GB2084755A - Optical comparator - Google Patents

Abstract

A compound microscope is arranged so that an image of the surface 16 and of a test subject 42 are formed by a common objective 14 and presented as juxtaposed images for comparison. Light of the same spectral composition (e.g. same source 22) is used and the test object may be one of a set of transparencies. The comparator is provided more especially for the inspection of surface finish of metal preparatory to painting, and a set of transparencies illustrates surface appearances from unsatisfactory through acceptable to highly satisfactory, so that an inspector can access surface finish with a high degree of consistency. <IMAGE>

Description

SPECIFICATION Improvements in or relating to optical comparators This invention relates to an optical comparator for comparing a given test surface under investigation with a standard representation of a surface, in such a manner that direct and simultaneous comparison is carried out by a user under substantially identical lighting conditions.

Large metal structures, for example bridges, may be preserved from deterioration by coating with paint or the like adherent substance which excludes, more or less completely, liquids and gases which can corrode unprotected metal. The adhesion of a coating to a metal surface is dependent on the cleanness of the surface, that is freedom from oxide, roughness and general dirt. Such contaminants are usually removed, and roughness smoothed, by shot or sand blast treatment. In practice, the results of blast treatment vary considerably, making subsequent inspection essential, before painting.

Consistency of inspection has been controlled by the provision for inspectors of photographic illustrations of surfaces representing varying degrees of goodness, from wholly unsatisfactory to wholly acceptable. A commonly used comparison standard consists of a series of magnified representations of standard surfaces, the representations being colour prints, about 15 times full size, but characterised by large grain size. These standard representations are arranged in a bookto protect them from light when not in use. An inspector must first look at an appropriate representation with the naked eye, and then at the test surface under inspection, with the aid of a magnifying device, and perhaps additional light, carrying in his mind a picture of the standard representation for comparison.The present invention avoids the difficulty of making a comparison with a mind picture, variation in magnification of the test surface, and discrepancy of illumination as between standard and test surface.

According to the invention an optical comparator, adapted to compare a test surface with a standard representation of a surface, has a microscope with an objective and an eyepiece; first supporting means for supporting the microscope at a predetermined distance from the test surface; second supporting means for supporting a standard representation of a surface in predetermined spatial relationship with the microscope; light directing means for directing light of substantially the same spectral distribution onto the said test surface and onto the said representation so that light from the said test surface and light from the said representation pass together into the objective of the microscope in such manner that an image of the test surface and an image of the representation each occupies substantially a respective half of the field of view of the microscope; and shielding means for shielding the test surface and representation, in use, from unwanted light.

The first supporting means and the shielding means may both be comprised in a box having an open side opposite to the objective of the microscope arranged in said box while the second supporting means may be adapted to support inside the box a standard representation in the form of a photographic transparency.

Preferably the light directing means includes means for adjustably positioning a light source or sources in relation to the test surface and the standard representation. The light directing means may include mirror means which directs light from a light source onto the test surface and onto the standard representation, and the mirror means may conveniently include an at least part frusto conical mirror which embraces the microscope tube. The light directing means may further include at least one optical prism which directs light from the standard representation into the microscope objective.

The comparator may include means for viewing through colour and/or polarising filters.

The invention extends to an optical comparison system including an optical comparator and at least one set of different standard representations of surfaces in the form of photographic transparencies.

The invention will be further described, by way of example only, with reference to the accompanying drawings in which Figure 1 is a sectional elevation of an optical comparator; Figure 2 represents a photographic transparency which can be used with the comparator in a comparison system.

Figure 3 is a diagrammatic illustration of a modified optical comparator; Figure 4 illustrates diagrammatically means for making one of a set of standard representations in the form of photographic transparencies.

Referring to Figure 1, the comparator includes a microscope, indicated generally by 10, having an eyepiece 12 and an objective 14. A test surface, to be inspected and graded with the aid of the comparator, is indicated by 16. The microscope is supported by the first supporting means in the form of a box 18, open on the side 20 for observation of surface 16.

