with reference to fig1 there is shown a thermal printer 10 constructed in accordance with the present invention . printer 10 comprises a pair of optical heads 14 which produce laser beams 16 and 17 . the beams 16 and 17 are modulated in accordance with an information signal . beam 16 is directed to a polarization beam splitter 19 through a collimating lens 22 and a half - wave plate 21 . beam 17 is directed to beam splitter 19 through a collimating lens 22 . the laser beams 16 and 17 are generally linearly polarized , and both of the beams are collimated and have similar beam profiles . the polarization of beam 16 is rotated 90 ° by passing through half - wave plate 21 , but the profile of beam 16 is not altered by the half - wave plate . since the polarizations of beams 16 and 17 are orthogonal at polarization beam splitter 19 , beam 16 is transmitted by beam splitter 19 and beam 17 is reflected by the beam splitter . beam splitter 19 is disposed to superimpose beams 16 and 17 , and the superimposed beams propagate coaxially and coangularly as a combined beam 27 . due to the orthogonal polarization of beams 16 and 17 , there is no phase interaction of these beams and only the amplitude is augmented . combined beam 27 is directed to a galvanometer 20 through beam shaping optics 24 , and a turning mirror 23 . beam shaping optics 24 includes a pair of cylindrical lenses 25 and 26 . galvanometer 20 is adapted to scan the beam onto a thermal print medium 30 . the beam 16 from galvanometer 20 passes through an f - θ lens 32 which functions in a well - known manner to maintain a flat field and a constant velocity of the scanned spot on the thermal print medium . the thermal print medium 30 is of a type in which a dye is transferred by sublimation from a donor element 34 to a receiver element 36 as a result of heating the dye in the donor . as shown in fig1 receiver element 36 is attached to a rotatable drum 40 for movement therewith , and donor element 34 is in the form of a web which extends between a supply roll 42 and a take - up roll 44 . receiver element 36 can be removably attached to drum 40 by any suitable means , for example , by means of a vacuum . drum 40 and rolls 42 and 44 can be driven , for example , by stepper motors ( not shown ) which are actuated in timed relation to the movement of galvanometer 20 . a thermal print medium which can be used to produce a transferred image in printer 10 can be , for example , a medium disclosed in u . s . pat . no . 4 , 833 , 124 , entitled &# 34 ; process of increasing the density of images obtained by thermal dye transfer ,&# 34 ; granted may 23 , 1989 . this patent is assigned to the assignee of the present invention . as disclosed in u . s . pat . no . 4 , 833 , 124 , the thermal print medium includes a donor element having a material which strongly absorbs at the wavelength of the laser . when the donor element is irradiated , this absorbing material converts light energy to thermal energy and transfers the heat to the dye to its vaporization temperature for transfer to the receiver element . the absorbing material may be present in a layer beneath the dye or it may be admixed with the dye . the laser beam is modulated by electronic signals , which are representative of the shape and color of the original image , so that each dye is heated to cause volatilization only in those areas in which its presence is required on the receiver element to reconstruct the color of the original object . a thermal print medium of the type which produces a retained image can also be used in printer 10 . in such a medium , no donor element is used , and a receiver element contains a dye layer . an image is formed by using the laser beam 16 to remove dye from selected areas on the receiver element . each of the optical heads 14 can be constructed as shown in fig2 . such a head is disclosed in detail in u . s . patent application ser . no . 238 , 225 , entitled athermalized optical head , filed aug . 30 , 1988 , in the name of thomas e . yates . the disclosure in application ser . no . 238 , 225 is expressly incorporated herein by reference . optical head 14 comprises a light source 52 and an optical device 54 , both of which are supported in a tubular support 56 . light source 52 includes a diode laser 58 , a thermal transfer plate 60 , a thermoelectric cooling element 62 , and a heat sink 64 . diode laser 58 is surrounded at an output side 66 by a cover 68 which is formed of an insulator material , such as no . 106 silicone , obtainable from the rtv corp . diode laser 58 is mounted by means of fasteners 26 to an insulator ring 70 which is made of glass - filled polycarbonate , for example , such a material sold under the trademark lexan 3414 by general electric co . insulator ring 70 is mounted to an annular laser mount 71 by means of fasteners ( not shown ). laser mount 71 can be , for example , copper . set screws 74 in support 56 are threaded into contact with insulator ring 70 to align light source 52 relative to optical device 54 . heat from diode laser 58 is transferred to heat sink 64 which expels the excess heat through a finned radiator ( not shown ) to the environment . optical device 54 includes a lens housing 85 which is adapted to receive collimating lens 22 and a threaded lens retainer 87 . diode laser 58 and lens 22 are mounted in optical head 14 such that the distance between the diode laser and the lens is maintained constant over a predetermined temperature range . a control system 89 for printer 10 is shown in fig3 . control system 89 comprises a frame store 90 for storing image data received from an image scanner ( not shown ) or from an image storage medium ( not shown ). the data stored in frame store 90 includes , for example , three 8 - bit values for each pixel , each value representing the red , green , or blue input for the pixel . a matrix multiplication circuit 92 multiplies the 8 - bit red , green , and blue values by a 3 × 3 matrix in order to effect desired color corrections . the output from circuit 92 is applied to ram lookup tables 91 which perform the necessary scaling for linearization and calibration . updated values for the lookup tables 91 can be provided by a central processing unit 93 . the digital outputs from lookup tables 91 are provided to a digital - to - analog ( d / a ) converter 94 , and the outputs from the d / a converter drive the voltage - to - current drivers 96 for the diode lasers 58 . a thermoelectric cooler for the diode laser 58 is controlled by a thermoelectric cooler servo 99 . in one embodiment of the present invention , the same information signal is provided to each of the drivers 96 . in certain applications , it is possible to drive one of the lasers 58 with a constant d . c . signal , which would be below the threshold necessary to produce a dye transfer , and to provide the information signal to the other laser 58 . it is also possible to drive one , or both , of the lasers 58 with a d . c . signal to preheat the thermal medium or to postheat the medium after an image has been formed thereon . a control and timing logic circuit 100 is provided to manage the data flow during the operation of printer 10 and to control the printer timing . circuit 100 accepts timing signals from a drum servo 112 , a galvanometer servo 110 , and an optical fiber line start sensor 102 , and uses these signals to synchronize the printing operations . these timing signals include a once - per - revolution pulse from drum servo 112 which receives inputs from an encoder 104 , a once - per - cycle pulse from servo 110 which receives inputs from an encoder 106 , and a line - start pulse that is generated when the laser beam crosses an optical fiber ( not shown ) in line start sensor 102 . upon receipt of these signals , a pixel clock is started and the data is clocked through the data circuits . also included in circuit 100 are a pixels - per - line counter for line length control and a counter for controlling the addressing of the lookup tables 91 . in one illustrative embodiment of the present invention , each of the diode lasers 58 is a model no . hl8351e , obtainable from the hitachi corp . ; this laser is a 50 mw single transverse mode coherent laser which emits radiation at 830 nm . each of the collimating lenses 22 is an nrc model f - l20 ; lens 22 has a focal length of 8 . 6 mm and a numerical aperture of 0 . 5 . half - wave plate 21 is a quartz half - wave plate for 830 nm , model no . 02wrqo25 , obtainable from melles griot co . polarization beam splitter 19 is a model no . 03pbs067 , obtainable from melles griot co . cylindrical lens 25 has a focal length of - 80 . 0 mm and is a no . 01lcn135 , obtainable from melles griot co . cylindrical lens 26 has a focal length of 250 . 0 mm , and is a no . 01lcp135 , obtainable from melles griot co . galvanometer 20 is a model no . 325 dt , manufactured by general scanning co . f - θ lens 32 has a focal length of 71 mm , and is a no . i - 4921 , made by d . o . industries . as noted above , each of the diode lasers 58 delivers 50 mw of coherent radiation in a single transverse mode . the gaussian outputs of the lasers 58 are combined by beam splitter 19 , and the combined beam 27 can be focused to a diffraction limited spot . this optical characteristic of the laser along with the disclosed optics makes it possible to obtain very high resolution in printer 10 . a very high resolution is needed in transparencies in order to obtain a desired sharpness in a projected image . in one exemplary use of the present invention , where the laser beam 27 is focused to a 7 μm ( fwhm ) spot on the medium 30 and the spots are written at a pitch of 6 μm , a resolution of 4000 spots per inch can be obtained . since the spot size can be varied , a higher or lower resolution can be obtained , if desired . it is also contemplated that lasers having a higher output could be used in the printer of the present invention , for example , lasers having an output of between 50 mw and 100 mw . after an image has been formed on a receiver as described herein , it is desirable for certain mediums to apply heat to the receiver to fuse the image . one suitable way to fuse the image is to apply hot air at 120 ° c . to the image for approximately two minutes . printer 10 can be used to form slide transparencies in a number of different ways . in the use of a medium of a type which forms a transferred image , a monochrome image can be produced by passing donor 34 in contact with receiver 36 during a single pass , that is , during one revolution of drum 40 . the receiver 36 , which in this case is a transparent film , would then be removed from drum 40 and mounted in a suitable slide mount . if a color image is desired , the donor 34 would include separate spaced sections , for example , cyan , magenta , and yellow sections , and these sections would successively contact receiver 36 in separate passes of the drum 40 . in the use of a medium of the type which forms a retained image , an monochrome image can be formed by one revolution of drum 40 . it is also possible to make a color image using either type of medium by forming three separate images , one for cyan , one for yellow , and one for magenta , on three separate receivers 36 ; the three receivers would then be laminated to form a slide . a second embodiment of the present invention is shown in fig4 and 5 . as shown in these figures , a thermal printer 10 &# 39 ; is generally similar to printer 10 and like elements in printer 10 &# 39 ; have been given the reference numeral of the corresponding element in printer 10 with a prime added . printer 10 &# 39 ; differs from printer 10 in that the beam shaping optics 24 &# 39 ; in printer 10 &# 39 ; are different from optics 24 in printer 10 . beam shaping optics 24 &# 39 ; includes cylindrical lenses l1 - l6 . the lenses l1 - l6 are used in a zoom lens to vary the spot size on the medium . in one representative example of printer 10 &# 39 ;, cylindrical lenses l1 - l6 can be lenses manufactured by melles griot co . which have the following focal lengths and part numbers : ______________________________________ focal lengthlens ( mm ) part no . ______________________________________l1 250 01lcp135l2 100 01lcp155l3 - 25 . 4 01lcn126l4 - 80 01lcn135l5 100 01lcp155l6 250 01lcp135______________________________________ this invention has been described in detail with particular reference to the preferred embodiment thereof , but it will be understood that variations and modifications can be effected within the spirit and scope of the invention . for example , although the present invention has been described herein in relation to a thermal printer , it will be apparent that the invention could be used in other applications where enhanced laser power is required .