GB2149200A

GB2149200A - Imaging and streaking tubes

Info

Publication number: GB2149200A
Application number: GB08410837A
Authority: GB
Inventors: Yoshiji Suzuki; Katsuyuki Kinoshita
Original assignee: Hamamatsu Photonics KK
Current assignee: Hamamatsu Photonics KK
Priority date: 1983-09-06
Filing date: 1984-04-27
Publication date: 1985-06-05
Also published as: JPS6056341A; GB8410837D0; GB2149200B

Abstract

In light-imaging tubes, such as imaging or streaking tubes wherein light incident on a photoelectric layer 4 causes photoelectrons to be emitted and to produce a light image on striking phosphor layer 9, background noise can be reduced by placing between the photoelectric layer 4 and phosphor layer 9 a separation wall 30 provided with an aperture 13 which is open during operation but is closed by lid means 14 during the formation of photoelectric layer 4 using alkali metal vapour from source 17. This prevents vapour reaching the phosphor layer. <IMAGE>

Description

SPECIFICATION Imaging and streaking tubes The present invention relates to imaging and streaking tubes, in particular to reduce background noise therein, and to a method of manufacturing such tubes.

Imaging tubes and the structurally closely related streaking tubes are well known in the art. Imaging tubes are used to amplify and observe low light intensity while streaking tubes can be used to analyze the light intensity distribution with time of incident light sources.

Conventional imaging and streaking tubes are illustrated in Figures 1 and 2 respectively of the accompanying drawings. In these figures features common to both tube types are indicated by common reference numerals.

In both tubes, light incident on window 1 causes emission of photoelectrons from photoelectric layer 2. The photo-electrons are focussed by focussing electrodes 6 and ultimately are incident on phosphor layer 9 causing a light image to be emitted through window 2.

In the case of the imaging tube shown in Figure 1, the focussed photoelectrons are multiplied by micro-channel plate 8 to produce an enhanced image whereas in the case of the streaking tube shown in Figure 2 the focussed photoelectrons pas s between deflection electrodes 108 across which a time dependent voltage is applied causing the photoelectrons to be deflected in the out-of-plane direction in Figure 2 and resulting in the emission through window 2 of a streak image in which the said out-of-plane dimension corresponds essentially to a time coordinate.

The streaking tube can convert an incident light pulse with a duration of 1 ns into a streak having a length of the order of several tens of millimeters on the phosphor layer, and it has an excellent time resolution of 2 pico seconds or less. The streaking tube is thus widely used for analyzing the waveforms of laser pulses.

Such conventional imaging and streaking tubes can be fabricated by the following method.

A hollow glass cylinder is constructed to form the wall of vacuum envelope 3. Next, first and second glass discs 1 and 2 on which photoelectric layer 4 and phosphor layer 9 respectively may be formed are constructed.

Discs 1 and 2 will form the light admitting and emitting end windows of the tube. The internal components of the tube, i.e. in the case of the imaging tube the elements used for making such electrodes as mesh electrode 5, focusing electrode 6, aperture electrode 7, and micro-channel-plate 8 and in the case of the streaking tube the elements used for making such electrodes as mesh electrode 5, focusing electrode 6, aperture electrode 7 and deflection electrodes 108 are prepared and then disposed within the glass cylinder 3. At that time, antimony metal contained within a tungsten coil to form an evaporation source of antimony is located against the photoelectric layer substrate.

Phosphor materials are coated on one surface of the second glass disc 2. First and second glass discs 1 and 2 are attached to the appropriate ends of the glass cylinder, and then the resulting envelope is exhausted to obtain a vacuum.

A branching tube is fastened to the side wall of the sealed envelope and an alkaline metal source is housed in this branching tube Air is then exhausted from the sealed envelope via the exhausting tube attached thereto.

A current is applied to flow through the tungsten coil so that antimony metal may be deposited onto the photoelectric layer substrate. The alkali metal is gradually fed from the branching tube into the envelope, while the sensitivity of the photoelectric layer is monitored, until the maximum sensitivity is obtained. Thereafter, the branching tube and the exhausting tube are removed to complete the imaging tube.

