 
   This application is related to U.S. patent application Ser. No. 10/929,652, filed on Aug. 30, 2004, and assigned to the same assignee as the present invention. 
   This application is related to U.S. patent application Ser. No. 10/929,300, filed on Aug. 30, 2004, and assigned to the same assignee as the present invention. 
   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The present invention relates to the testing of miniature digital camera modules and in particular to digital camera modules with electrical and/or optical zoom capability. 
   2. Description of Related Art 
   The digital camera is becoming a ubiquitous device. Not only are digital cameras replacing the traditional film camera, digital camera devices are being used in many other applications, such as small electronic devices, such as PDA (personal data assistant) and cellular phones. With the explosion of cellular phones, the ability to take a picture and then send that picture to another individual using a second cellular phone comes the need to produce inexpensive digital camera modules and efficiently test these modules in large quantities. This is further complicated by the many different module configurations that are emerging as a result of the many different application requirements, including fixed focus, manual focus and automatic focus as well as physical size. Some of these modules are very small and others have signal leads in the form of a flex filmstrip. The testing time for digital camera module, which can have mega-pixel capability, has traditionally been a relatively long process (approximately sixty seconds for a module with 0.3 mega pixels) to insure the integrity and picture quality of the camera. Quality testing at a low cost has become the utmost of importance. This necessitates a testing capability that is fast and insures the integrity and specification of the digital camera module while testing a large quantity of modules. 
   A patent application, Ser. No. 10/417,317 dated Apr. 16, 2003,is related to miniature cameras and their manufacturing methods that are used as built-in modules in hand held consumer electronics devices such as mobile phones and PDA&#39;s. In a second patent application, Ser. No. 10/434,743 dated May 8, 2003, a test system is described for digital camera modules used as built-in modules for consumer electronics, which performs electrical tests, adjustment of focus and sealing of the lens barrel with glue. 
   In addition there are a number of other prior art patents that are directed to testing of digital cameras: US 20040032496A1 (Eberstein et al.) is directed to a method of camera calibration and quality testing; EP 1389878A1 (Bednarz et al.) is directed to a method of camera calibration and testing camera quality; US 20040027456A1 (Pierce) directed to the use of calibration targets; EP 1382194A1 (Baer) is directed to dark current subtraction; JP 2003259126 (Keisuke) is directed to removing noise of an image; US 20030146976A1 (Liu) is directed to a digital camera system enabling remote monitoring; JP 2003219436 (Fuminori) is directed to adjustment of a pixel shift camera; US 2003142374 (Silverstein) is directed to calibrating output of an image output device; JP 2003179949 (Hidetoshi) is directed to a luminance level inspection apparatus; JP 2003157425 (Vehvilainen) is directed to improving image quality produced in a mobile imaging phone; JP 2003101823 (Kenichi) is directed to specifying a picture data area; EP 1286553 A2 (Baer) is directed to a method and apparatus for improving image quality; US 20030030648 (Baer) is directed to a method and apparatus for improving image quality in digital cameras; U.S. Pat. No. 6,512,587 (Marcus et al.) is directed to measurement method and apparatus of an imager assembly; US 20030002749 (Vehvilainen) is directed to a method and apparatus for improving image quality; US 20020191973 A1 (Hofer et al.) is directed to a method and apparatus for focus error reduction; WO 2002102060 A1 (Baer) is directed to a method and apparatus for smear in digital images using a frame transfer sensor; JP 2002290994 (Hidetoshi) is directed to a method and apparatus to detect foreign matter on the surface of a lens; JP 200223918 (Yanshinao) is directed to an image inspection device and method for a camera module; JP 2002077955 (Keisuke) is directed to a method and apparatus for evaluating camera characteristics; JP 2001292461 (Keisuke) is directed to a system and method for evaluating a camera; U.S. Pat. No. 6,219,443 B1 (Lawrence) is directed to a method and apparatus for inspecting a display using a low resolution camera; U.S. Pat. No. 6,201,600B1 (Sites et al.) is directed to a method and apparatus for inspection of optically transmissive objects having a lens; U.S. Pat. No. 5,649,258 (Bergstresser et al.) is directed to an apparatus and testing of a camera; EP 0679932 B1 (Kobayashi et al.) is directed to testing an electronically controlled camera; U.S. Pat. No. 5,179,437 Kawada et al.) is directed to an apparatus for color correction of image signals of a color television camera; JP 03099376 (Hiroshi) is directed to the quality of a display screen; U.S. Pat. No. 4,612,666 (King) is directed to a pattern recognition apparatus; and U.S. Pat. No. 4,298,944 Stoub et al.) is directed to a method and apparatus for distortion correction for scintillation cameras. 
