GB1600005A

GB1600005A - Electro-optical scanning

Info

Publication number: GB1600005A
Application number: GB1070377A
Authority: GB
Original assignee: Linotype Paul Ltd
Current assignee: Linotype Paul Ltd
Priority date: 1977-03-14
Filing date: 1977-03-14
Publication date: 1981-10-14
Also published as: DE2811021A1; JPS53137702A; DE2811021C2

Description

(54) IMPROVEMENTS IN OR RELATING TO ELECTRO-OPTICAL SCANNING (71) We, LINOTYPE-PAUL LIMITED, of Kingsbury Works, Kingsbury Road, London NW9 8UT, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an apparatus for and a method of producing a colour corrected image from an original.

In accordance with the invention, there is provided an apparatus for producing a colour corrected image from an original, the apparatus comprising means for sequentially scanning with radiation areas of the original, means for causing the radiation from each scanned area to be incident upon detector means in different wavelength bands in a predetermined time sequence, storage means for sequentially storing and simultaneously holding in store signals generated to correspond to the intensity of the radiation received by the detector means in each wavelength band, and means for modifying the stored signals received from each scanned area in one wavelength band by the stored signals received from the area in another wavelength band.

The invention also provides an apparatus for reproducing a colour corrected image from an original, the apparatus comprising means for sequentially scanning areas of an original having separate and distinct wavelength bands, means for generating in response to the scanning of an area sequential signals corresponding to the wavelength bands, storage means for simultaneously holding in store the generated signals, each stored signal corresponding to the area and to a distinct wavelength band, and means for correcting the stored signals in reponse to each other to produce a desired balance between wavelength bands of the area.

In accordance with another aspect of the invention, there is provided a method of producing a colour corrected image from an original comprising the steps of sequentially scanning areas of the original having separate and distinct wavelength bands, generating in response to the scanning of an area sequential signals corresponding to the wavelength bands, simultaneously holding the signals in store, each stored signal corresponding to a respective scanned area and to a distinct wavelength band, and correcting the stored signals by the other stored signal to produce a desired balance between the wavelength bands of the area.

In accordance with the invention there is also provided a method of producing a colour corrected image from an original, wherein areas of the original to be reproduced are sequentially scanned with radiation, radiation from each area of the image is supplied to a radiation detector in different wavelength bands in a predetermined time sequence, information corresponding to the intensity of the radiation received by the detector in each wavelength band for an area is simultaneously held in store, and the stored information received from each scanned area in one wavelength band is modified by the stored information received from the area in another wavelength band.

The wavelength bands may comprise respective light bands, each being of predominantly one colour.

In this way, for reproduction of a coloured original, only one detector is required to determine the intensity of the component wavelength bands or colours and only a single unsharp masking detector is required. Both detectors, which preferably are photomultipliers, can be arranged to receive the same wavelength band light simultaneously, and to receive light representing the wavelength bands sequentially. Preferably the colour components are blue, green and red. The colour components may conveniently be obtained from a white light source, for example by passing light through a prism so as to produce a spectrum. Alternatively, the splitting of the light into its component colours can be effected by passing white light through a rotating filter disc so as sequentially to transmit the different colour components. The splitting of the light may be carried out either before or after it as been used to illuminate the image, for example before or after transmission through a transparency or reflection from a print surface.

It is also envisaged that a plurality of light sources, each corresponding to a required colour component, may be used so as to avoid the necessity of beam splitting of composite-component light.

The method and apparatus of the invention will now be further explained, by way of example, with reference to the drawings accompaying the Provisional Specification.

Referring to Figure 1, a photographic colour transparency 2 is mounted on the curved surface of a rotatably-mounted cylindrical input drum 4. It is required to reproduce the image on the transparency 2 as monochrome separations on photosensitive material 6, for example photographic film. The material 6 is mounted on the curved surface of a further rotatably-mounted cylindrical drum, an output drum 8.

A light source 10 is arranged to shine light of blue, green and red components via an optical system 11 (as hereinafter described) sequentially along the axis of the input drum 4, and then via a prism, or other reflector 12 to a focus on a small area of the transparency 2.

