GB2049964A - Simultaneously projecting two images of a subject in register - Google Patents

Abstract

An image of an original (12) illuminated by a stationary radiating light source (15) and an image of that same subject (12) in raster form such as when generated by a scanning light source, for example a cathode ray tube (25) or a laser, are projected in register at (18). In order to eliminate mis- registering caused by vibrations use is made of a pair of positional indicators (13, 14) on the original (12), the images (19, 20) of which are projected onto corresponding light- sensitive detectors (21, 22) which generate an electric signal controlling the position of the raster image generated by the scanning light source (25). When a cathode ray tube (25) is used as the scanning light source, the signals control the current in the deflection coils (29X, 29Y) of the latter. Figs. 6, 7 (not shown) show circuits for controlling the CRT (25). <IMAGE>

Description

SPECIFICATION Method and apparatus for simultaneously projecting two images of a subject in register with each other.

This invention is concerned with exposure methods. More particularly, it is concerned with a method and an apparatus for simultaneously projecting two images of a subject to be recorded, e.g. a graphic original in register with each other.

The invention is of particular interest in the field of graphic arts techniques when simultaneously the image of a graphic original and a modified image of said original are projected onto a photographic recording material.

Such modified images are required in several graphic and reprographic reproduction techniques, as well in black and white as in colour work. This is done for the reason that in practically all cases a tone correction of an original has to be carried out.

Moreover, in colour work it is necessary to perform a colour correction in order to correct the undesired side absorbtions of the printing inks.

It is known in the prior art to carry out the above-indicated corrections with the help of socalled masks. These are separate photographic images which are held in register with the original for the step of imagewise exposing the photographic material in which the photographic reproduction is to be formed. The making of separate masks requires much skill and moreover the number of masks is greater the higher is the fidelity standard of the reproduction process. For high-quality work a large number of masks is required.

In order to reduce the amount of film material required in graphic/reprographic work, electronic devices have been designed which made it possible to carry out the necessary corrections in one step. These devices are in the art known as scanners. Scanners enable any desired correction to be made for each point of the original, thanks to the built-in programming facility. As a consequence, all desired information can be offered in a point-like fashion.

Another opto/electronic method makes use of two separate light sources. In this case, apart from a fixed image a second one is derived from the first one and is projected onto the same image plane for the purpose of obtaining a composite image incorporating the necessary corrections. In this system, in which the original is illuminated by a stationary, uniformly radiating light source, a second light source is used which is of the scanning type. The information relevant to the control of its intensity, so that it is appropriate for each point on the raster, is derived from the first image via a so-called by-pass channel.

A problem is however involved in the use of such said scanning light sources, because of their image instability, which is the cause of lack of proper registration between the fixed and raster images.

It is an object of the invention to provide a method for simultaneously projecting two images of a subject, e.g. a graphic original in register with each other under dynamic conditions.

According to the invention, there is provided: A method of exposing a light-image-receiver to a subject to be recorded, which method comprises the step of overall illuminating the subject, projecting light from the subject to form a lightimage (hereinafter called "fixed image") on said receiver, scanning the fixed image, or a corresponding secundary light image simultaneously derived from the illuminated subject, to produce image signals, modulating such signals in accordance with predetermined criteria for producing by means of a scanning light source a raster image of the subject which is projected onto said receiver in register with said fixed image, characterised in that:: first and second positional indicators are located so that fixed and raster images of each of said indicators are projected simultaneously with the formation of the said fixed and raster images of the object to be recorded.

first and second light-sensitive detectors are provided which are capable of receiving said images of said first indicator and of the second indicator respectively, said indicators are of such form that the output signals of the detectors are a measure of the registration error (if any) in two mutually perpendicular directions, between the fixed and raster images of such indicators, and output signals from such detectors are used to bring the raster images of the indicators on the detectors in registration with the fixed indicator images thereon and consequently to establish registration of the fixed and raster images on the receiver of the subject to be recorded.

