LASER-BASED IMAGE RECORDING SYSTEM TECHNICAL FIELD

This invention concerns the production of monochrome or colour images on photographic film from data provided by a digital computer. Although such images may take any form, the present invention is particularly suited to producing detailed colour composite images of land terrain from digital spectral data provided by LANDSAT and NOAA satellites. BACKGROUND ^* ART

Devices which produce images on film from data provided by a digital computer are known and in current use. The most relevant device of this nature for the production of colour composite images is believed to be that marketed by Optron¬ ics Corporation, USA, under the name "Colorwrite". A similar device for producing black and white imagery is produced by the same company and marketed under the name "Photowrite". These devices, which function effectively, are simple in concept and can be interfaced with a wide range of computers. An 8 inch by 10 inch film format (approximately 20.3 cm by 25.4 cm) is normally employed with these devices, and this makes them particularly suitable for recording LANDSAT and NOAA satellite imagery, as well as other pictures which are digitally processed. In these units, a focussed light beam from a low-voltage glow-discharge source (in the case of the "Colorwrite" equipment) or from a light emitting diode (in the case of "Photowrite" units) is made to scan across the film as it

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the light beam is modulated electronically in accordance with digital information fed to the unit from a digital computer so that a composite photographic image is produced on the film.. To produce colour images with this system, using the "Colorwrite" apparatus, white light from the low- voltage glow discharge tube is sequentially filtered with red, blue and green filters and each colour is separately recorded on this film. This means that three passes are required to produce a full colour image with this equipment. Each pass has to be made slowly because the light intensity of the glow discharge tube is low and the use of the colour filters means that the intensity of the red, blue and green light which reaches the film is even lower. The long time for each pass of the film leads to problems with light intensity stability in the apparatus.

An alternative method of producing colour images is to employ the "Photowrite" system,- which uses the more intense beam of a light emitting diode, with modified computer software to produce three transparencies on (say) red, blue and green films (without the use of filters) , which are then composited to produce a single colour image on another film. However,"there are registration problems when three component images are used, and these result in serious limitations on the pre¬ cision and speed of the image-forming technique. This approach is also more costly than when the "Colorwrite" equipment is used.

The use of lasers as alternative high- intensity light sources would clearly have the advantage of a speedier operation, but when lasers

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are used, there are major difficulties with respect to alignment and stability. The stability problem is in respect of both output intensity and alignment. Stability of the output is critical in the MHz regions. It appears that for these reasons the "Colorwrite" and "Photowrite" units do not lasers and,. to the applicant's, knowledge, satis¬ factory laser-based devices for producing colour images on photographic film have not been marketed, even though lasers and associated modulators have been commercially available for some years. DISCLOSURE OF .THE INVENTION

A principal object of the present inven¬ tion is to provide a laser-based system for pro- ducing images on film using data from a computer which will overcome or avoid the inherent problems of stability and/or alignment of the light beam or beams. With such a system, the advantages of the high-intensity laser output can be realised in more rapid scanning of the beam over the film, or improved picture quality, or both. In regard to multi-colour image systems, the present invention provides an arrangement which will allow beams from different lasers to be combined together and brought to a focus simultaneously at one spot on the film (or other image producing medium) so that further advantages of speed are realised without sacrificing fidelity.

According to one aspect, the present invention comprises apparatus for producing an image from digitally processed information, comprising at* least one light source, means for modulating the or each output beam of the

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light source or sources in accordance with the processed information and means for directing the modulated beam or beams on to an image- producing medium, ^: characterised in that :

(a) . the or each light source is a laser; and ^• (b) the or each modulated beam is directed on to a defined pixel area of the image-producing medium. Usually, the image-producing medium will be a photographic film.

The direction of the modulated beam on to a pixel (small picture element) of the image-producing medium may be effected by

^■ (a) directing the or each modulated beam on to a diffusing disc mounted in front of an aperture and focusing the diffused beam from the aperture on to a defined spot on the image-producing medium;

(b) directing the or each modulated beam on to a beam mixer which comprises a glass plate having a surface area typically 1.5 times the cross-sectional area of the or each beams, each surface of the plate being etched, and focusing the beam from the mixer on to a defined spot on the image- producing medium; or

(c) transmitting the or each modulated beam through a bundle of optical

fibres, combining the bundles of fibres in the case of a plurality of modulated beams, and focussing the light emana¬ ting from the fibre bundle or combined bundle of fibres on to a def ned ^* spot on the image- .producing medium. Using one of these techniques, minor align¬ ment errors in directing the modulated beam or beams on to the image-producing medium are avoided, without significantly affecting the intensity, shape or position of said illuminated pixel on the medium.