The transmitted light is collected by a converging lens 14 and focussed on to an optical stop 15. An aperture 16 in the stop 15 allows a certain amount of the light that has passed through the image 2 to fall on a photomultiplier 18. The remainder of the light that falls on the stop 15 is reflected or scattered from around the aperture 16, out of focus, on to a second photomultiplier 20. The light directed on to the photomultiplier 18 is indicative of the density of the small focussed area of image on the transparency 2 as seen through light of the instantaneous component colour, blue say. The light incident on the photomultiplier 20 is used for the unsharp masking. As the input drum 4 rotates information received by the photomultipliers 18 and 20 is representative of the density along one circumferential line of the transparency 2 as seen by illumination with the successive component colours. The information from the photomultipliers 18 and 20 is fed separately to a gain control circuit 22, and thence via a line 23 to a store 24.

The light source 10 directs through the transparency 2 blue, green and red light sequentially on each revolution of the drum 4, such that the density of each circumferential line of the transparency is detected in each case, and stored in the store 24 for production of respective monochrome prints or photographic plates.

The store 24 may be considered as comprising three sub-stores 24b, 24g and 24r, each of which has a store location corresponding to a particular point on the circumference of the input drum 4. The colour density of a circumferential line of the transparency 2 is scanned during one revolution of the drum 4 as illuminated by blue light, and the locations of the sub-store 24b filled. Sub-store 24r is filled on the next revolution of the drum 4. The store 24 is thus filled sequentially.

The input drum 4 is rotated at approximately three times the speed of the output drum 8, since three sets of information are required from each circumferential line of the colour transparency 2 to reproduce a single line of each monochrome separation on the drum 8.

Conveniently. the output drum 8 is rotated at 750 r.p.m. and the input drum at 2250 r.p.m.

The speed at which the density information is read into the store 24 along the line 23 is governed by the frequency of a pulse signal fed to the store 24 along a line 26 after passing through a frequency synthesiser 27 from a frequency generator 28 that is connected to the input drum 4. The frequency generator 28 can conveniently be provided by a light source, optical grating and a photocell. The speed at which information is taken from the store 24 is different from the read-in speed, and is determined by a similar frequency synthesiser 29, receiving pulses from a generator 30 driven by, and at the same speed as, the output drum 8. The store 24 is arranged to have the capacity to store information as to the optical density of the transparency 2 as seen through each of the illuminating component colours, for a single circumferential line of the input drum 4. Thus, when scanning of the transparency proceeds to an axially-displaced circumferential line, as hereinafter described, the new information is deposited in the same storage locations for read-out to produce the separations on the output drum 8.

The colour density information in the store 24 is fed to a computer 32, which may be analogue or digital. Although as explained above, the store 24 is filled sequentially, the information is passed on to the computer 32 in a parallel mode. That is to say, the computer 32 is supplied at any one time with the information stored in all of the sub-stores 24b, 24g and 24r from corresponding locations, and thus corresponding to a single circumferential point on the drum 4.

Colour correction is applied in the computer 32. For example, the density of any one colour component may be biassed in accordance with the density of the other two component colours so as to reproduce from the separations a final colour print of the same colouring as the original transparency 2. The apparatus may be modified in that the positioning of the store 24 and parts of the computer 32 may be interchanged. For example, tone correction and unsharp masking may be carried out before storing any information, since each of these adjustments is related to only one of the light components as opposed to a combination thereof. It should be noted, however, that colour correction has to take place after the individual colour component information has been removed from the store 24.

The information from the computer 32 is effective to control the intensity and duration of a light source 34 that is arranged to expose the photosensitive material 6 on the output drum 8.

It will be appreciated that the information obtained from the colour transparency 2 for the production of the separations may in addition, or alternatively, be stored othewise than as described for the store 24, on magnetic tape for example.

The computer 32, light source 34 and associated optical components for exposing the film or other material 6 on the drum 8, constitute an exposing head, and either a single exposing head can be used to produce the four separations end-to-end around the circumference of the drum 8, or two or more exposing heads can be employed so as to produce a greater number of separations side-by-side axially of the drum 8, as indicated by dashed lines and primed reference numerals in Figure 1. As a further alternative, a single computer 32 may be used to control several exposing heads.

The light-production and detection system including the reflector 12 associated with the input drum 4, is arranged to be driven axially of the drums at a constant speed, so that the whole surface of the transparency 2 is illuminated in a spiral form. This motion can conveniently be effected by a pantograph action controlled by the corresponding axial motion of the exposing head, or heads, along the output drum 8. Enlargement or reduction of the separations on the output drum 8 with respect to the transparency 2 in a direction axially of the drums is effected by relative adjustment between the scanning speeds of the transparency 2 and of the photosensitive material 6.

