US3872344A

US3872344A - Image pickup tube

Info

Publication number: US3872344A
Application number: US289278A
Authority: US
Inventors: Kazuo Shimizu; Okio Yoshida
Original assignee: Tokyo Shibaura Electric Co Ltd
Current assignee: Toshiba Corp
Priority date: 1972-09-15
Filing date: 1972-09-15
Publication date: 1975-03-18
Anticipated expiration: 1992-03-18
Also published as: NL7212747A; FR2199616B1; GB1386687A; DE2249457A1; FR2199616A1

Abstract

An image pickup tube comprises a cylindrical envelope, a target disposed at the front portion of said envelope, and an electron gun assembly so housed in said envelope as to shoot electron beams against said target, said target having a signal pickup electrode disposed outside the effective scanning region of the target.

Description

United States Patent Shimizu et a1.

1 1 IMAGE PICKUP TUBE [75] Inventors: Kazuo Shimizu; Okio Yoshida, both of Yokohama, Japan [73] Assignee: Tokyo Shibaura Electric Co., Ltd.,

Kawasaki-shi, Japan [22] Filed: Sept. 15, 1972 [21] Appl. No.: 289,278

[52] US. Cl. TY...:. ..Ii3f3/386 [51} Int. Cl. HOlj 31/26 [58] Field of Search 313/65 R, 65 A, 65 T, 68,

[56] References Cited UNITED STATES PATENTS 2,487,665 11/1949 Morton et a1. 313/65 R 2,774,002 12/1956 Carlo 2,851,625 9/1958 Ruedy et a1 3,020,442 Z/1962 Nicholson ct a1 313/65 A Mar. 18, 1975 3,136,909 6/1964 Cope 313/65 A 3,155,859 11/1964 Koert .1 313/65 R 3,218,505 11/1965 Yaggy 313/68 R 3,271,608 9/1966 Rome et a1. 313/89 X 3,230,357 10/1966 Nicoll 313/65 R 3,350,591 10/1967 Van Asselt 313/65 T 3,569,763 3/1971 Kiuchi et a1. 313/65 A 7 3,714,488 1/1973 Kato 313/65 R Primary Examiner-Alfred E. Smith Assistant Examiner-Saxfield Chatmon, Jr. Attorney, Agent, or F irm-Flynn & Frishauf [57] ABSTRACT An image pickup tube comprises a cylindrical envelope, a target disposed at the front portion of said envelope, and an electron gun assembly so housed in said envelope as to shoot electron beams against said target, said target having a signal pickup electrode disposed outside the effective scanning region of the target.

14 Claims, 13 Drawing Figures sum 2 of 2 FiG.9

FIG.8

FIG. H

.FiG.i3

FIGIO FIG12 IMAGE PICKUP TUBE The present invention relates to an image pickup tube, in particular, to one having a signal pickup electrode which is so improved as to reduce the residual image.

One of the TV image pickup tubes priorly known is a vidicon tube, wherein a transparent electric conductive layer is formed on the inner surface of the faceplate and functions as a signal pickup electrode, and a photoelectric conversion layer is vapor-deposited on said electric conductive layer and functions as a photoelectric conversion target. The transparent electric conductive layer of the target is impressed with positive voltage, while the photoelectric conversion layer is uniformly negative-charged as it is scanned with electron beams from the electron gun. Consequently, there appears an electric field about and perpendicular to said photoelectric conversion layer. The photoelectric conversion layer works as a capacitor because of its high resistivity, and its electrostatic capacity is inversely proportional to its thickness. Accordingly, the thicker the layer is made, the smaller its capacity will become and the more reduced will be the residual image on it. However, if the layer is made thicker, the image resolution is proportionally degraded.

The present invention aims to provide an image pickup tube which can minimize residual image and yet resolve the image effectively.

SUMMARY OF THE INVENTION The image pickup tube according to the present invention comprises a cylindrical envelope having a faceplate, a target disposed on the inner surface of said faceplate, and an electron gun so housed in said envelope as to emit an electron beam against said target. The target is comprised of a photoelectric conversion layer disposed on the inner surface of the faceplate, a high resistivity layer disposed on said photoelectric conversion layer and a signal pickup electrode disposed outside the effective scanning region of the target. Around this image pickup tube there will be arranged in a well-known manner a focus coil, an electron beam deflection coil and an alignment coil.

