GB2367687A

GB2367687A - Cathode ray tube and curved faceplate acting as a corrective lens

Info

Publication number: GB2367687A
Application number: GB0020451A
Authority: GB
Inventors: Terrence William Smith
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-08-18
Filing date: 2000-08-18
Publication date: 2002-04-10
Also published as: GB0020451D0

Abstract

A face plate 20 for bonding to a cathode ray tube 2 used within a film scanning system, the faceplate 20 having an inner surface profiled to be bonded to the cathode ray tube and an outer surface spaced from the inner surface, the outer surface being curved in such a way that it acts as a corrective lens to reduce the effects of the curvature of the film 12 being scanned.

Description

CATHODE RAY TUBES

This invention relates to flying spot telecine machines and the cathode ray tubes used therein and their face plates.

A flying spot telecine machine is used to convert the images in the emulsion on a cinematographic film into electrical signals for the purpose of, for example, broadcasting or data processing.

Typical examples of Flying Spot Telecine Machines are known as MK III, URSA and"C"REALITY. These machines are manufactured by Cintel International.

Typical examples of cathode ray tubes used in flying Spot Telecine Machines are known as the M-18-500 AFDU series and the GT-1800 LF series manufactured by New Century (London) Limited and Brimar Limited respectively.

A schematic of the elements of a flying spot telecine machine is at Figure 1.

In operation a spot of light having known luminance and chrominance is produced in the phosphor screen of the cathode ray tube 2, the spot of light being moved in a required manner on the phosphor screen 4 and focused by a lens 6 onto the image in the emulsion on the film 8.

The luminance and chrominance of the light allowed to pass through the film by the image in the emulsion is detected by suitable detectors thereby

generating an electrical signal which is the required analogue of the image on the film.

A signal generator (not shown) causes the electron beam of the cathode ray tube to traverse the phosphor screen in a controlled manner (for example a raster scan), so that the electrical signal which is generated by the detector system similarly corresponds to a scan of each frame in turn of the film passing over the skid plate, which can then be digitised, or format converted to a video signal, for storage on tape or disc or other processing.

In the schematic of the elementary flying spot telecine machine illustrated in Figure 1 the film is held in position by curved skid plates 10 and moved in a required manner by the film transport mechanism 12. The movement of the film being synchronised with the movement of the spot of light.

The reason for the curvature of the skid plates 10 rises from the manufacturing and developing processes necessary to produce a film whereby the film base may suffer lateral curvature. Similar curvature of the film base can develop during storage.

When used in conjunction with a simple flat faced cathode ray tube 2, skid plate curvature can lead to significant degradation of the resolution of the images produced by the machine depending upon the position of the spot in the phosphor screen of the cathode ray tube.

One purpose of this invention is to eliminate or reduce the degradation of resolution caused by skid plate curvature.

Referring again to the schematic of an elementary flying spot telecine machine at Figure 1, the use of a simple cathode ray tube in the manner illustrated suffers from the problem of flare caused by reflection of light within the faceplate.

To reduce the effect of flare it is known to bond a secondary thick glass faceplate (not shown in Figure 1) to the cathode ray tube 2. A further benefit of this technique is that dust settling on the faceplate of the cathode ray tube 2, which would otherwise be noticeable in use, is moved away from the object plane and, by becoming de-focused, is less noticeable.

A known example of such a thick faceplate is illustrated at Figure 2 where the thick faceplate is cylindrical in shape, a further known example has tapered sides with the thick faceplate being a section of a cone (Figure 3).

These examples are associated with cathode ray tubes which have a circular faceplate. Rectangular thick faceplates could be used on cathode ray tubes having rectangular faceplates.

One embodiment of this invention operates to reduce the degradation of resolution caused by skid plate curvature by providing optical correction between the plane of the phosphor and the plane of the image on the film, the optical correction being incorporated in the secondary thick faceplate which is usually bonded to the face of the cathode ray tube only for the purpose of reducing flare within the faceplate of the cathode ray tube and the effects of dust on the faceplate of the cathode ray tube.

Previously, the secondary thick faceplates were provided with flat outer surfaces. Typically the thickness of such plano/plano secondary thick faceplates being 50-75mm. In this invention, the outer surface of the thick faceplate, being either cylindrical, or the section of a cone in the case of a cathode ray tube having a circular faceplate, or a different shape, as required, for a cathode ray tube with a rectangular faceplate, is formed so that the outer surface of the thick faceplate becomes curved in one plane only (Figure 4 and Figure 5).

The curvature in one plane only of the secondary thick faceplate is obtained by the usual methods of grinding and polishing if glass is used.

Alternatively secondary thick faceplates with a curved outer surface could be manufactured by plastic extrusion and moulding processes if required.