In a preferred embodiment of the invention the output signals from the light-sensitive detectors are used to bring the raster images of the positional indicators into positions in which the light flux on the detectors is at a maximum.

As to the positional indicators, these may be of any form such that the projection of the raster images of the indicators onto the light-sensitive detectors causes the detectors to yield a series of pulses or pulse trains. This implies use of the detectors which exhibit density changes in at least one direction along and/or across the indicators.

The indicators may e.g. be in the form of small continuous tone wedges which are mounted outside the area where the subject is located during exposure as an alternative the indicators may be in the form of step wedges or showing a cheque-board pattern or analogous form.

In carrying out preferred embodiments of the method, initially only the light source forming the fixed image is energized. The light-sensitive detectors are then adjusted in order to obtain a maximum signal. Then, the scanning light source is energized and the projected raster images of the positional indicators are brought into registration with the fixed images. The system is then ready for use in exposing an image receiver, e.g. a lightsensitive photographic material, a mosaic or array of photodiodes or analogous devices, etc.

The light responsive elements are preferably silicon photocells in the form of a bar in front of which a diaphragm may be located. This diaphragm is of importance in the case the two light sources referred to form part of an enlarger.

In the latter case, the opening of the diaphragm is in direct proportion with the degree of enlarging.

In another preferred embodiment of the invention, the mean output light intensing of the scanning light source is equal to that of the light by which the subject and the positioned indicators are illuminated.

The invention also includes: An apparatus for exposing a light-imagereceiver to a subject to be recorded, such apparatus comprising a subject holder, a light source for uniformly illuminating a subject when it is in position in such holder, means for optically projecting a light-image (hereinafter called "fixed image") of said subject onto an image plane in which a said image-receiver may be located, means including a scanning light source for simultaneously producing a modulated raster image of said subject by electrical signals which are derived from said fixed image or from a corresponding secondary light image of the subject and are modulated in accordance with predetermined criteria, and means for projecting such modulated raster image onto said plane in register with said fixed image, characterised in that:: there is means for holding first and second positional indicators at positions such that they will be illuminated by said light source simultaneously with said subject and such that fixed and raster images of such indicators will be projected simultaneously with the fixed and raster images of the subject and by the same means as that used for forming such images, there are first and second light-sensitive detectors located for receiving the projected fixed and raster images of a said first positional indicator and of a said second positional indicator respectively when such indicators are held as aforesaid, such detectors serving, if appropriate indicators are used, to yield output signals which are a measure of the registration error (if any), in two mutually perpendicular directions, between the fixed and the raster images of such indicators, and said detectors are connected to means for influencing the position of the projected raster images of the positional indicators so that said output signals will maintain the fixed and the raster images of the positional indicators, and consequently the fixed and raster images of the subject, in register.

The scanning light-source is preferably a cathode ray tube, although other embodiments such as lasers, fibre optic tubes, and analogous devices may be used at will.

As positional indicators, small continuous tone wedges, having a density varying between 0 and 3 may be used with advantage. The images of the positional indicators are projected onto lightsensitive detectors which are preferably silicon resistors in bar form. In case the registration system is to be used in a graphic arts enlarger or camera in which a magnifying or reduction of the dimensions of the subject is carried out, the silicon resistors are located behind a triangular diaphragm, so that the area of the resistors which is exposed is function of the magnifying or reducing factor.

The signals generated by the light-sensitive detectors is used to control the raster position of the image generated by the scanning light source.

If the latter is a cathode ray tube, this may be carried out by influencing its deflection system such as by modulating the current going through the deflecting coils when magnetic deflection is applied. Alternatively, also the position of the mirror via which the image of the cathode ray tube is projected in the image plane may be used for carrying out the correction process. It will be clear that the latter solution is more appropriate when slow scan video systems are used, due to the inertia of mirror.

