GB2186991A - Optical examination of a body surface - Google Patents

Abstract

The surface (4) of a cellular body (1) which is flat except for a number of cavities (5) is examined by placing a transparent light refracting medium (7) against the surface and transmitting light (11) through the medium toward its interface with the body surface at such an angle of incidence to the interface that light incident on the interface at the body surface is refracted into the body and light incident on the interface at the cavities is totally reflected internally of the medium and thence outwardly thereof. The reflected light (12) produces an image having readily distinguishable zones of lightness and darkness indicative of the cavities and body material, respectively. The image is displayed in such manner as to enable either of the relatively dark and light zones to be counted. An optical coupling liquid preferably is interposed between the body surface and the refracting medium to compensate for small gaps or irregularities in the body surface. <IMAGE>

Description

SPECIFICATION Apparatus and method for facilitating the examination of a body surface This invention relates two an apparatus and a method for facilitating the examination of a body surface that is substantially flat exceptforthe presence of cavities. The invention is particularly useful in the determination ofthe size of cells in a body of cellular material.

In the manufacture of cellular products from foamed polymers the size of the cells has significant effects on many of the physical properties of such products. For example, thermal insulating and compressive strength properties are directly related to cell size. To establishe manufacturing procedures for the production of cellular products having the desired physical characteristics, therefore, it is important to be able to determine accurately the size of cells resulting from various production techniques. Once the cell size resulting from one production technique has been established, the technique may be maintained or varied, as required, to produce a cellular product corresponding to predetermined specifications.To ensure that the appropriate technique is being maintained, it is desirable to be able to monitor the production by making rapid examinations of samples ofthe productatfrequent intervals during its production run.

The importance of cell size to the physical properties of a particular product is well known and at leasttwo proposals have been made heretofore for determining cell size. Each proposal involves cutting a thin section from afoam sample and then, according to one proposal,transmitting light through a predetermined area of the sampleto produce an image and then projecting the image for visual inspection. In the second proposal a predetermined area of the surface of a thin sample is illuminated, and light reflected by such surface ofthe sample is transmitted to a microscope for visual examination.

Both proposals rely on the ability of a viewer two distinguish the contrast between the intensities of the light transmitted through or reflected by the sample at the walls of the cells and at the exposed cavities created by cutting ofthe sample. In many instances the intensities of the transmitted or reflected light are too nearly equal to enable the observer to discriminate accurately between the cell walls and the cell cavities, thereby resulting in significant errors in determining cell size. Theoretically, the problem in distinguishing between cells and cell walls may be overcome by forming a sample having athickness corresponding substantiallyto the diameter of a single cell. In practice, however, it is not possible to form such samples consistently.As a consequence cell walls or struts internal ofthe sample cannot be distinguished fromthewalls at the surface ofthe sample. The inability of an observer to distinguish between cell walls at and below the surface of a sample is particularly pronounced in those instances in which the sample is formed of a material which is transparent. Thus, it is virtually impossible to obtain accurate cell size determination using currently available methods.

Adistinctdisadvantage resulting from the slicing of a thin section sample from a body offoam material is thatthe slicing equipment presently available invariably causes the formation of irregularities in the exposed surfaces ofthe cell walls,thereby preventing the preparation of a truly representative sample. For example, many polymers are sofragilethatthe slicing operation results in the tearing away of sections ofawall and the formation of ragged edges. Particularly is this true in those instances in which the sample has athickness approaching that of only one cell diameter.

Afurther disadvantage of the known methods of examining samples of the kind referred to is thatthe inspection requires too great a timeto complete. Thus, if an inspection reveals deficiencies in a product, it is possible that a substantial quantity of the deficient product will have been produced during the time ittakes to complete the inspection and before corrective adjustments can be made in the production process.

Apparatus and methods according to the invention overcome the disadvantages referred to above and are applicable not only to the examination of opaque, translucent, and transparent cellular products, but to other objects as well.

