GB2218219A

GB2218219A - A reflective mask

Info

Publication number: GB2218219A
Application number: GB8907808A
Authority: GB
Inventors: Stewart Wynn Williams
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1988-05-04
Filing date: 1989-04-06
Publication date: 1989-11-08
Anticipated expiration: 2009-04-06
Also published as: GB2218219B; GB8810483D0; GB8907808D0

Abstract

A reflective mask for defining an image to be marked on an article using a laser beam (34) wherein the mask assembly (20, 40, 50) comprises reflective portions (32) defining the image. The mask (20) may comprise a plurality of individually selectable reflective elements (32) for defining an image, e.g. wheel members (26-29) each carrying several digits around its periphery, or wheel members each having a plurality of reflective elements around its periphery (Fig 4. not shown) or individually pivotable members arranged in arrays and each carrying a reflective element (Figs 5-8. not shown). <IMAGE>

Description

A MASK ASSEMBLY FOR DEFINING AN IMAGE This invention relates to a mask assembly for defining an image, particularly to such an assembly for defining an image to be marked on an article using a laser beam, and to a laser marking unit which includes such a mask assembly.

In conventional laser imaging applications, a mask is used in a simple transmissive mode. A conventional system will now be described with respect to Figures 1 and 2 of the accompanying drawings in which: Figure 1 is a diagram of a conventional laser marking system; Figure 2 is a plan view of a conventional laser imaging mask.

Figure 1 shows a conventional laser marking unit comprising a laser source 10, a mask 12, and lenses 14 and 16 which form part of a projection system for projecting an image onto an image plane 18.

Figure 2 shows the mask 12 which is formed from thin foil material in which apertures are etched or cut to define a desired image i . e. digits. Small supporting strips of the foil material remain to support the inner parts of the digits 8, 4, 5 and 0.

In use, a beam from the laser source 10 is shone onto the mask 12 and is transmitted through the apertures so as to define the image at the image plane 18. The image plane may be the surface of an article to be marked, such as cable.

One disadvantage of the conventional unit described above is that it is difficult to alter the image e.g. by changing digits.

Furthermore, if the required image is small this requires the use of a thin mask which is relatively fragile and which suffers heat dissipation problems in use, particularly as the source 10 typically is a pulsed CO2, excimer or Nd-YAG laser producing up to several Joules of energy per pulse with a resulting fluence on the mask of up to several Joules 1cm2 and average powers of up to several tens of watts.

According to one aspect of the present invention there is provided a mask assembly for defining an image to be marked on an article using a laser beam, we herein te mask assembly includes reflective portions defining the image.

The laser beam may comprise radiation falling within the infrared, visible and/or ultra-violet parts of the spectrum.

An advantage of a mask assembly according to the present invention is that it can be made to be relatively rugged e.g. the digits can be made as thick as desired e. g. 5mm as compared to the typical 0.5mum of a conventional transmission mask. This means that high fluences can be tolerated on the mask assembly without significant wear. Furthermore the elements of the image defined by the mask will be able to be completely formed because they can be given an underlying supporting structure. In addition, the larger mass of both the image elements and the supporting structure will increase their thermal mass and enable high average power lasers to be utilised without the need for extra cooling. Moreover because the mask assembly relys on reflection and not on diffraction the performance of the assembly is unaffected by the coherence of the laser.

Preferably, a mask assembly according to the present invention includes a plurality of individually selectable reflective elements for defining an image. A major advantage thus flows from providing a mask assembly in which image elements are separately supported in that these can then be individually selected as desired and the image can easily be modified.

A mask assembly according to the present invention may have a plurality of movable components each carrying one or more reflective elements for defining an image. These movable compcae-nts may comprise one or more wheel members each having reflective elements circumferentially spaced around the periphery thereof.

Alternatively, said movable components may each be in the form of a pivotally mounted member having a head portion forming or carrying the associated reflective element and a shank portion which is pivotally mounted.

Conveniently each pivotally mounted member is pivotable electromagnetically .

The reflective portions may be formed from polished metal or alternatively may comprise a dielectric coating.