Embodiments of the invention will now be described in greater detail with reference to the accompanying drawings in which: Figure 1 is a schematic diagram showing a flying spot telecine machine of the prior art, which is also suitable to be modified for use as an embodiment of the invention; Figure 2 is a side view showing schematically a cylindrical crosssection prior art thick face plate mounted on a round cathode ray tube; Figure 3 shows a prior art conical section thick face plate mounted on a round cathode ray tube;

Figure 4 corresponds to Figure 2 and shows a curved thick face plate according to a first embodiment of the invention having a cylindrical section mounted on a round cathode ray tube; Figure 5 corresponds to Figure 3 and shows a curved thick face plate according to a second embodiment of the invention having a conical section mounted on a round cathode ray tube; Figure 6 illustrates a curved thick face plate according to a third embodiment of the invention the curved thick face plate being profiled to match a rectangular cathode tray tube Figures 7a-7d are diagrams necessary to illustrate the calculations which are set out below.

Embodiments of the invention may make use of the telecine machine structure illustrated in Figure 1, and common integers will not be referred to further. Mounted upon the cathode ray tube is a thick face plate 20 having a curved outer surface as shown schematically in Figure 4 or Figure 5. Figures 4 or 5 are side elevations, showing the curvature in the vertical plane of the outer surface of the thick face plate. Where the cathode ray tube screen 4 is flat, the inner surface of the thick face plate 20 is likewise flat. In the horizontal plane, however, the thick face plate is not curved. Thus, the curvature of the thick face plate in these embodiments is in the same plane as that of the curved skid plates 10 and hence the film 8 running across them.

The thick face plate thus forms something similar to a cylindrical lens

(although it need not have an outer surface corresponding to a circular arc-in other words, an anamorphic lens).

Calculations to determine the radius of curvature required in the faceplate of a simple cathode ray tube (i. e. a cathode ray tube without a secondary thick faceplate) necessary to provide precision compensation for skid plate curvature are set out below.

Calculations For the simple aberration free lens of focal length f (Figure 7a) the textbook relationships are:

1 1 1 - 7 u v f

and the magnification (lateral) is :

u m ==... 2 v

In the special case where the image plane is a cylindrical surface, ray tracing will show that the object place is also cylindrical but of opposite curvature to the image plane in relation to the lens (Figure 7b).

Now consider the case where the object plane is a curved surface of radius r. The following analysis will show that the radius of curvature of the image plane is also r irrespective of the magnification of the lens.

Consider a point P at a distance h from the axis and displaced bu along the axis (Figure 7c). from I above

1 + =u+Ou v-8v f

Subtracting 3 from I

1111., -- ±u u+8u v v-8v 8u 8 v - 6u ±8v =0 u (u+8u) v (v-8v)

ifs 5u then v > > 8v and

å 8v - ±=0 U2 V2 U2 6u=-bv-... 4 V2 V 8u=-m2Sv... 5

This is the textbook result that the axial magnification is the square of the lateral magnification.

It will also be seen that r'=h'+ (r+8u)' =h2 +r2 +2rSu+8u2 provided that r 8 u then u2 may be ignored and

- 5u=... 6 2r

similarly

(H2) 8v=---... 7 2R

But from 5

ou=-rn20v 2-8v .. ri = Su

But the lateral magnification

h m= H and m2 = h and M2 =h2 H2 8u 8v h2 = 2r H2 2R 1 2 2 2r m =m-1 2R .. r=R

i. e. as stated above, the radius of curvature of the image plane is equal to the radius of curvature of the object plane regardless of the magnification of the lens.

The above calculations demonstrate that, in the case of a simple cathode ray tube, i. e. a cathode ray tube without a flare reduction plate, it is necessary for the faceplate to be curved to the same radius as the skid plate so as to avoid degradation of the performance of the optical system.

For practical reasons arising from the process of settling a high quality phosphor screen it is desirable that the phosphor plane of the cathode ray tube should be flat. In the interest of sustaining the quality of the phosphor screen it has previously been necessary to accept the degradation of performance of the optical system caused by the skid plate curvature when used in association with flat faced telecine cathode ray tubes.

It is the purpose of this invention to eliminate this degradation by curving the outer face of the flare reduction plate, the radius of curvature of the outer face of the flare reduction plate being calculated as follows: The textbook relationship between the thickness of glass and the

apparent displacement d of an object viewed through the glass is given by :

d= t (n-l) n n

where t is the thickness of the glass and n is the refractive index of the glass.

In the special case where the object is the phosphor plane on the surface of the glass then the apparent position of the phosphor plane is displaced d into the glass (Figure 7d).

t =t-SAG x SAG= 2Q x2 2Q

, =ff) d= t-2Q n -/1--f (n)-2Q n n-1 x' n 2R but d=t--- (n) 2R n J 2R 2Q n 2R 1 (n-l) Q n R Q n J R

Typically, for glass, the refractive index n=1. 5 and :

JJ 3Q R

In other words, for precision compensation for the effects of skid plate curvature, the radius of curvature of the outer face of a glass flare reduction plate should be three times the radius of the curvature of the skid plate.

