GB2191303A

GB2191303A - Optical mechanical scanning using polygonal prism having odd numbers of sides

Info

Publication number: GB2191303A
Application number: GB08026829A
Authority: GB
Inventors: Jacques Auge; Michel Duchet
Original assignee: Compagnie Generale dElectricite SA
Current assignee: Alcatel Lucent SAS
Priority date: 1979-08-31
Filing date: 1980-08-21
Publication date: 1987-12-09
Also published as: GB2191303B; IT8049564A0; FR2601464B1; IT1188990B; FR2601464A1; BE884997A

Abstract

The device includes a light source 1 and a right prism 4 disposed on the path of the beam 2 emitted by the source. The bases of the prism are polygonal and have an odd number of sides. The prism is made of a material which is transparent to light emitted by the source. Application to scanning the surface of a distant target. A motor 5 rotates the prism 4. The prism may bear a strip(s) 19 of light absorbent varnish or reflective metal. <IMAGE>

Description

SPECIFICATION A device for providing an alternating scan of a light beam in a plane The present invention relates to a device for providing an alternating scan of a light beam in a plane.

A known device for providing an alternating scan of a laser beam in a plane includes a light source which is capable of emitting a light beam in the scan plane and a right prism whose bases are regular n-sided convex polygons. The n plane side surfaces of said prism are reflecting surfaces. Said prism is placed on the path of the light beam so that the bases of the prism are disposed parallel to the scan plane on either side of said plane. When the prism is rotated about its axis, the light beam is reflected on the n side surfaces to provide an alternating scan of the plane. Said device has the disadvantage of deviating the beam through angles which are too great for some applications.

Preferred embodiments of the present invention overcome said disadvantage.

The present invention provides a device for providing an alternating scan of a light beam in a plane, said device including: - a light source which is capable of emitting said light beam in said plane; - a right prism made of a material which is transparent to said light, the bases of the prism being regular convex polygons having an odd number of sides, and the prism being placed on the path of the light beam so that both its bases are disposed parallel to said plane, one on either side of said plane; and - means for rotating the prism about its axis.

A particular embodiment of the device in accordance with the invention is described hereinbelow by way of an example and with reference to the accompanying drawings, in which: Figure 1 illustrates schematically an embodiment of the device in accordance with the invention; Figure 2 illustrates a cross-section showing the propagation of a ligh ray via a prism which forms part of the device illustrated in Fig. 1; and Figure 3 is a graph which illustrates the operation of the device of Figs. 1 and 2.

Fig. 1 illustrates a light beam source 1 such as a laser generator which emits, in a plane 3, a light beam represented schematically by a single ray 2. A right prism 4 is placed on the path of the beam 2. The bases of the prism 4 are regular convex polygons having an odd number of sides, e.g. seven sides as illustrated. The bases of the prism are parallel to the plane 3 and are disposed on either side of said plane. A motor 5 is linked to the prism 4 so as to rotate it about the axis 6 of the prism 4, said axis being perpendicular to the plane 3. The prism 4 is made of a material which is transparent to the light emitted by the source 1. The ray 2 enters the prism 4 through one of its rectangular side surfaces and leaves it through another surface 7 to form a beam 8 situated in the plane 3.Then, as illustrated, a beam 8 can pass through an afocal optical system formed by a diverging lens 9 and a converging lens 10.

Fig. 2 illustrates the path of the beam 2 through the prism 4 in the plane 3. The ray 2 strikes a side 12 of the prism 4 at a point 11 where it forms an angle i with respect to the perpendicular 13 to the side 12. The ray 2 is refracted in the prism 4 whose refractive index is n. A refracted ray 14 forms an angle r whose value depends on i and n to the perpendicular 13 and strikes another side 15 of the prism 4 at point 16. The ray 8 leaves the prism at point 16 and forms an angle i with respect to the perpendicular 17 to the side 15 at the point 16. The deflection angle D formed by the ray 8 with respect to the ray 2 is equal to D=i'-i-A where A is the angle between the sides 12 and 15. Since the cross-section of the prism is a polygon with an odd number of sides, the sides 12 and 15 are never parallel to each other.

Therefore, the deflection angle D is generally non-zero.

Preferably, the source 1 is directed with respect to the prism 4 so that whenever the beam enters the prism through the side 12 it always leaves the prism via the same side 15. For this to be so, the angle of incidence i must lie between two limit values which depend on the number M of sides of the polygon and on the refractive index n of the material from which the prism is made.

