GB2263344A

GB2263344A - Aspherical raster scanned camera iris

Info

Publication number: GB2263344A
Application number: GB9226279A
Authority: GB
Inventors: Kenneth Chassar Palfrey Moir
Original assignee: British Broadcasting Corp
Current assignee: British Broadcasting Corp
Priority date: 1992-01-20
Filing date: 1992-12-17
Publication date: 1993-07-21
Also published as: GB9226279D0; GB9201156D0

Abstract

An aspherical iris 4 for e.g. a television camera enables the resolution and depth of field to be increased in a direction transverse to the larger direction of the aperture of the elliptical iris. The iris is arranged such that the maximum dimension is transverse to the horizontal scanning direction of the camera. Circular lens 2 is shown. This iris may be used in any raster scanned camera or an HDTV camera. <IMAGE>

Description

TELEVISION CAMERA IRIS This invention relates to an iris for a television camera and in particular to an iris suitable for use in a high definition television (HDTV) camera.

Historically the television system has required equal horizontal and vertical definition. Conventional television does not require particularly high resolution and the simplest arrangement has always been to produce lens and iris arrangements which give equal horizontal and vertical definition. This has been achieved by using a circular lens and a circular iris.

When using conventional television cameras it is more important, and easier to focus on a vertical line rather than a horizontal line. This is because of the horizontal line structure of the raster scanned television picture which makes it somewhat difficult to focus on horizontal or near horizontal lines. The definition of a vertical line depends upon the horizontal resolution of the camera and lens.

In HDTV the system as a whole has a higher resolution than conventional television and lens resolution has to be as good as that of the system as a whole if this standard of resolution is to be maintained.

The problem of loss of resolution can occur when illumination levels are low and the television cameras have to be operated with large apertures. This leads to a loss of depth of field and resolution in the viewed scene.

Preferred embodiments of the present invention use an aspherical iris with its larger dimension transverse to the horizontal scanning direction of a television camera to increase the horizontal resolution of the television camera as compared with the vertical resolution. If the scanning directions of the television camera were other than horizontal then the larger dimensions of the aspherical iris would be transverse to that scanning direction.

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

An embodiment of the invention will now be described in detail by way of example with reference to the accompanying drawings in which: Fig. 1 shows a schematic diagram of a lens and iris for a television camera embodying the invention with the iris set to maximum aperture; Fig. 2 shows the lens and iris of figure 1 with the iris set to a medium aperture; and Fig. 3 shows the lens and iris of figure 1 with the iris set to a small aperture.

An embodiment of the present invention abandons the principle of equal resolution in the horizontal and vertical directions in a television camera. This is achieved by using an aspherical iris for the camera, the iris having its larger dimension in the vertical plane i.e. transverse to the horizontal scanning direction of the camera. The iris maybe is elliptical in shape with its larger dimension in the vertical direction, the ellipse being a true mathematical ellipse.

It will be appreciated that using such an arrangement will lead to much greater depth of field in the horizontal direction than in the vertical direction when the iris is used with a circular lens and the iris is "stopped down" to provide an aperture smaller than the maximum aperture of the lens.

Figures 1, 2, and 3 schematically show a circular lens 2 and an iris 4 for use with an HDTV camera with the iris set to various different apertures.

In figure 1 the iris is set to its maximum aperture. Such a setting would only be used in the very dimmest of light conditions where it is imperative that the illumination levels falling onto the camera tube targets are maximised. Using this particular setting the depth of field and resolution will be the same in both the horizontal and the vertical directions.

In figure 2 a medium aperture size is shown in which the vertical dimension of the aperture is the same as the diameter of the lens but the horizontal dimension of the aperture is smaller.

Resolution and depth of field in the vertical direction will be at substantially the same level as they were in the arrangement of figure 1 but the resolution and depth of field in the horizontal direction will have been increased. This type of setting would be used in relatively poor light conditions and would give an increase in horizontal resolution and depth of field over a circular aperture with an equivalent open area. The poor resolution and depth of field in the vertical direction will be concealed by the line structure in the television picture and will not be particularly noticeable to viewers.

Figure 3 shows a relatively small aperture in which the vertical length of the aperture is smaller than the diameter of the lens.

Again the resolution and depth of field in the horizontal direction is greater than that in the vertical direction but this difference is concealed by the line structure in the television picture.

The iris shown schematically in the figures can be manufactured using known techniques and would not present any significant problems in production. Conventional television cameras can then be converted by using such an iris and HDTV cameras can be fitted with such an iris as standard.

The invention can be applied to any raster scanned camera and is not limited to television cameras.

Claims

1. An aspherical iris for a raster scanned camera wherein the larger dimension of the iris aperture is transverse to the horizontal scanning direction of the camera.

2. An aspherical iris according to claim 1 in which the larger dimension of the iris aperture is substantially at right angles to the horizontal scanning direction of the camera.

3. An aspherical iris according to claim 1 or 2 in which the maximum aperture of the iris is larger than the aperture of a lens with which is is associated.

4. An aspherical iris according to claim 1, 2, or 3 in which the iris profile comprises two intersecting circular arcs.

5. An aspherical iris according to claim 1, 2, or 3 in which the iris profile comprises two intersecting elliptical arcs.

6. A raster scanned camera including an aspherical iris with its larger dimension transverse to the scanning direction of the raster.

7. A raster scanned camera according to claim 6 in which the maximum dimension of the iris is substantially at right angles to the scanning direction of the raster.

8. A raster scanned camera according to claim 6 or 7 in which the camera is a television camera.

9. A raster scanned camera according to claim 8 in which the camera is a high definition television camera.

10. An aspherical iris for a raster scanned camera substantially as herein described with reference to the accompanying drawings.