Different refractive mediums will give different results.

Curved flare reduction plates may also be used in conjunction with CRTs having a curved phosphor plane as a means of enhancing compensation for skid plate curvature in these circumstances the inner face of the curved thick face plate must also be curved to match the CRT face plate and the above calculations must be adjusted accordingly.

In practice, different film gates may have different skid plates with different curvature and, on those occasions when a curved flare reduction plate is used in conjunction with an unmatched film gate, the degradation of

resolution that would otherwise result from being used with a flat flare reduction plate is still reduced.

A further benefit of curving the outer face of the flare reduction plate is an improvement in flare reduction. Flare occurs where the plane of the reflecting surface is parallel to the phosphor plane. In the case of a flare reduction plate with a flat surface as illustrated at Figure 2 and Figure 3 the outer surface of the flare reduction plate is entirely parallel to the phosphor plane whereas in the case of a curved flare reduction plate as illustrated at Figure 4 and Figure 5 the plane tangential to the curved surface is only parallel to the phosphor plane at the centre of the flare reduction plate.

Claims

CLAIMS : 1. A thick faceplate for bonding to a Cathode Ray Tube (CRT) located within an optical system, the faceplate having an inner surface profiled to be bonded to the CRT and an outer surface spaced therefrom, the outer surface being curved so as to act as a corrective lens.
2. A faceplate according to claim 1 in which the faceplate acts as an anamorphic lens.
3. A faceplate according to claim 2 in which the outer surface is an approximately cylindrical section.
4. A CRT adapted to be located within an optical system, and having a thick faceplate according to claim 1.
5. A film scanner comprising a CRT according to claim 2 and an optical system.
6. A film scanner comprising a curved support for carrying a film in a curved position, a CRT for generating a scanning spot for scanning the film via an optical system, and a thick faceplate mounted to the CRT faceplate

for flare reduction, characterised in that the thick faceplate has a lens profile arranged to reduce the effect of the curve of the film.
7. A film scanner according to claim 3 or claim 4, in which the inner and outer surfaces of the faceplate are separated by a spacing greater than the depth of focus of the optical system.
8. A method of scanning a film comprising generating a spot using a CRT; illuminating a film with the spot; and transducing the spot as affected by the film to generate an electrical scan signal; characterised by passing the spot through a thick CRT faceplate acting as a lens to reduce the effects of curvature of the film.
9. A method of curving the outer surface of cathode ray tube flare reduction plates to eliminate the effects of skid plate curvature.
10. A method of curving the outer surface of cathode ray tube flare reduction plates to reduce the effects of skid plate curvature.
11. A method of curving the outer surface of a cathode ray tube flare reduction plate to improve the effect of flare reduction.

11. A method of curving the outer surface of a cathode ray tube flare reduction plate to improve the effect of flare reduction.

Amendments to the claims have been filed as follows

I. A thick faceplate for bonding to a Cathode Ray Tube (CRT) locatcd within an optical system, the faceplate having an inner surface profiled to be bonded to the CRT and an outer surface spaced therefrom, the outer surface being curved so as to act as a corrective lens.

2. A faceplate according Lo claim 1 in which the faceplate acts as an anamorphic lens.

3. A faceplate according to claim 2 in which the outer surface is f A an approximately cylindrical section.

4. A CRT adapted to be located within an optical system, and having a thick faceplate according to claim 1.

5. A film scanner comprising a CRT according to claim 2 and an optical system.

6. A film scanner comprising a curved support for carrying a film in a curved position, a CRT for generating a scanning spot for scanning the film via an optical system, and a thick faceplate mounted to the CRT faceplate

for flare reduction, characterised in that the thick faceplate has a tens profiie arranged to reduce the effect of the curve of the film.

7. A film scanner according to claim 3 or claim 4, in which the inner and outer surfaces of the faceplate are separated by a spacing greater than the depth of focus of the optical system.

8. A method of scanning a film comprising generating a spot using a CRT ; illuminating a film with the spot :, and transducing the spot as affected by the film to generate an electrical scan signal ; characterised by passing the spot through a thick CRT faceplate acting as a lens to reduce the effects of curvature of the film.

1-14-7-9. A method of. curving the outer surface of cathode ray tube Hare NE reduction plates to eliminate the effects of skid plate curvature.

1\ c e- ) e-T < +'c-o- R-2'c ?. < =10. A method of curving-the outer surface of cathode ray tube Hare " \e. < -. c & CL'-f reduction plates to reduce the effects of skid plate curvature.

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