WO2002099359A1

WO2002099359A1 - Method and device for determining the angle between two facets of a transparent object

Info

Publication number: WO2002099359A1
Application number: PCT/BE2002/000091
Authority: WO
Inventors: Johan Verbeke; Maxime Blanchaert; Peter Cooke
Original assignee: Wetenschappelijk En Technisch Onderzoekscentrum Voor Diamant, Inrichting Erkend Bij Toepassing Van De Besluitwet Van 30 Januari 1947
Priority date: 2001-06-05
Filing date: 2002-06-05
Publication date: 2002-12-12
Also published as: BE1014212A3

Abstract

The invention is relating to a method and a device for determining the angle between facets (6, 7, 9) of a transparent object (1) that is defined by these facets (7, 7, 9), whereby a so-called incident light beam (15) of a light source (22) enters the object (1) according to a determined orientation such that this light beam is reflected internally on a facet (6) that has to be measured ad leaves the object (1) in the form of a so-called outgoing light beam (20), whereby the orientation of this outgoing light beam (20) is measured and the orientation of said facet (6) to be measured is calculated on basis of the orientation of the incident light beam (15) and of the outgoing light beam (20) and of the refraction of these light beams (15, 20) at the surface of the object (1).

Description

METHOD AND DEVICE TO DETERMINE THE ANGLE BETWEEN TWO FACETS OF A TRANSPARENT OBJECT

The invention concerns a method and a device to determine the angle between the facets of a transparent object confined by these facets. This transparent object is for example a precious stone, such as a diamond.

The value of precious stones, such as diamonds, is largely determined by their cut. Thus, it is important that the facets have a mutually correct orientation during the cutting, and that a cut gemstone is obtained which is preferably almost perfectly symmetrical. For, the mutual position of its facets largely determines the sparkle of the gemstone.

Until now, however, it was very difficult to determine the mutual angles ofthe facets during the cutting ofthe gemstone. Indeed, the gemstone is to be clamped between polishing tweezers, such that the orientation of the facets of the gemstone situated on the side ofthe tweezers cannot be measured without removing the gemstone from the tweezers. When the gemstone is removed from the tweezers to perform such a measurement, however, it is practically impossible to clamp it between the tweezers again in exactly the same position.

According to the present state of the art, the angle between the different facets of a gemstone which has been removed from between the tweezers, can be measured by means of mechanical gauges, a proportion scope or a scanner.

When using a proportion scope, the shadow of the gemstone is projected onto a screen. The facet to be measured is then manually put in profile. By comparing the shadow to reference lines represented on the screen, the angle between the facet and a reference plane is measured. Scanner systems also make use ofthe shadow of a gemstone to carry out a measurement. The shadow is projected onto a camera via a lens. The gemstone is then rotated around an axis in small, discrete steps. For each position, an image of the object is registered by means of a computer. On the basis of these measurements is determined where the facets are situated, and thus it becomes possible to determine their angles in relation to a reference plane.

Consequently, these techniques do not allow to determine the angles between the facets of a gemstone clamped between polishing tweezers. Moreover, the angles between the facets can only be determined with little precision. The invention aims to remedy these disadvantages by proposing a device and method which make it possible to determine the angles between the facets of a gemstone in a very accurate manner, although it is held between polishing tweezers with certain facets. Thus, it is not necessary to remove the gemstone from between the tweezers to determine the mutual orientation of its facets, and it is possible to cut the gemstone with much more accuracy.

To this aim, what is called an incident light beam of a light source falls in on the object according to a specific orientation, such that this light beam is internally reflected on a facet to be measured, and leaves the object in the shape of what is called an outgoing light beam, whereby the orientation of this outgoing light beam is measured, and the orientation of said facet to be measured is calculated on the basis of the orientation ofthe incident light beam and the orientation ofthe outgoing light beam, as well as the refraction of these light beams on the surface ofthe object.