The present invention can be more fully understood from the following detailed description when taken in connection with the accompanying drawings, in which:

FIG. 1 is a sectional view of an image pickup tube according to the present invention;

FIGS. 2 to 4 are side sectional views of various targets of the image pickup tube;

FIG. 5 is a front view of the image pickup tube having either one of the targets shown in FIGS. 2 to 4;

FIG. 6 is a side sectional view of another target having a transparent electric conductive layer disposed outside the effective scanning region;

FIG. 7 is a front sectional view of the target illustrated in FIG. 6;

FIG. 8 is a front sectional view of another target;

FIG. 9 is a side sectional view of another target;

FIG. 10 is a side sectional view of still another target provided with a transparent electric conductive layer on the outer surface of the faceplate of the envelope;

FIG. 11 is a side sectional view of the front portion of the image pickup tube attached with the target shown in FIG. 10;

FIG. 12 is a side sectional view of another target having an insulation layer mounted on the central surface of the electric conductive layer deposited nearly all over the inner surface of the faceplate; and

FIG. 13 is a side sectional view of a target of multilayer type having a ring-shaped electrode and a central electrode layer.

Referring to FIG. 1, the image pickup tube comprises a cylindrical envelope 11 having at its front end a faceplate 12, a target 13 on the inner surface of said faceplate 12, an electron gun 14 so housed in said envelope 11 as to-emit an electron beam against said target 13. On the periphery of theimage pickup tube, a reflective coil 15, a focus coil 16 and an alignment coil 17 are arranged in a well-known manner.

In the target as illustrated in FIG. 2, a

cadmium selenide layer

18, 2 micrometers thick, is prepared on the inner surface of the faceplate 12. An electric conductive layer 19 serving as a signal pickup electrode covers the periphery and that surface of said cadmium selenide layer 18 not corresponding to the effective scanning region of the target 13 to be scanned with electron beams. On said cadmium selenide layer 18 and said electric conductive layer 19, 0.2-micrometer thick layer 20 of a high resistivity material such as arsenic trisulfide is vapor-deposited. A vidicon tube provided with a target of such construction was provided, upon repeated tests, to possess a high photoelectric sensitivity and an excellent effect of reducing any residual image. Owing to said good residual image reducing effect of the vidicon tube, the level of residual image signal in the third field at the end of scanning became 0% after the start of light-break and after start of-lightsupply. Notably, it was found that after the start of light-break, no residual image signal was observed in the first field upon the end of scanning.

Although it has yet to be theoretically explained why the target so greatly helps the vidicon tube to reduce residual image, it is supposed that said excellent residual image reducing effect results from the specific construction of the target according to this invention. Namely, the target of this invention, which is of multilayer type, has an electrostatic capacity smaller than that of a conventional target of uni-layer type. Further, it was found that the good residual image reducing effect'comes from the fact that the image signal pickup electrode covers not all the surface of the photoelectric conversion layer 18 but only on that portion of the layer 18 which does not correspond to the effective scanning region of the target; the capacity of the layer 18 determined by the diameter of the layer 18 is utilized for reduction of residual image in addition to the capacity determined by the thickness of the layer 18.

Another factor affecting the residual image reducing effect on a vidicon tube is the resistivity of the photoelectric conversion layer of the target. Upon the several experiments it was found preferable that the photoelectric conversion layer 18 of the target shown in FIG. 2 has a resistivity of less than IO Q-crn and that if the layer 18 has a resistivity as small as lOQ-cm, the target does not so function to reduce the residual image.

In view of these findings,.the target employed in the image pickup tube of this invention is provided with a signal pickup electrode on the periphery of its photoelectric conversion layer, and said photoelectric conversion layer has been selected to have a resistivity ranging from 10 O-cm to 10 Q-cm.

There will now be explained how the target of this invention as illustrated in FIG. 2 works.

