text: referring first to fig1 - 4 , the system provides a light source in the form of a laser 10 . a beam 12 from the laser 10 enters a telescope 14 where it is reduced in width . for example , a typical tea co 2 pulsed laser would provide a beam of square cross - section with a width of about 1 inch in each direction . the telescope 14 will reduce this beam to a square beam with sides each about half an inch in width . the thus narrowed beam 16 is directed in a direction x onto the surface of a flat circular mirror m1 that is controlled by a galvanometer g1 to rotate about an axis extending in a direction z perpendicular to the direction x . the mirror m1 reflects the beam 16 onto a second similar mirror m2 that is controlled by a further galvanometer g2 to rotate about an axis extending in the x direction . the mirrors m1 and m2 form a first or &# 34 ; transmitting &# 34 ; assembly , the scanning function of which will be more fuly described below . as seen in fig1 a parallel light beam 17 is projected by this transmitting assembly , generally in the direction z , and onto the surface of a fixed , spherically concave mirror m3 . the mirror m3 constitutes a &# 34 ; first imaging element &# 34 ; and serves to reflect a converging , focused beam 18 towards a second or &# 34 ; receiving &# 34 ; assembly that consists of further flat mirrors m4 and m5 . the mirror m4 which directly receives the focused beam from the spherical mirror m3 is controlled by a galvanometer g4 to rotate about an axis in a direction y ( perpendicular to both the directions x and z ), and reflects the beam onto the mirror m5 which is controlled by a galvanometer g5 to rotate about an axis extending in the direction z . during its passage from the spherical mirror m3 to the receiving assembly m4 , m5 , the beam 18 passes through a fixed mask 20 ( see fig5 ) to form a modified focused beam 19 , i . e . a beam that has been modified by having passed through a window in the mask and hence having adopted the shape of one of the character - defining windows . the mask 20 is generally opaque , except for rows and columns of windows , each in the shape of a character 22 , e . g . letters a , b , c etc . the geometry of the system is such , including the mask 20 being offset from the principal axis a of the mirror m3 ( as best seen in fig1 and 2 ) that the transmitted beam 17 misses the mask , while the return beam 18 must pass through it to become the modified beam 19 . the axis a extends in the direction z and intersects a line b joining the centre points of the mirrors m2 and m4 , i . e . the mirrors that respectively directly transmit the beam 16 to the mirror m3 and directly receive the beam 19 from the mirror m3 . these axes a and b define a plane that extends in the x and z directions . the axis b is spaced along the axis a from the mirror m3 by a distance 2f , where f is the focal length of the mirror m3 . the mask 20 is preferably spaced from the mirror m3 by a distance slightly less than the focal length f , e . g . by a distance approximately equal to 0 . 8f . as an alternative , the mask need not necessarily be located in the reflected beam 18 . instead it can be located in the transmitted beam 17 . as a further alternative , the transmitted beam need not necessarily be a parallel beam . it could be a converging or diverging beam , in which case the beam reflected by the concave mirror m3 could be converging , diverging or parallel . from the mirror m5 , the beam 19 , now conveniently referred to as a working beam 21 ( and normally now diverging ) passes to a fixed flat mirror m6 and then through a final lens l1 onto a workpiece w . the lens l1 constitutes a &# 34 ; second imaging element .&# 34 ; assuming that the workpiece w has a flat surface for receiving the beam , the lens l1 will preferably be a compound , flat field lens , i . e . one that will ensure that the image of the chosen mask character will be correctly focused on any selected area of the workpiece surface . should the workpiece surface be appropriately concave , a normal lens could be used . the function of the movable mirror m1 of the transmitting assembly is to scan the beam 17 in the x direction , e . g . between paths 17 and 17 &# 39 ; shown in fig1 . the mirror m3 ensures that the reflected beam 19 or 19 &# 39 ; ( focused and modified to carry the character data ) arrives at the mirror m4 regardless of whether the transmitted beam followed path 17 or 17 &# 39 ;, thus , in effect , regardless of which character in the mask was traversed by the beam . the mirror m4 of the receiving assembly is moved in synchronism with the mirror m1 of the transmitting assembly so as to seek the received beam 19 or 19 &# 39 ; and to ensure that it is always reflected as the working beam in the same path 21 extending towards the mirror m5 , subject to a superimposed incremental advance that will be described below . in a similar manner , the mirrors m2 and m5 synchronously scan and seek the transmitted and received beams in the y direction . see fig4 where the transmitted and received beams are shown coincident , i . e . assuming no simultaneous x direction scanning , the beams being shown as scanned from 17 ( transmitted ) and 19 ( received ) to 17 &# 34 ; and 19 &# 34 ;, rspectively . in practice , of course , scanning can take place in the x and y directions simultaneously , if desired . the galvanometers g1 , g2 , g4 and g5 are controlled for this purpose by a control computer c , the receiving assembly always seeking the received beam in the correct direction having regard to its transmitted direction and the ultimate working beam path 21 that it is to follow . in this manner , the beam received by the workpiece can be caused to pass through any selected one of the characters in the mask , without any need to move the mask . while in the illustrated embodiment the x scan mirrors come first in the direction of travel of the beam ( i . e . mirrors m1 and m4 ) and the y scan mirrors follow ( mirrors m2 and m5 ) this sequence can be reversed provided it is done in both the transmitting and