GB2203312A

GB2203312A - View scanning television camera

Info

Publication number: GB2203312A
Application number: GB08808024A
Authority: GB
Inventors: Ronald Arthur Awcock; Philip Galbraith
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1987-04-06
Filing date: 1988-04-06
Publication date: 1988-10-12
Anticipated expiration: 2008-04-06
Also published as: GB8708195D0; GB8808024D0; GB2203312B

Abstract

A television camera having a view scanning mechanism within its body has the ability to scan a wide field of view with minimum picture distortion and without a bulky external pan-and-tilt mechanism. The camera consists of a fisheye lens 2 mounted in the front of a camera casing which houses the camera 5, a solid state image sensor 6 having a target area 7 which is a fraction of the image area of the lens 2, and a view scanning mechanism 11 which moves the target area 7 within the image area. Flexible cables 10 connect the independently movable sensor 6 to the rest of the camera 5. Linear displacement of the sensor 6 in two mutally orthogonal directions perpendicular to the optical axis of the lens 2 is provided by orthogonally-mounted lead screws (22, 26 Fig. 3) each rotated by a separate reversible D.C. motor (23, 28). <IMAGE>

Description

View Scanning Television Camera This invention relates to a television camera having a view scanning mechanism and a wide angle objective lens.

The scanning function of television cameras, which are used to scan a field of view by remote control and are commonly referred to as closed circuit TV cameras, is conveniently carried out by an external scanning mechanism attached to the camera casing. The most common form of external scanning mechanism for a camera is a pan-and-tilt head which usually forms part of the camera mounting.

However, such mechanisms are costly, add considerably to the bulk of the camera because they must carry the weight of the camera and lens, require relatively large drive motors (which usually require mains electricity) to pan and tilt the bulk of the camera, cannot easily be used in confined spaces due to camera travel across the field of view, and are often difficult to protect against the environment in which the camera is designed to operate because their moving parts are difficult to seal against, for example, the ingress of water. Furthermore, an external scanning mechanism is an undesirable feature of a camera which is being used for surveillance purposes, for example inside shops and banks, because it renders the camera and its direction of view more conspicuous to an individual who wishes to evade detection.

These problems have in the past lead to the development of cameras having wide angle objective lenses and having scanning mechanisms within the camera housing, which are adapted for use in pipelines and other inaccessible regions. The camera has a elongated camera holder which is controllably pivoted about its mid region behind a fixed position fisheye lens. The holder contains a camera tube inserted at one end, and pivoting of the holder causes the photocathode target area of the tube to scan over the image area of the lens. In this way, the target area of the tube can internally scan the entire field of view of the lens without moving the camera casing. Since camera tubes can only be used with associated electromagnetic beam deflection coil equipment, then this equipment must also be present in the holder, preferably together with the rest of the camera electronics.This type of camera is disclosed in patent application DE-30919339-CI and GB2177278A. Although these patent specifications disclose relatively simple mechanisms whereby controllable positioning of relatively bulky and heavy camera tube equipment can be achieved, cameras of this type generally suffer from the disadvantage that the picture becomes increasingly distorted and lacking in clarity, to a greater or lesser extent depending upon the type of camera lens employed, as the tube is pivoted away from the optical axis of the lens. They can generally only be used effectively either over a fairly narrow field of view in front of the camera, or within the confined space of the interior of a pipe where the distance between target and lens about the periphery of the total field of view is very much smaller than the distance between target and lens down the length of the pipe.They require a focussing mechanism, partly in order to reduce the out-of-focus effects caused when the target area of the tube pivots away from the image area of the lens.

It is one object of the present invention to provide a TV camera having a scanning mechanism in which the aforementioned problems associated with picture distortion about the periphery of the field of view of the the camera lens and the need for a focussing mechanism are overcome or at least mitigated in part.

According to the present invention, there is provided a television camera having a view scanning mechanism, comprising a camera housing with a fixed wide angle objective lens having an acceptance angle of at least 1100, a movable television camera image sensor, consisting of a solid state imaging device, mounted within the camera housing, the image area of the lens being a multiple of the target area of the image sensor, and an electrically-controlled sensor drive means moving the target area of the sensor in a plane which is substantially parallel to the plane of the image area of the objective lens.