It can easily be understood from the description of the fabrication method that a small amount of alkali metal necessarily adheres to each electrode while alkali metal is being fed to the sealed envelope.

When an imaging or streaking tube fabricated in accordance with such a process is operated, the phosphor layer sometimes emits light even when no light is incident upon the photoelectric layer as a result of the decrease in the work function caused by the alkali metal.

When a high voltage is applied to microchannel-plate 8 in an imaging tube or when an RF voltage is repetitively applied to deflection electrode 108 in a streaking tube, this mode of light emission is especially enhanced.

This mode of light emission causes the S/N ratio to decrease af fe cting the background noise for the image or the streaking image, and it makes the dynamic range low.

In the case of the imaging tube, we have found that the phosphor layer emitted light without any incident light when a voltage was applied only to the phosphor layer or the micro-channel-plate unless voltages were also applied to the imaging section consisting of a photoelectric layer, a focusing electrode, and an aperture electrode. It was also found that the objectionable light emission was connected with the presence of the micro-channel-plate. Furthermore, it was found that the background sensitivity was not increased when a set of voltages was applied to the respective electrodes of an imaging tube having the same dimensions but in which no photoelectric alkali layer was provided.The above phenomena suggest that generated electrons increase the background sensitivity due to the following reasons: Alkali metal adheres to the inner surface of the micro-channel-plate which multiplies secondary electrons, while the photoelectric layer is being formed, and it decreases the work function of electrons at the surface. When a voltage is applied to the micro-channel-plate during operation, high localized electric fields are generated at microscopic locations of nonuniform areas on the inner surface thereof.

Interaction of both the low work function and high electric field causes the inner surface of the micro-channel-plate to emit electrons.

Electrons generated due to field emission are multiplied by the micro-channel-plate and, incident upon the phosphor layer, cause the unwanted background sensitivity to increase.

In the case of the streaking tube, study of the photoelectrons, from the photoelectric layer, which were generated due to light emitted by excitation or ionization of gaseous molecules or atoms which had collided with electrons, or by collision of electrons or ions with the sealed envelope, showed the main reason for unwanted photoelectric emission was caused by the effect of the deflection electrode 108 on the dynamic range.

It was found that, unless any voltage was applied across a pair of deflection electrodes although a high DC voltage was applied across photo-electric layer 4 and aperture electrode 7, light emission occurring in phosphor layer 9 was diminished in intensity while enhanced by the repetitive sweep voltage applied across the deflection electrode.

Objectives of the present invention are to provide imaging tubes and streaking tubes in which unwanted light emission is reduced and to provide a method of fabricating such tubes.

In one aspect, the present invention thus provides a light-imaging tube having a photoelectric layer capable of emitting photoelectrons on the incidence of light thereon and electron detecting means capable of detecting the photoelectrons, characterized in that said tube comprises a separation wall located between said photoelectric layer and said electron detecting means and provided with an aperture through which photoelectrons emitted from said photoelectric layer and travelling towards said electron detecting means may pass, a lid means movable between a first position whereat said apertur e is closed and a second position whereat said aperture is open, and means for moving said lid means into said second position.

The term "a light-imaging tube" is used herein to include both imaging and streaking tubes to refer to tubes capable of producing a light image following incidence of light thereon.

Preferably the electron detecting means is a phosphor layer capable of emitting light on the incidence of photoelectrons, preceded by a micro-channel plate in the case of an imaging tube or deflection electrodes in the case of a streaking tube.

The separation wall is preferably disposed within the light imaging tube with its aperture at or near the crossover point or focus point of the photoelectrons emitted from the photoelectric layer. With the lid means open, the aperture permits photoelectrons to pass through to be incident on the micro-channelplate where the tube is an imaging tube or to pass between the deflection electrodes if the tube is a streaking tube.

During fabrication of the tube however, the lid means may be maintained in the closed position while the photoelectrie layer is being formed thus effectively preventing components such as the micro-channel-plate or deflection electrodes which are on the phosphor layer side of the separation wall from being coated with the alkali metal.