The light source is an electric lamp 22 in a holder 24, and supplied through leads 26 from a conventional source (not shown) such as a battery. The lampholder is supported by, and can be adjusted in position along, a tube 28 projecting from the box 18. The position of the lamp holder can be clamped by conventional means such as a grub screw (not illustrated). Light is emitted from the lamp generally in the direction of the arrow 30. A substantial proportion of the light is reflected from the inner surface of a mirror 32 which embraces the microscope tube. The mirror is frusto conical, except for a cut-out portion 34, for the admission of more light from the lamp, and has an apex angle of substantially 90 . Light is reflected from the mirror 32 as indicated by the arrows 36, 38.Some of the light, as indicated by the arrow 36, reaches the surface 16 under inspection, and is reflected as indicated by arrow 40. Another portion of the light, as indicated by arrow 38, passes first through a photographic transparency in a mount 42, supported by second supporting means indicated by 44.

The photographic transparency is a full size colour photograph of a standard surface of a stated degree of goodness. Lightwhich has passed through the transparency enters a right angle prism 46 to be reflected back into the objective 14 of the microscope along a path indicated by 48, along with the lightfrom surface 16, indicated by the arrow 40. The prism is so arranged in relation to the objective 14 that substantially one half of the field of view of the microscope is occupied by an image of a part of the surface 16, and the other half is occupied by at least a portion of the standard representation of a surface on the transparency in the mount 42.The microscope 10, the second supporting means 44, and the prism 46 are mutually arranged in the box 18 so that, in use, the transparency in mount 42 and the surface 16 are both at substantially the same optical distance from the objective 14. Thus an observer through the eyepiece 12 of the microscope sees a two-part image, both parts with the same magnification, both illuminated with light of the same spectral distribution, and substantially of the same intensity. If there should, in use, prove to be any difficulty through difference of intensity, this could be corrected by means of a neutral filter at 50, which could, for example, be of the wedge kind so as to be adjustable. Thus the test is made objective to a very substantial degree, and does not involve a mental comparison with a remembered standard.

The transparency itself is indicated in Figure 2 at 52. The portion 54 is occupied by the photographic standard representation of a surface, and the portion 56 is opaque (so as to obscure any unwanted light).

The portion 54 is so exposed, in making the photo graph, that although.it occupies only about half the frame, and to one side, nevertheless, the centre of the portion 54 coincides substantially with the axis of the camera lens used when making the photograph.

This ensures that for a given quality of lens, the photograph is as sharp as possible. In the embodi ment described, it has been convenient to arrange each mount 42 with a cut-out portion 58, enabling the mount to clear the objective 14 of the micro scope. With different proportioning of the parts of the apparatus, eg a larger prism 46, the cut-pout 58 need not be made. Frames 42 are placed in and removed from the second supporting means 44 through a slot 60 in the side of the box 18.

Instead of being provided in separate mounts the standard representations 54 may be provided as consecutive frames in a roll of film, eg 35mm film.

The film may be passed through the second support ing means 44 through slots (not illustrated) at right angles to the slot 60, between roll film holders indicated diagrammatically at 62.

The lamp 22 is desirably of the Krypton filled kind, for a suitable spectral distribution of light output. In order to prolong life the lamp may be under-run by about 5% with reference to the stated working voltage. The objective of the microscope suitably gives a magnification of about X2, while the eyepiece has a magnification of the order of X5 to X10. In general, an overall linear magnification of about X15 is found to be suitable, but larger or smaller magnification may be provided if required, eg in relation to size of features on the surface under examination.

With some surfaces it may be found that improved results are obtained by viewing a surface (16), or the surface and a standard representation, through a coloured filter. In the first case the colour filter may be incorporated at 50, and in the second case at 64, adjacent the lamp 22. In some circumstances it may be desirable to view the surface in polarised light, in which case a polarising filter (or filters) may be located at 50. In conjunction with this it may also be desirable to employ a set of standard representations which have been made using polarised light for the photographic exposure.

The inspection of metal surfaces has been mentioned in connection with this invention, but it is equally possible to apply the invention to surfaces of other kinds which require critical visual examination, eg asto suitability for further treatment, or for performing a given function.

