phase shifting interferometry relies on using data from multiple interferograms to directly measure the phase of a wavefront under test . in one implementation , these interferograms are taken over time . for this specific application the tear film can change between each phase - shifted interferogram , so rapid data collection is required . a second and more preferred embodiment makes use of an instantaneous phase shifting system , where multiple interferograms are recorded concurrently so that there are no changes in the tear film during the measurement . a series of measurements may then record the time - evolution of the tear film . the phase shifting interferometer of the present invention provides an in vivo method of evaluating tear film dynamics on a human eye , both with and without contact lenses in place . in the in vivo system of the present invention , the subject will sit in front of the interferometer and the system will be aligned with the subject &# 39 ; s eye . at this point , the system will continuously collect interferometric data for the desired measurement duration . once the data is captured , algorithms are utilized to convert the phase shifted interferograms into first phase information and that phase information is then converted into a single surface measurement . this process is repeated for each measurement . the measurements are compared to determine how the tear film has changed over time . there are a number of ways of analyzing the data set of surface measurements and it is expected that other ways will suggest themselves to those skilled in the relevant art without departing from the spirit and scope of the invention . a first method is to just examine the unaltered or unedited measurements without any modification thereto . the unaltered or unedited measurements refer to the surface contour with only piston and tilt subtracted from the information . statistics describing the surface may be captured for each measurement frame and output to a database as needed . a second method relies on subtracting one “ reference ” frame from every measurement and analyzing that output . by referencing every measurement to one within the set , the changes in the tear film shape are easily seen and analyzed . in other words , any systematic or constant characteristics are filtered out . in one exemplary embodiment , the first collected surface is utilized as the reference surface . in an alternate exemplary embodiment , the surface corresponding to the time after blink , yielding the most stabile vision , ( approximately one to two seconds after blink ) is utilized as the reference surface . in yet another alternate exemplary embodiment , the most stable surface in the set , as determined by a particular metric , for example , rms , is used as the reference surface . a third method relies on subtracting a fitted polynomial surface from each measurement . this differs from the second method described above in that the reference surface being subtracted now changes for each measurement . by doing this , only the high order surface perturbations are seen . it is thought that tear film break - up happens in a way that may not be described by simple polynomials , so subtracting such a surface makes those features easily identifiable . in all the above described methods , software scripts or algorithms are used to modify , analyze and save the desired data . statistics may be saved to a database , and the visual surface representations may be combined to create a movie of the tear film evolution over time . in a preferred exemplary embodiment , the interferometer is a custom built polarization based twyman - green system where the patient is seated such that his or her eye is positioned in a test arm of the system . the preferred exemplary embodiment is illustrated in diagrammatic format in fig1 . in an alternate exemplary embodiment , the interferometer may be built on a laser fizeau configuration ; however , as stated above , the twyman - green system is preferred because of its ability to more easily phase shift using the single shot method . in addition , the polarizing variant more easily allows adjustment of laser power into each arm of the interferometer , allowing testing over a large range of reflectances . a beam expander and converger system is used in the test arm to expand the beam and focus it at the center of curvature of the tear film . in this method , the test beam is reflected back through the system where it is combined with a reference beam and directed to a camera . the resulting interference pattern between the two signals is used to calculate the phase of the wavefront reflected off of the tear film and this information may be used to calculate a topographic map of that surface . the light source 102 is a stabilized diode laser with a wavelength of 785 nm . this wavelength is desirable since it appears as a very dull light to most humans . therefore , the tear film will not be altered by a bright signal causing discomfort and potentially producing reflex tearings . the light from the diode laser 102 is passed through an isolator or a neutral density filter 104 , a high speed shutter 106 , a pick - off beam splitter 108 , an objective and spatial filter 110 and a collimator 112 wherein it is spatially filtered and expanded to an eighteen ( 18 ) mm diameter collimated beam . this beam is then directed to a fold mirror 114 and passes through a half - wave plate 116 which allows the intensity in the test and reference arms to be balanced , and is split into each arm with a polarizing beam splitter cube 118 . in the reference arm , the beam passes through a quarter - wave plate 120 , oriented with its fast axis at forty - five ( 45 ) degrees , is reflected off a reference mirror 122 , and again passes back through the quarter - wave plate 120 to the polarizing beam splitter cube 118 where it is directed toward a camera 126 , discussed in detail subsequently , through an imaging lens 124 and quarter - wave plate oriented with its fast axis at forty - five ( 45 ) degrees ( not shown ) associated with the camera 126 . in the test arm , the beam passes through a quarter - wave plate 128 oriented with its fast axis at forty - five ( 45 ) degrees , through an aberrometer beam splitter 140 , a partial null module 130 , discussed in detail subsequently , through a beam expander and converger 132 , discussed in detail subsequently , and is reflected off the tear film in the eye 134 . the beam then passes back through the beam expander and converger 132 , the partial null module 130 , the aberrometer beam splitter 140 , the quarter - wave plate 128 , the polarizing beam splitter cube 118 , the imaging lens 124 , the quarter - wave plate and into the camera 126 . the