EP2567538A1

EP2567538A1 - Imaging method

Info

Publication number: EP2567538A1
Application number: EP11723534A
Authority: EP
Inventors: Laurent Brouard; Cyrille Tourneur
Original assignee: Astrium SAS
Current assignee: Airbus Defence and Space SAS
Priority date: 2010-05-04
Filing date: 2011-04-28
Publication date: 2013-03-13
Also published as: US20130050494A1; FR2959904A1; US9413990B2; FR2959904B1; WO2011138542A1

Abstract

The invention relates to an imaging method, including reconstructing a final image (IF) from intensity values inputted by a limited selection of photodetectors (2). One out of every two photodetectors is selected along columns (CDET) and rows (LDET). The final image has a resolution that is double that of a picture within the plane of the photodetectors. With identical resolution, the modulation transfer function is improved relative to an image that would be inputted while using all the photodetectors. Additionally, an image-sensing optical system that is used for an imaging method according to the invention can be of reduced size and weight, having constant resolution and constant modulation transfer functions.

Description

IMAGING METHOD

The present invention relates to an imaging method that is intended to be implemented from an aircraft or a spacecraft, as well as an imaging system that is adapted to implement such a method.

When a photodetector array is used to capture intensity values of a view that is formed by a shooting optics, the final image that is provided to the user is typically constructed by assigning in English) directly the intensity values that have been captured at the pixels of an image matrix that reproduces the matrix of photodetectors. All the photodetectors then correspond one by one to the pixels of the image matrix, and intensity values that have been captured by two adjacent photodetectors are assigned to two pixels that are also adjacent in the matrix of the final image.

But there are other modes of image capture ("image capture mode" in English), and the intensity values that are entered are assigned to the pixels of the matrix of the final image in a way that is adapted at each input mode. In particular, when a scene is captured in push-broom mode, the final image is reconstructed by assigning pixels of different segments of the final image matrix, intensity values that have been captured at different times of the image. scene scanning by the camera system. Such a push-broom acquisition mode is used in a scanner or a photocopier, for example. In known manner, the push-broom mode is also used to photograph portions of the earth's surface from a satellite. In this case, the scanning of a portion of the terrestrial surface is achieved by scrolling the pointing direction of the optics.

However, in such a push-broom acquisition mode, pixels adjacent to the final image still reproduce elementary areas of the photographed scene that are separated by the size of a photodetector, divided by the magnification of the optical image. shooting for conditions shots used. For the terrestrial imaging application, this separation distance between two elementary areas of the Earth's surface that are reproduced on neighboring pixels in the final image array is referred to as the ground-based resolution or sampling distance, and designated by GSD for "Ground Sampling Distance" in English.

Another feature of an imaging method is the modulation transfer function, commonly referred to as FTM. It is equal to the quotient of the contrast of a periodic modulation in the final image, by the real contrast of this modulation in the photographed scene. The value of the modulation transfer function decreases as the spatial frequency of the modulation increases. The value of the modulation transfer function is limited by several effects, among which an effect of the viewing optics and the effects of the photodetectors which are used. In particular, an increase in the diameter of the pupil of the shooting optics makes it possible to increase the modulation transfer function. Conversely, nonzero individual dimensions of photodetectors and possible crosstalk that occurs between neighboring photodetectors contribute to reducing the modulation transfer function.

However obtaining a modulation transfer function which has high values is sought, especially for terrestrial imaging applications.

An object of the present invention is then to increase the modulation transfer function, with regard to the effects of photodetectors on the values of this function.

Another object of the invention is to reduce the dimensions of the image optics, with constant values of the resolution and the modulation transfer function. Such a reduction in the dimensions of the optics is intended to reduce the cost of this optics, its size and mass. Consequently, it is also a question of reducing the dimensions of an aircraft or a spacecraft on which this optics is installed, as well as the costs of launching such a spacecraft.

These aims of the invention relate both to an acquisition mode in which an array of photodetectors is used to acquire the two-dimensional image information in a single exposure, an acquisition mode in which image lines are captured one by one, or a push-broom mode of acquisition.

In general, the final image that is provided by a method according to the invention is composed of intensity values which are respectively assigned to pixels of a matrix of this final image, this image matrix consisting of columns. adjacent and adjacent lines of pixels.