   SUMMARY OF THE INVENTION 
   It is an objective of the present invention to test a zoom capability for a miniature digital camera module that has an electrical zoom capability. 
   It is also an objective of the present invention to test a zoom capability for a miniature digital camera module that has a mechanical zoom capability. 
   It is further an objective of the present invention to position a plurality of focus targets simultaneously at varying distances from a digital camera module to permit a zoom test to evaluate a full range of focus during the zoom test. 
   It is further another objective of the present invention to combine the plurality of focus targets into a single composite target. 
   It is still further an objective of the present invention to position the single composite target to provide optical depth for the zoom test. 
   In the present invention a miniature digital camera module with a zoom capability is tested for the ability of the zoom to maintain far and near focus over the range of the zoom. An optical system, comprising a light source, focus targets, a field lens, and an image magnifier is used to provide an image to a digital camera module under test (MUT) that can be changed to provide a view to the MUT that is optically closer or farther away from the lens of the MUT. The MUT has either an electrical controlled zoom or a mechanical zoom, the adjustment of which is controlled by a tester. 
   The tester controls the optical system contained within a test station to vary the image viewed by the MUT. A test fixture within the test station positions the MUT under the optics centerline and provides electrical contact between the MUT and the tester. The zoom adjustment of the MUT is varied over a complete zoom range and the image of the focus targets is measured for maintaining focus as the zoom is changed. When a MUT has an electrically controlled zoom, the tester controls the zoom through electrical signals to the MUT. When the MUT has a mechanically adjustable zoom, a zoom stepper mechanism physically contacts the zoom adjustment of the lens of the MUT, and the tester controls a stepper motor to vary the zoom of the MUT. 
   The focus targets used to test the zoom capability of the MUT form a composite image that is focused onto the lens of the MUT by the field lens. The composite image comprises a far, an intermediate and a close target that have been positioned simultaneously within the optical view of the MUT. The composite image allows the focus test of the zoom using one setup of the optics system and one set of test images captured by the MUT, which reduces the test time that would have been required for individual targets resulting in multiple setup and test. 
   A single composite target can also be used to provide the composite image to the MUT for testing the zoom capability. The composite target comprises a far, intermediate and close targets constructed within a single focus target. The composite target is held by a target holder within the view of the MUT and is either orthogonal to the axis of the optical system or at an angle less than ninety degrees with respect to the axis to provide a depth of field. 
   The focus targets comprise light and dark area, where the light areas are holes in the targets that permit light through. The preferred shape of the holes is round; however, other shapes, i.e. oval, rectangular, hexagonal, and octagonal, can be used. A far target uses large holes and a close target uses small holes with the intermediate targets having a hole size that is in between. Monitoring the brightness of the image viewed by the MUT provides the test of the focus of the targets over the zoom range. If the brightness remains within an acceptable predetermined range, the zoom capability of the MUT is tested as good. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This invention will be described with reference to the accompanying drawings, wherein: 
       FIG. 1  is a diagram of the optical system of the present invention, 
       FIG. 2A  is a diagram showing a plurality of focus targets mounted in the optical system of the present invention, 
       FIG. 2B  is a diagram of the image of the plurality of focus targets seen by the MUT, 
       FIG. 3A  is a diagram showing a composite focus target mounted in the optical system orthogonal to an optical centerline of the present invention, 
       FIG. 3B  is a diagram of the image of the orthogonal mounted composite of focus target seen by the MUT, 
       FIG. 4A  is a diagram showing the composite focus target mounted at a non-orthogonal angle to the optical centerline of the present invention, 
       FIG. 4B  is a diagram of the image of the non-orthogonal mounted composite of focus target seen by the MUT, and 
       FIG. 5  is a method of testing the zoom capability of a MUT. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In  FIG. 1  a miniature digital camera module under test (MUT)  30  with a lens cap  31 , which has an adjustable zoom, is mounted on a test fixture  32  containing positioning clamps  33 . When the MUT  30  is place onto the test fixture  32 , the MUT is clamped into place by the positioning clamps  33  and contact is made to the electrical I/O of the MUT (not shown). The test fixture  32  positions the MUT  30  under an optical centerline  46  of an optical system  20 . A tester  22  provides control data  23  to the optical system  20  and the MUT  30 , and receives test data  23  back from the MUT  30 . 
