GB2185166A - Imaging apparatus - Google Patents

Abstract

The sensitivity of the image sensor is controlled differently respectively during a first and a second plurality of field periods interspersed, for example alternating, one with another. The sensitivity during the respective periods may be respectively high and low so that, in effect, a greater dynamic intensity range of the sensor is obtained. The video signal portions associated with the respective periods may constitute interlaced fields on the display or may be displayed on two different monitors or in different portions of the screen of a single monitor. For missile guidance, the higher sensitivity signals display a relatively dull target while lower sensitivity signals show only the missile flare and this avoids confusion by cloud edges and the like. A CCD sensor 3 provides rapidly chargeable sensitivities, the charges from the radiation-sensing the "integration region" being clocked into the "store" by means of a pulse generator 7 which controls the timing of reverse and forward transfer of charges in such a way as to achieve control of sensitivity. Odd and even control loops 15, 16 provide for example higher sensitivity for odd lines than for even lines of the scan. <IMAGE>

Description

SPECIFICATION Imaging apparatus This invention relates to imaging apparatus and also to missile guidance systems comprising such apparatus.

More particularly, but not exclusively, the invention relates to a missile guidance system comprising a camera forforming an electrical video signal representative of a viewed scene containing a target and the flare of a missile being guided towards the target, the video signal being passed to electronic guidance apparatus which guides the missile within the field- of-view of the system, and also being passed to a display monitor so that an operator can maintain the system aimed at the target. The target itself and the viewed scene in general may be quite dull whilethe missile flare will usually be very bright. Also, the scene may contain some discrete fairly brightfeatures, notably cloud edges and the like.In orderto give the operator the best possible view ofthetarget on the display monitor and to provide the best possible signal-to-noise ratio ofthe system in the face of the constant, ie sensitivity independent, base level of noise generated within the image sensor of the camera, the camera sensitivity is best adjusted to be as high as possible by reference to the general brightness of the viewed scene.However, because of the limited dynamic range of available image sen- sors,thiswill almost certainlymeanthatthe missile flare image is well above the saturation limit ofthe camera so that, as far as the guidance apparatus is concerned, the apparent brightness of the aforementioned fairly bright scene features may approach or even equal that of the missilefiare and hence maybe confused with it. Ideally, the guidance apparatus should 'see' little or nothing more than the flare itself but if the camera sensitivity is reduced, eg if its aperture is stopped down, to achieve this then the operator will see little or nothing ofthetarget.

Although difficult, it may be possible to choose some intermediate value of sensitivity which gives a reasonably good target view on the monitor and reasonabiy good operation of the guidance apparatus; particularly if say the signal fed to the monitor is amplified. This is however a compromise solution and it does not give the best signal-to-noise ratio of the system. Another solution is to provide two cameras, one for the monitor and one for the guidance apparatus, and to set the respective camera sensitivities as appropriate. This may be expensive however, and it also introduces the problem of setting and maintaining the two cameras accurately boresighted one with the other.

According to one aspect of the invention, there is provided imaging apparatus inciuding an image sensor, sensor control means for causing the sensor to make available an electrical signal representative of a continuing sequence of raster scans of a scene imaged by the sensor, and sensorsensitivitycontrol means which is operable to control differently the sensitivity of the sensor respectivelywhilstthe sensor is forming the signal portions ofafirstplura- lity of said raster scans and whilst the sensor is for- ming the signal portions of a second plurality of said raster scans which are interspersed with the first plurality of raster scans.

Byway of example, the image sensor maycom- prise a frametransfer charge-coupled area imaging array with an anti-blooming structure within the integration region ofthe array or in which the ends of the shift register columns in the integration region remote from the storage region are terminated by signal sinks. In this case, sensitivity control of the sensor may be achieved by reverse clocking the integration region at some controlled instant during the integration so that the image representative signal pattern formed up to thattime runs out through the signal sinks or piies up atthestorage region remote end of the integration region and is hence drawn off by the anti-blooming structure. The pattern formed after this reverse clocking operation is then transferred to the storage region and read out in the normal way.This controllably reduces the effective integration period within each frame period ofthe sensor and hence controls its sensitivity electronically.

According to another aspect of the invention, there is provided a missile guidance system comprising imaging apparatus as described above, display monitor means and an electronic missile guidance means, said sensitivity control means being operable to control the sensitivity ofthe image sensor such thatduring each ofthefirst plurality of raster scans the image sensor sensitivity is controlled in dependence upon the general brightness of the viewed scene, the portions of the electrical signal which correspond to these scans being passed to the display monitor, and such that during each ofthe second plurality of raster scans the image sensor sensitivity is controlled in dependence upon the re quirements ofthe guidance means.

