GB2141004A - Streak video camera - Google Patents

Abstract

A streak camera comprises a video camera 1 having a substantially reduced field scan (eg. to 1% or less) so that only a slit image falling on the imaging surface 8 is converted to electrical signals. The scan line is fixed but may be rotated by applying the line scan signal to both field and line coils. The streak camera can be combined with a direct display system for an instantly available image, or with a recording system for later analysis. This is useful for recording high speed events such as race photo-finishes or the detonation of explosives, where rapid access to a continuous record of an event is required. <IMAGE>

Description

SPECIFICATION Streak video camera The present invention relates to the art of streak photography, such as is usedforexample in photofinish photographs of races, and in particular to a camera for use in streak photography.

One normally expects a photograph to portraytwo spatial dimensions (height and width) ofthe subject at a given instant in time. Acine film providestheillusion of continuous movement by means of a series of such photographstaken at regular intervals.

In some applications a conventional two-dimensional record is unnecessary andthe discontinuous nature ofcinematographic recording may be undesirable. In such cases a streak camera may be used to provide a continuous record of movemment in one axis only. The resultant streak photograph shows time as one dimension, leaving only one dimension for spatial information.

A description of the theory and practice of streak photography may be found in: "Applied Photography" byArnold, Rolls and Stewart; publisher: The Focal Press; 1971; ISBN 0 240 507231 sections:-2.2; 11.1; 11.4; 41; 12.3.4.

Existing streak camera systems may have one or more of the following disadvantages: a) Rapid access processing of the film to record the image is usually restricted to monochrome materials, and fast development generally entails expensive and bulky equipment or riskof processing errors or physical damage due to manual handling.

b)The recording time is limited bythefilmcapacity ofthe camera, and is typicallyofthe order of seconds.

c) Access to the camera is necessary for film changing.

This may greatly reduce the number of events which may be recorded in a given time.

d) Modification of the photographic image to assist analysis is not easy under field conditions.

e) The time resolution ofthe image is fixed atthe time of recording. Analysis of the overall pattern of the event may require a much lowertime resolution than that needed for some detailed aspects ofthe event.

An object of the present invention is to minimise or nullify the disadvantages inherent in known streak cameras which use photographicfilm as the recording medium. This is done by using a video camera instead offilm.

According to the presentinventionthere is provided a streak camera comprising avideo camera having a substantially reduced field scan so asto allow a slit image to be converted to electrical signals.

The video camera may be arranged to have a substantially stationary line scan so as to provide a horizontal slit image. Alternatively, the video camera may bearranged to have time base signals connected to the field coils so as to provide a vertical slit image.

As a further alternative, the video camera may be arranged to have the line time base signals connected to the line and field coils with a division thereof to provide a slit sampling image axis otherthan vertical or horizontal.

The streak camera may be used in conjunction with any suitable display system or recording system as required forthe particular application to which the streak camera isto be put, and such combined systems are also within the scope ofthe present invention.

In effect, the video camera has its circuits so arrangedthatthere is no field deflection ofthe line scan. The stationary line scan so generated performs a function analogous to the image slit of a photographic streak camera.

The signal from this video camera may be recorded by any suitable means (e.g. video tape) or inputto any suitable display device (e.g. storage tube monitor).

The video signal is displayed or printed as hard copy in rasterform; changes in the sampling plane arethus shown asvariations along the axis ofthe linescan, and the time dimension of the event is represented by the axis of the field deflection ofthe display.

In this application all references to camera tubes refer to the video imaging tubes used in conventional video cameras to convertthe optical image to an electrical signal. Unless otherwise specified all such references are to be taken to include functionally similar devices using different technologies, such as solid state imaging arrays (S.S.l.'s). Similarly, references to monitor display tubes shall be extended to functionally similar devices such as liquid crystal and plasma displays.

A description of the design and operation of conventional raster scan video camera and monitor system including an illustrated explanation ofthe scanning process is given in: "CLosed circuittelevision fortechnicians, part 1, industrial & commercial" by K. J. Bohlman, Pub. N.

Price; l.S.B.N. 0835801002.

A brief description of charge coupled devices as used in solid state imaging arrays may be found in the British Journal of Photography, 30 Oct., 1981.

