INFRARED CAMERA

Abstract: Infrared camera for depicting an object, comprising an objective for projecting the object on a focal plane upon which conversion means have been provided for converting photons in the infrared range emitted or reflected by the object into electrically charged particles as a function of position of the emission or respectively the reflection of the photons, and position detection means for detecting the position of the emission of the electrically charged particles, wherein the conversion means comprise a gaseous medium which contains particles for bringing into an excited electron state, for instance a Rydberg state, and is provided with excitation means, for instance a laser light source, for bringing particles in said excited electron state, in order to absorb the photon pulse and to emit the electrically charged particles.

Description: INFRARED CAMERA The invention relates to an apparatus for depicting an object which emits or reflects radiation in the infrared range, for instance an infrared camera, comprising an objective for projecting the object on a focal plane upon which conversion means have been provided for converting photons in the infrared range emitted or reflected by the object into electrically charged particles as a funtion of position of the emission or respectively the reflection of the photons, and position detection means for detecting the position of the emission of the electrically charged particles . Known are infrared cameras by means of which an infrared emitting or reflecting object is projected upon a projection plane which is provided with a layer of infrared sensitive material in the condensed phase, by which material photons in the infrared range are converted in an electron stream which can be detected as a function of position in order to construct an image of the original object in the visible part of the spectrum. The known infrared cameras have the drawback that their spectral range in the infrared region is limited, that their sensitivity is low, which means that the number of photons in order to generate a detectable signal is necessarily relatively high, that the freedom for the shutter time to be set, i.e. the time-resolution, is limited, and that the selectivity for the wavelength in the infrared range is little. It is an object of the invention to provide an infrared camera which has a large spectral range . It is further an object to provide an infrared camera which has a high sensitivity, and with which a very short shutter time can be realized. Another object is to provide an infrared camera having a high selectivity for the wavelength of the infrared radiation emitted or reflected by the object to be depicted. These objects are achieved, and other advantages are obtained, with an apparatus of the type stated in the preamble, wherein according to the invention the conversion means comprise a gaseous medium for absorbing the photon pulse to be detected and for emitting the electrically charged particles. The electrically charged particles are for instance electrons or ions. In an apparatus wherein according to the invention the pulse conversion means comprise a gaseous medium the spectral range of a photon pulse for detecting is not limited to the infrared region, but the spectral range can be extended as required to the wavelength region of the far-infrared light. In an embodiment of an apparatus according to the invention the apparatus is provided with excitation means for bringing particles into an excited electron state and the gaseous medium contains particles for bringing into this excited electron state in order in this state to absorb the photon pulse and to emit the electrically charged particles . The excited electron state is for instance a Rydberg state . By bringing particles, for instance atoms, into an excited electron state, for instance a Rydberg state, pulse conversion means are obtained for converting radiation in the infrared or far infrared region and therefore low-energy photons into a stream electrically charged particles. An atom in a Rydberg state, referred hereinafter to as Rydberg atom, has a high value of the main quantum number n, and therefore a relatively low binding energy E (E = -13.6/Ώ 2 eV) . As a consequence the relatively low energy of a far-infrared photon is sufficiently high to cause photo-ionization of an atom in a Rydberg state and to liberate a weakly bonded electron from that atom. Moreover, the active cross-section for photo-ionization is high for a gas with Rydberg atoms, so that relatively few photons are required for this process. A gaseous medium comprising particles for bringing into an excited state is for instance admitted into the apparatus via a gas supply line . In one embodiment the apparatus according to the invention comprises an evaporation oven for bringing into a gaseous state the particles for bringing into an excited electron state . Atoms for bringing into an excited electron state which are suitable for use in an apparatus according to the invention are for instance alkali atoms, in particular the elements Rb (rubidium) or Cs (caesium) . The atoms are brought into an excited electron state for instance by excitation using a laser light source. A laser light source for use in an apparatus according to the invention is for instance a dye laser pumped with an Nd:YAG(neodymium:yttrium-aluminium garnet) laser. The second harmonic of the light of an NdrYAG laser is particularly suitable for pumping the dye laser in such an apparatus . In another embodiment the apparatus comprises a diode laser. The light of the laser light source is projected into the focal plane of the object to be depicted according to a method known per se by means of for instance a cylindrical lens, in such a way that at the site of said focal plane in the gaseaous medium a plane comes into being in which particles are to be brought into an excited electron state, and particles outside said plane are not to be brought into said excited electron state. In an advantageous embodiment the apparatus comrises field generating means for generating an electric field in order to cause ionisation of a particle having an excited electron brought in a Rydberg state. In an apparatus which is provided with such field generating means it is possible to cause field ionization