Thus in a further aspect, the present invention provides a method of fabricating a lightimaging tube comprising a vacuum envelope containing a photoelectric layer and electron detecting means, with disposed between the said layers a separation wall provided with an aperture through which photoelectrons emitted from said photoelectric layer and travelling towards said electron detecting means may pass. a lid means movable between a first position whereat said aperture is closed and a second position whereat said aperture is open, and means for moving said lid means into said second position, said method comprising assembling the internal components of said tube prior to the formation of said photoelectric layer, exhausting said envelope on each side of said separation wall, forming said photoelectric layer with the deposition of alkali metal while maintaining said lid means in said first position, exhausting said envelope to remove therefrom excess photoelectric layer forming materials, and operating said means for moving said lid means to move said lid means into said second position.

In one preferred embodiment the method of the invention is a method of fabricating an imaging tube having in its envelope a microchannel-plate to multiply photoelectrons emitted from the photoelectric layer and usable to observe a diminished light image and comprising (a) an assembling process which comprises providing a lid covering an aperture in a separation wall to separate a first space which includes at least a focussing electrode and a surface on which may be formed a photoelectric layer from a second space which includes at least a micro-channel-plate and a phosphor layer, the separation wall being arranged with its aperture on the tube axis at or near the photoelectron crossover point; (b) an exhausting process to exhaust the first and second spaces; (c) a photoelectric layer form ing process to form a photoelectric layer while introducing alkali metal to form the photoelectric layer via a branching tube into said first space; (d) an ejection process to cut the branching tube, to exhaust the envelope while the envelope is being heated, and to eject the excess photoelectric layer forming materials, and (e) a removing process to remove the lid from the aperture after completion of exhausting operations.

In another preferred embodiment, the method of the invention is a method of fabricating a streaking tube having in its envelope a deflection electrode to deflect photoelectrons emitted from the photoelectric layer and to observe a diminished light image generated by the deflected photoelectrons and comprising (a) an assembling process which comprises providing a lid covering an aperture in a separation wall to separate a first space which includes at least a focussing electrode and a surface on which may be formed a photoelectric layer from a second space which includes at least a deflection electrode and a phosphor layer, the separation wall being arranged with its aperture on the tube axis at or near the photoelectron crossover point, (b) an exhausting process to exhaust the first and second spaces; (c) a photoelectric layer forming process to form a photoelectric layer while introducing alkali metal to form the photoelectric layer via a branching tube into said first space; (d) an ejection process to cut the branching tube, to exhaust the envelope while the envelope is being heated, and to eject the excess photoelectric layer forming materials; and (e) a removing process to remove the lid from the aperture after completion of exhausting operations.

The said first space is designed to be filled with alkali metal vapor for forming the photoelectric layer during fabrication, and the second space is designed to protect the microchannel-plate or the deflection electrode against being covered by alkali metal vapor during this period.

By disposing the aperture at or near the photoelectron crossover point, the size of the aperture can be kept to a minimum thus keeping low the travelling of alkali metal into the second space (and onto the micro-channel plate or the deflection electrode) during the operation of the tube.

Thus the tube is designed so that the microchannel-plate or the deflection electrode will not be contaminated by residual alkali metal during operation.

Even if light emission has occurred due to ionization near the micro-channel-plate or deflection electrode or due to collision of electrons to the inner wall of the sealed envelope, the tube is designed so that light does not arrive at the photoelectric layer thus preventing the phosphor layer from unwanted light emission.

In the operation of the imaging tube, a higher DC voltage is applied to the focusing electrode with respect to the photoelectric layer, and another higher DC voltage to the aperture electrode with respect to the focusing electrode. A DC voltage the same as or a little higher than that applied to the aperture electrode is aplied to the input electrode of the micro-channel-plate, and a higher DC voltage than that applied to the input electrode is applied to the output electrode of the microchannel-plate. A further higher DC voltage than that applied to the output electrode of the micro-channel-plate is applied to the phosphor layer.

Thus, if an optical image is incident onto the photoelectric layer, the photoelectric layer emits an electron image corresponding to the optical image, and the emitted electrons are accelerated and focused by the focusing electrode. They pass through both the aper ture electrode and the micro-channel-plate, and arrive at the phosphor layer to be focused thereon.