According to another aspect of the present invention, a device or system for producing images from digitally processed information on -an image- producing medium comprises at least one light- source, an electro-optic modulator associated with the light source or with each one of the light sources, and means for directing the output light beam from the or each light source through its respective modulator and focussing the beam or beams on to the image-producing medium, charac¬ terised in that :

(a) the or each light source is a laser; and

(b) a beam splitter is interposed in the path of the or each light beam between the laser and its respective modulator to divert a portion of the beam to photodetector means electrically connected to the modulator;

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wherevy variations in the output intensity of the or each laser may be compensated by control of said modulator or modulators.

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Preferably, the or each modulator is also controlled by electrical signals derived from the digitally processed information whereby the desired image is generated upon the film by moving it with respect to the focussed beam.

The above-defined aspects of the present invention may be employed singly or in combination in a device or system for recording digitally pro¬ cessed information. If a multi-colour image is to be formed on a photosensitive substrate (such as photographic colour film) , more than one laser will be employed and,' in accordance with another aspect of the present invention, the two (or more) modulated laser beams are combined into a single "multi-colour" beam, after each component beam has been individually modulated, by the Use of one or more dichroic filters (which transmit light beam of one colour and reflect light beams of other colours) to effect their combination into a single output beam. Thus, in a particular form, the above aspects of the present invention* may be combined to provide a multi-colour device or system which incor¬ porates three lasers, each capable of emitting a ^' beam of one of three primary colours (for example, red, blue and green) ,. and a separate modulator for modulating each beam, the device or system being characterised in that

(a) a pellicle beam splitter is interposed in each beam between the respective laser and modula¬ tor to sample each laser output and to direct a fixed proportion of the beam on to a corresponding

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photosensor;

(b) -electronic means are provided for comparing the output of each photosensor with, a respec¬ tive reference signal, said electronic means being connected to its respective modulator to bias said modulator to compensate for variations in the optical energy of the beam from the respective laser, thereby stabilising the beam passing through the modulator;

(c) an arrangement of front surfaced mirrors and dichroic filters is provided to combine the separate, stabilised and modulated beams into a single multi-colour beam; and

(d) means are provided in the path of the multi-colour beam to focus a portion of the multi-colour beam on to the film (or other photosensitive medium) as a spot of defined size and shape. Where it is desired to employ a stepped or continuously moving film (or other recording medium) , it may also be desirable to chop the modulated light beam so as to form discrete image pixels or spots on the film. It is envisaged that in the case of the present invention, this will be achieved by employing yet another electro-optic modulator, locate in the path of the modulated light beam (in the case of a single laser light source) or the combined

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modulated light beam (where a plurality of lasers is used) , to regularly interrupt the beam prior to its direction on to film or other recording medium. BRIEF DESCRIPTION OF DRAWINGS In order to further portray the nature of the present invention, examples of multi-colour laser-based image producing systems will now be described, by way of example, with reference to the accompanying drawings, in which : Figure 1 is a schematic diagram of a pre¬ ferred form of apparatus for producing images from information supplied by a digital computer, in which three lasers are used;

Figure 2 illustrates an alternative arrange- ment for projecting the combined laser beams of the apparatus of Figure 1 on to the surface of. the film of that apparatus; and

Figure 3 shows another way in which the three laser beams of the apparatus of Figure 1 may be combined and the combined beam projected on to the film. BEST MODE OF CARRYING OUT THE INVENTION

Referring now to Figure 1, the apparatus illustrated is a three-colour system employing three lasers as light sources. The laser 10 is a helium- cadmium laser having an output of 10 milliwatts at a wavelength of 0.442 micrometers. Laser 11 is a 50 milliwatt argon ion laser having an output wavelength of 0.5145 micrometers. Laser 12 is a helium neon laser having an output of 5 milliwatts at a wavelength of 0.6328 micrometers. High quality lasers with good long term output stability and minimal high frequency (MHz) fluctuations are most desirable for the system. Typical lasers are those provided by Spectra-physics

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International, Model No. 120 (helium/neon) and Model No. 122-03/262 (argon), and the Linconix laser Model No. 4110 (helium cadmium) .

As shown in Figure 1, a portion of the output of each laser is sampled by the use of a respective pellicle beam splitter 13, 14 or 15. The sampled portions of the laser beams are direc¬ ted on to the respective photo-diodes 16, 17 and 18, the outputs of which are compared with reference voltages 33, 34,35 established during the initial calibration of the equipment. The differences between the photo-diode outputs and the reference voltages are amplified and input to the power supplies 36, 37, 38 of the electro-optical- modula- tors 19, 20 and.21.

The main portions of the laser beams are passed through respective electro-optical modulators ^' 19, 20 and 21. Each of these modulators is controlled.by a drive circuit 26, which is in turn controlled by an interface network which is fed with digital electronic information from an elec¬ tronic computer (not shown). ^' The three analogue voltage outputs from drive circuit 26 are connected to the power supplies 36, 37 and 38 of the electro- optical modulators 19,20 and 21. These signals are the major controlling signals of the modulators. The analogue voltages from the photo diode/reference voltage combinations exercise a "fine-tuning" control of the transmittance of their respective modulators. The modulated red beam which emerges from modulator 21 is directed via a front surface mirror 22 through a red transmitting dichroic filter 23, and is then reflected from a blue transmitting dichroic filter 24.