Figure 2 shows schematically the pantograph system employed for driving an input carriage 100 which carries the light production and detection system associated with the input drum 4, and for driving an output carriage 102 which carries the exposing head, or heads, associated with the output drum 8. The output carriage 102 has one end of a connecting rod 104 fixed thereto, the other end of which is slidably mounted by means of a roller bearing 106 within a guide member 108 that is mounted for pivotal movement about a point 110. Only a single drive motor 111 is required, and this acts directly to effect movement of the output carriage 102 along a line 112 that extends axially of the input and output drums 4 and 8. A further arm 114 of the pantograph system is mounted for movement within the guide 108 by means of a roller bearing 116, and is rigidly connected to the input carriage 100. The system is thus effective to translate movement of the directly-driven output carriage 102 into parallel movement of the input carriage 100 and, by suitable positioning of the roller bearings 106 and 116 within the guide 108 is effective to produce movement of the input carriage that is precisely and predeterminedly scaled with respect to the movement of the output carriage. The scaling can be chosen so as to effect enlargement or reduction of the separations on the output drum 8 compared to the transparency 2 on the input drum 4.

Enlargement or reduction in a direction circumferentially of the drums 4 and 8 will now be considered.

As has been explained, each circumferential line of the transparency 2 must be scanned three times to provide information for one line of the photosensitive film 6 on the output drum 8 i.e. one line for each of the blue, green and red separations. Accordingly, the most efficient usage of the colour reproduction system occurs when the input drum is rotated at three times the speed of the output drum. This results in a fixed reduction by a factor of three when the information is written on to the separations at the same speed as it is read from the transparency 2.

The magnification M of the separations with respect to the original in this direction can be expressed as: N M N D x S ' where pulses are supplied to the store 24 along the line 26, to control the flow of information along the line 23, at a rate which is N times that at which they pass along the line 31, to control the deposit of information on the material 6 from the computer 32. These rates are expressed in terms of pulses/revolution of the respective input and output drums 4, 8, where the input drum 4 rotates at S times the speed of the output drum 8, and where the diameter of the input drum 4 is D times that of the output drum 8. Clearly, the magnification M can be greater or less than unity, depending on the values chosen for the parameters N, D and S. For practical convenience, D and S are preferably kept constant so that variation in the magnification is controlled by N, the ratio of the read and write rates.

It is also convenient although not essential to use a fixed picture element size on the output drum 8. If the store contains P words that is to say if the density information of a single line extending circumferentially of the drum 4 is stored in P locations, thus defining the resolution of the system, and if the photo-sensitive material 6 extends around the output drum 8 so as to substend an angle 6 at the centre, the rate of reading from the store 24 will be P x elements/revolution of the drum 8.

6 The rate of reading is synchronised to rotation of the drum 8 by the pulse signals on line 31, the signals being derived from a generator 30 which is locked to the rotation of the drum and multiplied to the required rate by a frequency synthesiser 29 the multiplication ratio being fixed at P x P X 2x R29 = L x 6 where L is the number of pulses per revolution from the generator 30. The input drum 4 is equipped with a similar system but the ratio of the frequency synthesiser 27 is arranged to be varied, and can be considered as R27 = K x E where K is a constant and E is variable by means of a control on the synthesiser 27. The constant K is chosen to be K = D x S x 2(3 x 0 L the ratio of the write to read rate (N) is now equal to

=DxSxE so M = DD Xx S x E = E Thus the magnification is equal to the controllably variable part of the analysis frequency synthesiser multiplication ratio.

As explained, a difference in the diameter or the relative speed of rotation of the input and output drums will manifest itself as an enlargement or reduction of the resulting separations with respect to the original transparency, so that a convenient enlargement or reduction value can be chosen.

If the ratio of the speeds of rotation of the input and output drums is three, then optimum resolution is obtained without introducing any redundancy into the store 24.

