GB2302612A - Image converter - Google Patents

Abstract

An image converter is capable of converting radiation outside the spectral range of human vision to radiation capable of being viewed by the human eye. The image converter consists of a thin film of metal or other material 4, suspended in a holding frame 6. The thin film of material can be slightly heated by a current. The whole system is suspended in a vacuum chamber 3 to which at one end is a window 2 to allow incoming radiation to be focussed onto the thin foil. The opposite side of the foil being coated or modified 5 to easily emit electrons which are then emitted as a function of the intensity of the incoming focussed radiation. These emitted electrons are accelerated under the influence of an electric field to impinge on a phosphor screen 11. The image of the radiation may be viewed through the window 10.

Description

IMAGE CONVERTER As is well known various detectors can convert one type of radiation to a visible image, such as that employed in night image converters, whereby long wave length infra red radiation is converted into a visible image using a solid state amplifier. Other examples are the use of fluorescence screens to convert X-rays to provide a visible image and the use of electrons to produce a visible image via a phosphor in television screens.

An objective of this invention is to produce a display system, whereby, radiation of a very wide band, ranging from centimetric radar, far infra red, visible radiation, ultraviolet and soft and hard X rays can be imaged onto a detector causing a visible image of that radiation to be produced. The detector would also be capable of producing a visible image of alpha, beta and gamma radiation enhanced by photoelectric emission in the case of X rays and alphas.

Such a device could possibly be used to image the long wave infrared emissions associated with the temperature fluctuations of wind shear in the atmosphere associated with clear air turbulence and give an aircraft pilot a visible picture, ahead of the aircraft, of clear air turbulence.

According to a first aspect of the invention there is provided a method of allowing the radiation from the image to be incident on a thin metal foil. the foil being heated by the passage of an electric current, the whole system being enclosed in a vacuum chamber. Radiation incident on the preheated thin film will cause local heating of the film in proportion to the intensity of the image, thus causing greater electron emission from the hotter parts of the film.

According to a second aspect of the invention there is provided a fluorescent screen whereby electrons released from the thin metal foil are accelerated under the influence of an electric field to impinge on the fluorescent screen and produce a visible image of the incident radiation on-the thin film.

According to a third aspect of the invention, several thin metal films may be placed one behind each other, with appropriate accelerating voltages, so that the electrons released from one film are accelerated onto the next screen producing a more intense image. This process would have the advantage of producing high gain for the detection of weak incident images on the first thin metal film. Increase in the number of electrons released and at lower thin film temperature, can be artificially increased by coating the surface facing the fluorescent screen with a low work function material as in the manner of the coating of cathodes in electronic valves.

The optimum system design may be determined through mathematical or computer modelling so that the device materials and type of controlling means employed may be suitably selected to give the optimum operating conditions.

This may include chopping the incoming radiation so that the thin metal film can be temperature normalised. The type of window material would have to specified according to the type of focussed radiation impinging on the detector thin film.

This invention allows the use of a preheated thin metal film to be used as a means of converting a focussed image, within a wide band of the electromagnetic spectrum, to an image in the visible spectrum. It may also be used to convert an image from one part of the electromagnetic spectrum to an image in another part of the electromagnetic spectrum. The device would have applications in surveillance technology, image conversion, astronomy, atmospheric turbulence, infra red and ultra violet to visible image conversion, focussing effect of lenses and mirrors in non-visible portions of the spectrum, etc. all in real time.

The present invention will now be described further, by way of example only with reference to the accompanying drawings in which: Figure 1 is a schematic cross-section representation of the basic design of the image converter and Figure 2 shows a schematic of the image converter with a possible arrangement of a focussing mirror.

Referring to Figure 1, focussed radiation 1 passes through a window 2 into the vacuum chamber 3 and impinges onto a thin film of metal 4. The back of the film 4 could be coated with a thin film 5 of low work function material. The film is bonded onto an insulating frame 6, such as quartz, by a conductive paint or sputtering 7. A current can be passed through the thin metal foil 4 by means of the contacts 8 which are isolated from the vacuum chamber 3 by means of an insulator 9.

Figure 2 illustrates, in more detail, the insulated support frame 6 for the thin metal foil 4 bonded to the support frame 6 by a conducting paint or other means 7, the whole frame 6 being connected to the current leads 8.

Referring to Figure 1, the rear window 10 consists of a sheet of glass or quartz having deposited on its inside face a fluorescent powder or material 11 to which an electrical connection can be made via the contact and feedthrough 12.