   The optical system contains a light source  45  and a light diffuser  69  to provide illumination  70  to the optical system  39 . The light source  45  comprises a plurality of serially connected strings of light emitting diode devices (LED). Each string of LED devices contains LED devices of a same color. There are strings of LED devices that produce colors comprising red, green, blue and infrared. Additional colors are also possible depending upon the test requirements. The tester  22  connected to the optical system controls the selection of a color of light and the intensity of the light. Any source of light that produces the colors required and can be switched on and off rapidly can be used. The LED devices, for example, produce a stable light that has a very fast on and off switching time and provides the colors that are required. 
   The optical system  20  contains two target wheels  40  and  41 . Within each target wheel are target holders  42  that can contain focus targets in a position  72  more distant from the MUT  30  and in a position  73  that is closer to the MUT. Each target wheel  40  and  41  contains six target holders  42 . The target holders  42  within a target wheel  40  and  41  are rotated into position over the optical axis  46  by stepper motors  75  and  76  controlled by the tester  22 . Since there are six target holders in each target wheel, the stepper motors  75  and  76  rotate the target wheels in sixty-degree increments. Target wheels with a fewer or a greater number of target holders than six and a corresponding stepper increment are within the scope of the present invention 
   Continuing to refer to  FIG. 1 , the positioning the target wheels  40  and  41  with respect to each other and the MUT  30  are critical in providing a focus image to the MUT. The positioning of the target wheels  40  and  41  along the optical axis  46  is done with stepper motors  77  and  78  under the control of the tester  22 . A field lens  43  is used to focus the image of targets contained within the target holders  42  onto the lens of the MUT  30 , and tester  22  controls a stepper motor  79  to position of the field lens  43  along the optical axis  46 . A magnifier lens  71  is used in the test of the zoom capability of the MUT  30  and is positioned over the optical centerline  46  with a stepper motor  81  under the control of the tester  22 . The magnifier lens  71  is positioned along the optical axis  46  by a stepper motor  80  under the control of the tester  22 . The magnifier lens  71  can also be placed above the field lens  43 , and the field lens can be exchanged with another field lens so that the optical distance of the targets contained within the target wheels  40  and  41  can be modified with respect to MUT  30  in order to perform the zoom test. 
   The MUT  30  can have either an electrically controlled zoom or a manually controlled zoom. The zoom capability of the MUT expands or contracts an image incident on the zoom lens to a light sensitive device contained within the MUT. If the zoom is electrically controlled, the tester  22  provides a control signal to adjust the zoom contained within the lens cap  31  of the MUT  30 . If the zoom is adjusted manually, the tester  22  controls a zoom stepper  82  in contact with the lens cap  31  to vary the zoom of the MUT  30 . The zoom stepper  82  is in addition to and similar to a focus stepper that is needed to focus the lens of the MUT  30   
   In  FIG. 2  is a diagram of the optics system  20  of the present invention in which four focus targets  105 ,  106 ,  107  and  108  are positioned in the target holders  42  of the target wheels  40  and  41 . Each target holder  42  has a target position  72  that is furthest from the MUT and a target position  73  that is closest to the MUT. The target holders  42  containing the four focus targets  105 ,  106 ,  107  and  108  are positioned over the optical centerline  46 , and the focus targets  105 ,  106 ,  107  and  108  are positioned within the target holders  42  such that an image of all for focus targets can be viewed simultaneously by the MUT. A far focus target  105  is located in the target position  72  of a target holder  42  in the target wheel  40  furthest from the MUT  30 . A first intermediate focus target  106 , which is closer to the MUT  30  than the far focus target  105 , is located in target position  73  of the target holder  42  of target wheel  40 . A second intermediate focus target  107 , which is closer to the MUT  30  than the first intermediate target  106 , is located in target position  72  of a target holder  42  in target wheel  41 . A close focus target  108 , which is closest to the MUT, is located in target position  73  of the target holder  42  in target wheel  41 . 
