EP3198625B1 - Bimode image acquisition device with photocathode - Google Patents

Description

TECHNICAL AREA

The present invention relates to the field of night vision image acquisition devices, comprising a photocathode adapted to convert a stream of photons into a stream of electrons. The field of the invention is more particularly that of such devices, using matrix color filters.

STATE OF THE PRIOR ART

In the prior art are known various devices for acquiring night vision images, comprising a photocathode.

Such a device is for example an image intensifier tube, comprising a photocathode, adapted to convert an incident flux of photons into an initial flow of electrons. This initial flow of electrons propagates inside the intensifier tube, where it is accelerated by a first electrostatic field towards multiplication means.

These multiplying means receive said initial flow of electrons, and in response provide a secondary electron flow. Each initial electron incident on an input side of the multiplication means, causes the emission of several secondary electrons on the side of the output face of these same means. Thus, an intense secondary electron flux is generated from a low initial electron flux, and thus ultimately from very low intensity light radiation.

The secondary electron flux is accelerated by a third electrostatic field in the direction of a phosphor screen, which converts the secondary electron flux into a photon flux. Thanks to the multiplication means, the photon flux provided by the phosphor screen corresponds to the flux of photons incident on the photocathode, but in more intense. In other words, each photon of the photon flux incident on the photocathode corresponds to several photons of the photon flux supplied by the phosphor screen.

The photocathode and the multiplying means are placed in a vacuum tube having an entrance window to let the incident photon flux enter the photocathode. The vacuum tube can be closed by the phosphor screen.

When the incident photon flux on the photocathode is converted into an initial electron flux, the information relating to the wavelength of the photons is lost. Thus, the photon flux provided by the phosphor screen corresponds to a monochrome image.

The document GB 2,302,444 proposes an intensifier tube of images making it possible to restore a poly-chromatic image.

A first primary color filter array is disposed upstream of the photocathode to filter incident photon flux before it reaches the photocathode.

A primary color filter is a spectral filter, which does not transmit part of the visible spectrum complementary to this primary color. Thus, a primary color filter is a spectral filter that transmits part of the visible spectrum corresponding to this primary color, and possibly part of the infrared spectrum, and even part of the near-UV spectrum (200 to 400 nm) or even UV (10 to 200 nm).

The first primary color filter matrix consists of red, green, and blue filters that draw primary color pixels on the primary color filter. photocathode. Thus, a photon flux incident on a given pixel of the photocathode corresponds to a given primary color. The electron flow supplied in response by the photocathode does not directly contain chromatic information, but corresponds to this given primary color.

At the output of the intensifier tube, the photon flux supplied by the phosphor screen corresponds to a white light, a combination of several wavelengths corresponding in particular to red, green and blue. This stream is filtered by a second matrix of primary color filters. This second matrix draws pixels of primary color on the phosphor screen. Thus, a flux of photons emitted by a given pixel of the phosphor screen is filtered by a primary color filter. At the output of this primary color filter, a flux of photons corresponding to a given primary color is obtained. The second matrix is identical to and aligned with the first matrix. The pixels of the phosphor screen are therefore aligned with the pixels of the photocathode. The image supplied at the output of the second matrix is thus composed of pixels of three primary colors, corresponding to an intensified image of the pixelated image at the output of the first matrix.

This produces a night vision intensifier tube providing a color image. However, because of the presence of the two matrices of primary color filters, this intensifier tube has high energy losses, detrimental in a field characterized by the need for a strong intensification of a photon flux.

An object of the present invention is to provide an image acquisition device for acquiring color images while minimizing the damage caused by energy losses.

STATEMENT OF THE INVENTION

• une photocathode, adaptée à convertir un flux incident de photons en un flux d'électrons ;
• un capteur constitué d'une matrice d'éléments, dits pixels ; et
• des moyens de traitement.

This objective is achieved with an image acquisition device comprising:

• a photocathode adapted to convert an incident flux of photons into a stream of electrons;
• a sensor consisting of a matrix of elements, called pixels; and
• processing means.