In some circumstances it may be found acceptable not to use all three sets of information from each line of the transparency for producing the separations. For example, for printing magenta, the red, blue and green information may be stored in the store 24, and read out in the following sequence: green, red, green, blue, green, red, green, blue, etc. This sequencing improves the resolution of the green component, which is the main component required to print magenta, to 1:2 from the ratio 1:3 that would be obtained if all three colours, were given equal attention. A lower resolution, of 1:4, is consequently obtained for the remaining, so-called correction, colours. Corresponding sequencing can be adapted where the main colour component is red or blue. Finally under these conditions, the optimum speed ratio of the drums becomes 2:1 rather than the previous 3:1.

The optical system for illuminating the transparency 2 will now be briefly described with reference to the schematic arrangement of Figure 3. The light source 10 comprises a xenon arc light, the light output of which is collected by a lens 42, directed through an aperture-defining plate 44, to a lens 46 and then through a further aperture-defining plate 48 before being directed as a parallel beam through a colour splitting prism system 50 that comprises a prism and two lenses. The light output from the final lens 51 of the system 50 is directed on to a filter disc 52 for transmitting blue, green and red light respectively.

The prism of the system 50 acts to divide the incoming white light into a spectrum, and the lens 51 focusses each spectral line on a different point of the disc 52, each point being an image of the aperture of the plate 44. A spectrum thus appears on the disc 52 extending from red R to blue B, and the disc 52 has three approximately 120C arcuate slots spaced apart axially thereof. The disc 52 is rotated in the light path by a motor 54 at precisely one third the speed of the input drum 4, so as to direct on to a collecting lens 56 the sequentially coloured light. The light transmitted by the lens 56 then passes to a further prism system 58, for collecting the sequentially-coloured light as a parallel beam and passing this on to a prism or other reflector 60 that is arranged to transmit the parallel beam axially of the input drum 4 to the prism 12 which is disposed therewithin. Thus, since the prism 12, which is mounted as described for movement axially of the drum 4, receives the sequentially-colured light as a parallel beam, the illumination of the transparency 2 does not change during scanning.

An alternative optical system for efficiently producing the separate colour components from white light involves the use of dichroic filters, and is shown schematically in Figure 4, the optical focussing and directing components being omitted for clarity.

White light 70 is directed obliquely on to a first dichroic filter 72. Substantially all the blue light is arranged to be reflected from the front surface of the filter 72 to provide a beam 74, one of the required beams, the rest of the light being transmitted to a second dichroic filter 76. The filter 76 is arranged to reflect substantially all of the incident red light so as to provide a second of the required beams, beam 78. The light transmitted by the filter 76 is then the third required beam, beam 80, and comprises substantially only green light. Each of the required beams 74, 78 and 80 comprise approximately 80% of the total individual colours of the incident white light beam 70.

Whatever optical system is employed to produce the individual colour components for illuminating the transparency 2, the individual pass bands can be arranged to correspond exactly to the response of the human eye to each of these components, so as to provide visually faithfull colour reproduction. Alternatively, and to the same end, the optical system can be arranged to introduce colour corrections, into the light incident on the transparency, that are related to the colour correction curves of the printing inks to be used in the final production of the colour print from the transparency 2.

WHAT WE CLAIM IS: 1. An apparatus for producing a colour corrected image from an original, the apparatus comprising means for sequentially scanning with radiation areas of the original, means for causing the radiation from each scanned area to be incident upon detector means in different wavelength bands in a predetermined time sequence, storage means for sequentially storing and simultaneously holding in store signals generated to correspond to the intensity of the radiation received by the detector means in each wavelength band for each area, and means for modifying the stored signals received from each scanned area in one wavelength band by the stored signals received from the scanned area in another wavelength band.

2. An apparatus as claimed in claim 1 wherein each wavelength band received by the detector means comprises light of predominantly one colour.

3. An apparatus for reproducing a colour corrected image from an original, the apparatus comprising means for sequentially scanning areas of an original having separate and distinct wavelength bands, means for generating in response to the scanning of an area sequential signals corresponding to the wavelength bands, storage means for simultaneously holding in store the generated signals, each stored signal corresponding to the area and to a distinct wavelength band, and means for correcting the stored signals in response to each other to produce a desired balance between wavelength bands of the area.