A suitable micro-channel-plate provides a secondary electron emitting surface of lead oxide on the inner walls of fine glass tubes.

The multi-channel-plate consists of a strand of the order of 106 Of these fine glass tubes, each having an inner diameter of 1 5 microns, and being 0.9 mm long. The strand has a diameter of 25 mm.

The incident electrons are multiplied by the micro-channel-plate and then the multiplied electrons are emitted from the micro-channelplate.

The multiplication factor depends on the voltage difference between the input and output electrodes. When the voltage difference between input and output electrodes changes from 1.3 kV to 1.9 kV, the multiplication factor goes from 103 to 3 X 106.

In operation of the streaking tube, a higher DC voltage is applied to the mesh electrode with respect to the photoelectric layer, another higher DC voltage to the focusing electrode 1 with respect to the mesh electrode, and a further higher DC voltage to the aperture electrode with respect to the focusing electrode. A DC voltage the same as or a little higher than that applied to the aperture electrode is applied to the phosphor layer.

Thus if a linear optical image which lies in the center of the photoelectric layer is incident onto the photoelectric layer, the photoelectric layer emits an electron image corresponding to the optical image, and the emitted electrons are accelerated by the mesh electrode and focused by the focusing electrode. They pass through both the aperture electrode and arr ive at the phosphor layer to be focused thereon.

While the linear electronic image is passing through a gap within the deflection electrode, a deflection voltage is applied to the deflec tion electrode. The electric field caused by this deflection voltage is normal to both the tube axis and linear electronic image. The field strength is proportional to the deflection voltage. The electron beam on the phosphor layer travels normal to the linear electronic image when scanned. A series of linear optical images are arranged onto the photoelectric layer in a direction perpendicular to the linear images in a ser ies of time, and thus a streaking image can be formed. Brightness change in the direction that a series of linear optical images are arranged or that scanning is being carried out indicates a change in intensity of the optical image incident on the phosphor layer.

Preferred embodiments of the tubes of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a cross-sectional view of the configuration of a conventional imaging tube, together with an interrelation between the photoelectric layer and optical image; Figure 2 shows a cross-sectional view of the configuration of a conventional streaking tube together with an interrelation between the photoelectric layer and optical image; Figure 3 shows a longitudinal cross-sectional view of an imaging tube during its fabrication in accordance with the present invention; Figure 4 shows a longitudinal cross-sectional view of a streaking tube during its fabrication in accordance with the present invention;; Figures 5A and B show transverse crosssectional views of the tubes of Figures 3 and 4 showing the lid means in the closed and open positions respectively; Figures 6 A and B and C show partial longitudinal and transverse cross-sectional views of further tuBes according to the invention having an alternative configuration of the separation wall and lid; Figures 7 A and B and C show partial longitudinal and transverse cross-sectional views of further tubes according to the invention having a third configuration of the separation wall and lid; Figures 8 A, B, C and D show partial longitudinal and transverse cross-sectional views of further tubes according to the invention having a fourth configuration of the separation wall and lid;; Figures 9 A and B show the dynamic characteristics for an imaging tube in accordance with the present invention as compared to those for an equivalent conventional imaging tube; and Figures 10 A and B show the dynamic characteristics for a streaking tube in accordance with the present invention as compared to those for an equivalent conventional streaking tube.

Figure 3 shows a sectional view of an imaging tube in the proeess of fabrication in accordance with the present invention. In the figure, the same numerals as in Figure 1 indicate the same elements in the imaging tube.

First, the configuration of the imaging tube shown will be described.

In the imaging tube in accordance with the present invention, separation wall 30 dissects a sealed vacuum envelope 3 into a space including photoelectric layer 4, mesh electrode 5, focusing electrode 6, and aperture electrode 7 and another space including micro-channel-plate 8 and phosphor layer 9.

Likewise, Figure 4 shows a sectional view of a streaking tube in the process of fabrication in accordance with the present invention.

In this figure, the same numerals as in Figure 2 indicate the same elements in the streaking tube.

First, the configuration of the streaking tube will be described.

