GB2096793A

GB2096793A - Visually examining a gem stone

Info

Publication number: GB2096793A
Application number: GB8210180A
Authority: GB
Original assignee: Gersan Ets
Current assignee: Gersan Ets
Priority date: 1981-04-06
Filing date: 1982-04-06
Publication date: 1982-10-20
Also published as: GB2096793B

Abstract

In order to examine the shape of a gem stone by sensing its edge or profile with great accuracy whilst enabling the central parts of the gem stone to be in sufficiently good focus for them to be examined visually for flaws, viewing means are used which provide images of the edge and of the central parts which differ substantially in nature, the images being separated. For instance, the stone 1 can be examined using a large numerical aperture lens 4; a small mirror 8 is placed on the optical axis so that the outer part of the lens 4 is associated with a first TV viewer 3 whilst the central part of the lens 4 is associated with a second TV viewer 9; the outer part of the lens 4 gives good resolution, and the viewer 3 can be focussed accurately on the plane of the edge of the stone 1; the central part of the lens 4 gives poorer resolution but a greater depth of focus, and the TV viewer 9 can be focussed roughly on the front face of the stone 1. The two images can be recombined to give a sharp dark line as the edge profile and detail of the central parts within the edge profile. Alternatively a chromatically unmounted lens can be used with light of different wavelengths to shift the focal point of the lens. <IMAGE>

Description

SPECIFICATION Examining a gem stone Background of the invention The invention relates generally to examining the shape and surface of a gem stone, and the invention can be used in conjunction with the methods described in GB 2 080 71 2A and GB 2 081 439A. In GB 2 080 71 2A, a rough gem stone can be centred prior to for instance bruting, or a bruted stone can be re-measured, or a cut stone can be centred and examined for quality control. In GB 2 081 439A, a gem stone can be examined by providing images normal to its axis and sensing the edges of the images, and there is a list of parameters that can be determined. The term "bruting" is explained in GB 2 080 71 2A.

In GB 2 080 71 2A and GB 2 081 439A, the gem stone is rotated relative to the viewer, and by sensing the images of the gem stone, the shape of the whole gem stone can be determined.

However, it is found convenient, particularly when making a visual examination either directly or using a TV screen, to examine not only the edge or profile or outline (which can be determined automatically as in GB 2 081 439A), but also to examine the central parts of the stone. In a rough stone, this enables the operator to look for cracks or naturals (also called nijven) in the central part of the stone image; in a cut or polished stone, this enables the operator to make a ready assessment of the quality of the facetting. However, the central parts of the stone protrude forwards from the plane of the edge of the stone: if the edge is in focus, then these central parts will not be in focus, and vice versa.

The invention The present invention provides methods as set forth in Claims 1 and 1 6 and systems as set forth in Claims 17 and 20. The remaining Claims set forth preferred features of the invention.

In general, the invention enables one to focus in the plane in which the edge of the gem stone is expected to appear (it would be the transverse diametrical plane through the gem stone for viewing normal to the axis of the gem stone) and provide an image with good resolution for accurate assessment or measurement of the stone, e.g. good intercepts with a reference shape for instance in accordance with GB 2 080 712A or GB 2 081 439A. However, one can also provide a reasonable image, i.e. an image in reasonable focus, of the central parts of the stone.

The term "differ in nature" is used because the images will in any case differ from one another as the edge looks different from the central parts.

Although the invention is primarily useful for visual examination, e.g. of the image on a TV screen, it would be possible to carry out the invention without providing any real image, performing the whole operation electronically without any screen.

Claims 2 to 6 claim various methods of using viewing systems which differ substantially and thereby provide images which differ substantially in nature. However, other methods are possible, specifically for producing different focal planes or producing different depths of focus. For instance, polarised light can be used or sensed, or illuminations can be used which are modulated at different frequencies, or alternating, strobed colours can be used.

It is not essential that the two images be separated, but in general the two images can be separated in any suitable manner, and can then be recombined if desired.

