GB1582844A

GB1582844A - Optical systems

Info

Publication number: GB1582844A
Application number: GB4627976A
Authority: GB
Original assignee: Elliott Brothers London Ltd
Current assignee: Allard Way Holdings Ltd
Priority date: 1976-11-06
Filing date: 1976-11-06
Publication date: 1981-01-14
Also published as: DE2700745B2; FR2370293B1; DE2700745A1; FR2370293A1; IT1091721B; NL7712007A

Description

(54) IMPROVEMENTS IN OR RELATING TO OPTICAL SYSTEMS (71) We, ELLIOTT BROTHERS (LONDON) LIMITED, a British Company, of Marconi House, New Street, Chelmsford, Essex CM1 1PL, do hereby declare the invention, for which we pray that a patent maybe granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to optical systems and in particular to optical systems in which a colour filter is provided to be selectively inserted in the optical path from a viewed scene.

As is known from the photographical art, the contrast between objects or features in a viewed scene may be increased in many instances by the use of selective colour (spectral) filters, and in particular the use of a very narrow band red filter enables contrast to be significantly improved when viewing a scene under conditions of a very low level of illumination.

'In applying the technique to a television system, and in particular a low light level television system, a problem arises, or more properly becomes increasingly evident, in view of the wide spectral transmission band employed. This problem is so-called longitudinal chromatic aberration, i.e. the change of the focusing plane for different colours due to the dependence of the refractive index upon wavelength of the optical components.

This effect is illustrated in the schematic diagram of Figure 1 of the drawing accompanying the Provisional specification which represents the case of a red filter inserted in the optical path.

Referring to Figure 1, an optical system is represented by an objective lens 1 on an optical axis 2. The lens 1 focuses collimated white light from the object (not shown) to the left of the Figure (as viewed) onto an image plane P1. The image distance from the image plane P to the lens 1 is shown as S.

If it is desired to introduce a, say, red filter into the optical path and, following usual practice it is positioned where indicated in dashed line outline at 3, i.e. on the object side of lens 1, the effective image plane will shift to P2 with a consequential increase d2 in the image distance.

Even in the absence of the red filter the red light present in the white light will not be focused at the image plane P1, but the effect is insignificant then,'since the most dominant colours present in the white light are correctly focused.

Thus, with a colour filter in use as described above, refocusing may be necessary. This effect may be minimised, and sometimes avoided, by the use of lens materials (usually glass) of differing dispersions. However, even so, correction can only be achieved exactly at two or three wavelengths within the normal bandwidth. Outside of this bandwidth, the change of image position increases rapidly and refocusing is still required for restricted bandwidths. The amount of image shift generally increases with the focal length of the lens.

The present invention seeks to provide an improved optical arrangement of the kind referred to, in which the above problem is reduced. The invention is particularly intended for use in television optical systems.

Acording to this invention an optical system includes a colour filter and a colourless filter, means for holding both filters and for selectively inserting one of the filters into a convergent or divergent portion of an optical path of the optical system, the thickness and refractive index of the colourless filter being chosen with regard to the thickness and the refractive index of said colour filter such that the position of the image plane in respect of the light passed by the filters is substantially the same for both filters.

Preferably, said colourless filter is a zero density filter.

Normally, both said colour filter and said further filters are parallel sided and arranged normally to the optic axis.

Preferably, said optical system is a television optical system, preferably of the low light level kind.

The invention is further described, with reference to Figure 2 of the drawing accompanying the Provisional specification which is a schematic diagram illustrating the principles behind the present invention.

Referring to Figure 2, in which like references are used for like parts in Figure 1, the upper portion (a) represents the optical system with a red colour filter 3 introduced and the lower portion (b) represents the optical system without a colour filter but with a colourless zero density filter 4 introduced in its place, all in accordance with the present invention.

Both the colourless filter 4 and the red filter 3 are parallel sided in the optical path.

It will be noted that both the red filter 3 and the alternative colourless filter 4 are both introduced into the optical path in a position where the light rays are convergent, i.e. on the image side of the lens 1.

Referring particularly to the lower portion (b) of Figure 2, assuming that the system is in air with a refractive index N = 1 it can be shown that the effect of introducing the colourless filter 4 into the optical path as shown causes a shift in the effective image plane of a white light image from the position P1 (corresponding to the image plane P1 in Figure 1, i.e. where no filter is interposed between the lens 1 and the image) to a position Pw of a red light image, with a consequential increase dw in image distance.

Referring particularly to the upper portion (a) of Figure 2, the effect of introducing the red filter 3 into the optical path as shown causes a shift in the image plane from the position P1 to a position PR which is a distance dR beyond the position P2 which would have been the position of the plane of the red image if, in accordance with Figure 1, the identical red filter 3 had been introduced on the object side of lens 1.

By suitably choosing the thickness tR of the red filter 3 in the optical path and the thickness tw of the colourless filter 4 in the optical path, the.positions of planes PR and Pw are made co-incident so that no change in the position of the image plane occurs when one filter is substituted for the other.