In the description that follows, it will be assumed that the apparatus according to the invention is incorporated in an enlarging device which makes use of a cathode ray tube for generating the raster image of the subject and the positional indicators. It is clear, however, that the apparatus of the invention may also be used in combination with other opto- or opto-electronic devices.

The invention will be better understood at the hand of a description of a preferred embodiment and in the light of following figures, in which: Fig. 1 shows the set-up of an enlarging apparatus incorporating the apparatus according to the invention, Fig. 2 is a top view of the image plane, Figs. 3-4-5 show the principle of controlling the vertical and horizontal position of the raster image of the subject by means of the raster images of the positioned indicators, Figs. 6 and 7 show the electronic circuitry for controlling the vertical and horizontal position of the image of the scanning light source.

As may be seen in fig. 1, an apparatus 10 according to the invention, here represented as an enlarger, comprises a subject holder 11 onto which a subject 12 and two positional indicators 13 and 14 may be located. All subjects in the original holder are illuminated by a uniformly radiating stationary light source 1 5 such as a xenon arc discharge lamp, a tungsten lamp, etc., and their images are projected, via objective 16, into the image plane 1 7 of the deivce. In the image plane 17 the following images appear: the image 18 of the subject 12 and the images 19 and 20 of the positional indicators 1 3 and 14 respectively. The positional indicators 13, 14 are in the form of small continuous tone wedges, so that their light distribution in the image plane has also a wedgelike aspect.

Located under the areas where the images 1 9 and 20 are generated, photosensitive detecting devices 21 and 22 are provided.

The image of the subject is captured by an imaging tube 23 via optical system 24 and semitransparent mirror 28a. The imaging tube 23 converts the optical signal into an electrical one which serves to modulate the scanning light source 25 in intensity. The scanning light source 25 is here represented as being a cathode ray tube although a laser provided with modulation and deflection system may also be used, if desired. It will be clear that the signal generated by imaging tube 23 is of too small amplitude to directly modulate the electron beam of the cathode ray tube 25 and that an amplifying stage 26 is required.

The image appearing on the screen of the cathode ray tube 25 is then, via objective 27 and semi-transparent mirror 28b, again projected in the image plane 17 of the enlarger so that in that plane two superimposed images of the subject and of each of the positional indicators is obtained. The configuarion of the semitransparent mirrors 28a and 28b may be varied. So mirror 28a may be located between the light source 15 and the objective 16 or may even coincide with the side of mirror 28b facing the objective 16.

Between the imaging tube 23 and the cathode ray tube 25, a programmer 26 is provided in order to preliminarly amplify the electric signal generated by the imaging tube 23 and to modify said signal in accordance with preset instructions or programs necessitated by the photographic reproduction process. As such programmer, however, forms no part of the invention, it is not explained in detail.

It will be clear to the skilled worker that the registering of the images generated by the cathode ray tube 25 and those generated by light source 15 may cause problems.

Therefore, the photosensitive detecting devices 21 and 22 which are located in the image plane 17 generate an electrical signal which acts upon the deflection system of the cathode ray tube 25 (here represented by the coils 29 as used in cathode ray tubes with magnetic deflection). In so doing a correction signal, representative for the position of the images in the image plane 1 7 may be generated which is used to control the current through the deflection system, thereby regulating the position of the raster of the cathode ray tube.

The current flowing through the deflection system 29 is generated by a controller 30 which contains the sweep generators for the deflection system of the cathode ray tube 25 and in the mean timbe controls the output of said sweep generators in such a way as to make the image generated by said cathode ray tube 25 to coincide with that generated by the original 12, the xenon arc discharge lamp 15 and objective 16.

In fig. 2 a top view of the image plane 1 7 is illustrated. In the image plane 17 is provided a baseboard 31 onto which a sheet of photosensitive material 32 is positioned. On said sheet are projected the images 18a generated by the subject 12 in the original holder and 18b (shown in dotted lines) generated by the screen of the cathode ray tube 25. Images 18a and 18b form the image 18 referred to in fig. 1. Due to imperfections of the optic systems, vibrations of the support and other causes, images 18a and1 8b do not necessarily exactly coincide during the whole period an enlarging cycle is carried out.