   The four focus targets are arranged in the target holders  42  such that all four focus targets  105 ,  106 ,  107  and  108  can be viewed by the MUT  30  at the same time. The target holders are round in shape and each focus target occupies approximately one fourth, or ninety degrees, of the target holder. Other shapes of target holder  42  are within the scope of the present invention depending upon the requirements of the optics system  20 . The tester  22  sets the distance of the target wheels  40  and  41  from the MUT to values predetermined in a focus test setup by adjusting the stepper motors  77  and  78 , and the tester adjusts the field lens to focus a composite image of the focus targets (shown in  FIG. 2B ) onto the lens of the MUT by controlling the stepper motor  79 . A magnifier lens  71  is moved over the optical centerline  46  by a stepper motor  81  under the control of the tester  22 . The tester controls a stepper motor  80  to vary the distance of the magnifier lens  71  from the MUT to provide different image sizes to the MUT  30  of the composite image shown in  FIG. 2B . The zoom of the MUT  30  is varied under control of the tester  22  for each setting of the magnifier lens  71 , and image data from the MUT  30  is coupled to the tester  22  to determine if the composite image maintains focus as the zoom of the MUT is changed. The tester  22  monitors the brightness of the composite image, shown in  FIG. 2B , to determine that the focus remains within acceptable focus limits as the zoom is varied. 
   Continuing to refer to  FIG. 2A , the tester  22  selects different colors of light from the light source  45  and the zoom test is repeated for each color to check the capability of the multiple element lens system of the MUT  30 . The MUT can have either an electrical controlled zoom or mechanical zoom. If the MUT  30  has an electrically controlled zoom, the tester  22  provides signals to control the zoom. If the MUT  30  has a manually adjustable zoom, a zoom stepper mechanism  82  ( FIG. 1 ) in contact with the lens cap  31  rotates the lens cap zoom adjustment under the control of the tester  22  to vary the zoom of the MUT  30 . 
   In  FIG. 2B  is shown the arrangement of the focus targets  105 ,  106 ,  107  and  108  as viewed by the MUT  30 . Each of the targets  105 ,  106 ,  107  and  108  comprise dark  84  and light areas  83 ,  86 ,  91  and  96 . The light areas are holes through the material forming the dark areas  84  that allow light to pass through from the light source  45 . The holes are preferably round; however, the holes can have other physical shape, i.e. oval, rectangular, hexagonal and octagonal. The arrangement and shape of the holes  83 ,  86 ,  91  and  96  are for illustrative purposes only and are not intended to explicitly define the focus targets  105 ,  106 ,  107  and  108 . In the far focus target  105  the light areas  83  are formed by large holes. In the first intermediate focus target  106  the light areas  86  are formed by holes that are smaller than the light areas  83  in the far target  105 . In the second intermediate target  107  the light areas  91  are formed by holes that are smaller than the light areas  86  of the first intermediate focus target  106 , and in the close focus target  108  the light areas  96  are formed by holes that are smaller than the light areas  91  in the second intermediate focus target  107 . 