If the technique used to provide viewing systems which differ substantially involves the use of lights of different colour, it is simplest if the two lights are both monochromatic; however, this is not essential and for instance one of the lights can be white or comparatively white; as long as the spectrum of one non-monochromatic light does not contain significant energy in the spectrum of the other light, whether monochromatic or not, results can be obtained. It is preferred that the blue light be used for viewing the edge of the stone as this gives high resolution while the central parts of the stone can be viewed in for instance white light or red light or (for direct visual examination) green light (the eye is best able to pick out detail in green).

Description of preferred embodiments The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figures 1 to 5 are schematic representations of five different systems in accordance with the invention; and Figure 6 shows the image on a screen which can be produced using any of the systems of Figures 1 to 5.

Figure 1 shows a rough diamond (sawn half) 1 on a dop 2 to which it can be stuck or on which it can be held by say light suction, depending upon the purpose of the examination. The dop 2 is rotated about a vertical axis and details of the rotation as well as details of the method for centering the diamond 1 or determining the shape of the diamond 1 can be taken from GB 2 080 712AorGB 2 081 439A.

In Figure 1, the diamond 1 is viewed by a TV camera 3 through a large numerical aperture lens 4 (which may form part of the viewer 3) mounted close to a filter 5. The filter 5 has a blue outer annulus (reject red, transmit blue) and a clear centre or a red centre (reject blue, transmit red).

The stone 1 is illuminated with nonmonochromatic light, which would normally be white light, but could be blue and red light. The lens 4 thus provides good resolution but short depth of focus for blue light, and poorer resolution but long depth of focus for white light or red light.

The blue light is used for examining the edge of the stone 1, diffracted light being important when making such an examination. The TV camera 3 shown in Figure 1 is a high resolution colour camera, enabling the two images to be separated by separating the two differently-coloured images. A colour camera has the advantage that if there is any distortion in the scanning circuits, it will be common to both images, which will therefore, register exactly when recombined (see below). An alternative is to use a single black and white TV camera 3 with an e.g. rotating coloured disc separating the two images, and take the frames sequentially. This would be of use in an automatic system where the extra time was not important and any extra flicker does not matter.

It is possible to use in Figure 1 a lens 4 having different focal lengths for different colours. More specifically, the edge of the stone 1 can be imaged with high resolution using the lens 4 of large numerical aperture focussed in the transverse diametrical plane of the stone 1; if the focal lengths for the different colours are sufficiently different, the front or central parts of the stone can be imaged at lower resolution with a larger depth of focus using the lens 4, with an interposed stop to give a lower numerical aperture, focussed substantially nearer than transverse diametrical plane of the stone 1, ideally at a point midway between the diametrical plane and the nearest point of the stone 1, to maximise the information that can be obtained by a visual observer.In this case, it is not necessary to separate the images (though it is better to do so) and a black-white TV camera 3 can be used.

Figure 2 shows a system similar to that of Figure 1, and is particularly suitable for a system where the edge of the stone is sensed automatically but the central parts are viewed by eye. The outline of the diamond 1 is imaged in bright field with e.g. collimated blue light (arrow 6) and imaged from the front with e.g. diffused red light (arrow 7). This provides a good sharp outline to the diamond 1, whilst providing good depth of focus for viewing the central parts.

Figure 3 shows that a small mirror 8 can be positioned on the centre line and viewed with a second TV camera 9 to provide an effect similar to that using the annular filter 5 of Figure 1 or 2.

In this case, the cameras 3 and 9 can be monochrome cameras, with independent focussing for the two images. The camera 3 is focussed in the transverse diametrical plane of the stone 1; the camera 9 is focussed at the midway point referred to above.

Figure 4 illustrates that a larger mirror 10 can be used as a beam splitter, the two parts of the stone 1 being viewed effectively through different ienses 11, 12 and filters 13, 14, to provide a sharp outline and a separate good front view without compromise. In this case, although filters 13, 14 are shown, it is not necessary to use lights of different colours as the camera 3 can be focussed precisely on the outline plane while the camera 9 can be focussed for instance at the midway point referred to above.

The system of Figure 5 is another variation, in which a single large numerical aperture lens 4 is used. This system is of limited use as advantage is not taken of different apertures.