The drive means of the view scanning mechanism preferably comprises two electrically-controllable motors, each provided for linear displacement of the sensor in either of two different, preferably mutually orthogonal, directions each perpendicular to the optional axis of the lens. A transmission means for each motor is preferably provided for converting the rotational motion of the motor into linear movement of the sensor. For example, the transmission means may comprise a rotatable lead screw which is rotatable about the axis of rotation of the motor.

As with previous cameras of this type, movement of the sensor across the image area of the wide angle objective lens enables the target area of the sensor to receive selected parts of the field of view of the lens without having to pivot or tilt the whole camera housing. However, the use of an image sensor comprising a solid state imaging device (SSID) in the present TV camera means that the sensor is very much smaller and lighter than a conventional camera tube and its associated equipment, and so movement of the sensor within the camera housing in a planar rather than a pivotal manner then becomes feasible. Solid state image devices, such as charge couple devices (CCDs), metal oxide semiconductors (MOSs) and charge injection devices (CIDs) are all self scanning image sensors, and so do not require associated beam scanning equipment.

The advantage of reduced picture distortion when employing parallel planar rather than arcuate sensor movement across the image area of the lens becomes more readily apparent the greater is the relative size difference between the image area of the lens and the target area of the sensor for a given target area size, since the greater the difference between the two the larger will be the degree of arcuate movement required of a corresponding pivoted sensor having the same image and target area parameters and so the greater will be the degree of distortion in the picture received from the periphery of the field of view of the lens for this latter type of camera. It is for this reason that the ratio of the image area of the lens to the target area of the sensor is preferably greater than 15:1, more preferably greater than 20:1, most preferably greater than 30:1.

The parameters governing the choice of lens and SSID format are the total field of view required, and the field of view of the camera at any one time. The first of these parameters is dependent on the focal length of the lens in relation to the diameter of the image circle it produces.

Wide angle camera lenses, including superwide-angle and fisheye camera lenses, having acceptance angles of at least 1100, are currently available which provide image circles designed to cover at least some of the following formats: 1/2 inch (12.7mm) camera tube 2/3 inch (17mm) camera tube 1 inch (25.4mm) camera tube 35mm film (still cameras) 6 x 4.5cm rollfilm cameras 6 x 6cm rollfilm cameras These lenses produce image circles of diameter 9, 12, 17, 44, 75, and 85mm respectively. Commonly available SSIDs have formats conforming to the 2/3 inch (17mm) and 1/2 inch (12.7mm) camera tube standards. These have photosensitive areas with the following dimensions: 17mm; height: 6.6mm, width: 8.8mm, diagonal: llmm.

12.7mm; height: 4.8mm, width: 6.4mm, diagonal: 8mm Using these currently available SSIDs and lenses, the selected objective lens for use in the present camera preferably has an image circle of at least 44mm, producing a lens image area to sensor target area ratio of at least 26:1 (ie the ratio which results from using a 17mm SSID format and a 35mm still film camera lens). Lenses of the same focal length designed for different formats produce image circles of different sizes and therefore different total fields of view.

The objective lens is preferably a fisheye lens having an acceptance angle (which is the angle spanning the total field of view of the lens) of between 110 and 220 degrees. The greater is the acceptance angle of the lens the wider will be the total field of view which can be scanned by the sensor without having to move the camera casing. For the purpose of this specification, a fisheye lens is a lens which has an acceptance angle within the range specified above and which is optically designed to produce an image by equisolid angular projection. This has the advantage in surveillance applications that the angular proportion of the total field of view of lens which is "seen" at any one time by the sensor is constant regardless of the position of the sensor within the focal plane of the lens.

Where an image focussing or distance adjustment function is required in the present camera, the sensor scanning means may be mounted within a camera rack which is itself slideably mounted within the camera housing in a longitudinally displaceable manner.

However, it is an advantage of the present invention that for most surveillance applications, a focussing or distance adjustment function is not required. This is because wide angle camera lenses and in particular wide angle camera lenses having acceptable angles 0 of at least 110 , usually have relatively short focal lengths, and so tend to have a much greater depth of focus than narrow angle, low-and-zero-distortion lenses since depth of focus of camera lenses tends to increase with decreasing focal length. By maintaining the sensor substantially within the focal plane of the lens at all times, the image sensor remains in focus for all objects in the camera's field of view down to an object-to-lens distance of typically less than 1 metre under daylight conditions.