For a better understanding ofthe invention, reference will now be made, by way of example, to the accompanying drawings, in which: Figure lisa diagrammatic plan view of a frame transfercharge-coupled area imaging array, Figure2 is a simplified circuit diagram of imaging apparatus including the array of Figure 1, Figure 3a is a timing diagram associated with the normal operation ofthe Figure 1 array, Figure 3b is a timing diagram associated with the operation ofthe Figure 1 array when it is incorporated in the apparatus of Figure 2, Figure4is a simplified circuit diagram of a sensor sensitivity control loop,two ofwhich are used in the Figure 2 apparatus, and Figures 5and 6 are timing diagrams associated with respective modes of operation of the Figure 2 apparatus.

As will be appreciated, the invention is not only limited in application to the form of missile guidance system referred to earlier, or even to missile guidance systems at all. Rather it is applicable in a number of imaging situations where a scene having a wide dynamic intensity range is to be imaged.

Television systems as normally envisaged have a limited dynamic range (approximately 50dB fora vidicon camera and upto 65dBfora solid state image sensor camera), which in certain applications is insufficient to provide an operatororsystem with the necessary scene information they require.

An example ofsuch a situation is a scene, poorly lit, which includes a dull target and a missile carrying a brightflare. The dynamic range of the scene is too great to enable clear images of both the target and missile to be generated simultaneously. Ifthe camera sensitivity is adjusted to provide clear images ofthe target, the missile flare will overload the image and produce distortion, but if the sensitivity is reduced to correctly image the missile flare, little or nothing will be seen ofthetarget.

In an example of imaging apparatus to be described herein with reference to the accompanying drawings, special electronic control is provided to a solid state image sensor ofthecharge-coupled device (CCD) variety, although similar techniques could be applied to the sensors of other varieties, to permit itto operate on alternatefieids of video with the camera sensitivity adjusted high then low or according to the specific requirements of any given application.

To achieve satisfactory performance it is preferable to adjustthe sensitivity of the sensor, notthe following video amplifiers because it is the sensor itselfwhich defines the system's signal-to-noise ratio for dynamic range, which may be extended by the amount of gain control appliedtothesensor.

It is preferred that solid state sensors, otherthan vidicons say, should be used because such solid state sensors may be more capable of a fast rate of change of sensitivity, which will permit,forexample, the dull targetto be correctly exposed on odd fields ofthetelevision signal and the bright missile flare to be correctly exposed on the even fields of this same television signal. In a practical missile guidance system, target video would probably be transmitted to an operator and missileflare electric images to an electronic tracking system. It should be noted that no collimation or boresight errors exist because a single sensor is employed.

In another example where it might be required to image a scene of very wide dynamic rangeforsurveil lance purposes it would be possible by doubling the field rate to display two images simultaneously on one CRT, the upper image being obtained for example with high sensor sensitivity and therefore providing full shadow detail whilstthe lower image might be obtained with low sensitivity and therefore provide the necessary image quality from the sky.

As stated above, the described embodiment makes use ofthe zero lag characteristics of sensors of the CCD variety, the ability to move charge atwill in a three-phase COD device and special readout techniques which permitthe sensorto integrate useful wanted charge for periods which may be varied from a full field to a period which may be small compared with a full field, and which may be varied quickly, either under manual or automatic control.

The mode of operation is described in conjunction with the RCA SID403 which is a frame transfer COD with anti-blooming drains, although other CCDs could be employed. The normal mode of readout of such a device is well known and is covered in detail by its manufacturer's data sheet. The described specifictimings of clocking wavefqrms aretypical.

Forfurther specific applications, different values might be employed.

The basic forms of sensitivity control are disclosed in Patent Application No. 8127959. The technique employed to demonstrate will be described in con- junction with Figures 1 to 6, but could also function satisfactorily with other control techniques described in Application No. 8127959.

Referring now to Figures 1 and 3a ofthe accompanying drawings, photon generated charge is permitted to integrate in region I of the COD area image sensorfor a full field period and is then transferred into region S from where it is slowly read out via region Bwhilstthe subsequentfield of charge is integrating in region I. This mode of operation provides the sensor with unvarying sensitivity.

Figure 3b illustrates a typical mode ofoperation according to the present embodiment ofthis invention. Photon generated charge is initially permitted to integrate as in Figure 3a, but after a period, which depends upon the sensitivity required and which is usually less than one field period, charge in region I is reverse clocked away from region S. This clocked charge accumulates atthe lower extremity of region I where it is removed by the anti-blooming structure of the sensor. In other sensors the charge removal could be by other means; a diode diffusion exists in the GEC sensor, for example. Having removed this charge by reverse clocking, integration is permitted to continue until the normal forward transfer re- gion S is initiated atthe end of a field period.Itwill be seen from Figure 3b that by varying the timing of reverse transfer, the usual integratiqn period may be changed from field to field and therefore the sensitivity may be changed from field to field. In the example shown the sensitivity of odd fields is low, whilst that of even fields is much higher and is slightlyvariable.