Reference is now made to the accompanying drawings, in which: Figure 1 illustrates the operation of a streak camera according to the invention; Figure 2 illustratestheformation of an imagebythe streak camera of Figure 1; Figure 3 is a block diagram of the main functional circuit elements of a streakvideo camera according to the invention; Figure 4 shows sample wave forms for video output voltage and line coil current; and Figure 5 illustrates a time base compression circuit which may be employed with the streak camera of the invention.

As shown in Figure 1, a video camera tube 1 has a fixed scan line 2 and, in association with a lens 3, provides a sampling plane 4. AsubjectS is shown with a direction of subject motion 6. The line scanned at the subject plane is thus shown at7 and an image is formed on the imaging surface 8 ofthe tube.

In orderto produce an electronic analogue ofthe fixed image slit plus moving film used in a photo graphic streak camera as described in "Applied Photography", mentioned above, the present streak video camera uses a video camera tube 1 in which the line scan is not deflected byfiled coils as in a conventional raster scan video camera, described in "Closed circuit television fortechnicians" mentioned above. This fixed scanning line 2 acts as an analogue oftheimageslitofa photographicstreakcamera as shown in Figure 1 ofthe drawings. The repeated scanning ofthe line provides a sequence of one (spatial)dimensional records ofthe sampling plane 4.

When displayed as a rasterthis sequence of line images forms a streak image in which the scan line axis provides spatial information and the axis perpendicularto it acts as the time base, as shown in Figure2 ofthe drawings (see raster image 9).

It may be found worthwhile in some applications to align a physical slit mask in front of the fixed scan line in orderto obtain a narrower or sharper-edged scan line.

Apart fro this slit mask, if used, the present streak video camera is optically similarto a raster scan video camera. The image tube may be any of those available for conventional video cameras, including colourtube systems and specialised sensors (e.g. infra-red).

However it must be understood that a tube which exhibits image streaking or ghosting at the required operating light level of the system is likely to give a worse degradation of image quality in a streakvideo system.

In general termsany lens oroptical accessory suitable for a conventional video camera with the sameimagetubediameterwill beavailableforstreak use.

The camera may be either a dedicated streak mode unit with no provision for raster scan, or be switchable between streak and raster modes.

The camera circuits, otherthan those involved in the control of the tube scan (i.e. the line and field sync., blanking and timebase circuits) may be any the circuits used in raster scan video cameras, such as those described in "Closed circuittelevision for technicians" mentioned above, except insofar as modifications to the line and field circuits may affect them.

The recorded time resolution of a video streak camera may be defined as the time from the start of one scanto the start ofthe next. This is equal to the reciprocal ofthe scanning frequency. This is analogous to the time resolution of a photographic streak camera which isthetimefortheslit imageto move a distance across the film equal to its own width; see "Applied Photography", section 11.1.2.

Records made at high scan frequencies of relatively slow events may result in changes between scans being imperceptible to the eye, although such changes may be measurable by means of electronic image processing methods. In such cases the time resolution ofthe displayed image may be reduced by scanning frequency division in orderto display only every n'th line where is the division constant.

The sequential nature of the scanning process in vacuum tube type video cameras means that a certain degree oftime parallax will exist between points along the scan line. To keepthistime parallaxto a minimum the active scan time should be as short as practicable. Some SSI technologies may permit the output of all the elements in a line arrayto be latched simultaneouslyforsubsequentscanning. Thus,there would be no time parallax in such systems.

Thecircuits herein described are for use in avacuum tube type camera, or in the control unit of a camera which uses external drives. Analogous circuits will be required for cameras using SSI orotherimaging devices.

A block diagram ofthe main functional circuit elements ofthe present streakvideo camera is shown in Figure 3 of the drawings. The following components are shown: syne pulse generator 11 which provides line blanking pulses 12 and line sync pulses 13, line timebase 14, camera tube 15with lens 16, video pre-amp 17, syne and blanking mixer18andcomp.

video output 19.

The fixed line scan requiredfor streak mode recording may be obtained in a conventional video camera bydisabling the field deflection, field sync.

and field blanking circuits, or by intercepting their outputs. In some casesthis rnay entailfurther modification if other camera functions depend on these circuits.

Astandardvideo camera so modifiedwould provide a scan frequency of 15,625 Hz (UK 625 line, 25Hz raster system) with an active linetime of 52 microseconds. Where a different scan frequency or line time may be required it will be necessaryto modify or replace the line circuits.