The present invention will now be described by way of example only with reference to the accompanying drawings in which Figure 1 shows a partly diagramatic longitudinal crosssection of a television camera having a view scanning mechanism, Figure 2 shows a longitudinal cross-section similar to that shown in Figure 1, in'which the camera shown in Figure 1 is modified to include a distance setting mechanism for focussing the image, Figure 3 shows a transverse cross-section of the camera of Figure 2 taken along line AA', illustrating the view scanning mechanism in more detail, Figure 4 shows a perspective view of the view scanning mechanism ilustrated in Figures 1, 2 and 3, and Figure 5 shows a schematic representation of the camera at the centralised setting and at an off-centre setting of the image sensor, when employing an objective lens consisting of a fisheye lens having an acceptance angle of 180 degrees.

The televsion camera of Figure 1 comprises a camera casing 1 having detachably mounted at one end a wide angle, fisheye lens 2 within a lens holder 3. The casing 1 houses a camera 5. The camera 5 has a planar charge couple device (CCD) integrated circuit image sensor 6 with a photosensitive target area 7. The sensor 6 is mounted within an integrated circuit carrier 8 which is itself mounted on a printed circuit board 9. The circuit board 9 is connected to the rest of the camera 5 by means of a multiplicity of flexible cables 10, one of which is shown by way of example in Figure 1. The sensor 6, carrier 8 and circuit board 9 are mounted on a view scanning mechanism 11 (shown in outline in Figure 1) which is fixed inside the casing 1 and holds the plane of the target area 7 of the sensor 6 within the focal plane of the lens 2.

In the television camera illustrated in Figure 2, the lens 2 and lens holder 3 are mounted at one end of a second casing 101.

The camera 5 is housed within a camera rack 104 which is itself slideably engaged within the second casing 101. The view scanning mechanism 11 is fixed within the open front end of the camera rack 104 and holds the plane of the target area 7 of the sensor 6 in parallel alignment with the focal plane of the lens 2. The displacement of the hollow camera holder 104 along the longitudinal axis of the casing 101, which is for the purpose of distance setting or focussing if required, is performed by means of a d.c. rack drive motor 112 the rotation of which is converted into a translatory displacement of a displacing spindle 113 by means of a transmission 114.

The view scanning mechanism 11 is illustrated in greater detail in Figures 3 and 4, and comprises a first carriage plate 20 which is moveable in two mutually orthogonal directions. The mechanism is fixed onto the camera rack 104 (or directly onto the casing 1 - see Figure 1) so that the two mutually orthogonal directions are both perpendicular to the optical axis of the lens 2.

The first plate 20 carries, on its front surface, the circuit board 9 upon which is mounted the integrated circuit carrier 8 and sensor 6. The first plate 20 is moved in the vertical Uyff direction between the opposing walls of a "U"-shaped carriage member 21 by means of a first lead screw 22 which passes through and engages a threaded passage (not shown) in the plate 20. A first d.c. motor 23, acting through a first reduction gearbox 24 mounted on one of the opposing sidewalls of the carriage member 21, is provided to rotate the lead screw 22 in a clockwise or in an anticlockwise direction.Parallel guide rails 25, which are affixed at either end to the opposing side walls of the member 21 and which pass through in slideable engagement with the plate 20 either side of the lead screw 22, prevent rotation of the plate 20 about the "y" direction when the motor 23 is actuated. A similar arrangement provides for movement of the plate 20 in the horizontal "x" direction. A second lead screw 26 is provided which passes between the opposing sidewalls of a second, larger "U"-shaped carriage member 27 and which passes through and engages a threaded passage (not shown) in the base of the first member 21 at right angles to the first lead screw 22. The second carriage member 27 is secured to the base of the camera rack 104. A second d.c. motor 28, acting through a second reduction gearbox 29 mounted on one of the opposing sidewalls of the second carriage member 27, is provided to rotate the second lead screw 26 in a clockwise or anticlockwise direction.

Parallel guide rails 30 which are affixed at either end to the opposing sidewalls of the second member 27 and which pass through in slideable engagement with the first member 21 either side of the second lead screw 26, prevent rotation of the plate 20 about the "x" direction when the second motor 28 is actuated. Free rotation of the lead screws 22 and 26 in their respective carrier members in ensured by the provision of ports 31 and 32 and bearings 33 and 34.