As shown in Figure 2, the embodiment may com- prise a master clock signal generator 1 which feeds a three-phase transfer signal generator 2 producing the signals for transferring the video signal line-byline from the output register ofthe image sensor 3to a video amplifier4. The master clock signals are also fed via a divider5 to a first input of a changeover switch 6, to a further switch 7 and to a transfer of readout pulse generator 8. The output pulses formed by generator 8 are supplied to a second input of switch 6, to a sequence logic unit 9 and a forward or reverse fasttransfer pulse generator7. The sequence logic circuits of 9 control the operation of switches 6 and 7 on the basis of the sensitivity control signals from logic units 15 and 16 and fieldtimingfocus, pulse generator 8. The sensitivity control signals are received alternativelyfrom two control loops 16) which are identical except for their amplitude detectors.

Loop 1 (odd field) responds to small intense sources, such as a missile flare, and is matched to the resolution limit ofthe CCD. Afast peak detection is therefore used as the oddfield amplitude detector 11.

Loop 2 (even field) responds to the scene average and uses an amplitude detector which integrates the full field readout 14. The time constant of the detector is matched to the number of lines being read out on the field of interest (134 typical, partial readout, 256 full readout).

The system diagram for a singleAGC loop is shown in Figure 4, and the sequencing fortwo loops with 100Hz partial field rate in Figure 5.

The control element is a pulse width modulator which compares a ramp voltage with the output of an integrator which d.c. level, following update, is prop ortionedtothe next required COD integration (orexposure) period. When the ramp and the integrator output are equal, a change of state occurs. This change causes the sequence logictoinitiatea reverse clock during integration of the next field. The required integration then takes place during the remainder ofthatfield. The detected video Vp forms an inputto the loop and it passes initially through a gain element k, used to set the level of video output from the system.The gain corrected video kVp is inputto the analogue computational block which takes as its input kVp1 and a voltage Vj which represents the actual integration period used in the previous field. Vj is derived by storing the previous updated integration voltage on a sample and hold, and subtracting from it a voltage which is proportional to the reverse clocking period.

An analogue dividercomparesthetwo inputs and calculates from them the change in integration time needed to achieve the required video level on the next field. Since charge collected (and hence signal level) is proportional to integration time, then div ision of a reference level by kVp will produce a ratio which when multiplied by Vi, isthetransferfunction shown, results in the required correction voltage VR for the next integration. During the update period the integration output changes by the amount calculated ie VR Then VCN+1 = VCN + VA,VR can of course be positioned on negative depending upon whether the previous field was under or over exposed.Following the integrate period, the new value of Vc is stored by the sample and held readyforthe next iteration.

Figure6showsthe sequencing fortwo loopsoper- ating with a 50Hz full field rate. In this application two field stores would be needed to store and repeat each video field through separate monitors in order to achieve temporal separation of the alternate video outputs.

The outputs of switches 6 and 7 are fed to respect ive three-phase transfer signal generators 12 and 13 producing signals for controlling transfers within the storage and integration regions Sand I respectively of the sensor3. Following a transfer of photo-signals from the integration region I to the storage region S of the sensor, the logic circuit 9 opens switch 7 and causes switch 6 to pass slow frame transfer pulses from generator 8 to three-phase transfer signal gene rator 12 and thereby causes the photo-signals in the storage region to be transferred line-by-line into the output register R.After a portion of the overall field period, controllers 15 or 16, depending upon whether the field is odd or even, causes logic circuit 9 to control switch 7so as to give a fast reverse transfer of the photo-signals then present in integration region I. Sensor3 is of the aforementioned kind in which the integration region I contains an antiblooming structure into which the reversetransferred photo-signals are dumped, then lost by recombination in the bulk of the silicon sensor.

Thereafter switch 7 is again opened and then, at the end of the field period, is closed to give a fast forward transfer of the contents ofthe integration region.

At the same time, switch 6 is changed over to pass to the generator 12, the fast transfer pulses from div ider S,so that the photo-signals from integration region I are properly sequenced into storage region S.

During the next field period the process is repeated except that the output from control logic 15 or 16, whichever was not employed during the previous field, is then used to control the sequence logic unit 9 to set the useful integration time and therefore sensitivity ofthe sensor.

