COMBINED DAY VIEWING AND NIGHT VIEWING TELESCOPE

Abstract: Combined day viewing and night viewing telescope, having one entrance aperture for both day viewing light and night viewing light. The day viewing light is spatially separated from the night viewing light after passing the entrance aperture and is directed out around an image intensifier for the night viewing light to the exit aperture.

Description: Title: Combined day viewing and night viewing telescope The invention relates to a combined day viewing and night viewing telescope, comprising a housing with an entrance aperture and an exit aperture, first light guide means for guiding light received through the entrance aperture and intended for night viewing to an input of an image converter; and second light guide means for guiding light produced by the image converter at an output thereof to the exit aperture. Such a combined day viewing and night viewing telescope, hereinafter described by the term "day/night viewing telescope", is known from US Patent Specification 4,629,295. A disadvantage of this known day/night viewing telescope is that a separate second entrance aperture is required for receiving light for day viewing. This known day/night viewing telescope is thus essentially composed of two telescope parts situated adjacent to or above one another, and the light produced by the image converter at the exit of the part designed for night viewing has to be coupled by means of the second guide means into the telescope part designed for day viewing. The above- mentioned US Patent Specification 4,629,295 describes, for example, a partially transmissive mirror 12 for that purpose. The object of the invention is to improve the known day/night viewing telescope. In particular, the object of the invention is to provide a day/night viewing telescope of reduced dimensions and a reduced weight. More par¬ ticularly, the object of the invention is to provide a day/night viewing telescope with a combined entrance a- perture for day viewing and night viewing. To this end, according to the invention the day/night viewing telescope of the above-mentioned type is designed to receive light intended for day viewing also through the entrance aperture intended for night viewing, and the telescope is provided with light separating means for separating said light for day viewing received through the entrance aperture from the light intended for night viewing and directing said day viewing to the exit aperture via an optical path running outside the image converter. In a first embodiment, the light separating means are designed for spatially separating the light intended for day viewing and the light intended for night viewing, and thereto comprise a first optical system with positive strength positioned in the optical path between the entrance aperture and the input of the image converter, the surface area of which system is smaller than the surface area of the entrance aperture. In a second embodiment, the light separating means are designed for separating in time the light intended for day viewing and the light intended for night viewing, and thereto comprise a displaceable first reflective element which in a night viewing position is active for transmitting the light received through the entrance aperture, and in a day viewing position is active for reflecting the light received through the entrance a- perture. In a third embodiment, the light separating means are designed for separating according to wavelength the light intended for day viewing and the light intended for night viewing, and thereto comprise a first wavelength- discriminating element which is fitted to be at least substantially transmissive for light within a first wavelength range which at least substantially corresponds to the sensitivity range of the image converter, and to be at least substantially reflective for light within a second wavelength range which at least essentially corresponds to the visible wavelength range. In a further embodiment, the output of the image converter faces the entrance aperture, and between the entrance aperture and the image converter a second wavelength-discriminating element is situated, which is designed to function as a positive reflection lens for light within a narrow third wavelength range substantially coinciding with the emission peak λ * of the image converter and to be at least substantially light- transmissive for light w' in at least a fourth wavelength range which at le& ^ .