Depending on the requirements ofthe particular application the linefrequency may be controlled in one oftwo principal ways:- a) by the provision of one or more accuratelyfixed frequency references (e.g. crystal oscillator) to control the scanning rate either directly or by frequency division; b) by means of a continuouslyvariable frequency source, (e.g. an astable multivibratorwith potentiometer control).

Either or both of these may befitted in the same camera if required.

The line timebase generator may be used unmod ifiedifitsoutputwaveformcan match the line scan frequency in use, and if there is no requirement for a non-standard waveform. A modified line timebase generatormay provide.

a} a fixed active linetime short enough to be suitable for all the line scan frequeneies possible on the camera; b) an active line time which is proportional to the line cycle time (=reciprocal of line frequency); c) a waveform which provides a period during each line scan cycle in which non-image data may be inserted into the video signal.

Sample waveforms are shown in Figure 4 of the drawings. Figure 4A shows the video output voltage and line deflection current plotted againsttimefora standard video line scan. Figures 4B correspondingly shows the voltage and current waveforms for combined video and data signals.

For some applications it may be convenient to direct the line waveform drive to the field coils (if fitted) of the camera tube in order to rotate the sampling plane through 90 degrees without physically moving the camera. By suitable division of the line drive current between the line and field deflection coils it is possible to align the sampling plane axis in any orientation, and by applying a preset bias to the drive currentthe scan line may be aligned to any position on the face of the tube. These controls permitthe sampling plane to be adjusted remotely if required.

The linetimebase waveform ofthe camera must be matched by the monitor linetimebase if an analogue monitor display isto be correct. This matching may be by: a) driving the camera and monitorfrom a common timebase; b) manual adjustment of camera and monitor presets; c) waveform identification data automatically trans- mitted in series or parallel with the video signal.

Obviouslythis matching is facilitated by the use of a standardised series of timebase waveforms.

Optional deviceswhich may be included in a system incorporating the present streak video camera include the following:- 1) non-image sensors 21 data recording interfaces 3) recorders 4) time base compressor 5) analogue displays 6) digital image handling systems.

In more detail these devices may operate as follows.

1) Sensors of non-image data which may be of value in the interpreting of an event include analogue transducers for converting continuously variable phenoma into analogue electrical signals (e.g. temperature, windspeed); discrete event sensors and counters which detect specific events orthresholds (e.g.

impacts, runners finishing a race).

2) The interfaces needed to condition the outputs of the above sensors may include: amplifiers or attenuators to match the sensor signals to the recorder; analogue to digital converters; serial data transmitters. The data from these sensor interfaces may be recorded in various ways including: separate data channels (e.g. the sound tracks oaf video recorder); time sharing the video channel so that a part of each line scan cycle is used for data recording; encoding data ontothevideo signal in the manner of colour encoding systems (PAL, SECAM); direct inputto a computer control system. In addition to sensor data it will usually be useful to record a real time reference onto the signal; this may be in the form of absolute time (e.g. GMT) or of elapsed time.

3) Subjecttothe limitations of particular models any recording device suitable for conventional video signals may be used for streak mode recording; however, in systems where the line scan frequency is appreciably higherthan 1 6KHz the bandwidth ofthe recording device may be a governing factor.

4) It is possible to compress the displayed time dimension of a streakvideo recording by scan frequency division using a sync. pulse counter coupled to a fast analogue switch. An outline of a suitable circuit is shown in Figure 5 of the drawings. The component shown are a sync pulse detector 21, counter 22 and switch 23, with switch control 24, video in 25 and video out 26.

A similar circuit using a changeover switch will permit the outputs of two different camerasto be recorded on one video tape.

5) To display a sequence of scan lines in rasterform and thus generate a streak image the monitorfield timebase must be sychronised to the line display rate so that each successive displayed line scan is displayed by one line width from the position ofthe preceding line. This scan synchronisation may be achieved by using a conventional timebase generator matched to the scan frequency or by incrementing the field deflection current by a discrete amountatthe startof each line scan. In either case the field drive must limit the display to only one screenful of lines without overwriting.Astoragecathode raytube may be used to holdtheimageforinspectionorthe image may be photographed onto polaroid type film for more permanent storage.