For a mode of acquisition in a single exposure of the two-dimensional image information, the invention proposes an imaging method which comprises the following steps:

IM embark a camera on an aircraft or spacecraft, this instrument comprising a shooting optics and at least one photodetector matrix which is arranged in a focal plane of the optical pickup views, this matrix of photodetectors being itself constituted of adjacent columns and adjacent lines of photodetectors;

121 using the camera, enter radiation intensity values from elementary areas of a scene; then

121 construct the final image by assigning some of the intensity values that were captured in step 121 to the pixels of the final image array.

The method of the invention is characterized in that:

the intensity values which are input at step 121 are restricted to a selection of one photodetector out of two along the columns and along the lines of the photodetector matrix, so as to constitute a surface selection of a four out of four photodetectors in this matrix of photodetectors; and

in step / 3 /, the pixels of the matrix of the final image to which the intensity values which have been input by the Selected photodetectors are adjacent to each other along the columns and lines of the final image matrix.

Thus, two photodetectors that are used to capture intensity values rendered in the final image are separated by at least one other intermediate photodetector in the photodetector array. In this way, no crosstalk occurs between photodetectors whose intensity values are used for the final image. The limitation of the modulation transfer function due to the crosstalk between neighboring photodetectors is thus eliminated. Furthermore, in a method according to the invention, the resolution in the focal plane of the shooting optics resulting from the size of each photodetector is twice as small as the resolution associated with each pixel in the image. final image. Then, to obtain a constant resolution of the final image, the matrix of photodetectors can be chosen with a step of photodetectors which is twice as small as in a method which does not use the invention. The average effect of the light intensity which is captured by each photodetector individually, resulting from its non-zero dimensions, is then reduced. In mathematical terms, the collector surface of each photodetector causes a convolution operation by a window function that is narrower. Thus, the decrease in the individual size of the photodetectors also contributes to increasing the value of the modulation transfer function, at a constant value of the resolution of the final image. More precisely, the modulation transfer function is increased by a multiplying factor of approximately 0.90 / 0.64, for the spatial frequency which corresponds to the resolution of the final image.

Finally, the intensity values which are not used to construct the final image, that is to say those which have been captured by those of the photodetectors which are not selected according to the invention, may not be read. . The step of reading the intensity values after each exposure of the photodetector matrix is then shorter, so that images can be captured at a higher frequency. Step 121 of a method according to the invention may therefore consist in first selecting the photodetectors whose intensity values will be read, then read only these intensity values of the selected photodetectors. Alternatively, the intensity values that are captured by all the photodetectors can be read first as a whole, then only those that have been captured by those photodetectors that belong to the selection are used or stored. In other words, the selection of the photodetectors according to the invention can take place before or after the reading of the intensity values entered.

For a line-by-line acquisition mode, the invention proposes an imaging method that comprises the following steps:

IM also board the recording instrument on board the aircraft or the spacecraft, the instrument comprising the optics and at least one photodetector line which is arranged in the focal plane of the camera. this imaging optics, the line of photodetectors being itself constituted by photodetectors which are adjacent and aligned in a longitudinal direction;

121 using the camera, enter radiation intensity values from elementary areas of the scene, during successive exposures that are performed so that a view of the scene is moved in the plane focal perpendicular to the longitudinal direction of the line of photodetectors; then

/ 3 / constructing the final image by assigning some of the intensity values that were captured in step 121 to the pixels of the final image array.

Such a method according to the invention is characterized in that:

in step 121, the intensity values that have been entered are restricted to a selection of one photodetector out of two along the longitudinal direction of the photodetector line, and the exposures are carried out so that the view of the scene is displaced in the focal plane by twice the width of the photodetectors measured perpendicularly to the longitudinal direction, between two exposures; and in step / 3 /, the pixels of the matrix of the final image to which the intensity values which have been inputted during the successive exposures by the selected photodetectors are assigned are adjacent to one another according to the columns and the lines of the final image matrix.

The advantages of the invention which have been enumerated above for a mode of acquisition in a single exposure of the two-dimensional image information are identical for such a line-by-line acquisition mode.