As will be a ppreciated, by way of example, one aspect of what has been described could be defined as solid state imaging apparatus comprising a radiation sensitive memberfor scanning radiation energy received thereby to form radiation representative electrical signals, control signal forming means for causing radiation representativesignalsformed by said memberto be transferred to signal terminal means ofthe apparatus at successive readout times separated by predetermined intervals, and sensitivity control means for ensuring that signals trans- ferred as aforesaid to said signal terminal means over alternate readout periods are representative of radiation energy assumed by the radiation sensitive member even though the range of energy levels falling upon the energy sensitive member may normally exceed the dynamic range ofthe energy sensitive member.

A second aspect could be defined as solid state imaging apparatus which may be used for example for control or tracking applications, which as a result of the increased dynamic range of imaging may permit two or more tasks to be conducted simultaneously, forexampleimaging a bright missile flare and a poorly illuminated target, and which because ofthe factthat only one sensor is needed forthe multiple tasks, does not suffer from any of the problems of optical boresighting or collimation.

Athird aspect could be defined as apparatus as above but which may be operated at various televi sion field and line rates, including techniques to partially readout the solid state image sensor in order that two halffields of video obtained at two sensitivity levels may be displayed on television monitoring apparatus simultaneously.

It is not essential that the sensitivity of alternate fields is differently controlled. Someothersequence could be chosen. For example, in the missile guidance embodiment as described, it may be that the guidance unit does not need to be updated atalter nate fields so several fields could be taken atthe higher sensitivity appropriate to the display monitor for each one field taken at the lower sensitivity appropriate to the guidance unit.

Claims (8)

CLAIMS 1. Imaging apparatus including an image sensor, sensor control means for causing the sensor to make available an electrical signal representative of a continuing sequence of raster scans of a scene imaged by the sensor, and sensor sensitivity control means which is operable to control differently the sensitivity ofthe sensor respectivelywhilstthe sensor is forming the signal portions of a first plurality of raster scans and whilstthe sensor is forming the signal portions of a second plurality of said raster scans which are interspersed with the first plurality of raster scans. 2. A missile guidance system comprising imaging apparatus according to claim 1, display monitor means and an electronic missile guidance means, said sensitivity control means being operable to control the sensitivity of the image sensor such that during each ofthe first plurality of raster scans the image sensor sensitivity is controlled in dependence upon the general brightness ofthe viewed scene, the portions of the electrical signal which correspond to these scans being passed to the display monitor, and such that during each ofthe second plurality of raster scans the image sensor sensitivity is controlled in dependence upon the requirements ofthe guidance means. 3. Imaging apparatus substantially as herein before described with reference to the accompanying drawings. 4. A missile guidance system substantially as hereinbefore described with reference to the accompanying drawings. Amendments to the claims have been filed, and have the following effect: New ortextually amended claims have been filed as follows : CLAIMS

1. Imaging apparatus including an imagesensor, sensor control means for causing the sensor to make available a video signal comprising successive field portions representative of a scene imaged bythe sensor, and sensor sensitivity control means which is operable to control differentlythe sensitivity of the sensor respectivelywhilstthesensor is forming a first plurality of said portions and whilst the sensor is forming a second plurality of said portions which are interspersed with the first plurality of postions.

2. Apparatus according to claim 1, wherein the portions of said first plurality alternate with those of the second plurality.

3. Apparatus according to claim 1 or2, herein said image sensor is a charge-coupled area imaging device and said sensitivity control means controls the sensitivity ofthe sensor by varying the effective integration time period of the sensor.

4. Apparatus according to claim 1,2 or 3, wherein said sensitivity control means comprisesfirstand second automatic sensitivity control loop circuits, and switch means for rendering the first loop circuit effective during formation of each of said field portions of said first plurality thereof and for rendering the second loop circuit effective during formation of each of said field portions of said second plurality thereof.

5. Apparatus according to claim 1 wherein said loop circuits comprise respective video signal amplitude detectors, one producing a signal indicative of the average brightness of said scene and the other a signal indicatie of a peak brightness ofthe scene.

6. A missile guidance system comprising imaging apparatus according to claim 1, display monitor means and an electronic missile guidance means, said sensitivity control means being operable to control the sensitivity of the image sensor sensitivity is controlled in dependence upon the brightness ofthe viewed scene, the portions of the video signal which correspond to said first plurality being passed to the display monitor, and such that during each of the second plurality of portions the image sensor sensitivity is controlled in dependence upon the requirements ofthe guidance means.

7. Imaging apparatus substantially as here- inbefore described with reference to the accompanying drawings.

8. A missile guidance system substantially as hereinbefore described with reference to the accompanying drawings.