referring now to fig1 there is shown the cross - section of a presently - preferred embodiment of the calibration standard of the present invention for optical gap measuring tools . the apparatus 100 comprises a substantially transparent first element 10 having a substantially flat surface 15 in contact with a convex spherical surface 25 of a second element 20 . elements 10 and 20 are maintained in their positions by an upper shell 30 and a lower shell 40 , which may be made of metal , plastic or glass . shell 30 fits tightly inside shell 40 , thus protecting surfaces 15 and 25 from external sources of dirt and dust . shells 30 and 40 are held together by an adhesive material 50 . those skilled in the art will appreciate that alternative mechanical means of maintaining elements 10 and 20 in position while protecting them from the external environment are possible without departing from the spirit and scope of the present invention . referring again to fig1 there is shown a region of contact 80 . geometrically the region of contact is a single point . physically , however , the contact region is substantially circular and extends over an area determined by the radius of curvature of surface 25 , the mechanical force applied by shells 30 and 40 , and other physical effects that tend to bring surfaces 15 and 25 into contact and enlarge the contact region . these effects are generally advantageous to the function of the apparatus as a calibration standard , since they serve to establish an obvious and unambiguous region over which the gap is zero . referring now to fig2 the calibration standard of the present invention is shown with such auxiliary equipment as is appropriate for performing a calibration . the equipment preferably comprises an optical gap measuring tool 101 of the type disclosed in my co - pending patent application entitled &# 34 ; optical gap measuring apparatus and method &# 34 ; filed mar . 22 , 1995 and bearing u . s . ser . no . 08 / 408 , 907 , and now u . s . pat . no . 5 , 557 , 399 issued on sep . 17 , 1996 . laser source 1 emits a beam 5 which is focused by a lens 3 and polarized by a linear polarizer 4 . beam 5 measures calibration standard 100 at a lateral position x . in a next step , a reflected beam 6 is imaged by a lens 7 and is divided into two beams by a beamsplitter 8 . an interference phase detector 13 and an intensity detector 12 send information to a conventional computer 99 which conventionally calculates the gap size z for the lateral position x . further details relative to the function of optical gap measuring tool 101 are taught in the aforementioned u . s . patent application ser . no . 08 / 408907 and now u . s . pat . no . 5 , 557 , 399 issued on sep . 17 , 1996 , the contents of which are specifically incorporated by reference herein . for the purpose of calibrating optical gap measuring tool 101 , a linear translator 102 displaces calibration standard 100 by means of a support arm 103 . the lateral position x is controlled by the computer 99 through a cable 104 . the graph in fig3 a shows the results of a calibration scan using substantially equivalent apparatus to that shown in fig2 . the data clearly shows a region of contact between the lateral positions of 4 mm and 8 mm , approximately . in this region , the gap size z is substantially equal to zero . outside this region , the data shows an approximately parabolic variation in gap size as a function of lateral position . this relationship may be expressed mathematically by the formula ## equ1 ## where r is a constant . referring now to fig1 elementary geometric analysis demonstrates that r is substantially equal to the radius of curvature of surface 25 . provided that elements 10 and 20 are of high optical quality , deviations of the measured data from the ideal parabolic curve are indicative of calibration errors . the presently - preferred data processing method consists therefore in fitting an ideal parabolic curve to those portions of the data that are outside of the contact region . the difference between this ideal curve and the data is the calibration error of the instrument , which may subsequently be used to verify the accuracy of the instrument or provide correction data . those skilled in the art will appreciate that the requirement of high optical quality is readily achieved for elements 10 and 20 shown in fig1 since surface 25 has a particularly simple and common spherical form , such as is used for lenses and mirrors . similarly , the fiat or plano surface 15 of element 10 is also particularly common and easily made using standard grinding and polishing procedures known in the art . it will also be appreciated that the interpretation of the calibration data shown in fig3 a according to the presently - preferred data processing method has the significant advantage that the variable x need only be a relative measure rather than an absolute measure of lateral position . for example , the addition of an arbitrary offset of x 0 to the abscissa x shown in fig3 a has no effect on the accuracy or utility of the calibration data . thus , it is not necessary to add complicated reference features to either surface 15 or 25 to establish an absolute lateral point of origin for the calibration data . referring now to fig3 b , there is shown a graph of the same data as fig3 a , but with a different scale to magnify the contact region . the graph in fig3 b shows that the presently preferred calibration standard provides detailed data for small gap sizes . the gap measurement region between 0 and 100 nm is most relevant to flying height testing . of particular interest to flying height testing is performance at 0 nm , which for the calibration standard of the present invention is provided by a large , obvious and unambiguous contact region . the existence of a well - defined contact region facilitates compensation for material phase change on reflection . for the presently preferred calibration standard , there is an obvious and unambiguous portion of the data curve for which the physical gap size is zero . thus , it is readily possible to distinguish between the phase change on reflection and the phase delay introduced by a non - zero gap size . an additional advantage of the presently preferred