The flexible cables 10 from the circuit board 9 are fed back to the rest of the electronics of the camera 5 through individual guide tubes 35 mounted adjacent the first motor 23.

In use, each of the motors 23 and 28 is connected to a reversible d.c. power supply (not shown) which may be switched on (forward or reverse mode) or off by the camera operator from a remote location. Actuation of the motor 23 and/or 28 in the forward or reverse direction rotates lead screws 22 and/or 26 as appropriate and so moves the centre of the target area 7 of the sensor 6 in a single plane to any point within the square area bound by broken line 36. Appropriate selection of the lengths of the lead screws 22 and 26 ensures that this area is sufficiently large to cover the total circular image area of the lens 2 shown bounded by broken line 37.Mechanically-operated limit microswitches (not shown), actuated by the plate 20 and first member 21 as they move close to their limits of travel on lead screws 22 and 26, cut off the supply of current to the motors 23 and 28, to prevent damage to the mechanical parts of the mechanism 11 or overload of the motors.

The beam path at the centralised and at a peripheral scan setting of the sensor 6 within the image plane of the lens 2 is illustrated in Figure 5. The solid arrowed lines in front of the lens 2, which are subtended by angle B', indicate that portion of the total field of view which the sensor 6 can "see" when situated at the centralised setting (solid representation). The solid arrowed lines to the rear of the lens 2, which are subtended by image angle B", correspond to that part of the total image circle area of the lens 2 which is "seen" by the target area 7 at the centralised setting.Similarly, the broken arrowed lines in front of the lens 2, which are subtended by the same angle B', indicate that portion of the total field of view of the lens which the sensor 6 can "see" when situated at a peripheral setting (broken representation), and the broken arrowed lines to the rear of the lens 2, which are subtended by image angle C ', correspond to that part of the total image circle of the lens which is "seen" by the target area 7 at the peripheral setting.

It will be seen from Figure 5 that the parallel displacement of the sensor 6 in the focal plane of the lens 2 from the centralised to the peripheral setting provides approximately 90 degrees of camera scan from the forward to a sideways line of sight.

Broken ray line p represents the maximum sideways extent of the field of view of the 1800 fisheye lens 2 (ie 900 to the optical axis of the lens). The distance r is, therefore, equal to the radius of the circular image area of the lens 2. Since the lens is of a type which produces an image by equisolid angular projection, the angular proportion of the total field of view of the lens which is "seen" at any one time by the sensor 6 is constant regardless of its position in the focal plane of the lens.

Claims

1. A television camera having a view scanning mechanism, comprising a camera housing with a fixed wide angle objective lens 0 having an acceptance angle of at least 110 , a moveable television camera image sensor, consisting of a solid state imaging device, mounted within the camera housing, the image area of the lens being a multiple of the target area of the image sensor, and an electrically-controlled sensor drive means for moving the target area of the sensor in a plane which is substantially parallel to the plane of the image area of the objective lens.

2. A television camera according to claim 1 wherein the objective lens comprises a fisheye lens, having an acceptance angle 0 0 of between 110 and 220 , which is optically designed to produce an image by equisolid angular projection.

3. A television camera according to claim 1 or claim 2, wherein the sensor drive means comprises two electrically-controllable motors, each provided for linear displacement of the image sensor in either of two different directions each perpendicular to the optical axis of the lens.

4. A television camera according to claim 3 wherein the said two different directions are mutually orthogonal.

5. A television camera according to claim 3 or claim 4 wherein each motor is provided with a transmission means for converting rotational motion of the motor into linear displacement of the image sensor.

6. A television camera according to claim 5 wherein the transmission means comprising a rotatable screw which is rotatable about the axis of rotation of the motor and the screw thread of which engages a support member for said sensor.

7. A television camera according to any one of the preceding claims wherein the ratio of the image area of the lens to the target area of the image sensor is at least 15:1.

8. A television camera according to claim 7 wherein the lens has a circular image area having a diameter of at least 44mm.

9. A television camera according to any one of the preceding claims wherein the camera electronics are contained within the camera casing.

10. A television camera substantially as hereinbefore described with reference to the drawings.