   In  FIG. 3A  is shown is a diagram of the optics system  20  of the present invention in which a composite focus target  110  is located in the close target position  73  of a target holder  42  in the target wheel  41 . The composite focus target  110  comprises a combination into a single focus target of the far, the first intermediate, the second intermediate and the close focus targets shown in  FIGS. 2A and 2B . The location of the composite focus target  110  is for illustrative purposes, and the composite target can be located in either target location  72  or  73  in target holders  42  in either target wheel  40  and  41  depending upon the initial test setup. The determination of the location of the composite target is made during initial focus setup when the settings of the optics system  20  are made. During the initial setup the distance settings of the stepper motors  77 ,  78  and  79  are made to provide the best focus test of the zoom capability of the MUT  30 . The tester  22  stores the initial setup distances setting of the stepper motors to be used in the zoom test of product. 
   During zoom testing of the MUT  30 , the magnifier lens  71  is positioned over the optical centerline  46  by the stepper motor  81  under control of the tester  22 . The magnifier lens  71  is moved during zoom test by the stepper motor  80  under control of the tester  22 . At a position closest to the lens cap  31  of the MUT  30  the image of the composite target  110  has the smallest magnification, and at a position furthest from the lens cap  31  the image of the composite target  110  is at a largest magnification. At the different distance locations of the magnifier lens  71  from the lens cap  31 , the zoom of the MUT  31  is varied under the control of the tester  22 , and the tester  22  monitors image data from the MUT. The brightness of the image data is used to determine that the focus of the MUT is maintained during the zoom testing. 
   Continuing to refer to  FIG. 3A , the tester  22  selects different colors of light from the light source  45  and the zoom test is repeated for each color to check the capability of the multiple element lens system of the MUT  30 . The MUT can have either an electrical controlled zoom or mechanical zoom. If the MUT  30  has an electrically controlled zoom, the tester  22  provides signals to control the zoom. If the MUT  30  has a manually adjustable zoom, a zoom stepper mechanism  82  ( FIG. 1 ) in contact with the lens cap  31  rotates the lens cap zoom adjustment under the control of the tester  22  to vary the zoom of the MUT  30 . 
   In  FIG. 3B  is shown the composite focus target  110  as viewed by the MUT  30 . The composite focus target is constructed of light areas  83 ,  86 ,  91  and  96  and dark areas  84 . The light areas  83 ,  86 ,  91  and  96  are holes through the material forming the dark areas  84  that allow light from the light source  45  to pass through to the MUT  30 , and the holes can have other physical shape, i.e. oval, rectangular, hexagonal and octagonal. The holes are of different size and represent the different distance targets  105 ,  106 ,  107  and  108  shown in  FIGS. 2A and 2B . The arrangement and shape of the holes  83 ,  86 ,  91  and  91  are for illustrative purposes only and are not intended to explicitly define the composite target. The larger target holes  83  represent the far target  105 , the target holes  86  represent the first intermediate target  106 , the target holes  91  represent the second intermediate target and the smallest target holes  96  represent the close target, similar to that shown in  FIGS. 2A and 2B . 
   In  FIG. 4A  is shown the composite focus target  111  positioned in a target holder  42  at a skewed angle to the optical centerline  46  of the optics system  20 . The skewed angle provides a depth of field to the view of the MUT  30  during zoom testing where the larger holes  83  are positioned “U” furthest from the MUT and the smallest holes  96  are positioned “D” closest to the MUT. The location of the skewed composite focus target  111  is for illustrative purposes, and the composite target can be located in a target holder  42  in either target wheel  40  and  41 . The determination of the location of the composite target is made during initial focus setup when the settings of the optics system  20  are made. During the initial setup the distance settings of the stepper motors  77 ,  78  and  79  are determined to provide the best focus test of the zoom capability of the MUT  30 . The tester  22  stores the initial setup distance settings of the stepper motors to be used in the zoom test of product. 
   During zoom testing of the MUT  30 , the magnifier lens  71  is positioned over the optical centerline  46  by the stepper motor  81  under control of the tester  22 , and the magnifier lens  71  is moved during zoom test by the stepper motor  80  under control of the tester  22 . The magnifier lens  71  positioned closest to the lens cap  31  of the MUT  30  causes the image of the composite target  111  to have the smallest magnification, and at a position furthest from the lens cap  31  the image of the composite target  111  is at a largest magnification. At the different distance locations of the magnifier lens  71  from the lens cap  31 , the zoom of the MUT  31  is varied under the control of the tester  22 , and the tester  22  monitors image data from the MUT. The brightness of the image data is used to determine that the focus of the MUT is maintained during the zoom testing. 