In addition, these two embodiments of the invention can be combined with a push-broom scanning mode. According to an improvement of the invention for the row-by-line acquisition mode, the camera may comprise at least one mat of photodetectors having several lines of photodetectors which are adjacent parallel to the longitudinal direction, in the focal plane of the optics of shooting. These photodetector lines may belong to at least one matrix which is adapted to capture the radiation intensity values in step 121 according to a TDI type acquisition mode, for "time delay integration" in English. According to the TDI mode, the image acquisition is carried out line by line, during a succession of exposure sequences of the matrix and simultaneous reading of the intensity values entered, these sequences being carried out each time the view of the scene is displaced in the focal plane by a distance equal to the individual width of the photodetector lines measured perpendicular to the longitudinal direction of the lines. In an implementation of the invention combined with the TDI mode, the intensity values that are assigned to the pixels of the final image are restricted to one out of two photodetector selection in each row of the array, and to a sequence of matrix exposure and read intensity values entered in TDI mode every two sequences.

The imaging methods according to the invention are particularly suitable for terrestrial imaging applications. For such applications, the camera is embarked on board an aircraft or a satellite orbiting the Earth, and the scene is constituted by a surface portion of the Earth. The improvements in the modulation transfer function that result from the invention, for the contribution of the photodetector crosstalk and that of the photodetector dimensions, can be used to reduce the size of the imaging optics. Thus, at equal values of the resolution of the final image and the modulation transfer function for the spatial frequency that corresponds to this resolution, the camera can have a footprint and a mass that are reduced. Manufacturing savings from this instrument result, as well as reductions in the cost of an aircraft or satellite on board the instrument.

The invention furthermore proposes imaging systems which are adapted to implement the methods characterized above, respectively for a mode of acquisition in a single exposure of the two-dimensional image information and for a mode of imaging. acquisition line by line, the latter possibly being of the TDI type.

Other features and advantages of the present invention will emerge in the following description of nonlimiting exemplary embodiments, with reference to the appended drawings, in which:

FIG. 1 is a perspective view of an imaging system that is adapted to implement a method according to the invention with a mode of acquisition in a single exposure of the two-dimensional image information;

FIG. 2 schematically shows a correspondence between photodetectors and final image pixels;

FIG. 3 illustrates a variant of the implementation of FIG. 1;

FIG. 4 illustrates an application of the invention to an acquisition mode of the push-broom type;

FIGS. 5a and 5b illustrate an implementation of the invention with an acquisition mode of the TDI type;

- Figure 6 is a diagram comparing a sizing of a shooting optics using the invention and without using it. Figures 1 to 5a and 5b are diagrams of optical principles, so that the dimensions of the elements that are shown correspond neither to real dimensions nor to real size ratios. In addition, identical references which are indicated in different figures designate identical elements or which have identical functions. Likewise, the numbers of columns and lines that are shown for the arrangement of photodetectors in the focal plane, as well as the numbers of columns and rows of the final image matrix, do not correspond to actual implementations. of the invention but have been limited for clarity of the figures.

Figure 1 shows schematically a satellite S in orbit above the Earth, denoted T. The satellite S may be of the geostationary type, but not necessarily. A recording instrument 10 is embarked aboard the satellite S, to capture at least one image of a portion F of the surface of the Earth, which constitutes the photographed scene. The image data is transmitted by a transmitter 40 which is also embarked on board the satellite S, towards a terrestrial station U of a user of the captured image. The user station U comprises a receiver 50 of the data transmitted by the transmitter 40. The transmission mode of these image data between the transmitter 40 and the receiver 50 may be arbitrary, for example of the wave type. radio or laser signals.

The camera 10 comprises, in a first mode of implementation of the invention:

- Shooting optics 1, which is adapted to form a view of the land surface portion F in a focal plane denoted PF;

at least one matrix of photodetectors 20, which is arranged in the focal plane PF, and of which each photodetector is individually referenced 2; and

a control unit 30, which is adapted to control a radiation intensity value input by each of the photodetectors 2. The shooting optics 1 may be a telescope of a type known before the invention. It is represented symbolically in the form of a convergent lens, without limitation with respect to its actual structure.

The photodetector array 20 consists of adjacent columns and adjacent lines of the photodetectors 2, and each photodetector 2 is adapted to capture the intensity value of the radiation that it receives from an elementary zone ZE of the surface portion. This elementary zone ZE of the portion F is thus optically conjugated by the optics 1 with the corresponding photodetector 2. Optionally, a steerable mirror 3 may be arranged on board the satellite S in front of the input of the optical camera 1, to orient the pointing direction DP of the optical lens 1 towards the terrestrial surface portion F. However, the steerable mirror 3 is not essential for the invention, and the pointing of the shooting optics 1 can be achieved alternately by using a steerable support for this optics on board the satellite S. This score can still be achieved suitably varying the attitude of the satellite S as a whole, in a manner known to those skilled in the art. In the latter case, the shooting optics 1 can be supported on board the satellite S with an orientation which is fixed with respect to the platform of the satellite.