aberrometer beam splitter 140 directs a portion of the beam to a wavefront aberrometer 138 , which is discussed in detail subsequently . it is important to note that both the reference and test beams pass through an imaging lens 124 which images the tear film onto the camera . the beams pass through an additional quarter - wave plate associated with the camera 126 . the camera 126 or detector in the system is a pixelated camera kit available from 4d technology corporation . this camera 126 comprises a ccd where a pixilated phase mask is aligned to the sensor . the purpose of the phase mask is to create four phase shifted interferograms on a single detector . in this way , instantaneous phase shifting inteferometry may be used to measure the dynamic tear film . alternately , there are simultaneous phase shifting systems that utilize multiple cameras and polarizers . in addition , the simultaneous system may be replaced with high speed cameras and rapid phase shifting . in other words , any suitable means for capturing the images may be utilized in conjunction with any suitable means for phase shifting . the main purpose of the converger 132 is to focus the laser at the center of curvature of the tear film , ensuring that the reflected signal follows the proper path back towards the camera 126 . the specific design of the converger 132 is driven by the desired diameter to be tested on the contact lens or eye tear film , along with the working distance between the contact lens or eye tear film and the converger . the image space f /# of the converger 132 must be less than or equal to the tear film radius of curvature divided by the test diameter , represented by the equation as the desired test area increases , the f # must become smaller . assuming a constant working distance , this means the lens diameter must become larger as well . in general , increasing the working distance or increasing the test area also increases the converger diameter . therefore , the desired test geometry must be balanced with practical manufacturing considerations . additionally , the effective f /# of the entire imaging system , including both the converger and the imaging lens must be sufficient to match the resolution of the camera . the cutoff frequency of the detector is the largest fringe frequency it is capable of resolving ( i . e ., smallest fringe spacing ). the cutoff frequency of the imaging system is the largest frequency it is capable of transmitting through to the detector . the limiting detector resolution is given by the equation where d pixel is the pixel pitch on the detector . the imaging lens cutoff frequency or wavelength is given by where λ is the wavelength and f /# w is the working , or effective , f /# of the imaging system . in order to achieve a detector limited system , which is preferable , the imaging lens cutoff frequency must be larger than the detector cutoff frequency . in the exemplary embodiment of the invention , the converger 132 was designed to test a six ( 6 ) millimeter diameter area on the eye and provide a working distance of seventy - seven ( 77 ) millimeters between the last element of the converger and the eye , and assumes nominal surface geometry with 7 . 8 millimeter radius of curvature and a conic constant of − 0 . 25 . based on these requirements , the image space f /# of the system must be 1 . 4 or faster and more preferably 1 . 33 or faster . in addition to focusing the beam at the tear film &# 39 ; s center of curvature , the converger 132 must also take into account the conic shape of the cornea . since the beam is reflected off of this conic surface back into the interferometer , many fringes would be observed if the conic shape of the cornea is not taken into account . in the exemplary system , beam expanding optics are also incorporated into the converger system in order to provide a large enough beam to achieve the required f /# and working distance . the optics first expand an input beam having a diameter of eighteen ( 18 ) millimeters and then bring it to a focus as the proper location . the beam is reflected off of the tear film before it actually reaches focus and travels back through the converger following its previous path in reverse . the length of the system from first to last lens is one hundred forty ( 140 ) mm and the largest element diameter is eighty - two ( 82 ) mm . fig2 is a diagrammatic representation of both the converger 132 and the partial null module 130 , which is shown to the left of the converger 132 in the drawing . the partial null module 130 within the interferometer corrects for corneal astigmatism that will likely be present in the eyes under test . corneal astigmatism occurs when the cornea &# 39 ; s radius of curvature varies along different planes . if not taken into account , the measurements will show astigmatism which may introduce overall measurement errors . to correct this , the opposite amount of astigmatism may be introduced into the system . this is done by using a pair of cylinder lenses with opposite radii of curvature in collimated space before the converger . when these lenses are rotated with respect to each other and the whole system , the proper amount of astigmatism can be introduced without added other aberrations to the system . essentially , the cylinder lenses work in the same manner as the cylindrical corrections in spectacles or contact lenses . in the partial null module 130 , the first cylinder lens is plano - concave with the concave side having a radius of 1500 mm . the second cylinder lens is plano - convex with the convex side having a radius of 1500 mm . each is made of bk7 glass and has a center thickness of five ( 5 ) mm . each lens is mounted in a rotation mount , and they are located ten ( 10 ) mm apart . fig2 illustrates these lenses , although the concave / convex surfaces appear mostly flat due to the long radius . when rotated as described , these lenses provide continuously variable corneal astigmatism accommodation providing up to roughly +/− 1 . 