   Continuing to refer to  FIG. 4A , the tester  22  selects different colors of light from the light source  45  and the zoom test is repeated for each selected color to check the capability of the multiple element lens system of the MUT  30 . The MUT can have either an electrical controlled zoom or mechanical zoom. If the MUT  30  has an electrically controlled zoom, the tester  22  provides signals to control the zoom. If the MUT  30  has a manually adjustable zoom, a zoom stepper mechanism  82  ( FIG. 1 ) in contact with the lens cap  31  rotates the lens cap zoom adjustment under the control of the tester  22  to vary the zoom of the MUT  30 . 
   In  FIG. 4B  is shown the composite focus target  111  as viewed by the MUT  30 . The composite focus target is constructed of light areas  83 ,  86 ,  91  and  96  and dark areas  84 . The light areas are holes through the material forming the dark areas  84  that allow light from the light source  45  to pass through to the MUT  30 , and the holes are preferably round but can have other physical shapes, i.e. oval, rectangular, hexagonal and octagonal. The holes are of different size and represent the different distance targets  105 ,  106 ,  107  and  108  shown in  FIGS. 2A and 2B . The arrangement, shapes and sizes of the holes  83 ,  86 ,  91  and  91 , in  FIG. 4B , are for illustrative purposes only and are not intended to explicitly define the composite target  111 . The larger target holes  83  represent the far target  105 , the target holes  86  represent the first intermediate target  106 , the target holes  91  represent the second intermediate target and the target holes  96  represent the close target shown in  FIGS. 2A and 2B . For illustrative purposes, the hole arrangement in target  111  is shown to be different than that of the target  110  (FIG.  3 A and  3 B) as a result of the skew of the target in the target holder  42  and the requirement for the hole size to be in descending size from the hole  83  furthest from the MUT to the hole  96  closest to the MUT. The letter “U” identifies the edge of the composite focus target that is furthest from the MUT and the letter “D” identifies the edge that is positioned closest to the MUT. 
   In  FIG. 5  is a flow diagram of the method of the present invention to test the zoom capability of a miniature digital camera module under test (MUT). Focus targets are placed into target wheels, which are controlled by a tester to position the targets over the optical centerline of an optical system of a test station  200 . There can be one focus target with the composite image, shown in  FIGS. 3A and 4A , or there may be as many as four targets positioned simultaneously over the optical centerline as shown in  FIG. 2A . The distance of the target wheel from the MUT is set  201  by stepper motors controlled by the tester to cause the target wheels to be at distances from the MUT predetermined during an initial setup of the zoom test. The field lens distance from the MUT is set by the tester  202  using a value predetermined in the initial setup of the zoom test. A light color is selected  203  and the zoom of the MUT is varied to check that the focus of the MUT is maintained over the range of the zoom  204 . If the zoom of the MUT is electrically adjusted, a signal coupled from the tester to the MUT changes the zoom setting. If the zoom of the MUT is manually adjusted, a zoom stepper mechanism in contact with the lens cap of the MUT is controlled by the tester to turn the lens cap and adjust the zoom of the MUT. 
   Continuing to refer to  FIG. 5 , a magnifier lens is inserted between the field lens and the MUT within the optical centerline of the optics system. The magnifier lens is then positioned along the optical centerline  205  and the zoom of the MUT is varied to check that the focus of the target image is maintained over the range of zoom of the MUT  206 . If an additional position of the magnifier lens is required  207 , the magnifier lens is repositioned  205  and the zoom of the MUT is again varied to check the focus is maintained over the range of the zoom  206 . If an additional position of the magnifier lens is not required  208  and a next light color is required  209 , a next light color is selected  203  and steps  204  through  208  are repeated. Changing the color of light checks the multiple element zoom lens for focus problems related to a particular color. If the zoom testing has been tested with all of the light colors  210 , the zoom testing is ended. 
   While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.