The control unit 30 controls in particular the reset, accumulation and reading operations of each photodetector 2 to obtain the value of the intensity of the radiation which is received by this photodetector at each exposure.

An image reconstruction unit 60 assigns, or assigns, the intensity values that are captured by the photodetectors 2 to the pixels of a matrix of the final image that is provided to the user. This assignment is performed according to the coordinates of each photodetector 2 in the matrix 20, and the coordinates of each pixel in the matrix of the final image. In the usual way, the pixels to which the intensity values that have been input are assigned are adjacent to each other according to columns and lines of the final image matrix. The reconstruction unit 60 can be embarked on board the satellite S, in which case the data of the final image is transmitted by the transmitter 40 to the receiver 50. Alternatively and in accordance with Figure 1, the reconstruction unit 60 may be located on Earth. In this case, the data that is transmitted between the satellite S and the earth station U may be the intensity values that are captured by the photodetectors 2.

According to the invention, the control unit 30 is adapted to select a single photodetector 2 out of two along the columns and along the lines of the photodetector matrix 20, so as to constitute a surface selection of one photodetector out of four in the matrix of photodetectors 20. After exposure of the matrix 20 to the radiation that comes from the terrestrial surface portion F, the unit 30 controls a reading of the intensity values that have been captured by those of the photodetectors 2 which are selected .

The operation of the image reconstruction unit 60 is then restricted to the intensity values that have been captured by the selected photodetectors, while affecting those intensity values inputted only by the selected photodetectors to pixels that are adjacent between them in the final image matrix.

The camera 10 and the reconstruction unit 60 which is adapted for such operation together form the imaging system of the invention.

FIG. 2 illustrates the assignment according to the invention of intensity values to the pixels of the final image.

The left-hand part of FIG. 2 represents the photodetector matrix 20. CDET collectively denotes the columns of this matrix of photodetectors 20, and L _D AND denotes the lines thereof. The crosses indicate which photodetectors 2 of the matrix 20 are selected according to the invention.

The right part of Figure 2 represents the pixel matrix of the final image that is provided to the user. This image matrix is designated globally by the reference IF, and PI denotes the individual pixels of the image matrix IF. CIF denotes the columns of pixels of the image matrix IF, and LIF denotes the rows of pixels of the same image matrix. The arrows show the assignment to the pixels PI of the image matrix IF, intensity values that are captured by the photodetectors 2 selected. Intensity values are thus assigned to all the pixels PI, these being adjacent to each other according to the columns C | _F and the lines L _IF . But these affected intensity values have been captured by the selected photodetectors which are not adjacent to each other along the columns C _D ET and lines L _D E _T , but which are separated by lines and intermediate columns of non-photodetectors. selected. However, the assignment of the intensity values entered to the pixels introduces no inversion in the order of the pixels relative to that of the photodetectors, following any directions in the focal plane PF.

The selection ratio of the photodetectors 2 is one in two in each column CDE _T and in each line LDET of the matrix 20, so as to obtain a surface selection ratio of one in four over the entire matrix 20. By Therefore, the matrix 20 of the photodetectors has twice as many columns and rows as the matrix of the final image IF. For this reason, the image that is formed in the focal plane PF is said to be oversampled with respect to the final image IF. For example, to obtain an IF final image with 640 columns C | _F of pixels and 480 lines L | _F of pixels, the photodetector matrix 20 must comprise 1280 columns CDET of photodetectors and 960 lines LDE _T of photodetectors.

In other words, the invention consists in tightening the rows and columns of the matrix 20, removing one out of two in both directions. The resolution R of the final image IF, that is to say the distance to the ground in the portion of terrestrial surface F which corresponds to the passage of a column or a line of pixels PI in the final image at the next column or pixel line is twice the resolution of the view which is formed in the focal plane PF on the photodetector matrix 20. In Fig. 2, x denotes the distances that correspond to this resolution R, on the matrix of photodetectors 20 and in the final image IF. In FIG. 1, the dimension of the sides of the elementary zone ZE is therefore R / 2.