Practically, said incident light beam comes in on a first facet of the object, such that this light beam is internally reflected in the object by a second facet and leaves the object, thus forming the aforesaid outgoing light beam whose orientation is measured, whereby the orientation of said second facet is determined on the basis of the refractive index (n) ofthe object, the orientation of said first facet, the orientation of the incident light beam and the orientation ofthe outgoing light beam.

In an advantageous manner, said light beam is internally reflected in the object by a third facet, and this light beam leaves the object via said first facet, thus forming the aforesaid outgoing light beam whose orientation is measured. The orientation of said third facet is then determined on the basis of the refractive index of the object, the orientation of said first facet, the orientation of the incident light beam, the orientation ofthe outgoing light beam and the orientation of said second facet.

According to a preferred embodiment of the method according to the invention, said incident light beam is made to fall in on said first facet of the object, such that this light beam is internally reflected in the object by a second facet, and subsequently leaves the object via a third facet, thus forming said outgoing light beam whose orientation is measured, whereby the orientation of said second facet is calculated on the basis of the refractive index of the object, the orientation of said first and third facet, the orientation of the incident light beam and the orientation of the outgoing light beam.

According to a special embodiment of the method according to the invention, said light source is moved somewhat in relation to the object in order to determine the orientation of said facets, whereby, in a first position, the light beam falls in on said first facet and, in a second position, this light beam falls in on this first facet and is reflected by said second facet, whereas in a third position, the light beam falls in on said third facet, whereby the angular displacement of the light beam and/or of the light source is measured.

The invention also concerns a device for measuring the mutual orientation of the facets of an object, with a light source which makes it possible to generate a light beam, whereby this light source is mounted such that it can be moved in relation to said object in order to allow said light beam to be oriented such that it falls in on one of the above-mentioned facets, such that the light beam is reflected, whereby means are provided to determine the orientation ofthe incident light beam on said facets on the one hand, and means are provided to measure the orientation ofthe reflected light beam on the other hand.

According to a preferred embodiment of the device according to the invention, said light source is formed of an autocollimator.

Other particularities and advantages of the invention will become clear from the following description of a few specific embodiments ofthe method and device according to the invention; this description is given as an example only and does not restrict the scope of the claimed protection in any way; the reference figures used hereafter refer to the accompanying drawings. Figure 1 is a schematic side view of a brilliant cut diamond.

Figure 2 is a schematic top view ofthe brilliant from figure 1.

Figure 3 is a schematic bottom view ofthe brilliant from figure 1.

Figure 4 is a schematic diagram illustrating the operation of an autocollimator.

Figure 5 is a schematic view in perspective of a transparent object mounted in the cap of polishing tweezers, together with an autocollimator, in a first position.

Figure 6 is a schematic view in perspective of a transparent object mounted in the cap of polishing tweezers, together with an autocollimator, in a second position.

Figure 7 is a schematic view in perspective of a transparent object mounted in the cap of polishing tweezers, together with an autocollimator, in a third position. Figure 8 is a schematic view in perspective of an orthogonal co-ordinate system in which is represented a plane.

Figure 9 is a schematic two-dimensional section of a transparent object with three surfaces, whereby the light beam falling in on the object and going out of it again, is represented by vectors. In the different figures, the same reference figures refer to identical or analogous elements.

The invention in general concerns a method for measuring the angles between the facets of a transparent object such as a gemstone, for example a diamond, by means of a light beam. Figures 1 to 3 represent a diamond in the shape of a brilliant 1. This brilliant successively has a table zone 2, a crown zone 3, a girdle zone 4 and a pavilion zone 5. The table zone 2 comprises a facet 6 forming what is called the table. The table zone 2 is laterally confined by facets of the crown zone 3 which connects to the girdle zone 4. The pavilion zone 5 has a number of facets converging in one point on the side ofthe brilliant 1 opposite to the table zone 2. The facets ofthe pavilion zone 5 are also called pavilion facets 7. The sparkle of a brilliant largely depends on the orientation of these pavilion facets 7 in relation to the table 6. While, in the cutting process of the diamond 1, the pavilion facets 7 are cut, it is being held, on the side ofthe table 6, as is represented in figure 5, in a cap 8 of polishing tweezers which are not represented in the figures. As a consequence, it is not possible to measure the orientation of the table 6 with conventional measuring techniques as long as the brilliant 1 is being held in polishing tweezers.