In order to compensate these unwanted effects, the baseboard 31 has also a built-in control system comprising two image sensing devices 33 and 34 onto which the images 19 and 20 of the positional indicators 13 and 14 is projected. The sensing devices comprise photo-electric sensors 22 and 21 such as, in this particular system, silicon photoresistors in bar form. The silicon photoresistors 22 and 23 are located behind a diaphragm 35 respectively 36, the triangular opening of which gradually screening-off or exposing a part of the area of the silicon photoresistors in proportion to the magnifying factor which is set for a particular enlarging cycle.

So when the magnifying factor is small, only a small area of the silicon photoresistors 21 and 22 is exposed, and vice versa.

Fig. 3 shows how the composite image 18 of the subject and that of the positional indicators 19 and 20 are formed in the image plane.

The image of the positional indicators 19 and 20 show a wedge-like distribution as indicated, O being the area of the wedges showing a low density and 1 being the area corresponding with a high density. Preferably the density ranges from 0 to 3. The image of the cathode ray tube is linewise formed according to the scan lines 37 from which only a few have been shown for the sake of clarity.

It will be clear that each time the spot of the cathode ray tube performs a scan line in horizontal direction, and the image 20 of the positional indicator for controlling the vertical position of the image 18 is sensed by the vidicon tube 23 (in fig.

1) a voltage pulse of square shape is generated.

On the contrary, in the case the image 19 for controlling the horizontal position of the image 1 8 is sensed, a saw-tooth or sweep-like voltage is generated.

The nature and shape of the generated voltages is illustrated in figs. 4 and 5.

So, fig. 4 shows the voltage configuration, generated at the output of the sensor responsible for the control of the vertical position of the image displayed by the cathode ray tube. Said voltage is composed of a DC-voltage V, onto which a varying one is superimposed. The varying voltage is composed of a plurality of pulses ar, a2...

the magnitude of which being in direct or in inverse proportion with the density of the section of the positional 20 which is scanned. The voltages delivered by the sequence of pulses is integrated so that finally a sweep voltage is obtained lasting, according to a realistic model, during 1/4 of the raster period when the latter equals the European Television standard.

For what concerns the signals controlling the horizontal position of the image 1 8, these are represented in fig. 5. For a normal line period of 64 ssusec the image 20 of the positional indicator 13 (see fig. 1) is scanned in about 20 Msec in lengthwise direction. Due to the fact that the positional indicator has a density distribution corresponding with a wedge a sweep-like voltage is obtained upon scanning, which is added to the voltage V2 which is generated when the photosensitive detector concerned is not exposed by the spot of the cathode ray tube.

The signal processing necessary to correct the normal X-Y deflection of the spot on the screen of the cathode ray tube, if necessary, is schematically represented in figs. 6 and 7.

So fig. 6 shows the scheme of the circuit 40 for controlling the vertical position of the light spor on the cathode ray tube.

The circuit comprises an input terminal 41 to which the signal Vv represented in fig. 4 is applied.

The signal Vv which is a mixture of a DC-signal and a sweep-like signal is integrated by integrator 42.

This integration occurs during a preset "window" which is determined by the integrator start/stop 43. The presetting of the "window" occurs via the intermediary of a pulse detector 44 and a pulse counter 45. The pulse counter 45 is therefore set on a pre-determined number of voltage pulses which is function of the size and the enlarging factor of the final reproduction. When the preset number in the pulse counter 45 have been detected, the integration is stopped.

The circuit 40 also comprises two sample and hold stages 46 and 47 the output of which being connected to a comparator 48 which directly drives the electronic circuitry for the vertical deflection 49. The electronic circuitry 50 for controlling the sample and hold functions is also controlled by the pulse couter 45.