Since the intensity values that are used according to the invention for constructing the final image IF come from photodetectors 2 which are never adjacent to each other in the matrix 20, no crosstalk intervenes which could disturb these values.

In addition, the size of the photodetectors 2 of the matrix 20 which is used to implement the invention is half that which would be used without the invention to obtain an identical resolution R of the final image IF with a correspondence one for one between the photodetectors of the matrix 20 and the pixels PI of the final image IF. This dimension of the photodetectors, which is reduced by the invention, improves the modulation transfer function, in a further way with respect to the suppression of inter-photodetector crosstalk.

Preferably, the intensity values that are captured by those of the photodetectors 2 that are not selected are not transmitted by the transmitter 40. Advantageously, the control unit 30 can be programmed so that these intensity values of the Non-selected photodetectors are not read at the end of each exposure of the matrix 20.

FIG. 3 illustrates a variant of the invention when the matrix of photodetectors 20 is replaced by at least one, for example one, line of photodetectors 2 which are adjacent and aligned in a longitudinal direction denoted DL. Such a line of photodetectors may be constituted by an autonomous strip 21 of photodetectors. In this figure, V designates the ground track of the satellite S as it moves on its orbit. At each instant of the scrolling of the satellite S, the shooting optics 1 forms, on the strip 21, the image of a band of the terrestrial surface portion F. The references B † to B 3 denote bands which correspond to at three different times. They are shifted parallel to the trace V. In this case, the image of the terrestrial surface portion F is taken line by line, bringing together the images of the bands of the portion F which are captured during successive exposures. For this variant embodiment of the invention, a photodetector 2 out of two is selected by the control unit 30 for all the exposures, along the direction DL, and the unit 30 further synchronizes the successive exposures of the bar 21 with the scrolling of the satellite S on the trace V. This synchronization is to trigger each exposure so that the ground strips Bi, B ₂ , B _3l ... are successively offset parallel to the trace V by twice the individual width R / 2 of these bands. Under these conditions, the image of the portion F in the focal plane PF is displaced perpendicularly to the direction DL by twice the width of the photodetectors 2 between two successive exposures, the width of the photodetectors 2 to be considered being also measured perpendicular to the direction DL Thus, the selection of one line in two in the photodetector matrix 20 of the embodiment of FIG. 1 is replaced by an appropriate frequency of the exposures with respect to the scrolling of the satellite S.

The image reconstruction unit 60 then assigns to the pixels PI of the final image matrix IF these intensity values that have been entered during the successive exposures by the selected photodetectors 2. As previously, the pixels PI to which the intensity values that have been input by the selected photodetectors of the strip 21 are still located are still adjacent to one another according to the columns C [ _F and the lines L | _F of the final image matrix IF. Nevertheless, two of the bands Bi,, B ₃ , ... of the portion F which are acquired during different exposures are shifted by a distance to the ground which is equal to 2 n times the resolution R / 2 of the image. final IF, n being an integer, while each band has an individual width which is equal to R / 2.

The embodiments of the invention described with reference to FIGS. 1 to 3 may be combined with a push-broom image acquisition mode. Such a mode of acquisition is supposed to be known, so that it is not necessary to recall the principle in detail. The main push-broom mode parameters are illustrated in Figure 4:

V again designates the ground track of the displacement of the satellite S in its orbit, around the Earth denoted T;

B is either the land surface portion F of Figure 1 or one of the bands B2, B3, · ■ · of Figure 3;

w is the elementary swath size, corresponding to each exposure and to the length of the matrix 20 or of the bar 21, in the longitudinal direction DL; and

AC is the accessible corridor for shooting on the surface of the Earth T, which is obtained by varying the pointing direction DP perpendicular to the ground track V.

The pointing direction DP can be varied between two successive exposures by changing the orientation of the mirror 3, or by switching the satellite S along its axis of rotation. But any other mode of variation of the pointing direction DP can be used alternately.