calibration standard is that it is readily modified to work with a variety of materials having a range of reflectivities and material properties . the simple and straightforward physical form of elements 10 and 20 means that these elements may be made of almost any solid material that can be ground and polished according to conventional optical fabrication techniques . for example , element 10 could be made of a common form of optical glass known as bk7 and element 20 could be of silicon carbide . alternatively , element 10 could be a high - density flint glass and element 20 could be made of solid silver . as another example , element 10 could be made of fused silica and element 20 could be made of a typical slider material and coated with a thin film of another material such as diamond - like carbon . the presently preferred calibration standard used in conjunction with an optical gap measuring tool additionally provides a method and means for determining the material phase change on reflection for a variety of materials . referring to fig1 and 2 , in a first step , optical gap measuring tool 101 is calibrated using a preferred embodiment of the presently preferred calibration standard 100 in which elements 10 and 20 have well known optical properties . in a next step , optical gap measuring tool 101 measures the gap profile of an embodiment of the presently preferred calibration standard 100 in which element 20 is a test object made of a material whose phase change on reflection is unknown . element 20 may be any solid material ground and polished according to known optical manufacturing techniques , or alternatively a solid substrate coated with a layer of the test material . since contact region 80 of the presently preferred calibration standard provides a clear and unambiguous zero - gap reference , any observed phase change detected within the contact region is equal to the material phase change on reflection for the test object . referring now to fig4 there is shown an alternative embodiment of the presently preferred calibration standard in which a substantially transparent element 110 , preferably having a convex spherical surface 115 , is held in contact with a substantially opaque element 120 , preferably having a substantially flat surface 125 . the physical principle and measurement method for this embodiment are similar to the preferred embodiment shown in fig1 . referring now to fig5 there is shown another alternative embodiment of the presently preferred calibration standard in which a substantially transparent element 210 , preferably having a convex spherical surface 215 , is held in contact with a substantially opaque element 220 , preferably having a convex spherical surface 225 . the physical principle and measurement method for this embodiment are also similar to the preferred embodiment shown in fig1 . referring now to fig6 there is shown yet another alternative embodiment of the presently preferred calibration standard specifically designed for use with optical flying height test equipment . as shown and preferred in fig6 a substantially opaque element 320 , preferably having a convex spherical surface 325 , is held in contact with a surface 315 of a substantially transparent glass disk 310 . element 320 is held in position by a support 330 and a loading mechanism 340 . the physical principle and measurement method for this embodiment are also similar to the preferred embodiment shown in fig1 . referring now to fig7 there is shown still another alternative embodiment of the presently preferred calibration standard in which elements 110 and 120 , referred to above in relation to fig3 have an adjustable gap . as shown and preferred in fig7 there are two translators 400a and 400b , which may be piezo - electric crystals or like devices for effecting small (& lt ; 10 μm ) mechanical motion . translators 400a and 400b are under the control of a conventional computer ( not shown ) and associated conventional electrical drivers ( not shown ). in this embodiment of the invention , the gap between elements 110 and 120 may be advantageously adjusted to gradually bring elements 110 and 120 into and out of contact , and to control the amount of mechanical pressure applied once contact is established . by way of example , some of the advantages of the all of the various embodiments of the presently preferred method and apparatus of the present invention are : ( 1 ) accurate calibration data for many important forms of optical gap measuring tools , including in particular optical flying height sensors ; ( 2 ) detailed calibration data for small gap sizes (& lt ; 100 nm ); ( 3 ) no need for absolute lateral position reference marks ; ( 4 ) large contact region for verifying performance of zero - gap size measurement ; ( 5 ) simple fabrication using common optical and mechanical manufacturing techniques ; ( 6 ) no need to characterize the optical elements of the calibration standard , apart from what is commonly performed in optical fabrication to verify quality ; ( 7 ) readily adapted for a wide variety of materials , including any material that can be ground and polished by common optical fabrication techniques ; and ( 8 ) unambiguous distinction between zero physical gap size and material phase change on reflection effects . those skilled in the art will appreciate that various omissions and substitutions and changes in form or detail of the disclosed methods and apparatus may be made without departing from the spirit and scope of the present invention . for example , transparent element 10 and opaque element 20 shown in fig1 could be replaced by two substantially transparent elements , provided only that both replacement elements reflect a portion of the light impinging upon their surfaces . further , a different surface figure other than convex spherical or planar could be employed for one or both of the contacting surfaces while retaining the advantages cited herein . those skilled in the art will also appreciate that modification of the mechanical aspects of the disclosed apparatus may be made , such as would be needed to adapt the invention to specific environmental or mechanical constraints , without substantially departing from the disclosed method of the invention .