The examination of a cellular body according to the invention for determining the size ofthe cells involves the cutting ofthe body at one side thereof along a plane to form a sample. This will cause the walls ofthose cells in the plane ofthe cutto be severed sothatthe remaining portions of such walls are substantially coplanarandterminate in the plane ofthe cut. Between the adjacent wall portions are gaps orcavitiesof varying widths, depths, and shapes, depending upon the size and particular portion of a cell traversed bythe cutting tool.

Atransparent prism having polished, planarfaces is applied to the sample so that a selected face ofthe prism confronts and bears upon those portions ofthe cell wallswhich lie in the plane ofthe cut. Theforce with which the prism bears on the sample is sufficiently lightto avoid significant compressive deformation of the sample. The area of the prism face is sufficientto span a number ofthe cavities formed by severing the cells. The exposed surfaces of the cavities are spaced from the confronting face ofthe prism and in a direction inward of the sample. The space between the cavity and the overlying prism face is occupied by air or other environmental gas.

The prism is illu minated by a beam of preferably white lightthat is directed toward the prism/sample interface at an angle of incidence which is at least as great as the critical angle of the environmental gas, but less than the critical angle of the sample. As a consequence, those portions of the beam incident on the prism/sample interface at the cell walls are refracted into the sample, whereas those portions of the beam incident on the prism/sample interface atthe cell cavities are totally reflected internally along a path leading outwardly of the prism.The reflected light forms an image wherein the cavities appear as relatively light zones andthe cell walls appear as relativelydarkzones. The contrast between the light and dark zones is substantial.The image may be magnified, if desired, and projected onto a screen ordisplayed on avideo monitor, for example, thereby easily enabling the number of severed cell walls atthe surface ofthe sampleto be counted. Once the number of cell walls in a predetermined distance is known, the cell size may be calculated.

To compensate forthe inevitable irregularities in the cell walls caused by cutting of a fragile sample, a thin coating of transparent liquid is interposed between the sample and that face of the prism which confronts the sample. The liquid fills the irregularities and forms an optical coupling between the severed cell walls ofthe sample and the prism.

The apparatus adapted for use in practicing the invention is illustrated in the accompanying drawings wherein: Figure lisa diagrammaticview, partly in elevation and partly in section, of an apparatus illustrative ofthe invention: Figure2 is a largely diagrammaticview illustrating an apparatus suitable for use in practicing the invention on a commercial scale; Figure 3 is a fragmentary, enlarged detail view of a portion ofthe structure shown in Figure 1; Figurelis an enlarged diagrammaticview illustrating the criteriato be considered when practicing the invention; Figure 5is a table of data calculated for several different kinds of prism materials; and Figure 6is another table showing critical angles at a liquid/sample interface of a range of refractive indices.

The apparatus ofthe invention, especially adapted for use in the examination of a cellular body 1 to measure the size of cells is shown in Figure 1. The body is resiliently compressible or deformable and is formed of a polymeric material that may be opaque, translucent, or transparent. The body may or may not have an outer skin. In any case, the body 1 has a plurality of cells 2 defined and spaced from each other by wails 3.

Preparatory to the examination, the body 1 is cut in a known manner along one side thereof to form a plane 4in which all ofthe edges of the severed cell walls 3 lie. Those portions of the cells 2 between adjacent severed wall portions form cavities or gaps 5 exposed to air or other environmental gas. Cutting of the body along the plane 4, therefore, results in the forming of sample 6 having a substantially flat or planar surface interrupted by a plurality of caviti es of different widths and depths. The width and depth of a cavity depends upon not only the diameter of the cell, but also upon that part ofthe cell through which the cut was made. The thickness of the sample is immaterial, but it should be such as to be convenientto handle.

The apparatus illustrated in Figure 1 includes a light refractive medium such as a transparent prism 7 having at leastthree planarfaces, one of which is adapted to confront the cut surface of the sample and bear sufficiently lightly upon the severed walls 3 of the cells 2to avoid significant deformation of the walls. The area ofthe prism face chosen to confront the sample should be sufficiently large to span a numberofcavities 5 representative of the cellular structure ofthe body.