In the streaking tube in accordance with the present invention, separation wall 30 dissects a sealed vacuum envelope 3 into a space including photoeleetric layer 4, mesh electrode 5, focusing electrode 6, and aperture electrode 7 and another space including deflection electrode 108 and phosphor layer 9.

Both in the imaging tube and the streaking tube shown in Figures 3 and 4, respectively, separation wall 30 is provided with an opening 1 3 which can mate with lid 14.

Figure 5 (A) shows lid 14 covering opening 13, and Figure 5(B) shows lid 14 not covering opening 13.

Lid 14 is revolvable around pin 15 fastened to wall 30. Lid 14 covers opening 1 3 during fabrication, as shown in Figures 3 and 5(A), and lid 14 is clamped by leaf spring 1 6 fastened to separation wall 30 after the fabrication processes are completed. The center of opening 1 3 ties on the tube axis, and it is arranged at or near crossover point 11 of the focused photoelectron beam.

Referring to Figures 3 and 4, the method of fabricating the imaging tube and the streaking tube will be described hereafter.

Exhausting tube 1 9 leading to a vacuum pump, not shown, is provided in the said first space wherein photoelectric layer 4, mesh electrode 5, focusing electrode 6, and aperture electrode 7 are arranged.

Exhausting tube 20 is provided in the said second space, within a sealed vacuum envelope, wherein micro-channel-plate 8 in Figure 3 or deflection electrode 108 in Figure 4 and phosphor layer 9 are arranged.

The said first and second spaces are separated by closing the opening 13 on the separation wall with the lid during fabrication.

Branching tube 1 7 to store alkali metal and branching tube 18 to store the antimony evaporation source are respectively connected together via the first space.

First, the respective spaces within the envelope are exhausted until a predetermined vacuum is obtained.

Second, the antimony evaporation source is taken out of branching tube 18 by means of magnetic force. Antimony is heated by current and evaporated onto photoelectric layer substrate 1.

Third, alkali metal is evaporated from branching tube 1 7 to react with antimony on photoelectric layer substrate 1.

Branching tube 1 7 for storing alkali metal is cut when the maximum sensitivity is obtained on the photoelectric layer during monitoring operations.

Fifth, branching tube 1 8 for storing the antimony evaporation source is cut.

Finally, the envelope is heated to stabilize the photoelectric layer. Excess alkali metal is thus exhausted from envelope 3. Thereafter, exhausting tubes 1 9 and 20 are cut. Then, the imaging tube is completed.

When the imaging tube face is inverted after the tube is completed, lid 14 is automatically moved from over opening 1 3 by the force of gravity. One end of lid 14 is clamped by leaf spring 1 6 and fastened there. Figure 5(B) shows the lid when clamped in the open position.

Figure 6 shows a second embodiment of the separation wall and lid of the imaging or streaking tube.

In this embodiment, lid 14 is fastened by bimetal 42 to a supporting rod 41 on separation wall 30. When the imaging or streaking tube is kept at room temperature, lid 14 does not cover opening 1 3 (see Figure 6(C). While alkali metal is being fed to the photoelectric layer, bimetal 42, heated at about 200"C, is bent as shown in Figure 6(B) causing lid 14 to cover opening 1 3.

Even though such configuration as described above is employed, lid 14 protects against alkali metal penetration into the said second space.

Figure 7 shows a third embodiment of the separation wall and lid of the tube.

Lid 14 is fastened to rod 51 and pivotally supported around rotation axis 50 on separation wall 30.

A head member 52 of a ferromagnetic material is fastened to the other end of rod 51, and it is kept at the position indicated by Figures 7(A and 7 (B) so as to cover opening 1 3. Head member 52 may be held at the different position where opening 1 3 is kept opened as shown in Figure 7(C) by means of leaf spring 53 when an external magnetic force is applied to the head member after completion of fabrication, or when the tube is placed in a different attitude.

Figure 8 shows a fourth embodiment of the separation wall and lid of the tube.

Figures 8(A) and 8(B) depict the state of the lid during fabrication, and Figure 8(C) depicts the state of the lid during use of the tube.