The final result will be that the electronic circuitry for the vertical deflection 49 delivers a varying DC-signal to the DC-bias circuit 51 for the vertical deflection which is applied to the deflecting coils of the cathode ray tube (not shown) together with the sweep voltage generated by a common sweep generator 52.

For what concerns the control circuit for the horizontal deflection of the spot of the cathode ray tube, a more or less analogous fashion may be applied. The circuit 60, illustrated in fig. 7 comprises, as in the one of fig. 6 an input terminal 61 at which the signal VH illustrated in fig. 5 is applied and an integrator 62 connected to sample and hold stages 66 and 77. Instead of a pulse counter as required for the vertical deflection, the circuit 60 is directly synchronized by the line synchronization signals applied at terminal 64, so that the supplementary pulse counter and pulse detector may be omitted.

The circuit in addition comprises an integrator start/stop switch 63, an electronic circuit 70 for controlling the sample and hold stages 66 and 77 and a comparator 68 driving the electronic circuitry 69 for the horizontal deflection delivering a a varying DC-signal to the DC-bias circuit 71 for the horizontal deflection, the said DC-signal being applied to the deflecting coils of the cathode ray tube together with the sweep voltage generated by line sweep generator 7a.

The device according to the invention works as follows: A subject 12 is placed in the subject holder 11 and the uniformly radiating light source 15 is switched on. The image 18 of the original is projected into the image plane 17.

Simultaneously, the images 19 and 20 of two positional indicators 13 and 14 which are also located in the subject holder 11 are also projected in the image plane 17. Then the position of the photosensitive detecting devices 21 and 22 is adjusted so that the images 19 and 20 of the positional indicators 13 and 14 give rise to a maximum signal output of the photosensitve detecting devices. This event may be controlled by means of a measuring instrument (not shown).

Then, the imaging tube 23, the scanning light source 25 (here represented as a cathode ray tube) and the electronic circuitry linking them is energized and the image appearing on the screen of the cathode ray tube is also projected, via mirror 28b, in the image plane 17 of the device.

The electronic circuitry 30 which is controlled by the photosensitive detecting devices 21 and 22 maintains the raster image of the positioned indicators 13 and 14 appearing on the screen of the cathode ray tube 25 in coincidence with the image of the positional indicators 13 and 14 as generated by the uniformly radiating light source 1 5. By the fact that two signals of analogous shape are added to each other, the electronic circuitry is capable of controlling registration.

Indeed, should one image be displaced relative to the other, the maximum value of the signal would be lowered, which event is detected by the sample-and-hold components forming part of the electronic circuitry. As a consequence, a correction signal is generated which restores the coincidence between the images concerned.

From the foregoing, it will be clear that the principles of the invention may be applied to other configurations than the one described in detail.

The unit may be adapted for the copying of reflex originals, for using transparent photosensitive materials, etc.

If desired the cathode ray tube may be replaced by a laser or other scanning light source, etc., and the photosensitive recording medium, apart from being a carrier onto which one or more layers of light-sensitive material is coated, may be a matrix or mosaic of photodiodes or analogous devices, etc.

Instead of controlling the position of the raster image by means of varying currents in the deflection system of the scanning light source, other methods, such as the control of the position of mirror 28b may also be applied. The last method is of interest when a slow scan video system is used for modulating the spot of the cathode ray tube.

It will also be clear that for certain types of positional indicators, e.g. those showing a cheque- board pattern, the photosensitive detectors may be located in such a way that in case correct registration between the two images exists, no light falls onto them and that the electronic circuitry may be adapted so that it is capable of detecting the absence of light and of correcting the possible misregistration in function of minimum signals generated by said photsensitive detectors.