Figures 5a and 5b illustrate an adaptation of the invention to the particular type of acquisition which is designated by TDI. The lower part of Figure 5a is an enlarged and schematic view of the TDI detector that is used. Items that have already been described and are not changed are not repeated. The matrix of photodetectors 20 is of the CCD-TDI type, for example with four adjacent lines of photodetectors 2, referenced 20a to 20d, and a transfer register, referenced 20z. Z denotes the reading output of the matrix 20. For example, each line 20a-20d of the matrix 20 itself comprises 1280 photodetectors 2 which are adjacent in the longitudinal direction DL. The selection of the photodetectors 2 in the lines 20a-20d, identical for all the exposures, is the same as that described with reference to the left part of FIG. 2, but the reading of the intensity values which are captured by these selected photodetectors is carried out in accordance with the TDI acquisition mode. Thus, the intensity values that are assigned to the pixels PI of the final image matrix IF are sums of elementary intensity values that are captured during successive exposures by photodetectors offset perpendicularly to the direction DL. FIG. 5b shows how each intensity value is acquired perpendicularly to the direction DL, that is to say in the direction of the columns of the matrix 20. The positions of the matrix 20 with respect to the image of the portion of surface F in the focal plane PF are represented at successive instants T ₀ + j tj, T ₀ being an initial moment, t, being the integration time TDI and j being an integer successively equal to 1, 2, 3 , ... The image of the ground strip Bi is formed on the line 20d of photodetectors at the instant T ₀ + tj, then on the line 20c at the instant T ₀ + 2-tj, then on the line 20b at the instant T ₀ + 3-tj, and finally on the line 20a at the instant T ₀ +4 tj. An exposure of the matrix 20 is triggered during each integration time, and then the resulting signals are transferred to the transfer register 20z at time T ₀ + 5 tj. Simultaneously, the ground band B ₂ is captured by the line 20d of photodetectors during the exposure which is carried out at the instant T ₀ + 3 ti, then by the line 20c during the exposure of the instant ΤΌ + 4- tj, then by line 20b at time Τ ₀ + 5 · ¾, finally by line 20a at instant Το + δ ί ,, and is transferred to register 20z at time T ₀ + 7 tj. Thus, between each exposure of the TDI acquisition mode, the view of the scene F has moved in the focal plane PF perpendicular to the longitudinal direction DL by one time the individual width of the photodetector lines 20a-20d, also measured. perpendicular to the DL direction. The intensity values which are thus transferred to the transfer register 20z at times T ₀ + 5 tj, T ₀ + 7 tj, etc., that is to say at one integration time out of two, are then only retained and assigned to adjacent LIF lines in the final IF image. Furthermore, within each of these lines L _iF , the intensity values that are assigned to the successive pixels PI result from a selection of one of two values that are delivered by the transfer register 20z to each sequence TDI playback.

In general, for all the modes of implementation of the invention, the shooting optics 1 may have a focal length that is greater than a focal length that would also produce the resolution R of the final image IF if this final image was composed of intensity values captured by adjacent photodetectors 2 in the focal plane PF, at constant size of the photodetectors. In other words, for the same resolution R of the final image IF and for the same value of the modulation transfer function that corresponds to this resolution, the focal length of the optics 1 can be increased compared to an imaging method as known before the invention.

In addition, the input pupil of the optical optics can be sized for the resolution value which corresponds to twice the pitch of the photodetectors in the matrix 20 or the strip 21, instead of a single one. times the value of this photodetector step as is usually applied.

Another advantage of the invention arises when sizing optics 1. In known manner, the components of this optics must have dimensions that are compatible with the resolution of the final image IF , and with the desired values for the modulation transfer function. In particular, a diameter of each mirror of this optic is selected so that the entrance pupil causes a diffraction which does not reduce the resolution of the final image IF, nor the modulation transfer function. Thanks to the improvements in the contributions of photodetectors to the modulation transfer function, provided by the invention, the dimensions of the optics 1 can be reduced, for example by a factor of 2. The price of the instrument Shooting is reduced accordingly, and its installation on board the aircraft or spacecraft is facilitated. In addition, the weight of the optics is also reduced, which is an important advantage for an instrument that is onboard a satellite, compared to the constraints that are related to the launch of the satellite.

In addition, the invention makes it possible to reduce the spectrum aliasing effect, which is known as aliasing in English. This effect results from the sampling of the image by the photodetectors, and intervenes in the spatial frequency decomposition of this image. In general, for the field of spatial imaging, ie for high resolution optical conditions that are desired, the diffraction that is produced by the optics of shooting causes low-pass filtering of the images. spatial frequencies. This diffraction reduces the amplitudes of the spatial frequencies which are folded, to a greater extent than the non-folded frequencies. By allowing a reduction of the pupil size of the imaging optics, the invention further decreases the amplitudes of folded spatial frequencies, so that the effect of spectrum folding is even less important.