The method according to the invention relies upon the optical phenomenon oftotal internal reflection which occurs when light passing through a first medium of relatively high refractive index impinges on the interface between the first medium and a second medium of relatively low refractive index and at an angle of incidence greaterthan the critical angle of the second medium.

For proper performance of the method according to the invention the prism 7 should be formed of a transparent material having an index of refraction so related to the refractive indices ofthe environmental gas and the material from which the body 1 is made that light incident on the prism/sample interface at a cell wall 3will be substantiallywholly refracted into the sample, whereas light incident on the prism/sample interface at a cavity Swill be totally internally reflected away from the sample.

The refractive indices are known for air and other gases, and each of a large number of materialsfrom which prisms and cellular bodies may be formed. The refractive indices of ther materials may be computed according to Snell's law. Materials suitable for use as prisms include zinc sulfide, zinc selenide, and sapphire, but other materials, including glass and quartz, may also be used. The refractive index of air and other gases is 1,the refractive index of glass is 1.53, of zine selenide is 2.3, of sulfide is 2.4, and of sapphire is 1.76. The refractive index of polystyrene, one polymer from which the body 1 may be formed, is 1.6. The refractive indices of other polymers and prism materials may be obtained from existing tables or computed.

Once the refractive indices of the prism 7 and the body 1 are known, the prism may be shaped so that incident light perpendicular to one of its shorterfaces will impinge upon the face forming the hypoteneus at an angle having a value between the prism's critical angles relative to the polymer and relative to air.

For example, the critical angle of zinc selenide relative to polystyrene is 44" and relative to air is 26".The configuration of a zine selenide prism for use with a cellular polystyrene sample, therefore, should be so chosen that its smaller angle is between 26 and 44'. An obtuse prism having a smaller angle of 30 thuswill be appropriate and easy to manufacture. Prisms having other angles may be used, however, depending on the relationship referred to above witch respect two critical angles. For example, if a sapphire prism is to be used in the examination of a polystyrene body, it is convenientto use a right angular prism whose smaller angles are 45 . Any prism used may be truncated to conserve space.

The prism 7 shown in Figure lisa right angular prism having angles of 45", 90 , and 45 . The prism has three planar, polished faces 8,9, and 10 and is so oriented to the sample 6thattheface8 confronts and bears lightly upon the coplanar, severed portions ofthe cell walls 3, The face 8 is of such area asto span a number ofthe cavities 5.

In the arrangement shown in Figure 1 a beam of, preferably, white light 11 is directed upon and perpendicularto the prism face 9. The beam passesthrough the prism toward face 8 which interfacesthe prism and the sample 6. The refractive index ofthe prism is such thatthe angle of incidence ofthe beam on the face 8 is less than the critical angle of the material forming the body 1, but greaterthan the critical angle of air. Thus, those portions of the light beam 11 incident on the face 8 overlying the cavities 5 are totally reflected internally and thence along a path extending outwardly ofthe prism through the third face 10. The reflected portions ofthe light beam are indicated by reference number 12. Those portions ofthe light beam 11 incident on the face 8 overlying a cell wall 3 will be refracted into the sample.Thus, the reflected light beam 12will be interrupted by laterally spaced apart, relatively dark zones indicated by reference number 13.

Since some portionsofthe light incidentonthe prism face 8 are totally reflected and other portions are refracted into the sample, the reflected light from the interface between the prism and the sample will form an image having relatively light and dark zones in which the lighter zones represent the cavities and the darker zones representthe severed walls between adjacent cavities 5. Interposed in the path ofthe reflected light is an oculardevice E orothersuitable collector by means ofwhichthe image may be viewed directlyor projected in a conventional manner onto a movie or other conventional screen (not shown).