To prevent unwanted reflections, the mask assembly may comprise means for inhibiting reflection of the laser beam other than from the reflective portions e.g. by coating the surfaces of the supporting parts with an enhanced light absorber.

Alternatively, surfaces of the mask assembly neighbouring the reflective portions may be angled differently from the reflective portions so as to reflect light in a different direction.

Embodiments of the present invention will now be described, by way of example, with reference to Figures 3 to 8 of the accompanying drawings in which: Figure 3 is a plan view of part of a laser marking unit incorporating a mask assembly according to a first embodiment of the present invention, Figure 4 is a plan view of an alternative mask assembly according to the present invention, Figure 5 is a diagrammatic plan view of a mask assembly according to a further embodiment of the invention, Figure 6 is a cross sectional view to an enlarged scale taken on the line A-A of Figure 5 of part of the assembly of Figure 5 forming part of a laser marking unit, Figure 7 is a cross-sectional view taken on the line B-B of Figure 5, and Figure 8 is a diagrammatic end view of the assembly of Figure 5.

Referring to Figure 3 of the accompanying drawings, a laser marking unit comprises a mask assembly according to a first embodiment of the invention indicated generally at 20, a projection lens indicated at 22 and an apertured screen 24 to block unwanted radiation from reaching an eventual image plane.

The mask assembly 20 comprises, for example, four wheel members 26, 27, 28 and 29 mounted on an axle 30. On its periphery, each of the wheel members 26-29 has raised portions carrying image elements 32 e.g. digits. The image elements 32 are reflective and may be formed from polished metal or may comprise a reflective dielectric coating. The upper reflective surface of each of the image element 32 is angled with respect to the supporting surface of the wheel member so that, in use, any reflections from the supporting surface of the wheel member propagate in a different direction to reflections from the image elements 32 themselves and are therefore blocked by the screen 24.

The wheel members 26-29 each have individual bearings and may be individually selectably rotated by any suitable means.

For example the wheel members may be provided with teeth and rotated by a gear arrangement. The wheel member rotation may be controlled manually or automatically as appropriate and may be computer controlled.

In use, an incoming laser beam indicated at 34 impinges on the mask assembly 20 and is reflected from the reflective image elements 32 so as to pass through the projection lens 22 and the aperture screen 24 onto an eventual image plane. The laser used may be, for example, an infrared laser or an excimer laser.

The lenses used in the projection system will be made from an appropriate material for the particular wavelength of laser light used and will have the focal length appropriate to produce the desired magnification of the mask image. During use, the wheel members 26-29 can be rotated so as to change any or all of the digits prior to each laser pulse.

To take account of the angle of incidence of the laser beam 34 on the reflective image elements 32, the image elements are enlarged by a factor of V2 in the plane of incidence in the laser beam so as to avoid distortion of the image. The degree of enlargement needs to be tailored according to the angle of incidence.

Turning now to Figure 4, an alternative form of mask assembly is indicated at 40 and comprises a set of T-section wheel members 42, 43, 44, 45 and 46 each of which has a plurality of reflective image elements 48 circumferentially spaced around its periphery. Each wheel member 42-46 has the head of its T section providing a wheel 42b to 46b and the stem of its section providing a shaft 42a to 46a for rotating the associated wheel 42b to 46b. A number of the shafts 42a to 45a are longitudinally axially bored and sized such that the shafts fit one over another to locate the wheels side by side. To this end the wheel members 42-45 are mounted on bearings 50 around the central wheel member 46 and each of the wheel members 42-46 is separately rotatable so as to alter the mask image pattern.

In use, the mask assembly 40 would be illuminated with a laser beam as shown in respect of the mask 20 in Figure 3.

Figures 5 to 8 show a mask assembly 50 according to a further embodiment of the invention and a laser marking unit 51 incorporating such an assembly. In the assembly 50 the movable components 52 are each in the form of a pivotally mounted member having a head portion 53 forming or carrying the associated reflective image element 54 and a shank portion 55 which is pivotally mounted on rocker pins 56. To this end each shank portion 56 is hooked intermediate to its ends as can be seen from Figure 6.