Lid 14 is maintained over opening 1 3 of separation wall 30 during fabrication by means of leaf spring 61. Frame 60 is disposed to accept lid 14 after completion of fabr ication. Leaf spring 61 has a claw at its tip 61 a which contacts the shoulder of lid 14 and protects lid 14 against moving back to the closed position.

The imaging or streaking tube in accordance with the present invention is arranged and fabricated in such a manner as described above. Thus, alkali metal cannot be fed to the micro-channel-plate or deflection electrode while the photoelectric layer is being formed.

Light emission occurring in micro-channelplate 8 or deflection electrode 108 seldom arrives at the photoelectric layer due to the existence of separation wall 30 while the tube is being used, and thus the problem of unwanted light emission can be solved by the technique in the present invention.

An image on the phosphor layer of an imaging tube fabricated in accordance with the present invention was compared with that on the phosphor layer of an imaging tube with the same dimensions fabricated in accordance with the prior art technique. The result of comparison will be described hereafter with reference to Figure 9.

A voltage of 1.3 to 1.9 kV was applied across input electrode 8a and output electrode 8b of microchannel-plate 8 with no light incident upon photoelectric layer 4, and an electron current (the dark current) flowing into phosphor layer 9 was measured.

Figure 9(B) shows a graph of dark currents for the imaging tube in accordance with the invention whereas Figure 9(A) shows a graph of dark currents for the conventional imaging tube with the same dimensions but not including the separating wall.

The conventional imaging tube as shown in Figure 9(A), had a dark current of 5 x 10 - 10A when a voltage of 1.4 kV was applied across input elecrode 8a and output electrode 8b of micro-channel-plate 8 and it had a dark current of 2 X 10-8A when a voltage of 1.9 kV was applied.

When the dark 'current became 10~9 A, a number of bright spots appeared over the entire surface of the phosphor layer. When the dark current became 2 X 10-8 A, light emission over the entire surface of the phosphor layer became saturated and the light signal could not be displayed even though incident upon the photo-electric layer.

The imaging tube in accordance with the present invention, as shown in Figure 9(B), had a dark current of 2 X 10-11 A when a voltage of 1.7 kV was applied across input electrode 8a and output electrode 8b of micro-channel-plate 8 and it had a dark current of 2 x 10-' A when a voltage of 1.9 kV was applied. The dark current was thus drastically decreased in comparison to that of the conventional imaging tube.

The photoelectric layer of a streaking tube in accordance with the present invention was irradiated by light pulses (from a mode lock dye laser emitting light at a rate of 1 30 MHz).

A sine wave voltage synchronizing with the light pulse was repetitively applied to the deflection electrode.

Figure 10(A) compares the output signal of the streaking tube in accordance with the present invention with that of the conventional streaking tube.

Brightness at the valley of the curve for the conventional streaking tube in Figure 10(A), which causes the background noise, is 90% of that at its peak.

On the other hand, brightness at the valley of the curve for the streaking tube in accordance with the present invention in Figure 10(B), which causes the background noise, is 1 % of that at its peak and the latter can be disregarded as compared with the former.

It is easily understood by the personnel skilled in the art that a two-dimensional device such as the charge coupled device (CCD) or position sensitive device (PSD) can be used in place of phosphor layer 9 to increase the S/N ratio, and that the former has the same effect on sensitivity as compared to the latter.

Furthermore, it is easily understood that alkali metal does not contaminate the internal j unction of the CCD or PSD and it does not degrade its electrical performance.

Claims

1. A light-imaging tube having a photoelectric layer capable of emitting photoelectrons on the incidence of light thereon and electron detecting means capable of detecting the photoelectrons, characterized in that said tube comprises a separation wall located between said photoelectric layer and said electron detecting means and provided with an aperture through which photoelectrons emitted from said photoelectric layer and travelling towards said electron detecting means may pass, a lid means movable between a first position whereat said aperture is closed and a second position whereat said aperture is open, and means for moving said lid means into said second position.

2. A light-imaging tube as claimed in claim 1 wherein the electron detecting means comprises a phosphor layer.

3. A light-imaging tube as claimed in claim 1 or 2 in the form of an imaging tube which uses a micro-channel-plate to multiply photoelectrons emitted from said photoelectric layer to observe a diminished light image, said separation wall having said aperture arranged on the tube axis at or near the crossover point for photoelectrons travelling between said photoelectric layer and said micro-channelplate.