In the image the severed cell walls at the prism/sample interface will appear as dark zones, whereas the cavities between the cell walls will appear as much lighter zones, regardless of whether the body under observation is opaque, translucent, orwholly transparent. Thus, the contrast between the cavities and the cell walls is substantial, thereby clearly delineating the cavities and the cell walls at the prism/sample interface, but noneofthe cell wallswhich lie inwardlyofthesample's surface will appear in the image. Itis fairly easy, therefore, to count eitherthe cells orthe cell walls of the projected image.Once the number of cell walls has been counted, the cell diameter D can be computed using the equation D = 1.62 LIN wherein N represents the number of cell walls counted in a straight line distance L.

Figure 2 illustrates an apparatus useful in the commercial practice of the method. The body 1 and the sample 6 are the same as described earlier, bu the light refracting medium is an obtuse prism 7a.The additional apparatus includes suitable means (not shown) for supporting the prism, a quartz-halogen bulb or othersuitable, preferablywhite light, source 14, and a diffuser 15through which a beam of the light 1 6 is directed perpendicularly onto the prism face 9a in the same manner described earlier. The diffuser prevents a concentration of light directly opposite the light source.

Light incident on the prism/sample interface atthe cell walls 3 is refracted into the sample, whereas light incident on the prism/sample interface at the cavities 5 is totally internally reflected along a path which extends outwardly through the prism face 10a as a light beam 17 similartothe light beam 12described earlier. If a sapphire or other bi-refringent material is used for the prism, the reflected beam 17 should be passed through a polarizer 18 and thence collected and magnified buy a microscope objective 19 and transmitted to a video camera 20. From the camera the image produced bythe reflected light is transmitted to a display unit such as a video monitor 21 having a screen 22 on which the image may be displayed.The displayed image will show the cell wall portions 3 atthe prism/sample interface as black or darkzones 3a and the cavities 5 as white or light zones 5a, thereby providing substantial contrast between the cell walls and cavities. If desired, the prism face 8a may be provided with a reticle scale which also is displayed on the screen 22, thereby facilitating counting of the number of cell walls in a selected distance.

It is not essential that white light be used as the incident light. Light of a selected color may be used in some instances. White light is preferred, however, because it enhances the contrast between cavities and the cell walls when the image is displayed on movie and video screens.

The foregoing explanation ofthe apparatus shown in Figures 1 and 2 assumes that the severed edges of the cell walls are perfectly flat and coplanar. In actual practice it is not presently possible to produce such edges because commercially available cutting devices are incapable of cutting the fragile cell-defining walls of a polymerwithouttheformation of small irregularitiesorgaps in the exposed surfaces ofthe cutwalls.

Typical gaps are shown at23 in Figure 3. Such gaps will cause some portions ofthe surfaces ofthewalls 3to lie below the plane 4 at the prism/sample interface, thereby enabling air to interface the prism at the gaps and causing light incident on the prism/sample interface atthe gapsto betotally reflected. These gaps thuswill appear in the displayed image as light or white portions in the dark zone representing the cell walls and could be misinterpreted as constituting small cellular cavities in the walls. Such gaps could be compensated forto some extent by urging the prism againstthe sample under sufficientforceto deform the cutwallsso that the entire surface of each wall will engage the prism. Such deformation, however, could collapse one or more of the cut walls and cause the prism face to bear against the base of one or more cavities, thereby causing the displayed image to indicate falsely the absence of such cavities.

In the preferred embodiment of the invention the presence of small irregularities in the severed walls ofthe cells is compensated for by interposing between the prism and the sample an optical coupling comprising a thin coating 24 of a transparent liquid such as paraffin oil, kerosene, or silicone oil ranging in viscosityfrom 200 to 10,000 cp. Other liquids may be used, however, as is explained hereinafter.

The presence ofan optical coupling liquid between the prism and the samplefillsthe gaps 23 inthecell walls caused by the cutting of the body 1 to form the sample and provides an optical path which permits light incident on the prism/sample interface at such gaps to pass into the sample, rather than undergoing total internal reflection. The coupling liquid, therefore, reconstitutes the cell walls so thatthey act optically as if they had been cut perfectly. Thus, the use ofthe coupling liquid enables an imperfectly cut wall to appear in the displayed image as a continuous surfacewithout having to apply excessive compressive forces on the sample.