In the method according to the invention, the orientation of two facets is measured whose surface is at least partially free and is thus not entirely covered by the cap 8. This is done by means of an incident light beam falling in on each of these facets, and by measuring the angle at which this light beam is reflected. In order to determine the orientation of the table 6, said light beam is made to fall in on a first facet whose orientation has been determined such that this light beam penetrates in the object and is reflected on the table 6. Next, this light beam leaves the object via the second facet whose orientation has already been measured as well. The angles of the incident and the outgoing light beam in relation to the respective facets are hereby determined.

Thus, the orientation of the table can be calculated on the basis of the orientation of the two facets and the angles of the light beam, which is reflected on the table, in relation to these facets, as well as on the basis of the angular displacement of the light beam when determining the orientation of these facets. Preferably, the light source generating said light beam is mounted such, via what is called an angle coder, that the angular displacement of the light beam, in order to make it fall in on the different facets, is easy to determine.

This light source is preferably formed of an autocollimator known as such. Figure 4 schematically represents the working principle of an autocollimator. It contains a light source 10 whose light falls in via a condenser lens 11 on a semitransparent mirror 12 forming an angle of 45° with the axis 13 of the condenser lens 11. The light ofthe light source 10 is reflected via this mirror 12 onto an objective lens 14 which transforms the stream of light into a parallel light beam 15. When this light beam 15 is made to fall in on a surface 16 which is perpendicular to the axis 17 of the objective lens 14, this light beam 15 is reflected vertically onto the objective lens 14 and it will fall upon an ocular 18 situated in the focus of the objective lens 14 via the semitransparent mirror 12. In this position is provided a mark 24 in the ocular 18. When the light beam 15 falls in upon a surface 19 which is not perpendicular to the axis 17 and forms an angle α with the perpendicular surface 16, the light beam 20 which is reflected on the surface 19 will fall in on the objective lens 14 at an angle 2 . This reflected light beam 20 is thus focused in the ocular 18 at a distance d from said mark 24. In order to make this position visible on the ocular 18, what is called a crosshair 21 is provided between the condenser lens 11 and the semitransparent mirror 12, of which an image is thus formed on the ocular after the light beam has been reflected on the surface 19. The distance d between said mark 24 and the formed image ofthe crosshair 21 is proportional to the angle α. Preferably, said ocular 18 is replaced by a camera, such that it becomes possible to represent the formed image on a screen and to process it with a computer.

Thus, the angle forming a plane with the axis 17 ofthe objective lens 14, or in other words the longitudinal axis ofthe autocollimator, can be measured by means of such an autocollimator. Figures 5 to 7 schematically represent three positions of an autocollimator 22 for measuring the orientations of the facets of a transparent object 1 with the facets 7 and 9 held in the cap 8 of polishing tweezers.

The autocollimator 22 is mounted such that it can rotate via an arm, which is not represented in the figures, in relation to an axis of rotation whose prolongation preferably cuts the diamond 1. Thus, it is possible to rotate the autocollimator 22 in relation to this axis of rotation in order to make the light beam 15, generated by the autocollimator 22, fall in upon the facet 7 or 9 of the object 1. The plane in which the light beam 15 can thus be rotated together with the autocollimator 22 is called the light plane. In a first position of the autocollimator 22, as is represented in figure 5, the light beam 15 falls in on a first facet 7 in order to determine its orientation. The light beam 20 which is directly reflected by this facet 7 forms an image 23 of said crosshair 21 on the ocular 18 consisting of a camera. The perceived image is then represented on a computer screen 25 as is schematically represented in figure 5. The image 23 of the crosshair 21 is hereby situated at a horizontal distance A and at a vertical distance B from the above-mentioned mark 24. Consequently, these distances A and B are proportional to the angle formed by the facet 7 and two orthogonal lines situated in a plane which is perpendicular to the incident light beam 15.