„ substantially corresponds to the combination of the visible wavelength range and the sensitivity range of the image converter, with the exclusion of the said third wavelength range, and a focusing reflective element is disposed opposite the input of the image converter. The invention will be explained in greater detail below by a description of preferred embodiments of the day/night viewing telescope according to the invention with reference to the drawing, in which the same or comparable parts are indicated by the same reference numerals, and in which: Figure 1 schematically shows a first embodiment of the day/night viewing telescope according to the inven¬ tion; Figure 2A schematically shows a second embodiment of the day/night viewing telescope according to the inven- tion; Figure 2B schematically shows a third embodiment of the day/night viewing telescope according to the inven¬ tion; and Figure 3 schematically shows a further embodiment of the day/night viewing telescope according to the inven¬ tion. A number of terms used will now be further explained. Day viewing is understood to mean the observation of a scene with the aid of light coming directly from the scene and visible to the human eye. Night viewing is understood to mean the observation of a scene in those circumstances in which it is desir¬ able to intensify the visible light coming from the scene and/or to supplement it by converting non-visible light coming from the scene, generally infrared light, into visible light. For the sake of clarity, only axial beams are shown in the figures. In Figure 1 a first embodiment of the day/night viewing telescope according to the invention is indicated by reference number 1. The day/night viewing telescope 1 comprises a housing 2 with an entrance aperture 3 for receiving light 100 from a scene not shown. In or before the entrance aperture 3 an objective system (not shown) can be fitted for focusing on the scene. In that case the beams 103, 109 and 116 shown in Figures 1, 2 and 3 will not run parallel to the axis 5. Although the invention is not limited thereto, such a day/night viewing telescope 1 can be, for example, a telescopic viewing for a gun. The day/night viewing telescope 1 is provided with an image converter 10 with an input 11 and an output 12, which image converter 10 can be of a standard type. The operation of the image converter 10 is assumed to be known and does not form part of the invention, for which reason a more extensive description of the image converter 10 is omitted. It is merely pointed out that the input sensitivity of the image converter 10 may extend over a wavelength window ranging from visible light until into the near infrared, or even into the far infrared. In a practical example the image converter 10 is sensitive to light with wavelengths between 600 nm and 1000 nm. The light produced at the output 12 may also come about through the lighting up of a phosphorescent layer which has at least one sharp emission peak at a wavelength λ * . By way of example, the phosphorescent layer may contain a phosphor, with a sharp emission peak at λ * = 543 nm. It is pointed out that the emission peak in practice is always of a certain width, and that one or more subsidiary peaks may occur in the spectrum of the phosphorescent layer. The day/night viewing telescope 1 is also provided with first light guide means 20 disposed opposite the input 11 of the image converter 10, in order to guide the light received through the entrance aperture 3, and intended for night viewing, to an input of an image converter. In this first embodiment, the first light guide means 20 comprise a first optical system 21 with positive strength positioned between the entrance aperture 3 and the input 11 of the image converter 10, the surface area of which system is smaller than the surface area of the cross-section of the entrance aperture 3. The part 101 of the light 100 entering through the entrance aperture 3 which reaches the first optical system 21 is imaged (102) by the first optical system 21 on the input 11 of the image converter 10, and is thus used for night viewing, while the part 103 of the light 100 entering through the entrance aperture 3 which passes around the first optical system 21 is used for day viewing. The first optical system 21 thus acts as a light separating means. The day/night viewing telescope 1 is also provided with second light guide means 30, in order to guide (105, 107) the light 104 produced by the image converter 10 at the output 12 thereof to an exit aperture 4 of the day/night viewing telescope 1. The second light guide means 30 comprise a second optical system 31 with positive strength which is designed to give the image produced at the output 12 of the image converter 10 an object plane which coincides with the object plane of the scene 100 as observed with the day viewing light 103. In the example shown, the second optical system 31 is designed to make the light rays 105 coming from the same image point run parallel. Furthermore, the second light guide means 30 comprise a third optical system 32 with positive strength which is designed to make the light 103 intended for day viewing and the light 105 produced by the image converter 10, and deflected by the second optical system 31, converge (106, 107) towards the exit aperture 4. The output image of the day/night viewing telescope 1 observed at the exit aperture 4, for example by means of an eyepiece system, is a superposition of the day viewing image and the night viewing image. If the magni¬ fication