Figure 6 is a diagram that compares the values of the function of modulation transfer, with identical resolution of the final image, for a final image that is acquired and reconstructed according to the invention (curve in solid lines) and without the invention (curve in dashed lines). Obtaining the final image without the invention corresponds to the reading of all the adjacent photodetectors which are located in the focal plane of the optics, and the assignment one by one of all the values of intensity read at the pixels of the final image. The diameter of the input pupil of the imaging optics is indicated on the abscissa, and the values of the modulation transfer function are indicated on the ordinate. This diagram shows in particular that for the value of 20% of the modulation transfer function, the diameter of the entrance pupil can be reduced from about 1700 mm to about 1050 mm.

Thus, for identical values of the resolution and of the modulation transfer function, the invention makes it possible to reduce in a large proportion the size of the entrance pupil of the optical optics, at the same time as its focal length can be increased. This optic therefore has an opening which is greatly reduced, and the optical aberrations which are related to this opening, such as spherical aberration, coma and astigmatism, are reduced accordingly. In addition, this reduction of the opening facilitates the realization of the dioptric and catadioptric surfaces which constitute the camera, by reducing the necessary radii of curvature.

It will be apparent to the skilled person skilled in imaging that the various embodiments of the invention which have been described in detail are in fact equivalent to the principle of the invention illustrated in FIG. 2.

It is understood that the invention can be reproduced by adapting these modes of implementation in various ways, depending on each application of the imaging system. In particular, the invention can be implemented as well from any spacecraft, of the satellite or space probe type, or from any aircraft, of the type of exploration aircraft or unmanned aircraft, in particular .

Claims

1. An imaging method comprising the following steps for providing a final image (IF) composed of intensity values respectively assigned to pixels (PI) of a matrix of the final image consisting of adjacent columns (CIF) and adjacent lines (LIF) of pixels:

/ 1 / embedding a camera (10) on board an aircraft or a spacecraft (S), said camera comprising a shooting optics (1) and at least one matrix photodetector arrangement (20) disposed in a focal plane (PF) of the imaging optics, said photodetector array consisting of adjacent columns (CDET) and adjacent lines (LDET) of photodetectors (2);

/ 2 / using the camera (10), enter radiation intensity values from elementary areas (ZE) of a scene (F); then

/ 3 / constructing the final image (IF) by assigning some of the intensity values captured at step 121 to the pixels (PI) of the final image matrix, the method being characterized in that:

in step 121, the intensity values entered are restricted to a selection of one photodetector (2) out of two along the columns (C _D AND) and along the lines (LDET) of the photodetector matrix ( 20), so as to constitute a surface selection of one photodetector out of four in said photodetector matrix; and

in step 131, the pixels (PI) of the matrix of the final image (IF) to which the intensity values captured by the selected photodetectors are assigned are adjacent to one another according to the columns (CIF) and the lines ( LIF) of said final image matrix.

An imaging method comprising the following steps for providing a final image (IF) composed of intensity values respectively assigned to pixels (PI) of a matrix of the final image consisting of adjacent columns (C | _F ). and adjacent lines (LIF) of pixels:

IM boarding an instrument (10) on board an aircraft or a spacecraft (S), said camera comprising a shooting optics (1) and at least one line of photodetectors (21) disposed in a focal plane (PF) of the imaging optics, said line of photodetectors consisting of adjacent photodetectors (2) aligned along a longitudinal direction (DL);

/ 2 / by using the camera (10), to enter radiation intensity values from elementary zones (ZE) of a scene (F), during successive exposures carried out so that a view of the scene is moved in the focal plane (PF) perpendicular to the longitudinal direction (DL) of the photodetector line (21); then

131 constructing the final image (IF) by assigning some of the intensity values captured at step 121 to the pixels (PI) of the final image matrix, the method being characterized in that:

in step 121, the seized intensity values are restricted to a selection of one photodetector (2) out of two along the longitudinal direction (DL) of the photodetector line (21), and the exposures are realized so that the view of the scene (F) is displaced in the focal plane (PF) by twice a width of the photodetectors (2) measured perpendicularly to said longitudinal direction, between two of the exposures; and

in step 131, the pixels (PI) of the matrix of the final image (IF) to which the intensity values entered during the successive exposures by the selected photodetectors are assigned are adjacent between they according to the columns (C | _F ) and lines (L _tF ) of said final image matrix.