Thus, on the basis of these distances A and B, it is possible to calculate the orientation of the facet 7 in relation to said light plane by means of plane trigonometry known as such.

Next, as represented in figure 6, the autocollimator 22 is rotated in the light plane over an angle p into a second position in which the light beam 15 falls in on said facet 7, such that this light beam is internally reflected in the object 1 by a facet 6, which is not visible as it is situated in the cap 8. This last-mentioned facet 6 forms for example the table of a brilliant. The thus reflected light beam 20 then leaves the object 1 via the facet 9 which connects to the aforesaid facet 7. The outgoing light beam 20 hereby has another orientation than the incident light beam 15 due to the refraction of the light beam as it penetrates the object 1 and as it leaves it, and due to the internal reflection on the facet 6. Thus, the orientation of a virtual plane standing at right angles to the outgoing light beam 20 is measured by means of the autocollimator 22. The orientation of this virtual plane is determined by the horizontal distance A' and the vertical distance B' between the image 23' of the crosshair 21 and the mark 24 represented on the screen 25.

In order to determine the orientation ofthe facet 9, the autocollimator 22 is rotated from the second position into a third position over an angle ψ, such that the light beam 15 ofthe autocollimator 22 falls in on this facet 9, as is represented in figure 7. The orientation of the facet 9 is determined in an analogous manner as that of the facet 7. On the screen 25, the orientation of the facet 9 is thus represented by the distances A" and B" ofthe image 23" ofthe crosshair 21 in relation to the mark 24. The orientation of the non- visible facet 6 of the object 1, or in other words of the table 6, is then calculated on the basis of the orientation of said virtual plane, the refractive index of the object 1, the orientation of the two measured facets 7 and 9 and the displacement p and ψ of the autocollimator 22. On the basis of the orientation of the non- visible facet 6 and of the two facets 7 and 9 is then determined the angle between these facets.

Although, in the description of the method according to the invention, referring to figures 5 to 7, we speak of a first, a second and a third position of the autocollimator 22, it is clear that these positions can also be applied in random order according to the method ofthe invention.

Moreover, the light beam 15 must not necessarily fall in on several facets of the object 1. Thus, it is possible for example to let the light beam 15 fall in on one and the same facet according to three different orientations. For each of these orientations ofthe light beam 15, the orientation ofthe reflected beam 20 is measured.

In a first orientation of the light beam, it will fall in on a visible facet of the object 1, and the orientation of this facet is determined on the basis ofthe orientation ofthe light beam 20 reflected on this facet. A second orientation of the incident light beam is for example selected such that it is internally reflected in the object by a first, non- visible facet, whereby the light beam leaves the object 1 via the above-mentioned visible facet. The orientation of this outgoing light beam is measured in order to calculate the orientation of said first non-visible facet. A third orientation of the incident light beam 15 is ^"selected such that the light beam is internally reflected in the object 1 upon said first non- visible facet and upon a second non-visible facet. The orientation of this light beam is measured as it leaves the object 1 via the visible facet. Thus, the orientation of the second non- visible facet is calculated on the basis of the orientation of the first non-visible facet, the orientation ofthe incident and the outgoing light beam and the orientation of the visible facet.

What follows is a theoretical calculation, made to determine the angles between two visible facets and a non- visible facet of an object 1 on the basis of the measurements made with the autocollimator 22 for its three positions, represented in figures 5 to 7.