By way of example, when M=7 and n=1.5, the angles of incidence 1B and ic of the rays striking the side 12 at its end points B and C must satisfy the following conditions: -0.34031 radians < i,(0.34031 radians -1.20892 radians < i, < -0.34031 radians the angles jB and ic being counted positively in the anti-clockwise direction.

The angle of incidence may then vary within a range of about 35".

When the prism 4 is rotated about the axis 6, the ray 2 remains stationary and it is observed that the ray 8 scans an angular sector of the plane 3. Considering an elementary rotation of the prism 4 where the rotation corresponds to a shift of point 11 from point B to point C, the ray 8 scans the angular sector in both directions. Fig. 3 represents the curve of variation in the deflection angle D as a function of the angle of incidence i during said elementary rotation. It is seen that said curve is parabolic and that the deflection D passes through a minimum. It is evident that the disposition of the source 1 in the preferential direction discussed hereinabove makes it possible to avoid any discontinuity in scanning during said elementary rotation. In the case where M=7 and n=1.5, the maximum amplitude of the scan obtained is about 40 milliradians in both directions.

When the prism rotates about its axis, the beam 8 therefore scans M times in both directions per turn of the prism.

The afocal device 9-10 illustrated in Fig. 1 may make it possible to match scan to the maximum required amplitude. In the case illustrated in Fig. 1, the maximum scanning amplitude is reduced.

The prism 4 preferably includes means to prevent light emitted by the source 1 from passing through the prism 4 when, during the rotation of the prism, the point of incidence 11 of the beam 2 is in the immediate neighbourhood of the edges such as 18 of the prism 4 which are parallel to the axis 6. This is done by coating a portion 19 of the side surface of the prism surrounding the edge 18 for example with a varnish which absorbs light emitted by the source 1. As shown in Figs. 1 and 2, said portion is in the form of a short strip whose width 20 lies preferably between once and twice the maximum dimension of the cross-section of the beam 2 parallel to the plane 3. The layer of absorbent varnish may be replaced by a metal layer which is capable of reflecting light from the source. Of course, the prism includes strips such as 19 on all its edges parallel to the axis 6.

Said absorbent or reflecting strips prevent the beam 2, whose cross-section has a small surface which is always greater than zero. From dividing into two parts when it illuminates simultaneously two contiguous surfaces of the side of the prism 4.

The device in accordance with the invention therefore makes it possible to scan an angular sector of the plane 3, said sector having a very small angle, e.g. of about ten or so milliradians.

Said very small scan angle can be obtained with difficulty by prior art devices which operate by reflection and in which rotation of the reflecting prism causes double rotation of the reflected beam.

Taking into account the small scan angle, the device in accordance with the invention may advantageously be applied to scanning a distant target. However, since the device in accordance with the invention only provides scanning along a line on such a target, a mirror generally disposed on the path of the beam 8 is added thereto and is rotated about an axis parallel to the plane 3 so as to transform the line scan into an area scan of the target. If the photo-electric sensor disposed so as to receive light emitted by the source 1 and reflected by the target is disposed beside the device in accordance with the invention, it is possible to form an image of the surface of the target.

Claims

1. A device for providing an alternating scan of a light beam in a plane, said device including: - a light source which is capable of emitting said light beam in said plane; - a right prism made of a material which is transparent to said light, the bases of the prism being regular convex polygons having an odd number of sides, and the prism being placed on the path of the light beam so that both its bases are disposed parallel to said plane, one on either side of said plane; - means for rotating the prism about its axis.

2. A device according to claim 1, including means for absorbing that part of said light which reaches those portions of the sides of the prism, which are adjacent those edges of the prism which are parallel to said axis.

3. A device according to claim 1, including means for reflecting that part of said light which reaches those portions of the sides of the prism which are adjacent those edges of the prism which are parallel to said axis.

4. A device according to claim 2, wherein said means for absorbing said part of said light include a layer of absorbent varnish deposited on the prism along said edges.

5. A device according to claim 3, wherein said means for reflecting said part of said light include a metal layer deposited on the prism along said edges.

6. A device according to claim 2 or 3, wherein said portions are strips which are parallel to the edges, the widths of said strips lying between once and twice the maximum dimension, parallel to said plane, of the cross-section of the light beam.

7. A device according to claim 1, further including an afocal optical system disposed on the path of the light beam at the output of the prism.

8. A device according to claim 1, wherein said light source is a laser generator.

9. A device for providing an alternating scan of a light beam in a plane, substantially as herein described with reference to the accompanying drawings.