The two images can be recombined in any suitable manner. Figure 6 illustrates a TV screen 15 on which is shown the image of the diamond 1 and part of the dop 2. The edge of the first image is enhanced and the centre is suppressed, for instance using video-slicing and detecting only the edges. The best discrimination is obtained with the edge dark against a light field. The second image is placed in the suppressed centre of the first image and has tone gradations from light to dark to enable the nature of the central parts of the stone 1 to be examined by eye.

For all the Figures, the overall numerical aperture of the lens 4 can be f/l to f/4, preferably about f/2, and the stopped-down aperture can be f/50 to f/SO. preferably about f/60. The distance between the lens 4 and the stone 1 can be 100 to 500 mm, though this depends on the sizes of the stones 1 being examined; a reasonably long working distance is desirable in order to reduce perspective distortion; at short working distances errors in the horizontal plane could be compensated in an automatic system, though not errors in the vertical plane. The magnification ratio can be between 1 and 4. if a single source of white illumination is used, a 150 Watt quartz/halogen bulb can be used. Suitable low light level TV cameras are available for picking up the stopped-down image.

Claims

1. A method of examining a gem stone, comprising simultaneously viewing the edge and the central parts of the stone using viewing means which provide images of the edge of the stone and of the central parts of the stone which differ substantially in nature from one another.

2. The method of Claim 1, wherein the edge of the stone is viewed with a large viewing aperture to give good resolution of the edge and the central parts of the stone are viewed with a small aperture to give good resolution but a greater depth of focus.

3. The method of Claim 1 or 2, wherein the stone is illuminated with non-monochromatic light, the edge being imaged with a first colour and the central parts with another colour.

4. The method of Claim 3, wherein the stone is viewed through a colour filter adjacent a large numerical aperture lens, the filter permitting lights of different colour to pass through the peripherai area and the central area of the lens.

5. The method of any one of the preceding claims, wherein the stone is viewed through a lens having different focal lengths for different colours, and images in two different colours are viewed.

6. The method of any one of Claims 3 to 5, wherein the stone is imaged in bright field with light of a first colour, for viewing the edge, and is imaged from the front with light of a different colour, for viewing the central parts.

7. The method of Claim 6, wherein the light of the first colour is collimated and the light of the different colour is diffused.

8. The method of any one of the preceding claims, wherein blue light is used for viewing the edge of the stone.

9. The method of any one of the preceding claims, wherein red light is used for viewing the central parts of the stone.

10. The method of any one of Claims 1 to 8, wherein green light is used for viewing the central parts of the stone.

1 The method of any one of Claims 3 to 10, wherein a colour TV camera is used for viewing the images of the edge and of the central parts of the stone.

12. The method of any one of Claims 3 to 10, wherein two monochromatic TV cameras are used, viewing along a common axis with a beam splitter and at least one filter, thereby separating the images of the edge and of the central parts of the stone.

13. The method of Claim 1, wherein a beam splitter is used, and the beams after splitting are examined with lenses or lens systems focussed respectively on the edge and on the central parts of the stone, thereby separating the images of the edge and of the central parts of the stone.

14. The method of any one of Claims 1 to 11, wherein the images of the edge and of the central parts of the stone are separated.

1 5. The method of any one of Claims 12 to 14, wherein the images of the edge and of the central parts of the stone are separated and then superimposed.

16. A method of examining a gem stone, substantially as herein described with reference to, and as shown in, any one of Figures 1 to 5 of the accompanying drawings, optionally also as described with reference to, and as shown in, Figure 6 of the accompanying drawings.

17. A system for examining a gem stone, comprising viewing means for providing an image of the edge of the stone and for simultaneously providing an image of the central parts of the stone which differs substantially in nature from the image of the edge of the stone.

18. The system of Claim 17, and further comprising means for separating the two images.

19. The system of Claim 17, and arranged for carrying out the method of any one of Claims 2 to 16.

20. A system for examining a gem stone, substantially as herein described with reference to, and as shown in, any one of Figures 1 to 5 of the accompanying drawings, optionally also as herein described with reference to, and as shown in, Figure 6 of the accompanying drawings.