It is hereby assumed that the orientation of each facet is determined by two angles. This is schematically represented in figure 8, showing an orthogonal coordinate system xyz and a plane V. The plane V intersects the x-axis in point A, the y- axis in point B and the z-axis in point C. Further, it is assumed that the above- mentioned light plane in which the incident light beam 15 is situated coincides with the plane formed by the x-axis and the y-axis. As a result, the z-axis is parallel to the axis of rotation around which the autocollimator 22 can be rotated. The orientation ofthe plane V is then determined by a first angle α and a second angle β, whereby α is the angle between the x-axis and the normal line OD on the line AB, and whereby β is the angle of inclination of the plane V in relation to the light plane xy. The line AB hereby coincides with the intersecting line between the plane V and the light plane xy.

The angles α and β coπespond to the values measured with the autocollimator 22. These are proportional to the shifting of the image 23 of the crosshair 21 in relation to the mark 24 when the orientation of the plane V is measured with the autocollimator 22. If H is the distance between the intersection O ofthe axes x, y and z and the intersection C ofthe z-axis with the plane V, the following equations can be made: for the triangle ODC :

OD ^ι&

OD

OC for the triangle ODB : tg β = or tg β = OC

OD ^& OD

OC OC for the triangle ODA : tg β = or tg β =

OD ° OD

The equation ofthe plane V in the co-ordinate system xyz is then:

tg β

OD

or more in general x • tgβ • cos α + y • sin β • cos α + z • cos β = K

whereby K = H . cosβ _r ~

I sinβ.cosα ' the vector sinβ.sinα hereby forms the normal on the plane V. _^ cosβ When two pavilion facets 7 and 9 of a brilliant are thus measured with the autocollimator 22 whose orientation is represented by the angles oi_l, βi and α₂, β₂ in a co-ordinate system which is fixed for the brilliant, the cosine ofthe angle between these pavilion facets 7 and 9 will equal: cos αi . cos ₂. tg βi . tg β₂ + sin αi . sin α₂. tg βi . tg β₂ + 1 cos θ =

[ (cos² αi . tg² βi + sin² αi . tg² βi + l).(cos² α₂ . tg² β₂ + sin² α₂. tg² β₂ + 1) f or in other words cos θ = | cos ( _\ - α₂) . sin βi . sin β₂ + cos βi . cos β₂1

In order to determine the orientation of the non-visible facet, i.e. of the table 6, the orientation ofthe light beam in the object 1 is calculated first. The plane 16, which is perpendicular to the longitudinal axis of the autocollimator 22, and as a result perpendicular to the incident light beam 15, is hereby determined by the angles α₀ and β₀. The angles α₃ and β₃ determine the orientation of the above-mentioned virtual plane.

Figure 9 schematically represents the table 6 and two facet planes 7 and 9 of a transparent object 1 in two dimensions. Said plane 16 which is perpendicular to the incident light beam 15 and said virtual plane 26 are schematically represented by means of a dashed line.

The incident light beam 15 is represented by a vector I, while the outgoing light beam 20 is represented by a vector U. Further, the reflection ofthe light beam on the table 6 is represented by the vectors Ti and T . The vectors N₁ and N₂ form the normal on the facet 7 and the facet 9 respectively.

Thus, the orientation of the table 6 is determined by its normal T which equals T = Ti + T₂.

The vector for the incident light beam 20 is - 1 =

'^''"sinβi.cosαi^ while the normal on the pavilion facet 7 equals Ni

Thus, the cosine of the angle between Ni and -I equals the scalar product of the following vectors :

COSΘQI = -I.Ni = sinβi . cos i . sinβo . cosα₀+ sinβi . sinαi . sinβo . sinα₀ + cosβr . cosβ₀ or COSΘQJ. = sinβi. • sin βo cos (αi -α₀) + cosβi . cosβo The vector Ti ofthe light beam refracted on the facet plane 7 is determined by the formula Ti = -n . I - (n . cos θ₀₁ - [1 - n² (1 - cos² θoi)]¹⁴ ) . Ni

For this formula we refer to the manual "An Introduction to Ray Tracing", published by Andrew Glassner (Academic Press, 1989).

