Method and apparatus for assessing the characteristics of precious stones

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METHOD AND APPARATUS FOR ASSESSING THE

CHARACTERISTICS OF PRECIOUS STONES

שיסה והתקן להערכת הסגולות של אבני חן

Dihaco Diamanten Handels Compagnie Establishment

Inventor:- Gernot Klaus Bruck . METHOD AND APPARATUS FOR ASSESSING THE . .

: CHARACTERISTICS OF PRECIOUS STONES.

The invention relates: to a method of assessing the characterו sties of precious stones , whereby the stone is scanned with a 5 concentrated Beam of light of a cross section of the, same order of. magnitude as the smallest occlusions to.be detected, the scanning taking place over its entire cross-sectional plane perpendicular , to the incident beam and along scanning trajectories separated from one another by. a. distance not exceeding the beam cross10 section,the intensity of the beam being measured after it has. ‘ been subjected׳ to the influence of the. stone.

• By this method which is the subject matter of our co-pending patent application No.48.106, the size and position of an occlusion is being indicated in a plane perpendicular to the incident beam, e.g.

15. in a plane parallel to the baseplane of a precious stone. By. applying X-Y-coordinates to this plane it is possible to indicate the position of every occlusion in respect of these coordinates, but the method does not indicate the spatial location of an occlusion, e.g. the Z-value in a three-dimensional X-Y-Z - coordinate'system, 20 In this connection it should be borne tn mind that for assessing the true characteristics of precious stones it is highly important to know . the.exact position of occlusions in׳ three-dimensional coordinates.

It is., therefore, the object of the present invention to enlarge the above scanning method to the effect,of measuring, in addition to 25 the X-Y - coordinates of an occlusion, the value of the Z-coordinate . as well. According to the Invention, this value is obtained by an additional scanning process subsequent to the aforedescribed method of determining the X-Y -,coordinates of a flaw, which comprises causing a beam of parallel, widely spread light rays to converge into < 30 a focus, directing the focus to a predetermined point in the stone interior, moving said focus in a direction perpendicular to the cross-sectional plane'.on the coordinates of the previously detected occlusion and measuring the light intensity after its passage through the' stone.

According to the invention the initial stage of the method comprises defining the X-T - coordinates of the occlusions, while the second stage comprises: measuring, the spatial depth, of each occlusion perpendicular to the first-defined coordinates. In this connection it has been found advantageous to use laser-emitted parallel rays in a beam of a diameter of 0.3 to 1 mm, to enlarge or spread it into a beam of large diameter and subsequently to focus, it to a point, whereby an occlusion will cause a substantial variation of light intensity'whenever its position coincides with the actual focus. The focus- intensity increases with wider spreading of the beam, and accordingly exact focussing becomes a function of the beam diameter.

In a preferred embodiment of the invention shifting of the focus is obtained by constant to-and-fro motion of at least one optical system in the direction perpendicular to the crosssectional plane. On the other hand, shifting of the focus may. likewise‘be obtained by continuous to-ahd-fro or oscillatory motion of the precious stone perpendicular to.the crosssectional plane. Decrease of light intensity at any ' amplitude of the oscillatory motion reckoned from a given zero position, compared with the light intensity indicated while the beam passes through the stone unimpeded, is the . indication of the position of an occlusion on the Z-coordinate at right angles to the cross-sectional plane in the X-Y - coordinate system.

An additional feature of the invention is a method .of assessing the characteristics ofa coloured stone» especially a diamond, which comprises irradiating the entire cross-sectional area of a precious stone by a parallel light beam perpendicularly.

.incident on its cross-sectional plane, and measuring the intensity

30׳ of the rays emerging from the stone.

It is the object, to provide a simple apparatus.for carrying this method into effect by permittinga rapid through-put of stones without the need of complicated adjustment.־ By means of the apparatus in accordance with the invention a fixedly positioned

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stone is irradiated by a beam of parallel rays, and the . intensity of the rays is measured after their passage through'the stone.in a point to point relationship. By this method it is possible to determine the position of .' a flaw by its shadow without the need of adjusting.or shifting either the stone or the optical system. . The accuracy of measurement depends obyiously on tHe resolving power of the light sensor system adapted to record the light intensity and is a direct function of the distance between . sensor points on a cross-line plane. By coordinating such plane with a rectangular coordinate system it is possible to define the position.of every occlusion in respect of this plane. In this connection it is advantageous to position the stone centre into the coordinate origin?!!

' In a .further embodiment־of the inventionthe stone is ׳ irradiated by a beam of monochromatic, light and the intensity ' of a ray passing through the stone, preferably through its . centre, is being compared with that of.a ray not passing through the stone material־at all;' this permits the simultaneous .

assessment of the stone's colour and its shade, by comparing . .them with, the aid of a standard spectral curve.

The above methods can be further improved by completely . .immersing the stone'in a liquid and scanning it therethrough, preferably ,using a liquid of the same refractive index es that 25 of the stone for the wave length of the light used for scanning.