in the optical path of the first optical system 21 up to and including the second optical system 31 is equal to 1, the day viewing image and the night viewing image match each other exactly, so that, for example, during twilight the two pictures can be observed simultaneously without a disturbing difference in size. Such a magnification can be achieved by a suitable combination of the first optical system 21, the image converter 10, and the second optical system 31, while the image converter 10 may be provided with an inverter ("twister") if desired, as will be clear to an expert. In the day/night viewing telescope l the light entering through the entrance aperture 3 is spatially separated by the first optical system 21 into a part intended for day viewing and a part intended for night viewing, the division ratio being defined by the ratio between the surface area of the entrance aperture 3 and the surface area of first optical system 21. A consequence of this is that information suitable for night viewing is present in the light part intended for day viewing, but is not used for the former, while information suitable for day viewing is present in the light part intended for night viewing, but is not used for the former. An improvement of the light intensity of the night viewing image through enlarging the first optical system 21 results in a reduction of the light intensity of the day viewing image, and vice versa. Figure 2A shows a second embodiment 40 of the day/night viewing telescope according to the invention, having an improved light intensity over the day/night viewing telescope 1, both for the day viewing image and for the night viewing image. For this purpose, the first optical system 21 and the entrance aperture 3 have such dimensions that all the light entering through the entrance aperture 3 can strike the first optical system 21. For this purpose, a beam-impander 41 can be fitted, for example at or near the entrance aperture 3, and/or the surface area of the first optical system 21 can be i.ncreased. Furthermore, the day/night viewing telescope 40 comprises, in the optical path between the entrance aperture 3 or the beam-impander 41 and the input 11 of the image converter 10, light separating means 50 which are designed to separate the light intended for day viewing and the light intended for night viewing. For the sake of clarity, the ray path for day viewing is shown in the top half of Figure 2A, and the ray path for night viewing is shown in the bottom half of Figure 2A. In the example shown in Figure 2A the light separating means 50 comprise a displaceable first reflective element 51. When using the day/night viewing telescope 40 the user opts for a night viewing mode or a day viewing rode by selecting a position of the first reflective element 51. In the day viewing mode the first reflective element 51 is positioned in the optical path between the entrance aperture 3 or the beam-impander 41 and the input 11 of the image converter 10, as shown in Figure 2. All the light 100 entering through the entrance aperture 3 is reflected (108) by the first reflective element 51 to a second reflective element 52, and from there is reflected (109) to the third optical system 32 for use for day viewing in a manner such as described above with reference to Figure 1. In the example shown, the first reflective element 51 and the second reflective element 52 cooperate in order to reflect (109) light rays incident on the first reflective element 51 from the second reflective element 52 past the image converter 10 to the third optical system 32, for the purpose of being focused thereby in an intermediate image. The second reflective element 52 is preferably a mangin mirror with a central recess which is aligned with the entrance aperture 3. In the night viewing mode the first reflective element 51 is removed fr .m the optical path between the entrance aperture 3 or the beam-impander 4 and the inlet 11 of the image converter 10. All the light entering through the entrance aperture 3 then strikes the first optical system 21 and is used for night viewing in a manner such as described above with reference to Figure 1. For displacing of the first reflective element 51 and for controlling such displacement, any desired suitable means can be used, as will be clear to an expert. Since the design of said means does not form part of the present invention, these means are not described further here, and for the sake of simplicity are not shown in Figure 2. It is merely pointed out that switching over from the night viewing mode to the day viewing mode and back can take place automatically, without specific action by the user if said means comprise a control device such as a motor and a light intensity detector for controlling the control device. In the day/night viewing telescope 40 the light 100 entering through the entrance aperture 3 is separated in time by the light separating means 50 into a part intended for day viewing and a part intended for night viewing, the