First, the light from the object enters the cadmium selenide layer 18 through the faceplate 12 made of,

transparent glass. The light then controls the electrons and holes in cadmium selenide layer 18 so that the electrons radially moves toward the signal pickup electrode 19 and that the holes are trapped and arranged in the forbidding band of layer 18 in accordance with their energy levels. In proportion to the number of the trapped holes, the surface potential of the arsenic trisulfide layer 20 is elevated. An electron beam scanned on the surface of said layer 20 and the holes in layer 20 re-unify with the electrons of the electron beam, whereby discharge current flows through the target. In this case, the trapped holes do not re-unify with the electrons of the beam right after impinging the beam. A first coming electron does not re-unify with a hole but reaches signal pickup if it penetrates into the layer 20 within the time necessary for forming one picture element; it is a second coming electron that re-unifies with a hole. Thanks to such electron-hole reunification, two electrons enter the target per hole. As a result, a vidicon tube with the target as shown in FIG. 2 attains a sensitivity favorably doubled.

.The number of electrons which enters the target from the beam is proportionate to that of holes in the effective scanning region of the-target. Namely, not only the holes trapped in the target during the time required for forming one frame, but those energized by the light from the object before and during the electron beam scanning contribute effectively to the enhancement of the signal output. In the image pickup tube according to this invention, the holes other than those accumulated in cadmium selenide layer 18 also help effect a vidicon-operation and therefore contribute to residual image reduction. This means that it is desirable that layer 18 possess a high electronic conductivity and that the holes remain in their original places or move toward and be trapped beneath the scanning surface of cadmium selenide layer 18.

More specifically, in the target shown in FIG. 2, upon the electron beam scanning, there is formed a strong electric field perpendicular'to the target 13 and the holes, before being trapped in cadmium selenide layer 18, moves to the border between cadmium selenide layer 18 and arsenic trisulfide layer 20, or further move into arsenic trisulfide layer 20 and are trapped in the layer 20 or beneath the surface of thelayer 20. Consequently, no hole moves toward signal pickup electrode 19.

Another target according to the present invention as illustrated in FIG. 3 comprises a cadmium selenide layer 18, a cadmium selenite layer 21 formed on said cadmium selenide layer 18, an arsenic trisulfide layer 20 formed on said cadmium selenite layer 21, and a signal pickup electrode 19 covering the peripheries of cadmium selenide layer 18 and cadmium selenite layer 21 and the circumferential portion of the surface of cadmium selenite layer 21.

The target of this invention shown in FIG. 4 is, like that shown in FIG. 3, constituted of three

layers

18, 20 and 21, but provided with a signal pickup electrode 19 formed between cadmium selenide layer 18 and faceplate 12 and outside the effective scanning region of the target. Because of this specific disposition of signal pickup electrode 19, electrons in the cadmium selenide layer 18 are easily moved toward the electrode 19.

The signal pickup electrode 19 may be made of either transparent tin oxide (SnO or opaque aluminium or tin.

The photoelectric layer 18 may be made of photoelectric conversion materials other than cadmium selenide such as cadmium sulfide and cadmium sulfoselenide-all the material having a resistivity ranging from 10 Q-cm to l0 Q-cm.

Similarly, the layer 20 may be formed of, in place of arsenic trisulfide, any one of such materials as control the electron flow from the electron beam into the target to lessen the dark signal current and sustain the image resolution of the image pickup tube. Suitable materials are, for example, arsenic disulfide, zinc sulfide, arsenic trisulfide, germanium selenide, arsenic triselenide, zinc selenide and antimony trisulfide.

The targets shown in FIGS. 2, 3 and 4 each having a signal pickup electrode disposed outside the effective scanning region are quite effective in residual image reduction, but are not free from the following phenomenon. When a rectangular white object witha black frame in the white background is photographed by vidicon, the white object appears gray when it is reproduced on the screen of the reproduction monitor. This undesirable occurrence is herein referred to as blackframephenomenoniThis phenomenon is inevitably observed also when a black rectangular object with a white frame in the black background is photographed by a vidicon tube.

It is well presumed that the black-frame phenomenon takes place in the following process. When a white object with a black frame is imaged on the photoelectric conversion layer of the vidicon tube, a greater part of the voltage impressed across the target is applied to that surface of the photoelectric conversion layer on which the black frame is imaged. That surface of the photoelectric conversion layer on which the white object is imaged is impressed with only a smallpart of the voltage, and inevitably degraded in photoelectric sensitivity. Consequently, the image signals from said surface of the layer become feeble.

There will now be described other targets according to the present invention which are free from the above discussed black frame phenomenon, and yet effective in residual image reduction.

