GB2272815A - Flying spot telecine - Google Patents

Abstract

A flying spot telecine is formed from a cathode ray tube (CRT) scanning means 10 with a curved face plate 2. A cinematographic film 12 which is being scanned is constrained by a film gate 16 to move in a curved plane. The axis of curvature of the CRT is arranged to match that of the film in the film gate such that all lines scanned by the CRT are focused on the film plane. <IMAGE>

Description

IMPROVEMENTS RELATING TO ELECTRO-OPTIC IMAGE TRANSDUCERS, E.G TELECINES Background of the Invention This invention relates to flying spot telecines.

In this type of telecine the signal to noise ratio in the generated video signal improves as the light intensity from the flying spot cathode ray tube (CRT) increases. Because of this the aperture of the objective lens is chosen to be as large as possible, whilst maintaining sufficient resolution with regard to the depth of focus. The larger the lens aperture the smaller the depth of focus. Therefore, to maintain a good signal to noise ratio, the depth of focus tends to be small.

To ensure that the film plane is accurately defined in relation to the scanning direction whilst the film is moving at a constant velocity, it is common practice to curve the film plane along the line of motion to enhance its rigidity. The film will, therefore, move in a cylindrical plane. There is, therefore, no or very little loss of resolution due to the curvature of the film plane when the film is moving if the scanning amplitude in the direction of film motion is small. The film will normally be moving for most applications.

Sometimes it may be desirable to produce a "freeze frame" display.

For this the film remains stationary. When the film is stationary the curvature of the film will cause loss of resolution and it is, therefore, usual to include a cylindrical correction element in the film gate to correct for this error. For perfect correction the correcting element would need to be in contact with the film. This is not practical because of the motion of the the film.

Satisfactory results are obtained with a small spacing, the only disadvantage is that dust and dirt from the film tend to collect on this correction element-which is close to the focal plane. This makes the dust and dirt very visible.

Similar problems arise if the scanning direction is the same as the direction of film motion.

Such an arrangement is sometimes used because a small vertical scan on the CRT can result in a shortening of the life of the tube. The small vertical scan also results in the phosphor loading on the CRT being nearer to saturation point so that variations in scan amplitude can produce variations in brightness and colour balance.

Another disadvantage of a small vertical scan is the effect of dirt and dust on the CRT face plate and blemishes in the phosphor which become elongated vertically. These are therefore much larger in the resulting video image.

The main disadvantage of the use of larger scans in the direction of film motion is the need for the correcting element close to the film path.

Si-ary of the Invention In a preferred embodiment of the invention the film moves in a cylindrical plane in the scanning aperture. Frames of the film may be scanned from bottom to top rather than top to bottom. The CRT is provided with a curved front face to ensure that the flying spot is focussed on the film throughout its vertical scan in the scanning aperture.

In one embodiment the CRT front face may be flat with a cylindrical lens attached thereto.

The invention is defined with more precision in the appended claims to which reference should now be made.

Brief Description of the Drawings The invention will be described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic side view of a first embodiment of the invention where the CRT face plate is curved in a cylindrical plane; and Figure 2 is a similar view of a second embodiment using a cylindrical convex lens attached to a flat face plate CRT.

Detailed Description of the Preferred Embodiments To obtain a large vertical scan frame of a film passing through a flying spot scanner frames may be scanned vertically from bottom to top instead of top to bottom. This may also arise when special effects are used. For a moving film this results in the vertical scanning amplitude being larger than that required when the film is stationary. For a moving film the vertical scanning amplitude will, for example, be approximately twice that required for a stationary film. This is achieved by altering the scanning control circuits in a straightforward manner.

Figure 1 shows how this may be accomplished whilst retaining good resolution over the whole picture. A cathode ray tube (CRT) 10 is provided with a curved face plate 22. The radius of curvature of the internal and external faces of the CRT are the same and so no light is condensed or dispersed by it. Its axis of curvature is parallel to the film plane and perpendicular to the direction of film motion. A capstan 20 transports a film 12 with a constant velocity over a roller 14 and past a film gate 16. The film is constrained by the gate 16 to move in a curved plane.

Objective lenses are normally more easily designed when assuming that image planes are flat and at right angles to the optical axis.

This is assumed in the present case. Horizontal scan lines on the CRT which scan the top and bottom of a frame of film are equidstant from the optical axis and lie on the flat plane 28. Images of these lines can be accurately focused on a flat image plane 32, which passes through the film, by the objective lens 18. The horizontal line scanning the centre of the film lies on a flat plane 30 on the CRT face plate and is focused on the film plane 34 by the objective lens 18. This assumes that the radius of curvature of the CRT face plate has been chosen to obey the simple optical equations of the objective lens 18. Scanning lines between the top and middle, and, bottom and middle of the CRT lie on planes intermediate of the planes 30 and 28 and are focused on planes intermediate of planes 32 and 34 on the film.Thus every line which can be scanned by the CRT raster is focused onto part of the curved film plane and the image of the CRT raster will remain in focus over the whole of the film frame.

To maintain optical focus over the whole film frame is a question of physical optics and no electronic manipulation can compensate.

Optical focus has been achieved whilst ignoring any magnification errors that arise by the use of the curved face plate CRT. These can be quite substantial and would be unacceptable in a purely optical system. Electronic manipulation can easily solve this problem by electronically modulating the length of the scanning lines and the distance from the optical axis.

Magnification error correction is already standard practice on CRT for a different reason. This is because the distance of the CRT phosphor from the centre of deflection is not constant with scanning angle. This gives rise to a similar kind of problem. The CRT shown in figure 1 will have a similar error because of the cylindrical face plate. This results in magnification errors due to the CRT and objective lens being combined. The scanning coil current of the CRT can be modulated to compensate for this using well known technology.

Although a CRT with a cylindrically curved face plate would be the preferable solution it is not a standard product and therefore an alternative is proposed as shown in figure 2. In this arrangement a CRT 10 with a flat face plate 26 has a cylindrical lens 24 cemented to its front thereby creating a curved front face. This lens is thicker in the centre and thus shortens the object distance in the centre, i.e. it has the same function as the curved face plate of figure 1. The presence of the additional high refractive index medium in the optical path requires that the objective lens 18 should be altered to compensate for the extra medium, The accuracy of the cylindrical lens 24 is not as critical as that required in the film gate and close to the film and therefore this lens could be fabricated from a plastics moulding.

Claims (10)

1. A flying spot telecine comprising a film gate having a curved film plane on which a cinematographic film is scanned, and a CRT scanning means having a curved front face whereby lines scanned on the CRT are focused on the curved film plane in the film gate.

2. A flying spot telecine according to claim 1 or 2 in which the vertical scanning amplitude of the flying spot is substantially twice that required to scan a stationary frame of the film.

3. A flying spot telecine according to claim 1 in which the curved front face of the CRT comprises a cylindrical lens attached to a flat faced CRT.

4. A flying spot telecine according to claim 3 in which the lens is formed from plastics material.

5. A flying spot telecine according to claim 4 in which the curved front face of the CRT comprises a curved plate.

6. A flying spot telecine according to claim 5 in which the plate is cylindrically curved.

7. A flying spot telecine according to any of claims 1 to 6 including means to compensate for magnification errors.

8. A flying spot telecine according to claim 1, 2, 3 or 4 wherein the cinematographic film moves through the scanning means with a substantially constant velocity.

9. A flying spot telecine substantially as herein described with reference to figure 1 or the accompanying drawings.

10. A flying spot telecine substantially as herein described with reference to figure 2 of the accompanying drawings.