In achieving this condition the following relationships apply: S + dw = S + d2 + dR dw dw = d2 + dR Nw NN - x tw = d2 + NN 1 x tR, Nw mm since d = N - 1 xt N Let Nw - 1 = 1 and NR 1 1 Nw kw NR - KR tR Then tW = d2 * KR Kw tR = KR tW - KR d2 Kw To obtain values for this equation KR and Kw are calculated from Nw and NR as tabulated in Glass Tables. The value for d2 may be calculated for any particular filter or measured by setting up the filter as shown in Figure 1. Initially the thickness tw for the colourless filter 4 is usually estimated (a thickness of approximately 3 mm is typical) and a value for the thickness tR of the red filter calculated.

In practice the two filters 3 and 4 are mounted in a magazine so that normally filter 4 may be in use whilst filter 3 may be substituted therefor when this is required to enhance contrast.

If desired additional colour filters of different wavelength may be provided in the magazine for substitution for the colourless filter 4. In each case, the thickness in the optical path through each respective filter would be calculated to provide co-incident image planes so as to avoid refocusing.

WHAT WE CLAIM IS: 1. An optical system including a colour filter and a colourless filter, means for holding both filters and for selectively inserting one of the filters into a convergent or divergent

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Preferably, said colourless filter is a zero density filter. Normally, both said colour filter and said further filters are parallel sided and arranged normally to the optic axis. Preferably, said optical system is a television optical system, preferably of the low light level kind. The invention is further described, with reference to Figure 2 of the drawing accompanying the Provisional specification which is a schematic diagram illustrating the principles behind the present invention. Referring to Figure 2, in which like references are used for like parts in Figure 1, the upper portion (a) represents the optical system with a red colour filter 3 introduced and the lower portion (b) represents the optical system without a colour filter but with a colourless zero density filter 4 introduced in its place, all in accordance with the present invention. Both the colourless filter 4 and the red filter 3 are parallel sided in the optical path. It will be noted that both the red filter 3 and the alternative colourless filter 4 are both introduced into the optical path in a position where the light rays are convergent, i.e. on the image side of the lens 1. Referring particularly to the lower portion (b) of Figure 2, assuming that the system is in air with a refractive index N = 1 it can be shown that the effect of introducing the colourless filter 4 into the optical path as shown causes a shift in the effective image plane of a white light image from the position P1 (corresponding to the image plane P1 in Figure 1, i.e. where no filter is interposed between the lens 1 and the image) to a position Pw of a red light image, with a consequential increase dw in image distance. Referring particularly to the upper portion (a) of Figure 2, the effect of introducing the red filter 3 into the optical path as shown causes a shift in the image plane from the position P1 to a position PR which is a distance dR beyond the position P2 which would have been the position of the plane of the red image if, in accordance with Figure 1, the identical red filter 3 had been introduced on the object side of lens 1. By suitably choosing the thickness tR of the red filter 3 in the optical path and the thickness tw of the colourless filter 4 in the optical path, the.positions of planes PR and Pw are made co-incident so that no change in the position of the image plane occurs when one filter is substituted for the other. In achieving this condition the following relationships apply: S + dw = S + d2 + dR dw dw = d2 + dR Nw NN - x tw = d2 + NN 1 x tR, Nw mm since d = N - 1 xt N Let Nw - 1 = 1 and NR 1 1 Nw kw NR - KR tR Then tW = d2 * KR Kw tR = KR tW - KR d2 Kw To obtain values for this equation KR and Kw are calculated from Nw and NR as tabulated in Glass Tables. The value for d2 may be calculated for any particular filter or measured by setting up the filter as shown in Figure 1. Initially the thickness tw for the colourless filter 4 is usually estimated (a thickness of approximately 3 mm is typical) and a value for the thickness tR of the red filter calculated. In practice the two filters 3 and 4 are mounted in a magazine so that normally filter 4 may be in use whilst filter 3 may be substituted therefor when this is required to enhance contrast. If desired additional colour filters of different wavelength may be provided in the magazine for substitution for the colourless filter 4. In each case, the thickness in the optical path through each respective filter would be calculated to provide co-incident image planes so as to avoid refocusing. WHAT WE CLAIM IS:

1. An optical system including a colour filter and a colourless filter, means for holding both filters and for selectively inserting one of the filters into a convergent or divergent

portion of an optical path of the optical system, the thickness and refractive index of the colourless filter being chosen with regard to the thickness and the refractive index of said colour filter such that the position of the image plane in respect of the light passed by the filters is substantially the same for both filters.

2. A system as claimed in claim 1 and wherein said colourless filter is a zero density filter.

3. A system as claimed in any of the above claims and wherein both said colour filter and said further filter are parallel sided and arranged normally to the optic axis.

4. A television system including an optical system as claimed in any of the above claims.

5. A television system as claimed in claim 4 and which is of the low light level kind.

6. An optical system substantially as herein described with reference to Figure 2 of the drawing accompanying the Provisional specification.