When an optical coupling liquid is used, two conditions must be satisfied if the projected image isto display accurately, the cavity/cell wall relationship ofthe sample under examination. First, there must betotal internal reflection (TIR) at the interface between the liquid and the air in a cavity and, second, there must not beTIR at the interface between the liquid and a wall 3. The necessary relationships to ensure satisfaction of these two conditions are explained hereinafter with reference to Figure 4wherein the reference characters therein denotethefollowing: R = A ray of white light.

6 = Angle of incidence for the light ray within the prism approaching the prism/liquid interface. This angle also is thy vertex angle of the prism for normal external incidence.

6o = Angle of refracted ray in the coupling liquid coating. This angle is also equal to the angle of incidence at the liquid/sample interface and the angle of reflection atthe liquid/air interface.

6, = Angle of refracted ray in the sample.

#a = angle of refracted ray in the air.

= = Critical angleforthe prism/liquid interface.

6cpa = Critcal angle for the prism/air interface.

Loco, = Critical angleforthe liquid/sample interface.

coa = Critical angle for the liquid/air interface.

Ov = Vertex angle of the prism.

np = Refractive index of the prism material.

nO = Refractive index of the coupling liquid.

= = Refractive index of the sample.

The incident light ray R shown in Figure 4 is perpendicularto one of the two shorter faces of the prism and impinges on the prism/liquid interface at an angle of incidence 61 which is less than the critical angle 6Cp0 ofthe coupling liquid. The ray Rthus is refracted into the liquid at the angle 00 and proceeds toward and impinges upon the liquid/sample interface at an angle of incidence which is less than the critical angle Ocos of the body material, but greaterthan the critical angle loco, of air.Thus, if the liquid/sample interface at the ray R is formed bya section of a cell wall,the raywill be refracted into the body at the angle 05. However, if the liquid sample interface atthe ray R is formed by air in a cavity, the ray will bye totally reflected at the angle 60 and redirected through the liquid to the prism. The reflected ray is refracted by the prism at the angle 0B and directed outwardly thereof throug h its second shorter face.

The criteria necessary to produce the desired results ofTIR at a liquid/cavity interface and no TIR at a liquid/sample interface may best be explained by the following applications ofSnell's law.

Since the refractive indexofairis 1.0, the application of Snell's lawtothe light ray in Figure 4 results inthe following equation: npSin0; = nOSinHO = n,Sin6, = Sin#a, (Equation 1) For the light ray Rat a liquid/air interface to be subjected to TIR, angle #a must equal or exceed 90 .Thus, Sin0,1 When equation 1 is rewritten to include this requirementthe result is: npSin0; = noSin#o# 1 (Equation 2) which indicatesthatthe minimum possible values for 60 in the coupling liquid and #iin the prism are: ZOmin = Sin-(1/no)= Oco, (Equation 3) and O1min = Sin-1(1/np) = Ocpa (Equation 4) Notethat equations 3 and 4 correspond to two ofthe critcal angles shown in Figure 4.

For the light ray Rat a liqu id/sam ple interface to be refracted into the sample, thereby satisfying the condition that the ray not be subjected to TIR at such interface, the angle of refraction 0, must be less than 90 or stated differently, Si nB,1 When equation 1 is rewritten to include this requirement the result is:: npSin0; = noSino ' n, (Equation 5) Thus, 6o is restricted to the value #o#Sin-(ns/no) (Equation 6) and #i is restricted to thevalue 0jsSin-1(ns/np) (Equation 7) The maximum value of the incident angle #i given by equation 7 is subject two the condition that no TlRtake place atthe interface between the prism and the liquid.This requires the angle of refraction 0o in the liquid to be less than 90' with the result that Since 1. Rewriting equation 1 to reflect this requirement yields a second restriction on the angle of incidence: Ojsin-1(nO/np) (Equation 8) Thus, the angle of incidence for the light at the prism/liquid interface must lie between the extremes given in equations 4,7 and 8: Sin-(1/np) < #i < Sin-1 (n/np) (Equation 9) wherethe maximum allowablevalueforthe angle of incidence #i is: Oj max = Sin-1(n/np) (Equation 10) n being the smaller of nO and n,.