FIG. 6 shows a target which is, like the target of FIG. I

3, comprised of a cadmium selenide layer 18, a cadmium selenite layer 21, a layer 20 of a high resistivity material and a signal pickup electrode 19, and, unlike the target of FIG. 3, further comprised of a transparent electric conductive layer 22 sandwiched between the layer 18 and the faceplate 12 and insulated from the electrode 19. This transparent electric conductive layer 22 has a surface area at least as large as that of the effective scanning region of the target. Since layer 22 is insulated from electrode 19, it functions merely to keep any portion of photoelectric conversion layer 18 at the same potential. The specific use of the transparent electric conductive layer 22 makes the target free from the black-frame phenomenon.

The target of FIG. 6 works to reduce the residual image for the following reason. Almost all the picture elements of a raster formed on the effective scanning region of the target are resolved thanks to photoelectric conversion layer 18 whose electrostatic capacity is small, and the residual picture elements are reduced by the electrostatic capacity between signal pickup electrode 19 and transparent electric conductive layer 22. The capacity between electrode 19 and layer 22 is in reverse proportion to the sum total of the thickness of the target (measured from layer 22 to the scanning surface) and the minimum distance between electrode 19 and layer 22. Electronic conduction may occur during a long operation of the target between signal pickup electrode 19 and transparent electric conductive layer 22. If this is the case, the capacity of photoelectric conversion layer 18 grows larger than that of a target provided with no transparent electric conductive layer 22. However, the small capacity in the minimum space between electrode 19 and layer 22 further reduces the residual image.

Accordingly, the target of FIG. 6, as it is provided with among other things a transparent electric layer 22, makes the target free from any black-frame phenomenon and reduces the residual image.

FIGS. 7 and 8 show two different arrangements of the signal electrode 19 and the transparent electric conductive layer 22 employed in the target of FIG. 6.

In the arrangement of FIG. 7, a disc-shaped electrode 19 with a rectangular window surrounds a rectangular electric conductive layer 22. In the arrangement of FIG. 8, a ring-shaped electrode 19 surrounds a discshaped electric conductive layer 22. In both arrangements, the electrode 19 and the layer 22 can overlap to some extent one upon the other since there is formed between them a photoelectric conversion layer having a great resistivity.

FIG. 9 shows still another target according to the present invention which is identical with that as illustrated in FIG. 4 except that it is provided with a transparent electric conductive layer 22 deposited on the inner surface of the faceplate 11. The front sectional view of this target is therefore similar to FIG. 8.

In the targets shown in FIGS. 6, 7, 8 and 9, the transparent electric conductive layer 22 is an electrically low resistive layer of chemical compound such as a tin oxide (SnO layer (Nesa layer) or a metal or chemical compound layer through which specific light beams can pass into the photoelectric conversion layer 18. It is possible to control the photosensitivity of these targets by adjusting the voltage to be impressed on the transparent electric conductive layer 22. Further, it is possible with these targets to penetrate lights other than those from the object into that portion of layer 18 between electrode 19 and layer 22, or scan the electron beam on the electrode 19 topenetrate electrons of the beam into said portion layer 18, thereby to vary the resistivity of said portion of layer 18 and ultimately control the photosensitivity of the target.

Another target of this invention, as shown in FIG. 10, is of multi-layer type similar to that of FIG. 3 in that it is made up of cadmium selenide layer 18, rign-shaped electrode 19, a high resistivity layer 20 and a cadmium selenite layer 21. This target, however, is characterized by a transparent electric conductive layer 22 mounted on the outer surface of the faceplate 12. The target is, as illustrated in FIG. 11, fitted to the front end of the envelope 11 by means of a stainless steel ring 23 fixed on the outer periphery of electrode 19. Said stainless steel ring 23 is covered with a ring 24 made of a flat indium bar, which attaches the target assembly airtightly to the envelope l1.

In the target as illustrated in FIG. 12, a transparent electric conductive layer 22 is deposited on the inner surface of the faceplate 12, and an insulation layer 25 is formed on said layer 22. This target is characterized in that it is provided with an insulation layer 25 formed on that surface of the layer 22 which corresponds to the effective scanning region of the target. On the

layers

22 and 25, a cadmium selenide layer 18, a cadmium selenite layer 21 and an arsenic trisulfide layer 20 are successively formed.