4. An apparatus as claimed in claim 1, 2 or 3 wherein the original image is illuminated

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. the following sequence: green, red, green, blue, green, red, green, blue, etc. This sequencing improves the resolution of the green component, which is the main component required to print magenta, to 1:2 from the ratio 1:3 that would be obtained if all three colours, were given equal attention. A lower resolution, of 1:4, is consequently obtained for the remaining, so-called correction, colours. Corresponding sequencing can be adapted where the main colour component is red or blue. Finally under these conditions, the optimum speed ratio of the drums becomes 2:1 rather than the previous 3:1. The optical system for illuminating the transparency 2 will now be briefly described with reference to the schematic arrangement of Figure 3. The light source 10 comprises a xenon arc light, the light output of which is collected by a lens 42, directed through an aperture-defining plate 44, to a lens 46 and then through a further aperture-defining plate 48 before being directed as a parallel beam through a colour splitting prism system 50 that comprises a prism and two lenses. The light output from the final lens 51 of the system 50 is directed on to a filter disc 52 for transmitting blue, green and red light respectively. The prism of the system 50 acts to divide the incoming white light into a spectrum, and the lens 51 focusses each spectral line on a different point of the disc 52, each point being an image of the aperture of the plate 44. A spectrum thus appears on the disc 52 extending from red R to blue B, and the disc 52 has three approximately 120C arcuate slots spaced apart axially thereof. The disc 52 is rotated in the light path by a motor 54 at precisely one third the speed of the input drum 4, so as to direct on to a collecting lens 56 the sequentially coloured light. The light transmitted by the lens 56 then passes to a further prism system 58, for collecting the sequentially-coloured light as a parallel beam and passing this on to a prism or other reflector 60 that is arranged to transmit the parallel beam axially of the input drum 4 to the prism 12 which is disposed therewithin. Thus, since the prism 12, which is mounted as described for movement axially of the drum 4, receives the sequentially-colured light as a parallel beam, the illumination of the transparency 2 does not change during scanning. An alternative optical system for efficiently producing the separate colour components from white light involves the use of dichroic filters, and is shown schematically in Figure 4, the optical focussing and directing components being omitted for clarity. White light 70 is directed obliquely on to a first dichroic filter 72. Substantially all the blue light is arranged to be reflected from the front surface of the filter 72 to provide a beam 74, one of the required beams, the rest of the light being transmitted to a second dichroic filter 76. The filter 76 is arranged to reflect substantially all of the incident red light so as to provide a second of the required beams, beam 78. The light transmitted by the filter 76 is then the third required beam, beam 80, and comprises substantially only green light. Each of the required beams 74, 78 and 80 comprise approximately 80% of the total individual colours of the incident white light beam 70. Whatever optical system is employed to produce the individual colour components for illuminating the transparency 2, the individual pass bands can be arranged to correspond exactly to the response of the human eye to each of these components, so as to provide visually faithfull colour reproduction. Alternatively, and to the same end, the optical system can be arranged to introduce colour corrections, into the light incident on the transparency, that are related to the colour correction curves of the printing inks to be used in the final production of the colour print from the transparency 2. WHAT WE CLAIM IS:

1. An apparatus for producing a colour corrected image from an original, the apparatus comprising means for sequentially scanning with radiation areas of the original, means for causing the radiation from each scanned area to be incident upon detector means in different wavelength bands in a predetermined time sequence, storage means for sequentially storing and simultaneously holding in store signals generated to correspond to the intensity of the radiation received by the detector means in each wavelength band for each area, and means for modifying the stored signals received from each scanned area in one wavelength band by the stored signals received from the scanned area in another wavelength band.

from a white light source and the white light is divided into the wavelength bands before reaching the original.

5. An apparatus as claimed in claim 4 comprising optical means having a prism arranged to produce spatial separation of the component wavelengths of white light of the source, and an optical shutter arranged to transmit different wavelength bands in time sequential manner.

6. An apparatus as claimed in any preceding claim wherein the storage means is arranged sequentially to store information received from successive areas of the original.

7. An apparatus as claimed in any preceding claim comprising a cylinder having means for mounting the original on a surface thereof, the cylinder being rotatably mounted about its axis, and the radiation being focused on the surface of the cylinder.

8. An apparatus as claimed in claim 7 wherein the sequencing of the radiation and the rotation of the cylinder are such that radiation of the different wavelength bands is focused on the surface of the cylinder during respective revolutions thereof.

9. An apparatus as claimed in claim 7 or 8 wherein the surface of the cylinder is transparent, and the radiation is directed through the surface from inside the cylinder.

10. An apparatus as claimed in claim 7, 8 or 9 having an imaging device to reproduce the original on an image receiving surface comprising the curved surface of a cylinder rotatably mounted on its axis.