The refractive index of the liquid should be between 2 and 2.4 for diamonds, particular preference being given to a refractive index of at least 2.2. The immersion of the stone in this type of .' liquid offers the additional advantage of preventing the regis30 tration of a continuous background noise during recording the intensity of the rays passing through the stone unimpeded by any obstruction, but indicating a maximum signal as soon as an Occlusion is encountered. The light passet through־the immersed stone unimpeded, even near its outer edge since reflection on the stone surfaces is almost completely prevented during passage

- •3 from the liquid to the stone. However, when a light ray meets an obstruction in the shape of an occlusion, it is dispersed and a reduced intensity is: being.registered By the light , sensitive device. To obtain Best results it is proposed to position the stone with its flat.base facing the direction of the light beam destined to scan .or to irradiate it, especially when usinga laser beam. ׳ ׳ ׳ גי ’

In the.accompanying drawings which illustrate, by way of example only, two embodiments of the invention; ־

Figure 1 is a diagrammatic view of an apparatus adapted to. indicate the position of occlusions in one plane,

Figure 2 illustrates an apparatus for carrying out the , method in accordance with, the present invention,

Figure 3 illustrates, partly in side view and partly in 15 section, another apparatus according to the. present invention, and

Figure. 4 is a’perspective view of a portion of the apparatus ¹ illustrated־in Figure 3. . . ;.

With reference to Figure 1, an apparatus for carrying out . the method .of measuring the. position of occlusions in respect of 20 X-Y - coordinates, comprises a light source 1, such as a laser source or a light-emitting diode, an optical system.2^, of three lenses and a. rectangular chamber 3 provided with a transparent front wall 4. A precious stone is attached by its flat base to the inside of the front wall 4. -The rear wall of the chamber .

contains a light-sensitive device 7 which may be composed of a plurality of single photocells, the area of the device being chosen so as to permitthe arrangement of even the largest stone .to be examined within the projection area on the rear wall. The chamber is filled with, an immersion liquid 8 of .a refractive index 30. corresponding to the refractive index of the stone to be examined.

The stone keeps its position on the inside surface of the front wall by virtue of the adhesive forces acting on the adjoining planes of both stone and wall., The optical system 2 is designed' to produce a light beam which in the portion entering and passing 35 through the stone', is of a cross section commensurable with the size of the smallest occlusion to be measured.

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The chamber is'adapted to be moved in horizontal and vertical directions (arrows 8 and 9) by a. mechanism not visible in the drawing, ' in such, a manner that the incident light ray passes over'and.along' the stone along parallel trajectories in meander or raster fashion, 5 scanning the stone interior on parallel cross-sectional planes.

Figure ? illustrates an apparatus for carrying out the method which is the subject of.the present invention and which similarly comprises a.l ight source 1 in the form of. a laser or a l ightemitting diode. Γη contradistinction to the optical system 2 10 the present.system 10 is designed to initially spread'the parallel rays: emitted by the light source by means of at least one dispersion lens 11. and to focus the spread beam by means of at least one converging lens 12 into a focus directed into the precious stone 5. The lenses are annealed and made specially transparent for laser 15 light, thus attaining up to 100% transparency.

The chamber 3 which is similar to the chamber 3 of Figure 1׳ is, however, additionally movable in the direction of the beam (arrow 15), permitting the constant.shifting of the focuswithin the stone interior. Whenever the focus meets an occlusion in the , 20 stone, the beam is dispersed and the intensity of the light emerging from the stone and hitting the chamber rear wall decreases, which decrease is registered by the light-sensitive device-7 and recorded by a recording apparatus (not shown) in the form of breaks . in the continuously measured maximum signal.׳

The .spatial position of an occlusion can be determined, after > first recording its position in. the X-Y - plane by means of the device illustrated in Figure 1, by the position of the chamber . ' . relative to a sero position which preferably coincides with the.

focus position, e.g. in the base plane of the stone 5., . The . ' 30 shifting mechanism for the chamber in or opposed to the beam • ׳' direction may be'in the shape of a'.finely threaded .screw drive, either motor driven or manually operated.־ Alternatively the converging lens 12 may be similarly movable in the beam .' ׳ direction. ..'.

' י . ־ ן .

A further apparatus for assessing the characteristics of :. , a coloured stone, as illustrated in Figure 3, comprises a light source 21, preferably emitting a continuously adjustable coloured, laser beam, an optical system 22 composed 'of an orifice 23, a dispersion lens 24 and. a converging lens 25, ,.this system being adapted to emit ai. parallel, widely spread beam after its passage through, the lastlens¹ 25. A chamber 26'is positioned within the cross-sectional area of this beam, and contains a precious stone.28 attached to its front wall 27. The chamber 26 is filled with an 10 immersion liquid 29 of .a refractive index corresponding to the refractive index of the stone, and the liquid should be transparent for most of the visible spectrum. The stone clings to the front wall by virtue of the adhesive forces acting on the. adjoining planes.

־ A matrix 30 of photosensitive cells is positioned adjacent to the 15 rear of the chamber, which preferably is of a mesh density of 1.000 x 1000, offering a.resolving power of 1/100 mm. The size of the matrix corresponds to the size of the chamber, so as to permit the collection of rays passing along the outside of the largest stone to be immersed for examination; this in order to permit 20 comparative intensity measurement of rays passing through the stone, preferably through׳ its centre, and of‘rays passing through, the chamber outside the contours of the stone.

As illustrated in Figures 3 and 4, the complete apparatus is mounted op a slotted base 31, each part being separately movable and .25 ׳ adjustable in longitudinal direction. The photocell matrix is additionally movable׳and adjustable in a rail 32 fastened at right angles to the slotted base 31. This arrangement is necessary . whenever the matrix is composed of only one row of photocells and . scanning of the entire stone width requires constant transverse.

30, motion of the light sensitive device. The photocell matrix is fastened to a sled 33 by means of an angle^piece. 34, and the sled is adapted to slide in the rail 32 by means of a tongue and groove arrangement. The sled can be moved along the rail in constant longitudinal and oscillating motion by an electric motor (not shown) actuating a push rod 35 35 pivotally attached to the said sled.