6) Formoststreakvideoapplicationstheideal display system is a digitally converted and stored image. Suitablevideo signal digitisersarecommercially available or may be purpose built Digital conversion allows easier display, analysis, storage and transmission of video images. In particular the image of a short event may be converted and displayed in real time with no appreciable lag between event and analysis.

Substituting a streakvideo camera and electronic recording system as described above for the film used as the recording medium in a conventional streak camera offers the following advantages: a) The time lag between the event and the display of the streak image may be greatly reduced, even for colour systems.

b) recording capacity may be such greater (hours ratherthan minutes or seconds).

c) Physical access to the camera is not required.

d) Manipulation of the image can be performed in real time using electronic signal processing to permit, for example, the analysis of the image into various luminance levels.

e) The time axis ofthe displayed image may be compressed relative to the recorded time resolution.

f) The video signal may be easily transmitted to any remote location.

g) Data other than image information may be recorded in either analogue or digital form and usedto assist in the analysisof the event.

h) It is possible to multiplex the outputs of several cameras onto one recording.

Thefollowing fields illustrate some ofthe various uses to which a system incorporating the present streakvideo camera may be put.

Photo-finish recording of races; either for official use bythetime keepers, or as a complementto conventional action replayed used by broadcast television.

Track logging of road or rail surfaces, performed by mounting the camera in a vehicle so that the axis of vehicle travel is perpendicular to the sampling plane.

The analysis of shock waves and similar phe nomena due to impacts and explosions. The long recording capacity permits the unsynchronised recording of even unpredictable events.

The recording of cyclic events such as pistons or the peripheries of wheels; either with asynchronous timebase, or with a standard line frequency.

Systems incorporating the present streakvideo camera may be configured to suit particular applica tions.Timing devices and non-image sensors will be chosen to conform with the data logging requirements ofthe application, and recording and display systems will be selected based on the amount of data to be recorded at different frequencies, the recording capacity required and the access time and image format requirements ofthe operator.

As far as the camera itself is concerned the operator may select the tube type, optics, scanning frequency, scan duty cycle and extent of remote control facilities.

For some applications it may be useful to use a restricted field scan so that more than one line is scanned in orderto provide a wider "slit" image, e.g. a field deflection of 1% of normal would give an effective slit approximately 6 lines wide. In such an event it may decided to select one or more of the lines forsubsequent recording or analysis.

Claims (14)

1. Astreakcamera comprising a video camera having a substantially reduced field scan so as to allow a slit image to be converted to electrical signals.

2. Astreakcameraasclaimedinclaim 1 wherein the video camera is arranged to have a substantially stationary line scan so asto provide a horizontal slit image.

3. Astreakcamera as claimed in claim 1 wherein thevideocamera is arranged to have linetimebase signals connected to the field coils so asto provide a vertical slit image.

4. A streak camera as claimed in claim 1 wherein thevideocamera is arranged to havethe linetimebase signals connected to the line and field coils with a division thereofto provide a slit sampling image axis otherthan vertical or horizontal.

5. Astreak camera as claimed in any preceding claim wherein a physical slit mask is located in the optical image-forming path ofthe video camera so as obtain a sharper image.

6. A streak camera as claimed in any oneofclaims 1 to 4wherein the video camera is switchable between a streak mode, providing a slit image as claimed in any one of the preceding claims 1 to 4, and a raster scan as provided in a normal video camera.

7. Astreakcamera as claimed in any preceding claim wherein the video camera comprises a vacuum tubetypevideo camera.

8. Astreakcamera as claimed in any of claims 1 to 6 wherein the video camera comprises a solid state imaging array.

9. Astreakcamera as claimed in any preceding claim wherein the video camera has its timebase signals controlled by a fixed frequency reference.

10. Astreakcamera as claimed in any of claims 1 to 8 wherein the video camera has its timebase signals controlled by means of a continuouslyvariable frequency source.

11. Astreakcamera as claimed in any of claims 1 to 8 wherein the video camera utilised has a modified linetimebase generator.

12. Astreakcamerasystem includingastreak camera as claimed in any preceding claim and including anyone or more ofthefollowing: non-image sensors; data recording interfaces; recorders; a timebase compressor; analogue displays, or digital image handling systems

13. A streak camera substantially as hereinbefore described with referenceto Figs. 1 to 4 of the accompanying drawings.

14. Astreakcamerasystemsubstantiallyas he- reinbefore described with reference to the accompanying drawings.