of particles which have been brought into a Rydberg state by means of photo-excitation or photo-de-excitation. Photo-excitation is said to be there when the bonding energy of an electron in a Rydberg state is greater than the enery of in incoming photon in the infrared region, so no photo-ionization can occur, but the electron is driven by the infrared photon to a Rydberg state which has a higher energy level. By now exposing the particles having the electron in the particular Rydberg state to such an electric field, that particles having an electron in the initial Rydberg state do not ionize, but particles having an electron in the particular Rydberg state with a higher energy do ionize, the particular particle will ionize according to the mechanism of field ionization to a positive ion and an electron. This implies that the conversion means in this preferred embodiment are only sensitive to infrared radiation having wavelenghts which give rise to photo-ionization of the Rydberg particle. Moreover, the excited state should have an energy sufficiently high in order to give rise in the static electric field to be generated to the formation of a positive ion and an electron. The electric field in this preferred embodiment can be chosen in such a way that the conversion means are only sensitive to particular wavelengths in the infrared region, and therefore only sensitive to objects which emit or reflect light having said particular wavelength. Photo-de-excitation is said to be there when an incident photon in the infrared region drives an electron which is in an initial Rydberg state, whereby field ionization can occur, to another, de-excited Rydberg state with a lower enery level, having a greater bonding energy. The electric field in this preferred embodiment can be chosen in such a way that the conversion means are only sensitive to wavelengths in the infrared region which bring the particles in the conversion means into a de-excited Rydberg state, whereas particles in the de-excited state which have not absorbed a photon by field ionization do give rise to the formation of a positive ion and an electron. The electric field in this preferred embodiment can be chosen such that the conversion means are just insensitive to certain wavelengths in the infrared region, and therefore give rise to creation of a negative xmage . In yet another embodiment the apparatus comprises field generating means for generating a magnetic field in order to tune the energy level of an electron in the Rydberg state. Because the energy level of an electron in a Rydberg state is tunable by means of a magnetic field (just as it is by means of an electric field) , the transition of an electron to this state can be made selective for a photon to be absorbed in the infrared region, in such a way that only photons having an energy which corresponds to said transition are absorbed and the apparatus is insenstive to photons having a different energy. With a camera for the infrared region according to the invention objects emitting or reflecting infrared radiation with a wavelength up to for instance about 100 micrometers can be observed with a high resolution, for instance 10 micrometers, with a very fast shutter times, for instance 1 nanosecond. The invention will be elucidated hereinbelow on the basis of an embodiment and with reference to the drawing. In the drawing fig. 1 shows a schematic view of an embodiment of an infrared camera according to the invention. Shown are an infrared camera 1 for making by means of an optical element 7 a picture of an object 6 which emits or reflects infrared radiation, with gastight casing 2, which comprises voltage grids 3, 4 (connection and supply of which are not shown) , channel plates 8, 9 with high-voltage feed- throughs 16, 17 respectively, phosphor screen 9, oven 10 and window 11. The infrared camera 1 further comprises a CCD camera 14 coupled to a computer 13 and a diode laser 15. When the infrared camera 1 is in operation, the object 6 emitting or reflecting infrared light is projected by means of the optical element 7 upon the plane 21. De infrared radiation passes a grid 3 and is absorbed by gas present in the casing 2, which gas is excited by laser light (represented by arrow 22) from diode laser 15 via window 11 and is in a Rydberg state. Because of photo-emission a gas particle disintegrates into an electron and a positively charged ion. The voltage difference over the grids 3, 4, which is applied via connectors not shown, accelerates either the ions or the electrons (depending on the sign of the voltage difference) in te direction of the channel plates 8, 9. A single incident electrically charged particle on the channel plates 8 , 9 results in 10 7 electrons which strike the phosphor screen 19, where the electrons are converted into photons at an amplification factor of 10. The multiplication factor 10 7 will only be attained if the voltage difference over the sides of the channel plates 8, 9 which are directed away from each other is sufficiently high (typically 2 kV) . This voltage difference is applied by means of the electrical feed-througs 16, 17. Thus an image 20 of the object 6 is made on the phosphor screen 19. This image 20 is read using CCD camera 14 and processed using computer 13. In the distance 18 of the way of the electrically charged particles, optics can be placed between the focal plane 21 and the first channel plate 8, in order to make the image of the charged particles on the first channel plate 8 as sharp as possible. Infrared camera 1 is not sensitive to infrared radiation before diode laser 15 has excited the gas particles to a Rydberg state. This excitation process is comparable with the opening of a shutter in a prior art camera . As soon as the voltage difference between the feed-throughs 16, 17 has dropped below a certain critical value (for instance 1500 V) , incident ions or electrons will no longer be detected. The camera is in that case not sensitive to Rydberg particles which are ionized after the voltage difference between the feed-throughs 16, 17 has been dropped, taking into account the transit time of the electrons or ions from the photosensitive focal plane 21 and the front channel plate 8. This process is comparable to the closing of a shutter in a prior art camera . Because a diode laser can be