The windows are held in place via a flange, O-ring, face plate and screw 13 or the window may be sealed directly to the vacuum chamber 3 with a low vapour pressure epoxy adhesive.

Figure 3 is a schematic of the image intensifier cell 15 situated at the focus of a parabolic mirror 14 as an example of one particular application.

Referring to Figure 1 radiation 1, assumed in this case to be of a long wave infra red nature as an example, is brought to a focus through the window 2, which has the characteristic of being only slightly absorbing to the wavelength of interest, to impinge on the thin metal foil 4. The foil 4 is preheated by the passage of an externally variable current via the electrodes 8 and the conducting paint 7, this current will cause the foil 4 to heat up and in doing so will cause the low work function emitter 5 to also heat up.

The radiation impinging on the thin foil 4 will cause the foil 4 to be preferentially heated in relation to the distribution of energy in the focussed incoming radiation 1. This will cause a two dimensional temperature profile to be generated in the thin metal foil 4 consequently heating the thin layer of low work function material 5 to heat up in direct relationship to the temperature distribution in the thin metal foil 4. Thus there will be a profile of electrons emitted from the low work function emitter 5. The greater the number of electrons emitted corresponding to the higher temperature of the focussed radiation on the thin foil 4.

An uniform electron accelerating potential is applied between the thin foil 4, via the connectors 8 and the phosphor 11, via the connector 12, so that the electrons emitted from the low work function surface 5 are accelerated to impinge on the phosphor 11, thereby causing the phosphor 11 to emit visible light in relationship to the number of electrons arriving. This emitted light from the phosphor 11 can be viewed through the transparent window 12.

Thus the intensity of the incoming radiation 1 is converted to an electron emission current via the thin film 4 and the low work function surface 5. The electron current is then accelerated by a voltage gradient applied between the thin film 4 and the phosphor 11 to cause visible light to be emitted and viewed through the transparent window 11.

Thus. it will be appreciated that the present invention is not intended to be restricted to the details of the above described embodiment which is described by way of example only.

Initial calculations are presented to show that the overall concept is theoretically viable.

Preliminary calculations on broad band image converter From the Richardson Dushman equation for the thermionic emission from a heated metal in a vacuum

High Temperature Operation From the thermal balance of the thin film assuming that the heat loss is entirely due to radiation and electron cooling, then Heat input = heat output W= H+ + where j repesents the electron cooling by thermionic emission.

The sensitivity, S, is proportional to the change in emitted current to proportional change in heat input, is given by

Low Temperature Operation Operating at low temperature means that the heat losses, in the limit, would be dominated by conduction as radiation and thermionic cooling are strongly temperature dependent ThusW = B(T-T0) where B is the overall heat transfer coefficient.

The term This determined by the minimum current that can be detected and is only slightly sensitive to the kT choice of materiaL Assume in the limit that one needs one electron emitted per tenth of a second (persistence of vision) per 0.25 x 104 m2 (pixel on tv screen). This gives a current of 6.4x 10-l2A m2. For tungsten this is equivalent to an operating temperature of about 1000K (--52.4, tD =4.5ev). For a low work function kT emitter the corresponding temperature is about 440K, kT being the same.

kT This gives dj z 54.4 dT for all materials T i.e. at maximum sensitivity (minimum temperature) ATi 1K, gives a 12% change in current. At maximum sensitivity

Conclusions 1. It is possible to calculate the maximum sensitivity to a reasonable accuracyin a way that is substantially independent of the emitter because of the dominance of the term kT for both the high and low temperature operation.

2. In practice, a minimum temperature of around 440K is possible with a good oxide emitter such as thoriated tungsten or an alkaline earth oxide coated emitter having typically 4 > = 1 to 2 eV.

3. The small rise in temperature needed to achieve an initial current of even a thousand times higher than assumed would not significantly diminish the sensitivity.

Claims (6)

1. An image converter including an input window, an image detection foil, an electron accelerating field and an output phosphor coated window.

2. An image converter as claimed in Claim 1 whereby focussed radiation incident on the detection foil allows preferential heating of the foil.

3. An image converter as claimed in Claim 1 or Claim 2 where the focussed radiation impinging on the foil causes electron emission from the back face of the foil.

4. An image converter as claimed in Claim 3 where the electron emission from the back face of the foil is related to the intensity of the radiation incident on the front face of the foil.

5. An image converter as claimed in Claim 4 where the emitted electrons are accelerated to impinge on the phosphor and produce a visible image on the output window in direct relation to the radiation focussed onto the foil.

6. An image converter substantially as herein described and illustrated in the accompanying drawings.