separation ratio being defined by the ratio between the time during which the first reflective element 51 is in the position associated with the day viewing mode and the time during which the first reflective element 51 is in the position associated with the night viewing mode. Although it is possible for the first reflective element 51 to be moved to and fro with a relatively high repetition frequency between these two positions, in practice the first reflecting element 51 will be held stationary in one of the two positions. A consequence thereof is that in the night viewing mode no day viewing image is available, and vice versa. In a third embodiment 60 of the day/night viewing telescope according to the invention shown in Figure 2B, the advantages of the day/night viewing telescopes 1 and 40 have been combined. For this purpose, the light separating means 50 comprise a first wavelength- discriminating element 53 which is designed to be at least substantially light-transmissive within a first wavelength range which at least substantially corresponds to the sensitivity range of the image converter 10, and to be at least substantially light-reflective within a second wavelength range corresponding at least substantially to the visible wavelength range. An example of such a wavelength-discriminating element 53 is an edge filter. The operation of the day/night viewing telescope 60 works can be compared to that of the day/night viewing telescope 40. For light within the second wavelength range the first wavelength- discriminating element 53 fulfils the function of the first reflective element 51 in the position associated with the day viewing mode, while for light within the first wavelength range the first wavelength-discriminating element 53 is apparently not present and is therefore comparable to the reflective element 51 situated in the position associated with the night viewing mode. In this embodiment of the day/night viewing tele¬ scope 60, the first wavelength-discriminating element 53 separates the light 100 entering through the entrance aperture 3 according to wavelength into a part intended for day viewing and a part intended for night viewing. In this case all information suitable for night viewing can be used for forming a night viewing image, while at the same time all information suitable for day viewing can be used for forming a day viewing image. It is pointed out that the first wavelength range does not have to coincide exactly with the sensitivity range of the image converter 10. If the first wavelength range is less extended relative to the sensitivity range of the image converter 10, this is, however, accompanied by a reduced light intensity for night viewing. The same applies if the transmissivity of the wavelength- discriminating element 53 within the first wavelength range is less than 100%. If the first wavelength range relative to the sensitivity range of the image converter 10 is more extended towards the visible wavelength range, this is at the expense of the light intensity for day viewing, without, however, the light intensity being improved for night viewing. It has, however, been found that a loss of light the wavelength of which is near the upper limit of the visible wavelength range, means hardly any loss of image and/or colour information for the human eye. A suitable wavelength-discriminating limit of the wavelength-discriminating element 53 is approximately 620 nm. Figure 3 shows a further embodiment of the day/night viewing telescope according to the invention which has the advantages of the day/night viewing telescope 60, without a beam-impander being necessary, while the spatial dimensions of the housing can be retained. This makes possible a saving in weight and costs. The centre of gravity of the day/night viewing telescope can also be placed closer to the exit aperture 4, and thereby closer to the user. For the sake of clarity, the ray path for day viewing is shown in the top half of Figure 3, and the ray path for night viewing is shown in the bottom half of Figure 3. In the day/night viewing telescope 70 the input 11 of the image converter 10 faces away from the entrance aperture 3. For directing the light 113 received through the entrance aperture 3, and intended for night viewing, towards the input 11 of the image converter 10, the first light guide means 20 comprise at least one focusing reflective element 71 which is set up opposite the input 11 of the image converter 10, and of which the surface area at least substantially corresponds to the surface area of the entrance aperture 3. The focusing reflective element 71 is preferably a Mangin mirror with a central recess which is aligned with the exit aperture 4. Situated in the optical path between the focusing reflective element 71 and the input 11 of the image converter 10 are light separating means 72, comprising a first wavelength-discriminating element 73 of which the wavelength-discriminating action corresponds to that of the first wavelength-discriminating