The pivotally mounted members are arranged in side by side rows to form arrays as can be seen in Figure 5 and are held in these arrays by a keeper frame 57. Each reflective element 54 may form part of a desired image such as a character or digit to be reflected on to an article to be marked and to this end is pivotable by an amount sufficient to deflect that part of the laser beam 58 passing through a focusing lens 59 away from the optical path. Thus depending on the reflecting angle of the reflective elements 54 an image pattern can be reflected via a projection lens 60 on to an article to be marked (not shown).

The pivotally mounted members are selectively pivotable to vary the angle presented by their reflective elements 54 to the laser beam 58 by electromagnetic means or in any other suitable manner. As illustrated each pivotally mounted member is a permanent magnet and has associated therewith its own rod 61 and coil 62. Each rod 61 may selectively be magnetised by passing a current through its associated coil 62 in either direction, depending upon which, the reflector element 54 carried by the head portion 53 may be fitted or pivoted one way or the other. Means may be provided as in the previous embodiments for absorbing or deflecting unwanted laser radiation from the reflective elements such as in the direction 63.

In all embodiments of the invention the reflective portions or elements for defining an image may have rounded edges instead of straight edges. This is of advantage if using a CO2 laser with high spacial coherence as it produces less diffraction.

Claims

1. A mask assembly for defining an image to be marked on an article using a laser beam, wherein the mask assembly includes reflective portions defining the image.

2. A mask assembly according to claim 1, including a plurality of individually selectable reflective elements for defining an image.

3. A mask assembly according to claim 2, having a plurality of movable components each carrying one or more reflective elements for defining an image.

4. A mask assembly according to claim 3, wherein the movable components comprise one or more wheel members each having reflective elements circumferentially spaced around the periphery thereof.

5. A mask assembly according to claim 4, wherein each wheel member is substantially T shaped in longitudinal cross-section, with the head of the section providing a wheel and with the stem of the section providing a shaft for rotating the associated wheel.

6. A mask assembly according to claim 5, wherein a number of the wheel member shafts are longitudinally axially bored and sized such that said shafts fit one over another to locate the wheels side by side.

7. A mask assembly according to claim 3, wherein the plurality of movable components each carrying one or more of said reflective elements for reflecting the marking laser beam on to the article to be marked are individually selectively movable, said reflective elements defining one or more portions of the image.

8. A mask assembly according to claim 7, wherein said movable components are each in the form of a pivotally mounted member having a head portion forming or carrying the associated reflective element and a shank portion which is pivotally mounted.

9. A mask assembly according to claim 8, wherein the pivotally mounted members are arranged in side by side rows to form an array with the pivotally mounted members being selectively pivotable to vary the angle presented in use by their reflective elements to a laser beam so that by selection of the appropriate pivotally mounted members a required form of image may be made up.

10. A mask assembly according to claim 9, wherein each pivotally mounted member is pivotable electromagnetically.

11. A mask assembly according to claim 10, wherein each pivotally mounted member is a permanent magnet and has associated therewith its own rod and coil, operable such that the rod may be magnetised by current passed through the coil with the current direction determining the direction in which the pivotally mounted member pivots.

12. A mask assembly according to any preceding claim, wherein the reflective portions are formed from polished metal.

13. A mask assembly according to any one of claims 1 to 12, wherein the reflective portions comprise a dielectric coating.

14. A mask assembly according to any preceding claim, comprising means for inhibiting reflection of the laser beam other than from the reflective portions.

15. A mask assembly according to any one of claims 1 to 13, wherein surfaces neighbouring the reflective portions are angled differently from the reflective portions so as to reflect light in a different direction.

16. A mask assembly according to any one of claims 1 to 15, wherein the reflective portions and/or reflective elements have rounded edges.

17. A mask assembly for defining an image to be marked on an article using a laser beam, substantially as herein described with reference to, and as illustrated in, Figure 3, Figure 4, or Figures 5 to 8 of the accompanying drawings.

18. A laser marking unit including a mask assembly according to any one of claims 1 to 16.