The vertex angle Ov of the prism must be chosen to give a valueto Oj which satisfies the boundary conditions represented by equation 9. For light incident normal to the prism face, the vertex angle is just equal toO, so that #v can be substituted for 0 in equation 9. The incident angle üj is determined primarily by the available prism geometry options. It is clear fro equation 9 that, for a given sample index of refraction and prism geometry, the prism material itself is not limited to a specific value of refractive index. This means that it is possible to selectthe prism material on the basis of characteristics other than the refractive index only.A compilation of some actual valuesforthe angles shown in Figure 4 is presented in the Tables of Figures 5 and 6 for a range of prism, coupling liquid, and sample refractive indices.

One important non-optical characteristic ofthe prism material to be considered in the construction of apparatus for practicing the method is abrasion resistance. As samples are repeatedly interfaced with the prism and adjusted slightly to obtain a uniform liquid coupling between the prism face and the sample, considerable scratching of the prism face can occur if the prism is formed of a soft material such as zinc sulfide. It is preferred, therefore, to use a prism formed of a harder material. Sapphire, because of its extreme hardness and relatively high refractive index, is an excellent material to use for the prism.

Equations 4,7, and 8 indicate that the choice of #i becomes severely limited as the refractive index nO ofthe liquid coupling approaches the refractive index of air, i.e., 1. Since most liquids have refractive indices at least 30 percent greater than that of air, the liquid index nO has little practical effect on the choice of #i (and thus on the prism geometry) for a particular application. Consequently, the coupling liquid can be chosen on the basis of its wetting characterisitcs, chemical inertness to the sample material, low volatility, and the like, rather than its refractive index. For example, the refractive index nO of the coupling liquid can be either greater or lesserthan thatofthe sample and still enablethe method to be performed. This is becausethelight rays refracted atthe liquid/sample interface into the sample enterthe latter atthe same angle as would bethe case if no liquid were present. This can be demonstrated from equation 1 by rewriting it as: Sin#s = (no/ns)Sin#o = (no/ns) [(np/no)Sin#i] (Equation 11) Equation 11 can be rearrangedto: SinO, = (np/ns)Sin#i (Equation 12) which isSnell's lawforthe case in which there is no optical coupling liquid.

Similar results are obtained for the ray R that its totally reflected from the liquid/air interface. Defining 0R as the angle ofthe reflected ray entering the prism from the liquid layer, and applying Snell's law: nOSinûO = npSinûR (Equation 13) which is equivalentto: SinOR = (n,/n,) (np/nO)Sinû; (Equation 14) and which, upon simplifying, becomes: ÛR= Ûj (Equation 15) Again, this result corresponding to that which would be obtained if no optical coupling liquid were interposed between the prism and the sample.Thus, the only function of the coupling liquid is to displacethe airinthesmall gaps or irregularities produced by cutting ofthe cell walls and thereby enablethe lightto be refracted into the sample. The result is an image which conforms to the appearance that the cell wall would exhibit if the cut were perfect.

Asapphire prism having a vertexangle of4S'will have light incidentatthe liquid/air interface at an angle 00 that is greater than the critical angle for that interface for all liquids having refractive indices ranging from 1.3 to more than 3. However, the angle ûcOa is less than the critcal angle 0c0,for samples having indices ranging from 1.3 to 3, as can be seen from the values in Tables land II. Thus, the sapphire prism will producethe desired image for a wide range of combinations of sample and liquid refractive indices, even in those cases in which the refractive index of the sample is less than that of the coupling liquid.