element 53 described above. The combination of the focusing reflecting element 71 and the light separating means 72 is configured in such a way that the light intended for night viewing forms a focused image on, or at least near, the input 11 of the image converter 10. Disposed opposite the output 12 of the image converter 10 is a second wavelength-discriminating element 74 which is designed to act as a positive reflection lens for light within a narrow third wave¬ length range substantially coinciding with the emission peak λ * of the image converter 10, and designed to at least substantially transmissive to light within at least a fourth wavelength range at least substantially corresponding to the combination of the visible wavelength range and the sensitivity range of the image converter 10, leaving out the above-mentioned third wavelength range. The second wavelength-discriminating element 74 is preferably disposed in such a way that the object plane thereof coincides substantially with the output 12 of the image converter 10, in which the object plane is the plane of which the rays after reflection by the element 74 run in the same way as the rays 116. In practice, it can be acceptable for the third wavelength range to differ slightly from tne emission peak wavelength. As will be clear from the descriptic which follows, a greater extension of the thir-*. wavelength range results, however, in a reduced light intensity for day viewing, while a smaller extension of the third wavelength range results in a reduced light intensity for night viewing. The day/night viewing telescope 70 operates as follows: Light 100 received through the entrance aperture 3 passes (111) the second wavelength- discriminating element 74 essentially without refraction and/or loss, with the exception of the light within the third wavelength range, which is reflected (not shown) in a diverging manner. The light 111 is then reflected (112) in a converging manner by the focusing reflecting element 71 to the second light separating means 72. Light the wavelength of which is greater than the separation wavelength of the first wavelength-discr: nating element 73, in other words the light 113 intended for night viewing, is imaged in focus on the input 11 of the image converter 10. Light the wavelength of which is smaller than the separation wavelength of the first wavelength-discriminating element 73, in other words light 114 intended for day viewing, is focused by the light separating means 72 into a real intermediate image which can be viewed through the exit aperture 4 with an eyepiece. Light rays 115 of wavelength λ * emitted by the output 12 of the image converter 10, since they are in the third wavelength range, are reflected (116) by the second wavelength-discriminating element 74, so that they run in the same manner as the input rays ill associated therewith, and are reflected (117) also in a converging manner by the focusing reflective element 71 to the second light separating means 72. Since the wavelength λ * is smaller than the separating wavelength of the first wavelength-discriminating element 73, these night viewing rays are reflected (118) by the light separating means 72 to the exit aperture 4, parallel to the day viewing rays 114 associated therewith, so that at the exit aperture 4 a night viewing image is produced coinciding with the day viewing image. The combination of the focusing reflective element 71 and the light separating means 72 can in this case be configured in such a way that at the exit aperture 4 a real intermediate image is produced which can be viewed with an eyepiece. In an alternative embodiment the focusing reflective element 71 is designed to form on, or at least in the immediate vicinity of, the input 11 of the image converter 10 a focused image of the light 111 received through the entrance aperture 3 and the light 116 emanating from the output 12 of the image converter 10 and reflected by the second wavelength-discriminating element 74 . The first wavelength-discriminating element 73 is designed to be at least substantially diffusely reflective for light within the second wavelength range, and can also be flat shaped. For the rest, the way in which this alternative embodiment of the day/night viewing telescope according to the invention operates is comparable to the day/night viewing telescope 70. A real night viewing image and a real day viewing image are thus projected coincidingly on the first wavelength- discriminating element 73, which real images can be viewed through the exit aperture 4 by means of an eyepiece. The second wavelength-discriminating element 74 is preferably formed by a holographic optical element. Such an element can be made with a relatively small transverse dimension and therefore with a relatively small weight, whereby the total weight of the day/night viewing tele¬ scope 70 can be reduced, while at the same time the centre of gravity of the day/night viewing telescope 70 can be shifted in the direction of the exit aperture 4.