3 diopters of dynamic range . in interferometry it is crucial to stabilize the surface under test . therefore , a sturdy chin rest and head rest will be used to stabilize the subject . once the person is stable in this mount , the system may then be accurately aligned to the eye . in order to keep the eye steady , a fixation system or module 300 is used . this fixation system presents a collimated signal to the opposite eye under test . for example , if measuring the tear film on the left eye , the fixation target will be presented to the right eye . by asking the subject to focus on this target , eye movement will stabilize . by moving the target , the gaze direction of the subject can also be changed . the optical system comprises a target , collimating lens 302 and fold mirror 304 . the target is adjusted to provide targets of differing visual angles or geometries . the collimating lens 302 may be adjusted by the user to adjust proper focus taking into account the patient &# 39 ; s vision . fig3 and fig4 illustrate the optical and mechanical designs of the fixation module 300 . the system or module 300 also integrates an off - the - shelf eye tracking system to track eye movements and ensure that changes observed in the measurements are the result of tear film changes and not eye movement . the supplier of this system is arrington research . the laser 102 included in the system normally operates at class 3 b , which may not be eye safe . unfortunately , low power sources at the desired stabilized wavelength are not commercially available so the optical power in the system must be reduced to safe levels . a variable neutral density filter is used to set the output power , and a pick - off mirror or pick - off beam splitter 108 ( see fig1 ) is used to continuously monitor the power output via a power meter 136 ( see fig1 ). interlocks are used to ensure the power is not altered once set at safe levels . if these interlocks are broken , the system shuts down . while the measured fluid layer surfaces may be described with traditional optical metrics such as rms surface height or aberration terms , these metrics do not adequately describe the fluid layer topography . features such as pits in the tear film are an early indication of tear film breakup , and their presence is not quantified with the aforementioned metrics . it is important to know precisely when and where these artifacts begin . the presence of these artifacts in the tear film indicates the tear film is beginning to degrade , which on the eye can cause discomfort or vision degradation . therefore , a “ blob analysis ” routine was developed to analyze the measurements in order to identify and quantify holes or pits in the tear film . blob analysis is a simple form of texture analysis and generally involves various methods of extracting textural features from images . it is important to note that the analysis is preferably done over multiple and differently sized apertures because a stated rms value for a given measurement aperture may be arrived at by surfaces that might be characterized as smooth or rough . multiple measurements over different aperture sizes are thus needed to remove the ambiguity , to better determine the spatial frequencies of the surface within the test aperture . fig5 shows the four major steps in the blob analysis routine developed in conjunction with this instrument . the first step of the blob analysis procedure is to import the surface measurement into idl ( itt visual information solutions , boulder , colo .). in step two an un - sharp mask is applied to the measurement which effectively amplifies the high frequency areas on the surface corresponding to the perimeter of the blobs . the original measurement is then subtracted from the unsharp masked version leaving data only at those areas enhanced by the unsharp mask , and a smoothing kernel applied to remove noise , leaving only areas of interest ( the blobs ) in the fluid layer visible . at this point ( end of step 3 ) the processed measurement is a binary data array consisting of high values where the blob exists and low values everywhere else . in the final step perimeters are fit to every individual blob and the number of blobs along with their areas and locations are recorded . other methods of analysis include wavelet analysis , fractal analysis and fourier transform analysis . in accordance with another aspect , a wavefront aberrometer may be utilized in combination with the tear film interferometer of the present invention . a wavefront aberrometer is a diagnostic instrument which utilizes wavefront technology to measure wavefront errors or aberrations ( optical imperfections , both low and high order ) of the eye . in this case , the aberrations measured would include any imperfections in the eye , the contact lens and the tear film . in the exemplary embodiment illustrated in fig1 , the wavefront aberrometer 138 is synchronized to the operation of the interferometer such that both changes in the surface of the tear film interferometry and the corresponding changes to the ocular wavefront may be collected . an aberrometer beam splitter 140 is positioned between quarter wave plate 128 and the cylinder null 130 . in order for the aberrometer 138 to be used in conjunction with the tear film interferometer , appropriate optical elements must be provided in the aberrometer 138 so that the separate aberrometer light source produces a collimated beam at the eye 134 . the combination of tear film interferometry and wavefront aberrometry has the benefit of being able to attribute a portion of the changes in the wavefront to the tear film . more specifically , the data may be utilized to differentiate or compare tear film changes versus changes due to contact lens movement . this information may then be utilized to optimize the design of the contact lens . for example , two major components of temporal changes to the ocular wavefront are typically the tear film and contact lens movement . without the combination of an interferometer and aberrometer , there is currently no way of separating the two factors . in a preferred exemplary embodiment , a fiducialized contact lens is utilized to track contact lens position . essentially , a fiducial in optics is an object used in the field of view of an imaging system which appears in the image produced and is used as a point of reference . contact lens position ; namely , centration and rotation , is encoded in the wavefront aberrometer &# 39 ; s data via the shadow cast by the fiducials of the contact lens . although shown and described is what is believed to be the most practical and preferred embodiments , it is apparent that departures from specific designs and methods described and shown will suggest themselves to those skilled in the art and may be used without departing from the spirit and scope of the invention . the present invention is not restricted to the particular constructions described and illustrated , but should be constructed to cohere with all modifications that may fall within the scope of the appended claims .
an in vivo method of characterizing dynamic tear films has been developed using a near infrared phase - shifting interferometer . this interferometer continuously measures light reflected from the tear film , allowing precision analysis of the dynamic surface topography . movies showing the tear film behavior may be generated along with quantitative metrics describing the tear film surface as it changes in time .