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The modulated green beam which emerges from modulator 20 is reflected from the red transmitting dichroic filter 23. The laser 11 and its associated modulator 20 are positioned so that on reflection . from filter 23, the green beam is aligned to combine with the red beam from mirror 22. ^" The green com¬ ponent ^* , of this combined beam is ^* also reflected from the blue transmitting dichroic filter 24.

The modulated blue beam which emerges from modulator 19 is transmitted through the blue trans¬ mitting filter 24. Laser 10 and its associated modulator 19 are positioned so that the blue beam is aligned to combine with the red/green beam which has been reflected by filter 24 to produce a combined red/green/blue beam.

This combined red/green/blue beam is then passed through an electro-optical modulator 25 which is adapted to provide chopping of the combined red/ green/blue beams. The chopping action of modulator 25 is controlled by its associated power supply 32, which receives signals from the drive circuit 24 to synchronise with the timing requirements of the film recording device.

In the embodiment of Figure 1, the chopped beam emerging from modulator 25 is focussed by lens 27, via a front surface mirror 28, on to a planar diffusing plate 29. Diffusing plate 29 is etched on both planar surfaces and scatters and diffuses the modulated laser beams. ^* An aperture plate 30 attached to or formed integrally with the holder for diffusing plate 29 determines the size and shape of the diffuse light beam emanating from the plate. The emergent light beam is focussed by lens 31 on to the film surface 39, which is moving on a supporting

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drum of the recording device to form the desired multi-colour image. ^"

Suitable colour separation filters 22, 23, 24 may be Obtained from Optical Coating Laboratory Inc., of Santa Rosa, California, USA.

Suitable electro-optical modulators may be obtained from Interactive Radiation Incorporated of New Jersey, or from Isomet Corporation in the USA.

An alternative method of, and apparatus for, processing the combined red/green/blue beam is shown in Figure 2. This alternative uses a glass beam mixer 43 which comprises a glass cylinder of diameter approximately 1.5 times the cross-sectional diameter of the combined red/green/blue beam 44, each end face of the cylinder being etched to provide a ^* translucent surface. The beam 44, directed on to the first sur¬ face of the cylinder by mirror 28, is scattered into 'the cylinder, internally reflected from the sides of the cylinder, and further scattered on transmission through the second diffusing surface. In effect, the coherent ^* , highly linear, laser beams are converted into an incoherent, non-linear light "source" at the output end of the cylinder. This "source" can then be treate as a -conventional "point source", and focused through an aperture in aperture plate 40, which lies on or closely adjacent to the mounting plate 42 of the mixer 43, using a microscope objective 41,on to the film surface 39.

Figure 3 illustrates an alternative laser beam combination arrangement. In the arrangement of

Figure 3, optical fibre bundles 50,51 and 52 are used to transmit the modulated laser beams emanating from modulators 19, 20 and 21, respectively. The fibre bundles 50, 51 and 52 are brought together, and their

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individual optical fibres are interwoven to form a combined optical fibre bundle 53, supported by mounting plate 54.which also supports an aperture plate 55. Preferably, the combined and mixed optical fibres form, on a micro scale, sets of red-blue- green "triplets" (similar to masking on television screens to give three colour spots for each pixel) . ^* " Coloured light emanating from the end of the combined optical fibre bundle 53 is then focussed in a similar manner to the light from the aperture in plate 40 of the embodiment illustrated in Figure 2, on to the film surface. The fibre optic bundle approach has the limitation"of difficult fabrication to produce bundles of red-green-blue "triplets", but has the advantage of simpler alignment and movement along the rotating film drum.

^• Any chopping of the beams in the case of the 'modified arrangement of Figure 3 is effected either (a) after the combined laser beam has been transmitted through the aperture in plate 5*5, or (b) by synchron¬ ised chopping using modulators 19, 20 and 21, controlled by signals from drive circuit 26.

It will be clear that although arrangements using three lasers have been described, one or two of those lasers (and associated modulators) could be removed, and additional lasers could be added, provided all laser beams are combined for focussing on to the film surface. INDUSTRIAL APPLICABILITY It will be seen from the. above description that a colour imaging device has been provided which provides a relatively high intensity optical signal and, in addition, creates a composite three-colour beam which can be used to expose a photographic film

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is a single pass to produce a three colour image. This substantial advantage not only allows the production of images with a higher precision of detail more quickly, but also obviates the need for the use of special high speed photographic films and their attendant processing difficulties. It is also, particularly suited for the production of images from digital signals obtained from LANDSAT and NOAA satellites. Nevertheless, it will be appreciated by those skilled in this art that many modifications and additions can be made to the particular example described without departing from the principles ^* and scope of the invention.

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