11. An apparatus as claimed in claim 10 comprising means for correlating the speeds of rotation of the cylinders so as to control the magnification of the reproduction of the original.

12. An apparatus as claimed in claim 11 comprising a frequency generator connected to each cylinder for providing an output to the storage means to control the rate at which image density information enters and leaves the storage means and thereby to control the magnification of the reproduction of the original.

13. An apparatus as claimed in claim 10, 11 or 12 comprising drive means arranged to rotate both cylinders about their respective axes, the drive means comprising a motor arranged to drive one of the cylinders and a pantographic system for transmitting the drive to the other cylinder thereby to rotate other cylinder at a controllably variable speed with respect to the one cylinder.

14. An apparatus as claimed in any one of claims 1 to 9 wherein the storage means is arranged to store the signals in storage locations associated with respective ones of the wavelength bands until an area only of the original has been scanned, and wherein means is provided to reproduce that area of the original on an image receiving surface of an imaging device whilst a further area thereof is being scanned.

15. An apparatus as claimed in any one of claims 1 to 9 comprising means responsive to the corrected signals from the storage means to reproduce the original as an image on an image receiving surface of an imaging device.

16. An apparatus as claimed in any one of claims 10, 11, 12, 14 and 15 having a pantograph system for transferring parallel movement between the scanning means and the imaging means,.

17. An apparatus as claimed in claim 16 wherein the pantograph system comprises first and second roller bearings connected respectively to the imaging and to the scanning means, and a guide means pivoted at one end and guidingly receiving the roller bearings.

18. An apparatus as claimed in claim 1 or any claim dependent thereon comprising means to focus the radiation onto the detector means. further detector means arranged to receive radiation out of focus from the image, and means arranged to combine together the outputs of the two detector means to thereby increase the sharpness of the reproduced images.

19. An apparatus as claimed in claim 1 or any claim dependent thereon wherein the or each detector means comprises a photomultiplier.

20. A method of producing a colour corrected image from an original comprising the steps of sequentially scanning areas of the original having separate and distinct wavelength bands, generating in response to the scanning of an area sequential signals corresponding to the wavelength bands, simultaneously holding the signals in store, each stored signal corresponding to a respective scanned area and to a distinct wavelength band, and correcting the stored signals by the other stored signals to produce a desired balance between the wavelength bands of the area.

21. A method of producing a colour corrected image from an original, wherein areas of the original to be reproduced are sequentially scanned with radiation, radiation from each area of the image is supplied to a radiation detector in different wavelength bands in a predetermined time sequence, information corresponding to the intensity of the radiation received by the detector in each wavelength band for the area is simultaneously held in store, and the stored information received from each scanned area in one wavelength band is modified by the stored information received from the area in another wavelength band.

22. A method as claimed in claim 20 or 21 wherein the different wavelength bands correspond predominantly to respective colours of the optical spectrum.

23. A method as claimed in claim 22, wherein the wavelength bands comprise predominantly blue, green and red light respectively.

24. A method as claimed in claim 20, 21, 22 or 23 wherein the original is illuminated sequentially by light of different wavelength bands.

25. A method as claimed in claim 24 wherein the original is scanned by illuminating radiation so that successive portions thereof are subjected to the sequential illumination.

26. A method as claimed in any of claims 20 to 25 wherein the illuminated original is mounted at the surface of a cylinder that is rotated about its axis, and a light beam is directed to the image to illuminate a portion thereof extending circumferentially of the cylinder.

27. A method as claimed in claim 26 wherein the light beam is arranged to move axially of the cylinder so as to illuminate successive axially-displaced area of the original.

28. A method as claimed in claim 26 or 27 wherein the original is reproduced at the curved surface of a further rotatably-mounted cylinder, the magnification of the reproduction being controlled by the relative speed of rotation of the cylinders.

29. A method as claimed in claim 21 or any claim dependent thereon wherein radiation from the original is focused onto the detector and is detected out of focus by a second detector, the outputs of the two detectors being combined so as to increase the sharpness of reproduction of the image.

30. An image apparatus for producing a colour corrected image from an original substantially as hereinbefore described with reference to the drawings accompanying the Provisional Specification.

31. A method of producing a colour corrected image from an original substantially as herein described with reference to the drawings accompanying the Provisional Specification.