The optical coupling liquid may be brushed, sprayed, or otherwise applied to the sample orto that face of the prism which confronts the sample. In either eventthethickness of the liquid coating should be insufficient to fill the cavities 5.

Distinct advantage ofthe invention over the prior cell counting techniques referred to earlier is that, when using the method according to the invention, it is not necessartto cut a thin sample from the body under examination. It is only necessary to ensure that the body is cut along one side thereof to provide a substantially planar surface against which one face of the prism may bear. Thus, a body need be cut only once to form a sample, and the sample may be of such size that it conveniently may be held and manipulated by hand. These characteristics, coupled with the ease of distinguishing between cell walls and cavities, enablethe preparation and examination of a sampleto be completed considerably more quickly than has been possible heretofore. Thus, the need for modificatin in a process for the production of a cellular product can be detected and corrective action taken with less delay than formerly has been the case.

Although the foregoing description has been concerned primarily with the examination of cellular bodies formed of foamed polymers, the applicability of the method and apparatus according to the invention is by no means so limited. The apparatus and method are adaptable to the examination of hairlinefractures, woven fabrics, and other objects ofthe kind wherein the interface between any such object and a light refracting medium comprises a substantially planar surface interrupted by one or more cavities.

Claims (15)

1. A method of examining a body surface that is substantially flat exceptforthe presence of a numberof cavities therein containing a gas, comprising the steps of establishing an interface between a selected area of said body surface and aflatface of a transparent, light refracting medium; directing lightthrough said medium toward the interface at an angle of incidencetothe latterwhich is greaterthan the critical angle of said gas and less than the critical angle of said body, whereby light incident on the interface at the surface of said body is refracted into the latter and light incident on the interface at any of the cavities is totally reflected internallyofsaid medium along a path leading outwardly thereof to form an image having relatively lighter and darker zones indicative of the cavities and said flat surface, respectively; and displaying said image.

2. A method as claimed in Claim 1, including the step of interposing between the surface of said body and the flat face of said medium a coating oftransparent liquid to form an optical coupling therebetween said coating having a thickness less than the depth of said cavities.

3. A method as claimed in Claim 2, wherein said coating comprises an oil having a viscosity of from 200to 10,000 cp.

4. A method as claimed in any one ofthe preceding claims, including the step of counting the numberof darker zones in a selected iength of said image.

5. A method as claimed in any one of the preceding claims, including the step of magnifying the image priorto displaying it.

6. A method as claimed in any one of the preceding claims, wherein said transparent, light refracting medium is a prism.

7. A method as claimed in any one of the preceding claims, wherein the light is a white light.

8. A method as claimed in any one of the preceding claims, wherein the body surface is a planar cut surface of a foamed polymer.

9. A method as claimed in Claim 1 and substantially as hereinbefore described with reference to Figures 1 and 2.

10. A method as claimed in Claim 2 and substantially as hereinbefore described with reference to Figures 1 and 4.

11. Apparatusfor use in examining a body surface having a number of cavities therein containing a gas and spaced from one another by substantially coplanarwalls, said apparatus comprising a transparent, light refracting medium having a planarface forming an interface with said body surface and spanning a selected area thereof; means for directing light onto and through said medium toward said interface and at such an angle of incidence to said interface that at said walls the light is refracted into said body and at said cavities the light is totally reflected internallyofsaid medium along a path leading outwardly of said medium to produce an image having relatively light and dark zone indicative of said cavities and said walls, respectively; means in said path for collecting said reflected light; and means for displaying said image.

12. An apparatus as claimed in Claim 1 1,wherein said transparent, light refracting medium is a prism.

13. An apparatus as claimed in Claim 12, wherein said prism is formed of a material selected from zinc selenide, zinc sulfide, sapphire, quartz and glass.

14. An apparatus as claimed in any one of Claims 11 to 13, wherein the said planarface ofthe refracting medium is provided with a reticle scale.

15. An apparatus as claimed in Claim 1 and substantially as hereinbefore described with reference to Figure 1 or Figure 2.