A some what modified form of the comparator is shown in Figure 3. (Reference numbers which are the same as those in Figure 1 indicate corresponding features of the two versions of the comparator). In this version two Krypton filled lamps, 66,68 are provided, substantially identical with each other.

Comparison may also be effected using a single lamp located midway between the transparency in mount 42 and the surface 16 to be inspected. Two prisms, 70,72, are provided to conduct light from the transparency to the objective 14 of microscope 10.

As in the embodiment of Figure lithe two halves of the field of view of the microscope are occupied by the test surface and the standard representation respectively, as indicated by the rays 40 and 48.

Initial focussing is effected by means of the adjustable eyepiece 12 to make the image of the surface 16 sharp. Final sharpening of the image of the standard representation is carried out by adjusting the position of the prism 70 through a Knob 74 and an arrangement of (for example) cams (not shown).

Suitably, the microscope objective 14 is of 1.5 x magnification, has afocal length of 78 mm and an aperture of 12 mm. It is supported at 200 mm from the focussing eyepiece 12 of 10 x magnification; so that the overall magnification is 15 x. The depth of field provided is about 1.0 mm, which is found to be adequate to allow both the surface 16, and the bottom of pits therein to remain simultaneously in focus. Focussing of the microscope may be by sliding the eyepiece in the microscope tube, with fine focussing by rotation of the outer component of the eyepiece on a thread. Alternatively, rack and pinion focussing may be provided.

Figure 4 illustrates a suitable arrangement of apparatus for preparing a standard representation in the form of a photographic transparency. A standard surface is indicated by 80. Supported above the surface, by conventional means (not shown) is a 35 mm film camera 82 of usual single lens reflex kind.

The camera has an extension bellows 84, and a good quality lens 86 of 55 mm focal length, provided with a shallow lens hood 88. The camera is focussed on the standard surface 80, using the viewfinder 90. A suitable film, for colour stability and absence of grain, is Kodachrome 25 (RTM). Light is provided by two similar flash lamps 92, 94, without reflectors, placed about 300 mm from the surface, and at an angle which will give flat illumination at the surface.

Asuitable exposure is a total of 500 joule atf 16 aperture.

Claims (10)

1. An optical comparator, adapted to compare a test surface with a standard representation of a surface, having a microscope with an objective and an eyepiece; first supporting means for supporting the microscope at a predetermined distance from the test surface; second supporting means for supporting a standard representation of a surface in predetermined spatial relationship with the microscope; light directing means for directing light of substantially the same spectral distribution onto the said test surface and onto and from the said representation so that light from the said test surface and light from the said representation pass together into the objective of the microscope in such manner that an image of the test surface and an image of the representation each occupies substantially a respective half of the field of view of the microscope; and shielding means for shielding the test surface and representation, in use, from unwanted light.

2. An optical comparator according to claim 1 in which the first supporting means and the shielding means are comprised in a box having an open side opposite to the objective of the microscope arranged in said box and in which the second supporting means is adapted to support inside the box a standard representation in the form of a photographic transparency.

3. An optical comparator according to claim 1 or claim 2 in which the light directing means includes positioning means for adjustably positioning at least one light source in relation to the test surface and the standard representation.

4. An optical comparator according to any one of the preceding claims in which the light directing means includes mirror means to direct light from a light source onto the test surface and onto the standard representation.

5. An optical comparator according to claim 4 in which the mirror means includes an at least part frusto conical mirror which embraces the microscope tube.

6. An optical comparator according to any one of the preceding claims in which the light directing means includes at least one optical prism which directs light from the standard representation into the microscope objective.

7. An optical comparator according to any one of the preceding claims having filter means which provide for viewing through a colour filter.

8. An optical comparator according to any one of the preceding claims having filter means which provide for viewing through a polarising filter.

9. An optical comparator substantially as hereinbefore described with reference to either of Figures 1 and 3 of the accompanying drawings.

10. An optical comparison system including an optical comparator according to )any one of the preceding claims and at least one set of different standard representations of surfaces in the form photographic transparencies to be viewed in the comparator.