In the image pickup tube having such a target as is shown in FIG. 12, the transparent electric conductive layer 22 acts as a signal pickup electrode and is therefore impressed with target voltage. The electric field about the insulation layer 25 is uniform in potential in the radial direction of the target. It is supposed that the electric field of a uniform potential prevents the electrons coming into the target upon the electron beam scanning from penetrating into the electric conductive layer 22 through the insulation layer 25. Consequently, the electrons move through the cadmium selenide layer 18 in the direction of arrows in FIG. 12 toward the periphery of layer 22.

FIG. 13 shows another target according to the present invention which is almost identical with that illustrated in FIG. 12. The target is different from thetarget of FIG. 12 in that its transparent electric conductive layer 22 formed on the faceplate 12 is covered up with an insulating layer 26. Because of this specific construction, electrons coming from the electron beam into the target move toward the periphery of the photoelectric conversionlayer 18. As the electrons are all picked up by the electrode 19, residual image is greatly reduced and no black-frame phenomenon results from the image pickup tube according to the present invention.

What we claim is:

1. An image pickup tube comprising a vacuum envelope containing an electron gun, a target disposed on the front of the envelope and a signal pickup electrode, said target having an effective scanning region which is scanned by the electron beam emitted by said electron the improvement wherein:

said target comprises a first photo-conductive layer made of cadmium selenide disposed on the inner surface of the faceplate of said envelope, a second layer disposed on said first photo-conductive layer, a high resistivity layer disposed on said second layer and a transparent electrically conductive layer disposed on the outer central surface of said first photo-conductive layer corresponding to said effective scanning region; an

said signal pickup electrode is located so as to reduce the residual image on said effective scanning region of said target, said signal pickup electrode being disposed only on the edge portion of said target and not on said effective scanning region of said target, said signal pickup electrode surrounding said transparent conductive layer and being spaced from said transparent conductive layer.

2. An image pickup tube according to claim 1 wherein said signal pickup electrode is disposed on the 5 peripheries of said first and second layers and on the edge portion of said second layer but not on the central surface corresponding to said effective scanning region.

3. An image pickup tube according to claim 1 wherein said signal pickup electrode is disposed on the outer surface of said photo-conductive layer in such a manner as to surround said transparent electrically conductive layer with a space therebetween.

4. An image pickup tube according to claim 1 wherein said transparent electrically conductive layer is a rectangular layer the entire surface of which corresponds to said effective scanning region, and said signal pickup electrode is a disc with a rectangular window and surrounds said transparent conductive layer with a space therebetween.

5. An image pickup tube according to claim 1 wherein said transparent electrically conductive layer is a disc the surface area of which is larger than that of said effective scanning region, and said signal pickup electrode is a ring and surrounds said transparent conductive layer with a space therebetween.

6. An image pickup tube according to claim 1 further comprising an insulating layer disposed on and covering up said transparent electrically conductive layer.

7. An image pickup tube according to claim 1 wherein said high resistivity layer is formed of one of the group consisting of arsenic disulfide, zinc sulfide, arsenic trisulfide, germanium selenide, arsenic triselenide, zinc selenide and antimony trisulfide.

8. An image pickup tube according to claim 1 wherein said second layer is formed of cadmium selenite, and said high resistivity layer is formed of arsenic trisulfide.

9. An image pickup tube comprising a vacuum envelope containing an electron gun, a target disposed on the front of the envelope and a signal pickup electrode, said target having an effective scanning region which is scanned by the electron beam emitted by said electron the improvement wherein:

said target comprises a first photo-conductive layer made of cadmium selenide disposed on the inner surface of the faceplate of said envelope, a second layer disposed on said first photo-conductive layer, a high resistivity layer disposed on said second layer and a transparent electrically conductive layer disposed on the outer surface of the faceplate; and

said signal pickup electrode is located so as to reduce the residual image on said effective scanning region of said target, said signal pickup electrode being wherein said second layer is formed of cadmium selenite, and said high resistivity layer is formed of arsenic trisulfide.