Claims: CLAIMS 1. Combined day viewing and night viewing telescope, comprising: a housing with an entrance aperture and an exit aperture, first light guide means for guiding light received through the entrance aperture, and intended for night viewing, to an input of an image converter; and second light guide means for guiding light produced by the image converter at an output thereof to the exit aperture; characterised in that the telescope is designed to receive light intended for day viewing also through the entrance aperture intended for night viewing, and in that the telescope is provided with light separating means for separating said light for day viewing received through the entrance aperture from the light intended for night viewing and for directing said day viewing light to the exit aperture via an optical path running outside the image converter. 2. Device according to Claim 1, characterised in that the light separating means are designed for spatially separating the light intended for day viewing and the light intended for night viewing, and thereto comprise a first optical system with positive strength positioned in the optical path between the entrance aperture and the input of the image converter, the surface area of which system is smaller than the surface area of the entrance aperture. 3. Device according to Claim 1, characterised in that the light separating means are designed for separating in time the light intended for day viewing and the light intended for night viewing, and thereto comprise a displaceable first reflective element which in a night viewing position is active for transmitting the light received through the entrance aperture, and in a day viewing position is active for reflecting the light received through the entrance aperture. 4. Device according to Claim 1, characterised in that the light separating means are designed for separating according to wavelength the light intended for day viewing and the light intended for night viewing, and thereto comprise a first wavelength-discriminating element which is fitted to be at least substantially trans issive for light within a first wavelength range which at least substantially corresponds to the sensitivity range of the image converter, and to be at least substantially reflective for light within a second wavelength range which at least substantially corresponds to the visible wavelength range. 5. Device according to Claim 4, characterised in that the wavelength-discrimination limit of the first wavelength-discriminating element is approximately 620 nm. 6. Device according to at least one of Claims 3 5, characterised by a second reflective element to reflect again the light reflected by the first reflective element or the wavelength-discriminating element respectively. 7. Device according to Claim 6, characterised in that the second reflective element is a Mangin mirror with a central recess which is aligned with the entrance aperture. 8. Device according to Claim 4 or 5, characterised in that: - the output of the image converter faces the entrance aperture; - situated between the entrance aperture and the image converter is a second wavelength-discriminating element which is designed to function as a positive reflection lens for light within a narrow third wavelength range substantially coinciding with the emission peak λ* of the image converter, and at least to be substantially transmissive for light within at least a fourth wavelength range which at least substantially corresponds to the combination of the visible wavelength range and the sensitivity range of the image converter, with the exclusion of the said third wavelength range; and - a focusing reflective element is disposed opposite the input of the image converter. 9. Device according to Claim 8, characterised in that the focusing reflective element is a Mangin mirror with a central recess which is aligned with the exit aperture. 10. Device according to Claim 8 or 9, characterised in that the light separating means comprise a first wave¬ length-discriminating element which is fitted to be at least substantially transmissive for light within a first wavelength range which at least substantially corresponds to the sensitivity range of the image converter, and to be at least substantially reflective for light within a second wavelength range which at least substantially corresponds to the visible wavelength range. 11. Device according to at least one of Claims 8 10, characterised in that the combination of the focusing reflective element and the light separating means is configured in such a way that the light intended for night viewing is imaged in focus on, or at least near, the input of the image converter. 12. Device according to at least one of Claims 8 10, characterised in that the focusing reflective element is designed to image in focus on, or at least in the immediate vicinity of, the input of the image converter the light received through the entrance aperture and the light emanating from the output of the image converter and reflected by the second wavelength-discriminating element. 13. Device according to Claim 12, characterised in that the first wavelength-discriminating element is designed to be at least substantially diffusely reflective for light within the second wavelength range. 14. Device according to Claim 13, characterised in that the first wavelength-discriminating element is flat shaped. 15. Device according to at least one of Claims 8 - 14, characterised in that the object plane of the second wavelength-discriminating element substantially coincides with the output of the image converter. 16. Device according to at least one of Claims 8 - 15, characterised in that the second wavelength- discriminating element is designed as a holographic optical element.

Inventors: KRUIZINGA BOB, KRUIZINGA, BOB
Applicants: OPTISCHE IND DE OUDE DELFT NV, B.V. OPTISCHE INDUSTRIE 'DE OUDE DELFT'
Application year: 1991
Main classification: G02B17
IPC codes: G02B23