receiving assemblies . the fixed spherical mirror m3 serves the dual functions of condenser and imager . as the so - called first imaging element , it images the transmitting assembly of mirrors m1 , m2 onto the receiving assembly of mirrors m4 , m5 . the final lens l1 , on the other hand , the so - called second imaging element , is spaced from the mask and the workpiece and has a focal length such as to ensure that it images the selected character in the mask 20 onto the workpiece w . a spherical shape for the mirror m3 is preferred , because it is the cheapest to manufacture , but other concave shapes , such as ellipsoidal , could also be used . assuming that the system is to be used to mark a series of letters extending along at least one row on the surface of the workpiece w , i . e . one letter for each successive pulse of the laser 10 , the control c will superimpose on the above described movements of the mirror m4 ( for x direction movement ) and of the mirror m5 ( for y direction movement ), i . e . those necessary to keep the path 21 extending in a fixed direction , further incremental movements that move the path 21 in the manner necessary to achieve the desired distribution of the characters on the workpiece surface . calibration of the positions of the mirrors m1 and m2 of the transmitting assembly can be achieved by means of a plurality ( preferably four ) sensors 24 ( fig5 ) located at convenient places , e . g . the corners , of the mask 20 . the galvanometers g1 and g2 will initially be programmed by the control c to scan until each one of these sensors 24 has been energised , thus serving to identify various orientations of the transmitted beam . to calibrate the positions of the mirrors m4 and m5 of the receiving assembly , a set of holes 26 is provided in the mask . as the beam passes through each such hole and is received and detected downstream , e . g . by a sensor ( not shown ) located at a datum point in the workpiece plane , the orientation of the working beam and hence of the mirrors m4 and m5 becomes known . if the main laser 10 is unsuited for this calibration purpose , e . g . is too powerful , or of an inappropriate wavelength , a subsidiary laser or lamp 28 can be used , its beam 29 being fed into the system by a beam combining element 30 , e . g . a germanium flat , as shown in fig6 . a low power , helium neon , continuous wave laser , emitting visible red light , is a convenient choice as the subsidiary laser . the invention is not dependent on the use of coherent light as the source , although this will normally be necessary when marking a workpiece . when using the system for display purposes or for marking photographic film the laser 10 can be replaced by a lamp . as a further alternative , the mirror m3 can be replaced as the first imaging element by a lens l2 , as shown in fig7 with the two mirror assemblies m1 , m2 , m4 , m5 then located symmetrically and equidistantly on opposite sides of such lens , with the mask 20 preferably , but not necessarily , located downstream of the lens . the preferred distance for each of the mirrors m2 and m4 from the lens l2 will be 2f , where f is the focal length of the lens . this arrangement has the advantage that the mask can be centered on the principal axis a of the lens . however , a significant advantage of the mirror m3 is that it will usually result in a more compact mechanical configuration . one of the advantages of the present invention is that it enables effective scanning of a fixed mask to select a desired character and the positioning of an image of this character at a chosen location on a workpiece , or screen , or other surface . the term &# 34 ; work surface &# 34 ; is used in the claims that follow to refer generically to these various possibilities . however , while the ability of the present invention to operate with a fixed mask is normally advantageous ( for mechanical simplicity as well as for speed of operation ), of even more value is its ability to operate with both a fixed work surface and a fixed final lens . indeed , there are some embodiments of the invention in which there is an advantage in having the mask movable . for example , fig8 shows a mask in the form of a disc 32 rotatable about an axis 33 and carrying two ( or more ) peripheral series 34 , 35 of characters . selection of a desired character is achieved by spinning the disc 32 and energising the laser when the selected character is in line with the beam . increased speed of operation can be achieved by increasing the speed of rotation of the disc 32 , or by repeating the alphabet ( or other sequence of characters ) around the disc . with this movement of the mask , which is assumed to take place in the direction y where it intersects the beam , it no longer becomes necessary to scan the mirrors of the transmitting assembly in the y direction . a single flat mirror rotatable about an axis extending in the direction y , but otherwise similar to the mirror m1 , will be used to provide scanning of the transmitted beam in the x direction . however , both x and y direction scanning of the galvanometer operated mirrors m4 and m5 of the receiving assembly remain necessary in order to provide means for projecting the received beam into the desired location on the work surface . fig9 shows a further simplification that is especially appropriate for use with embodiments of the invention described in connection with fig8 namely the substitution of a series of flat mirror surfaces 36 arranged in an approximately spherical array to furnish a mirror m3 &# 39 ; that can be used to replace the mirror m3 as the first imaging element . the reason why this concave array of flat mirror surfaces can be used instead of a true concave surface , is that in the fig8 embodiment there will be a relative small number of accessed character positions through which the beam must be scanned . fig1 shows an alternative device for use as either the transmitting assembly or the receiving assembly or both , namely an acousto - optical device 40 for scanning either beam in the x and y directions .