turned on within a time period of 10 "9 sec, and the voltage across the channel plates 8, 9 can drop within a similar time period, shutter times shorter than 1 nanosecond can be realized, so that the so called dark current of the infrared camera according to the invention is very little. It is possible to use as a shutter only a difference in switching voltage across the feed-throughs 16, 17. In a rest situation the voltage across the feed-throughs 16, 17 is for instance 1500 V, and incident charged particles will not be detected. When next the voltage difference is enlarged by means of a voltage pulse of 500 V for instance to 2000 V, the charged particles will reach the phosphor screen 19. This implies that the camera 1 is only sensitive during the latter pulse, and that the shutter time corresponds with the pulse width of the switchpulse, for instance in the order of magnitude of 1 nanosecond. Under particular conditions it is possible to apply the infrared camera 1 wavelength-selective . This opportunity arises if the Rydberg particles under influence of the incident photons in the infrared region do not ionize, but rather are driven from the initial Rydberg state to a different Rydberg state with a higher or lower energy level. By raising the voltage difference across the grids 3, 4 to a limited extent after irradiating the Rydberg particles with the infrared radiation, only particles in said different (non-initial) Rydberg state will still ionize. Depending on the polarity of the voltage difference across the grids 3, 4 electrons or ions will yet reach the channel plates 8, 9 and give rise to a detectabele image of the object 6. In this application the infrared camera 1 is only sensitive to infrared radiation the wavelength of which matches with the transition from the initial Rydberg state to the other Rydberg state, and besides time- and position-resolving performances there is also a matter of wavelength- selectivity. It should be noticed that the embodiment described above only serves to illustrate the invention, and not to limit the scope of the idea of the invention. Where for instance in the above description an optical element 7 comes up, this should be understood that broad, that besides simple objectives from infrared cameras which are known per se for making images of terrestrial objects also advanced and complex telescope systems for astronomical purposes are comprised therein.

Claims: CLAIMS 1. Apparatus for depicting an object which emits or reflects radiation in the infrared range, for instance an infrared camera, comprising an objective for projecting the object on a focal plane upon which conversion means have been provided for converting photons in the infrared range emitted or reflected by the object into electrically charged particles as a funtion of position of the emission or respectively the reflection of the photons , and position detection means for detecting the position of the emission of the electrically charged particles, characterized in that the conversion means comprise a gaseous medium for absorbing the photon pulse to be detected and for emitting the electrically charged particles . 2. Apparatus as claimed in claim 1, characterized in that the apparatus is provided with excitation means for bringing particles into an excited electron state and the gaseous medium contains particles for bringing into this excited electron state in order to absorb the photon pulse and to emit the electrically charged particles. 3. Apparatus as claimed in claim, characterized in that the excited electron state is a Rydberg state. 4. Apparatus as claimed in claim 2 or claim 3 , characterized by an evaporation oven for bringing into gaseous state the particles for bringing into an excited electron state. 5. Apparatus as claimed in any of the claims 2-4, characterized in that the particles are alkali metal atoms. 6. Apparatus as claimed in claim 5, characterized in that the alkali metal atoms comprise one of the elements Rb (rubidium) or Cs (caesium) . 7. Apparatus as claimed in any of the claims 2-6, characterized in that the excitation means comprise a laser light source. 8. Apparatus as claimed in claim 7, characterized in that the laser light source is a dye laser pumped with an Nd:YAG(neodymium:yttrium-aluminium garnet) laser. 9. Apparatus as claimed in claim 7, characterized in that the laser light source is a diode laser. 10. Apparatus as claimed in any of the claims 7-9, characterized in that this comprises projection means for projecting the light of the laser light source into the focal plane of the object to be depicted. 11. Apparatus as claimed in any of the claims 2-10, characterized in that this comprises field generating means for generating an electric field in order to cause ionisation of particles brought into an excited electron state. 12. Apparatus as claimed in any of the claims 3-10, characterized in that this comprises field generating means for generating an electric field in order to tune the energy- level of an electron in the Rydberg state. 13. Apparatus as claimed in any of the claims 3-10, characterized in that this comprises field generating means for generating a magnetic field in order to tune the energy level of an electron in the Rydberg state. 14. Apparatus as claimed in claim 11 or claim 12, characterized in that the field generating means comprise at least two grids, over which grids an electric potential is to be applied in order to accelerate the electrically charged particles . 15. Apparatus as claimed in any of the foregoing claims, wherein the position detection means comprise at least one channelplate for generating an electron stream in response to the impact of electrically charged particles upon this plate and to accelerate the electron stream, characterized in that a pulse shaped electric potential is to be applied over this channelplate .

Inventors: NOORDAM LAMBERTUS DOMINICUS, NOORDAM, LAMBERTUS, DOMINICUS
Applicants: NOORDAM LAMBERTUS DOMINICUS, SEED CAPITAL INVESTMENTS, STICHTING FUND OND MATERIAL, STICHTING TECH WETENSCHAPP, NOORDAM, LAMBERTUS, DOMINICUS, SEED CAPITAL INVESTMENTS - 2 (SCI - 2) B.V., STICHTING VOOR DE TECHNISCHE WETENSCHAPPEN, STICHTING VOOR FUNDAMENTEEL ONDERZOEK DER MATERIE
Application year: 1997
Main classification: H01J31
IPC codes: G01J1, G01J5, H01J29, H01J31, H01J47, H04N5