Thus

Ti = I n . sin βo . cos α₀ - (n . cos θ₀₁ - [ 1 - n (1 - cos θ₀₁) ] sin βi . cos αi)

9 2 V n . sin βo . sin α₀ - (n . cos θ₀₁ - [ 1 - n (1 - cos θ₀₁) ] ² sin βi . sin α

2 2 '/ ji . cos βo - (n . cos θ₀₁ - [ 1 - n (1 - cos θ₀₁) ] cos βi ) , whereby n is the refractive index for the object 1 concerned. For diamonds, for example, this refractive index equals n = 2.42.

On the basis ofthe orientation ofthe outgoing light beam 20, which is perpendicular to the above-mentioned virtual plane, the vector T can be determined in an analogous manner coπesponding to the light beam reflected on the table 6.

The cosine ofthe angle θ₂₃ between the vector U representing the outgoing light beam 20 and the normal N₂ on the pavilion facet 9 is obtained by calculating the scalar product of these vectors :

Consequently, cosθ₂₃ = U.N₂ or cosθ₂₃ = sinβ₂ . cosα₂ . sinβ₃ . cosα₃ + sinβ₂ . sinα₂ . sinβ₃ . sinα₃ + cosβ₂ . cosβ₃ = sinβ . cos (α - α₃) + cos β . cosβ₃

The vector T₂ is then calculated by means ofthe following formula:

T₂ = n . U - (n . cos θ₂₃ - [1 - n² (1 - cos² θ₂₃)]'^/2 ) . N₂ (see the manual "An Introduction to Ray Tracing", published by Andrew Glassner (Academic Press, 1989)). Thus,

T₂ ^: n . sin β₃ . cos α₃ - (n . cos θ₂₃ - [ 1 - n (1 - cos θ₂₃) ] Yl sin β₂ . cos α )

n . sin β₃ . sin α₃ - (n . cos θ ₃ - [ 1 - n (1 - cos θ₂₃) ] sin β₂ . sin α₂) n . cos β₃ - (n . cos θ₂₃ - [ 1 -

cos β₂ )

whereby n is the refractive index ofthe transparent object 1.

The table 6 is, as mentioned above, perpendicular to the vector T = Ti + T₂.

Consequently, with the arithmetic methods known as such, it is possible to calculate the angle between the pavilion plane 6 and the facet planes 7 and 9.

In the preceding theoretical derivation, the angles α₀ and β₀, αi and βi, α₂ and β₂ and α₃ and β₃ represent the orientation of the respective planes of facets in a co-ordinate system being measured which is fixed for the object. These angles are calculated on the basis ofthe angular displacement p and/or ψ ofthe incident light beam

15 and the respective distances A and B, A' and B' or A" and B" read on the screen 25 ofthe autocollimator 22.

The preceding theoretical calculation was given as an example for a measurement in which the object has two visible facets 7 and 9 and a non- visible facet 6. Naturally, this calculation can be easily adapted to other situations, for example when the light beam is made to fall in on a single facet according to several orientations in order to determine the orientation of other facets ofthe object.

The invention is by no means restricted to the above-described embodiments of the method and device according to the invention represented in the accompanying drawings. Thus, for example the autocollimator can be replaced by a light source generating a laser beam whereby the orientation of the different reflections ofthe laser beam is then measured in order to determine the orientation of a non- visible facet ofthe transparent object.

Nor is it necessary for the angles p and ψ to be situated in one and the same plane, and the light source or the autocollimator may be movable according to one or several translations or rotations in order to make the light beam fall in on a facet of the object. One or several sensors or encoders can hereby be provided to measure the movement ofthe light source or ofthe light beam.

All sorts of techniques can be applied to measure the orientation of the incident and ofthe outgoing light beam. The use of an autocollimator in what precedes is merely given as an example.

The method according to the invention can be applied to cut gemstones entirely as well as to cut gemstones or semiprecious stones only partially.