12. An image pickup tube comprising a vacuum envelope containing an electron gun, a target disposed on the front of the envelope and a signal pickup electrode, said target having an effective scanning region which is scanned by the electron beam emitted by said electron the improvement wherein:

said signal pickup electrode is located so as to reduce the residual image on said effective scanning region of said target, said signal pickup electrode comprising a transparent electrically conductive layer disposed on the inner surface of the faceplate of said envelope, the central surface of said transparent conductive layer corresponding to said effective scanning region and being covered with an insulating layer;

said target comprises a first photo-conductive layer made of cadmium selenide disposed on said signal pickup electrode, a second layer disposed on said first photo-conductive layer and a high resistivity layer disposed on said second layer; and

said signal pickup electrode further comprising electrically conductive electrode means disposed only on the edge portion of said target and not on said effective scanning region of said target.

13. An image pickup tube according to claim 12 wherein said high resistivity layer is formed of one of the group consisting of arsenic disulfide, zinc sulfide, arsenic trisulfide, germanium selenide, arsenic triselenide, zinc selenide and antimony trisulfide.

14. An image pickup tube according to claim 12 wherein said second layer is formed of cadmium selenite, and said high resistivity layer is formed of arsenic trisulfide.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 2 3,872,344

DATED 1 March 18 mvrmomsr Kazuo SHIMIZU, et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 8, line 7, change "sulfied" to sulfide-.

Signed and Sealed this twenty-eight D :1 Of October 1975 [SEAL] A ttest:

RUTH C. MASON C. MARSHALL DANN Arresting 0mm Commissioner oj'Parenrs and Trademarks

Claims

1. An image pickup tube comprising a vacuum envelope containing an electron gun, a target disposed on the front of the envelope and a signal pickup electrode, said target having an effective scanning region which is scanned by the electron beam emitted by said electron gun; the improvement wherein: said target comprises a first photo-conductive layer made of cadmium selenide disposed on the inner surface of the faceplate of said envelope, a second layer disposed on said first photoconductive layer, a high resistivity layer disposed on said second layer and a transparent electrically conductive layer disposed on the outer central surface of said first photoconductive layer corresponding to said effective scanning region; an said signal pickup electrode is located so as to reduce the residual image on said effective scanning region of said target, said signal pickup electrode being disposed only on the edge portion of said target and not on said effective scanning region of said target, said signal pickup electrode surrounding said transparent conductive layer and being spaced from said transparent conductive layer.

2. An image pickup tube according to claim 1 wherein said signal pickup electrode is disposed on the peripheries of said first and second layers and on the edge portion of said second layer but not on the central surface corresponding to said effective scanning region.

9. An image pickup tube comprising a vacuum envelope containing an electron gun, a target disposed on the front of the envelope and a signal pickup electrode, said target having an effective scanning region which is scanned by the electron beam emitted by said electron gun; the improvement wherein: said target comprises a first photo-conductive layer made of cadmium selenide disposed on the inner surface of the faceplate of said envelope, a second layer disposed on said first photo-conductive layer, a high resistivity layer disposed on said second layer and a transparent electrically conductive layer disposed on the outer surface of the faceplate; and SAID signal pickup electrode is located so as to reduce the residual image on said effective scanning region of said target, said signal pickup electrode being disposed only on the peripheries of said first and second layers and only on the edge portion of said second layer but not on the central surface thereof corresponding to said effective scanning region.

10. An image pickup tube according to claim 9 wherein said high resistivity layer is formed of one of the group consisting of arsenic disulfide, zinc sulfied, arsenic trisulfide, germanium selenide, arsenic triselenide, zinc selenide and antimony trisulfide.

11. An image pickup tube according to claim 9 wherein said second layer is formed of cadmium selenite, and said high resistivity layer is formed of arsenic trisulfide.

12. An image pickup tube comprising a vacuum envelope containing an electron gun, a target disposed on the front of the envelope and a signal pickup electrode, said target having an effective scanning region which is scanned by the electron beam emitted by said electron gun; the improvement wherein: said signal pickup electrode is located so as to reduce the residual image on said effective scanning region of said target, said signal pickup electrode comprising a transparent electrically conductive layer disposed on the inner surface of the faceplate of said envelope, the central surface of said transparent conductive layer corresponding to said effective scanning region and being covered with an insulating layer; said target comprises a first photo-conductive layer made of cadmium selenide disposed on said signal pickup electrode, a second layer disposed on said first photo-conductive layer and a high resistivity layer disposed on said second layer; and said signal pickup electrode further comprising electrically conductive electrode means disposed only on the edge portion of said target and not on said effective scanning region of said target.