The invention relates to a method for the electronic production of thermal image reproduction of an object on a monitor whereby a temperature image of the object is reproduced by means of an infra-red objective on a signal electrode of a camera tube, which electrode responds to temporal changes in temperature, and whereby in said camera an electron beam scans the signal electrode, which operates according to the pyroelectric effect, by lines and images or half-images.

The practice is known of producing a thermal image reproduction by means of camera tubes based on the pyroelectric effect. Relative to FLIER (Forward Looking Infrared) equipment this kind of equipment has the advantage that it requires no cooled detector elements and little or only a very simple mechanism.

Since the pyroelectric effect responds only to changes in temperature, a heat flow modulator must be employed to obtain modulation of the heat radiation. Hitherto, a gear-tooth disc type of modulator has been employed immediately before the signal electrode. In order to obtain a steady image impression the practice is also known of synchronising the geartooth disc with the vertical deflection. In this case the rotary speed of the gear-tooth disc is advantageously determined in such a manner that in addition the flank of the gear-tooth moves at least approximately in time with the vertical deflection of the electronic beam.

Such equipment produces an irregular image impression and a considerable, in part continuous, interference background. It has therefore already been proposed that the image signal occurring when the modulator opens and that occurring when the modulator closes should be stored, that the oppositely polarised image signals thus obtained be added, and that only the differential signal thus obtained be conveyed to the monitor. An excellent picture can thus be obtained but care must be taken that the image modulation proceeds not only synchronously but also parallel with the electronic image scanning. This requires in practice a very large gear-tooth disc as heat flow modulator. Since the temperature resolution of such equipment is not wholly satisfactory in many cases, the practice is also known of increasing the time for vertical scanning from 20 milli-seconds to 40 milliseconds. This type of image signal is however no longer TV-compatible.

The object of the present invention is to provide a method and also an apparatus which do not have the above-mentioned disadvantages.

The invention comprises a chopper modulation alternately inhibits and facilitates transmission of thermal energy from a subject to the television camera head viewing the subject. Video information produced during separate first and second time intervals is separately stored. Video information produced during a third time interval between the first and second time intervals is not used. Video information produced during a forth time interval subsequent to the second time interval is also not used. A signal is produced which is a function of the difference between the stored video information corresponding to the first and second time intervals. This difference signal is employed, in combination with appropriate synchronization signals, to drive a television monitor to produce a thermal image of the viewed subject.

Since the picture objects normally occurring have mostly a much higher temperature relative to the heat flow modulator, in order to avoid temperature displacement of the signal electrode, it is of advantage if the time, during which only the heat flow of the object becomes effective on the signal electrode, is smaller, preferably at least half as small as the time during which only the heat flow of the modulator becomes effective on the signal electrode.

In order that a camera can be constructed without any single moving mechanical part, the heat flow modulator is preferably accommodated within the imaging objective.

An especially neat arrangement can be attained if the heat flow modulator is constructed as a rotating heat flow interrupter, in which case the rotary axis is disposed vertical to the optical axis and the cross-section of the heat flow interrupter as seen in a plane containing the rotary axis possesses a profile of at least approximately U-shape, the lateral limbs of which form in each case a section through one of the interrupter surfaces.

The invention further relates to an apparatus for carrying out the method according to the invention, which is characterised in that the heat flow modulator comprises liquid crystal elements. It is of advantage if the heat flow modulator is synchronised with the image or half-image scanning.

The invention will now be described with reference to an example illustrated in the drawings in which:

FIG. 1 is a diagrammatic sketch showing an arrangement for carrying out the method according to the invention;

FIG. 2 shows the action of the heat flow modulator of the arrangement shown in FIG. 1 upon the heat flow;

FIG. 3 shows the effect of the heat flow modulator of the arrangement shown in FIG. 1 upon the signal electrode temperature;

FIG. 4 is a longitudinal section through an arrangement of a heat flow modulator shown by way of example, and

FIG. 5 is a cross-section on the line V--V, FIG. 4.

FIG. 1 shows an arrangement for the thermal reproduction of an object 1 on a monitor 2.

For this purpose an image of the object is produced, by means of an objective 3 operating in the frequency range 8`14 μm, on the signal electrode 4 of a camera tube 5.

The heat flow of each image element of the object 1 on the corresponding image element of the reproduction on the signal electrode 4 is periodically interrupted and released by a rotating sector disc 6.

Since the signal electrode 4 is of pyroelectrically sensitive material and is provided on the side disposed towards the object 1 with an electrically conductive coating, the pyroelectric charge can be converted by means of an electron beam scanning system 7 into a signal current which is converted in the amplifier 8 into an image signal.

Since the signal electrode 4 is highly insulating, it must be provided with a base charge in order to reproduce not only the warmer but also the colder object portions. For production thereof the cathode 9 of the electron beam system is reduced to negative values suring the line return movement. The electrons of the electronic beams thus produced have such a high velocity that secondary electrons are produced and a base voltage U_o of the image signal U(t) is created.

By means of the motor 10 and the circuit 11, and phase-locked to the position sensor 12, the heat flow modulator is synchronised with the vertical deflection V.

Since the heat flow modulator 6 performs one rotation during the time of four vertical scans, an image signal U(t) appears at the point 13.

By means of the switch 14 the image signal U(t) is read into the stores 15 and 16 alternately during the scanning intervals P1 and P3 and is also synchronised with the vertical deflection. The stores 15 and 16 pass the registered image signals U(P1) and U(P2) directly and thrice repeated to the differential circuit 17, which in turn passes the differential signal U(P1)-U(P2), which no longer contains the base voltage U_o, to the monitor 2 for image reproduction.

This differential signal U(P1)-U(P2) possesses an increased amplitude compared with the individual signals U(P1) and U(P2). Even a decrease however would be more than compensated by the very much stronger acting elimination of the base voltage and of the standing noise or interference.

FIG. 2 show (dotted) the effect of a conventional heat flow modulator, and (continuous line) the effect of the modulator used according to the invention on the heat flow which reaches the signal electrode 4 in FIG. 1.

Whereas the heat flow modulator at present in use produces a heat flow which is symmetrical in time, the modulator 6 according to the invention produces only a short-time effect of the heat flow of the object 1, and, as compared therewith, a much longer action of the heat flow leaving the modulator 6.

FIG. 3 shows, on the same time scale as FIG. 2, the effect (broken line) of a modulator as hitherto employed, and (continuous line) the effect of a modulator 6 operating according to the invention.

For an object element at a temperatur of 40° C. the modulator at present in use, which may have a temperatur of 20° C., produces a temperature course of the corresponding image point on the signal electrode 4 following the curve A--B--C--D--E--F.

The modulator 6 according to the invention on the other hand produces a distinctly more strongly modulated temperature course A--G--H--I for an object point of the same temperature.

Since the pyroelectric effect increases with the temperature, with a camera fanctioning in this way much greater sensitivity is obtained by means of the method according to the invention than was hitherto possible.

A method of execution of a modulator 6' mounted in the objective 3', 3', 3'is shown in FIG. 4.

In this case, in order to enhance the mechanical rigidity thereof, the modulator 6' is in the form of a double-curved surface, preferably as seen in FIGS. 4 and 5, as a section of the surface of a sphere which is rotated around the rotary axis 18.