Claims

1. Method to determine the angle between facets (6,7,9) of a transparent object (1) which is confined by these facets (6,7,9), characterised in that what is called an incident light beam (15) of a light source (22) is made to fall in on the object (1) according to a specific orientation, such that this light beam is internally reflected on a facet (6) to be measured and leaves the object (1) in the shape of what is called an outgoing light beam (20), whereby the orientation of this outgoing light beam (20) is measured and the orientation of said facet (6) to be measured is calculated on the basis of the orientation of the incident light beam (15) and that of the outgoing light beam (20), as well as the refraction of these light beams (15,20) on the surface of the object (1).

2. Method according to claim 1, characterised in that said incident light beam (15) is made to fall in on a first facet (7) ofthe object (1), such that this light beam is internally reflected in the object (1) by a second facet (6) and leaves the object (1), thus forming the above-mentioned outgoing light beam (20) whose orientation is measured, whereby the orientation of said second facet (6) is determined on the basis of the refractive index (n) of the object (1), the orientation of said first facet (7), the orientation ofthe incident light beam (15) and the orientation ofthe outgoing light beam (20).

3. Method according to claim 2, characterised in that said light beam is internally reflected in the object (1) by a third facet (9) and leaves the object (1) via said first facet (7), thus forming said outgoing light beam (20) whose orientation is measured, whereby the orientation of said third facet (9) is determined on the basis of the refractive index (n) of the object (1), the orientation of said first facet (7), the orientation of the incident light beam (15), the orientation of the outgoing light beam (20) and the orientation of said second facet (6).

4. Method according to claim 2 or 3, characterised in that said incident light beam (15) is made to fall in on said first facet (7) of the object (1), such that this light beam is internally reflected in the object (1) by a second facet (6), and subsequently leaves the object (1) via a third facet (9), thus forming the aforesaid outgoing light beam (20) whose orientation is measured, whereby the orientation of said second facet (6) is calculated on the basis of the refractive index (n) of the object (1), the orientation of said first and third facet (7,9), the orientation of the incident light beam (15) and the orientation ofthe outgoing light beam (20).

5. Method according to claim 4, characterised in that said light source (22) can be moved somewhat in relation to the object (1) in order to determine the orientation of said facets (6,7,9), whereby the light beam (15) falls in on said first facet (7) in a first position, and this light beam (15) falls in on this first facet (7) in a second position and is reflected by said second facet (6), whereas the light beam (15) falls in on said third facet (9) in a third position, whereby the angular displacement of the light beam (22) and/or ofthe light source (22) is measured.

6. Method according to any of claims 1 to 5, characterised in that the orientation of said facets (7,9) upon which the incident light beam (15) falls in directly, is determined by measuring the angle between this incident light beam (15) and the light beam (20) which is reflected by this facet (7,9). 7. Method according to any of claims 1 to 6, characterised in that the orientation of said facets (6,

7,9) is determined in relation to a co-ordinate system which is fixed for said object (1).

8. Method according to any of claims 1 to 7, characterised in that the orientation of said incident beam (15) is determined.

9. Method according to any of claims 1 to 8, characterised in that an autocollimator (22) is used for said light source which makes it possible to measure the angle between the incident light beam (15) on the object (1) and the light beam (20) reflected by the latter.

10. Device for measuring the orientation of the facets (6,7,9) of a transparent object (1) which is confined by these facets (6,7,9), in particular a device for applying the method according to any of the preceding claims, with a light source making it possible to generate a light beam (15), characterised in that said light source (22) is mounted such that it can be moved in relation to said object (1) in order to allow said light beam (15) to be oriented such that it falls in on one of the above-mentioned facets (7,9), such that the light beam is reflected, whereby means are provided to determine the orientation of the incident light beam (15) on said facets (6,7,9) on the one hand, and means are provided to measure the orientation ofthe reflected light beam (20) on the other hand.

11. Device according to claim 10, characterised in that said light source comprises an autocollimator (22).

12. Device according to claim 10 or 11, characterised in that it comprises means for mounting polishing tweezers holding an object (1) to be measured.