4. A light-imaging tube as claimed in claim 1 or 2 in the form of a streaking tube which uses a deflection electrode to scan the photoelectrons emitted from said photoelectric layer and to observe a diminished light image, said separation wall having said aperture arranged on the tube axis, being arranged on the tube axis at or near the crossover point for photoelectrons travelling between said photoelectric layer and said deflection electrode.

5. A light-imaging tube as claimed in any one of claims 1 to 4, wherein said lid means is supported to be at least partially rotatable relative to said separatation wall, whereby said aperture may be closed by said lid means in said first position by the force of gravity, and whereby said lid means may be moved to be clamped in said second position by spring means attached to said separation wall when the attitude of the said envelope is changed.

6. A light-imaging tube as claimed in any one of claims 1 to 4, wherein said lid means is supported from said separation wall by a bimetal, said bimetal being such as to cause said lid means to be placed in said first position at the temperatures at which said photoelectric layer is formed and to be placed in the said second position at ambient or operating temperatures.

7. A light-imaging tube as claimed in any one of claims 1 to 4, wherein said lid means is slidably attached to said separation wall, and is provided with spring means to retain it in said first position.

8. Light-imaging tubes substantially as hereinbefore described with particular reference to Figures 3 to 8 of the accompanying drawings.

9. A method of fabricating a light-imaging tube comprising a vacuum envelope containing a photoelecric layer and electron detecting means, with disposed between the said layers a separation wall provided with an aperture through which photoelectrons emitted from said photoelectric layer and travelling towards said electron detecting means may pass, a lid means movable between a first position whereat said aperture is closed and a second position whereat said aperture is open, and means for moving said lid means into said second position, said method comprising assembling the internal comonents of said tube prior to the formation of said photo-electric layer, exhausting said envelope on each side of said separation wall, forming said photoelectric layer with the deposition of alkali metal while maintaining said lid means in said first position, exhausting said envelope to remove therefrom excess photoelectric layer forming materials, and operating said means for moving said lid means to move said lid means into said second position.

10. A method as claimed in claim 9 for fabricating an imaging tube having in its envelope a micro-channel-plate to multiply photoelectrons emitted from the photoelectric layer and useable to observe a diminished light image and comprising (a) an assembling process which comprises providing a lid covering an aperture in a separation wall to separate a first space which includes at least a focussing electrode and a surface on which may be formed a photoelectric layer from a second space which includes at least a microchannel-plate and a phosphor layer, the separation wall being arranged with its aperture on the tube axis at or near the photoelectron crossover point; (b) an exhausting process to exhaust the first and second spaces; (c) a photoelectric layer forming process to form a photoelectric layer while introducing alkali metal to form the photoelectric layer via a branching tube into said first space; (d) an ejection process to cut the branching tube, to exhaust the envelope while the envelope is being heated, and to eject the excess photoelectric layer forming materials; and (e) a removing process to remove the lid from the aperture after completion of exhausting operations.

11. A method as claimed in claim 9 for fabricating a streaking tube having in its envelope a deflection electrode to deflect photoelecrons emitted from the photoelectric layer and to observe a diminished light image generated by the deflected photoelectrons and comprising (a) an assembling process which comprises providing a lid covering an aperture in a separation wall to separate a first space which includes at least a focussing electrode and a surface on which may be formed a photoelectric layer from a second space which includes at least a deflection electrode and a phosphor layer, the separation wall being arranged with the aperture on the tube axis at or near the photoelectron crossover point; (b) an exhausting process to exhaust the first and second spaces; (c) a photoelectric layer forming process to form a photoelectric layer while introducing alkali metal to form the photoelectric layer via a branching tube into said first space; (d) an ejection process to cut the branching tube, to exhaust the envelope while the envelope is being heated, and to eject the excess photoelectric layer forming materials; and (e) a removing process to remove the lid from the aperture after completion of exhausting operations.

1 2. A method of fabricating a light-imaging tube substantially as hereinbefore described with particular reference to Figures 3 to 8 of the accompanying drawings.