3. Method according to claim 2, wherein the camera (10) comprises several lines (20a-20d) of photodetectors (2) adjacent and parallel to the longitudinal direction (DL) in the focal plane (PF). , said lines forming at least one matrix (20), and according to which the radiation intensity values are entered at step 121 according to a TDI type of acquisition mode, the intensity values which are assigned to the pixels (PI) of the final image (IF) being restricted to a selection of one photodetector out of two in each row of the matrix, and restricted to a sequence of exposure of the matrix and reading the intensity values entered in TDI mode every two sequences, the TDI mode exposure and playback sequences being performed each time the scene view (F) is moved in said focal plane by a distance equal to the individual width of the photodetector lines measured perpendicularly at said longitudinal direction.

The method of any one of claims 1 to 3, wherein a focal length of the imaging optics (1) is greater than a focal length producing an identical resolution of the final image (IF) if said final image was composed of intensity values captured by photodetectors (2) adjacent in the focal plane (PF), constant dimension of said photodetectors.

5. Method according to any one of the preceding claims, wherein the camera (10) is on board an aircraft or a terrestrial satellite (S), and wherein the scene (F) is a surface portion of the Earth.

An imaging system comprising a camera (10) for embarkation on board an aircraft or spacecraft (S), said camera comprising:

- Shooting optics (1) adapted to form a view of a scene (F) in a focal plane (PF); at least one photodetector array (20) disposed in the focal plane (PF), said photodetector array consisting of adjacent columns (CDET) and adjacent lines (LDET) of photodetectors (2), and each photodetector being adapted to entering a radiation intensity value from an elementary zone (EZ) of the scene (F); and

a control unit (30), adapted to select one photodetector (2) out of two along the columns (CDET) and along the lines (L _D AND) of the photodetector matrix (20), so as to constitute a surface selection of one photodetector out of four in said matrix of photodetectors, and for controlling a reading of the intensity values entered by the photodetectors selected after the same exposure,

the imaging system further comprising an image reconstruction unit (60) adapted to assign the intensity values captured by the selected photodetectors to pixels (PI) of an image matrix (IF), said pixels to which the intensity values captured by the selected photodetectors are located adjacent to each other according to columns (CIF) and lines (LIF) of the final image matrix.

- Shooting optics (1) adapted to form a view of a scene (F) in a focal plane (PF);

at least one line of photodetectors (21) arranged in the focal plane

(PF), said line of photodetectors consisting of adjacent photodetectors (2) aligned in a longitudinal direction (DL), and each photodetector being adapted to capture a radiation intensity value from an elementary zone (ZE) of the scene (F); and a control unit (30), adapted to select one photodetector (2) out of two along the longitudinal direction (DL) of the line of photodetectors (21), and to control a reading of the intensity values entered during a same exposure by the selected photodetectors, then at successive exposures controlled when the view of the scene (F) is moved in the focal plane (PF) perpendicularly to the longitudinal direction of the photodetector line (21) twice a width of said photodetectors (2) measured perpendicularly to said longitudinal direction, between two exposures,

the imaging system further comprising an image reconstruction unit (60) adapted to assign to pixels (PI) of an image matrix (IF) said intensity values captured during successive exposures by the selected photodetectors, said pixels being assigned said intensity values captured by the selected photodetectors being adjacent to each other according to columns (CIF) and lines (LIF) of the final image matrix.

8. System according to claim 7, wherein the camera (10) comprises several lines (20a-20d) of photodetectors (2) adjacent and parallel to the longitudinal direction (DL) in the focal plane (PF). , said lines belonging to a matrix (20) adapted to capture the radiation intensity values according to a TDI type acquisition mode, the control unit (30) being adapted to select one out of every two photodetectors in each line of the matrix, and to select those of said intensity values which are entered in TDI mode during a sequence of exposure of the matnce and reading said entered intensity values performed both exposure sequences and for reading, said exposure and read sequences being controlled whenever the view of the scene (F) is moved in said focal plane by a distance equal to the individual width of the photodetector lines measured perpendicularly. culairement to said longitudinal direction.

The system of any one of claims 6 to 8, wherein a focal length of the imaging optics (1) is greater than a focal length producing an identical resolution of the final image (IF) if said final image was composed of intensity values captured by photodetectors (2) adjacent in the focal plane (PF), constant dimension of said photodetectors.