Claims (16)

1. A method of exposing a light-image-receiver to a subject to be recorded, which method comprises the steps of overall illuminating the subject, projecting light from the subject to form a light-image (hereafter called "fixed image") on said receiver, scanning the fixed image, or a correspondingly secondary light image simultaneously derived from the illuminated subject, to produce image signals, modulating such signals in accordance with predetermined criteria for producing by means of a scanning light source a raster image of the subject which is projected onto said receiver in register with said fixed image, characterised in that:: first and second positional indicators are located so that fixed and raster images of each of said indicators are projected simultaneously with the formation of the said fixed and raster images of the subject to be recorded, first and second light-sensitive detectors are provided capable of receiving said images of the first indicator and of the second indicator respectively, said indicators are of such form that the output signals of the detectors are a measure of the registration error (if any), in two mutually perpendicular directions, between the fixed and raster images of such indicators, and output signals from such detectors are used to bring the raster images of the indicators on the detectors into registration with the fixed indicator images thereon and consequently to establish registration of the fixed and raster images on the receiver of the subject to be recorded.

2. A method according to claim 1, in which the positional indicators are in the form of continuous tone wedges.

3. A method according to claim 1 or 2, in which the output signals from the light sensitive detectors are used to bring the raster images of the positional indicators into positions in which the light flux on the detectors is at a maximum.

4. A method according to any preceding claim, in which the mean output light intensity of said scanning light source is equal to that of the light by which the subject and the positional indicators are illuminated.

5. A method according to any preceding claim, in which said raster images are formed by a cathode ray tube and the output signals from the light-sensitive detectors are used to influence the deflection system of the tube.

6.An apparatus for exposing a light-imagereceiver to a subject to be recorded, such apparatus comprising a subject holder, a light source for uniformly illuminating a subject when it is in position in such holder, means for optimally projecting a light image (hereafter called "fixed image") of said subject onto an image plane in which a said image-receiver can be located, means including a scanning light source for simultaneously producing a modulated raster image of said subject by electrical signals which are derived from said fixed image or from a corresponding secondary light image of the subject and are modulated in accordance with predetermined criteria, and means for projecting such modulated raster image onto said plane in register with said fixed image, characterised in that there is a means for holding first and second positional indicators at positions such that they will be illuminated by said light source simultaneously with said subject and such that fixed and raster images of such indicators will be projected simultaneously with the fixed and raster images of the subject and by the same means as that used for forming such subject images, there are first and second light-sensitive detectors located for receiving the projected fixed and raster images of a said first positional indicator and of a said second positional indicator respectively when such indicators are held as aforesaid, such detectors serving, if appropriate positional indicators are used, to yield output signals which are a measure of the registration error (if any), in two mutually perpendicular directions, between the fixed and raster images of such indicators, and said detectors are connected to means for influencing the positions of the projected raster images of the positional indicators so that said output signals will maintain the fixed and raster images of the positional indicators, and consequently the fixed and raster images of the subject, in register.

7. An apparatus according to claim 6, in which said scanning light source is a cathode ray tube.

8. An apparatus according to claim 6 or 7, in which positional indicators in the form of continuous tone wedges are present in said indicator holding means.

9. An apparatus according to claim 8, in which said wedges have a density range of O to 3.

10. An apparatus according any of claims 6 to 9, in which said light-sensitive detectors are silicon resistors in bar form.

11. An apparatus according to any of claims 6 to 10, in which displaceable diaphragms determining exposure windows are mounted in front of said light-sensitive detectors.

12. An apparatus according to claim 11, in which said windows are triangular.

13. An apparatus according to any of claims 6 to 12. in which the said light-sensitive detectors are connected to means for influencing the raster position of said scanning light source.

14. An apparatus according to claim 13, in which said output signals from the light-sensitive detectors influence said raster position by influencing the deflection system of the scanning light source.

15. An apparatus for exposing a light-imagereceiver to a subject to be recorded, such apparatus being substantially as herein described with reference to the accompanying drawings.

16. A method of exposing a light-imagereceiver to subject to be recorded, substantially as herein described with reference to the accompanying drawings.