• le dispositif comprend une matrice de filtres élémentaires, chacun associé à au moins un pixel du capteur, ladite matrice étant disposée en amont de la photocathode, de sorte qu'un flux initial de photons traverse ladite matrice avant d'atteindre la photocathode ;
• la matrice comprend des filtres de couleur primaire, un filtre de couleur primaire ne transmettant pas une partie du spectre visible complémentaire de ladite couleur primaire, et des filtres transmettant l'intégralité du spectre visible, dits filtres panchromatiques ; et
• les moyens de traitement sont adaptés à :
  • calculer une grandeur, dite grandeur utile, pour déterminer si au moins une zone du capteur est dans des conditions de faible ou de fort éclairement, la grandeur utile étant représentative d'un flux surfacique moyen de photons ou d'électrons détecté sur un ensemble de pixels dits panchromatiques du capteur, chaque pixel panchromatique étant associé à un filtre panchromatique ;
  • uniquement si ladite zone est dans des conditions de fort éclairement, former une image couleur de ladite zone à partir des pixels de cette zone associés à des filtres de couleur primaire.

According to the invention:

• the device comprises a matrix of elementary filters, each associated with at least one pixel of the sensor, said matrix being arranged upstream of the photocathode, so that an initial photon flux passes through said matrix before reaching the photocathode;
• the matrix comprises primary color filters, a primary color filter not transmitting a part of the visible spectrum complementary to said primary color, and filters transmitting the entire visible spectrum, called panchromatic filters; and
• the treatment means are adapted to:
  • calculating a quantity, called useful quantity, to determine if at least one zone of the sensor is in conditions of low or high illumination, the useful quantity being representative of an average surface flux of photons or electrons detected on a set of so-called panchromatic pixels of the sensor, each panchromatic pixel being associated with a panchromatic filter;
  • only if said zone is in conditions of high illumination, forming a color image of said zone from the pixels of this zone associated with primary color filters.

According to an advantageous embodiment, the photocathode is disposed inside a vacuum chamber, and the matrix of elementary filters is located on an inlet window of said vacuum chamber.

In a variant, the photocathode is placed inside a closed vacuum chamber by a bundle of optical fibers, and each elementary filter of the matrix of elementary filters is deposited on an end of an optical fiber of said bundle.

• des moyens de multiplication, adaptés à recevoir le flux d'électrons émis par la photocathode, et à fournir en réponse un flux secondaire d'électrons ; et
• un écran phosphore, adapté à recevoir le flux secondaire d'électrons et à fournir en réponse un flux de photons, dit flux utile de photons, le capteur étant agencé pour recevoir ledit flux utile de photons.

The sensor may be a photosensitive sensor, the processing means may be adapted to calculate a magnitude representative of an average surface flux of photons, and the device may furthermore comprise:

• multiplication means, adapted to receive the electron flow emitted by the photocathode, and to provide in response a secondary electron flow; and
• a phosphor screen, adapted to receive the secondary electron flux and to provide in response a photon flux, said useful photon flux, the sensor being arranged to receive said useful photon flux.

In a variant, the sensor may be an electron-sensitive sensor adapted to receive the electron flux emitted by the photocathode, and the processing means may be adapted to calculate a magnitude representative of an average electron surface flux.

Preferably, the panchromatic filters represent 75% of the elementary filters.

où R, G, B représentent respectivement des filtres de couleur primaire rouge, vert, bleu, et W représente un filtre panchromatique, le motif étant défini à une permutation près de R, G, B.The matrix of elementary filters is advantageously generated by the two-dimensional periodic repetition of the following pattern: $$\mathrm{M}=\left\{\begin{array}{cccc}R& W& \mathrm{BOY\; WUT}& W\\ W& W& W& W\\ \mathrm{BOY\; WUT}& W& B& W\\ W& W& W& W\end{array}\right\}$$

where R, G, B represent red, green, blue primary color filters, and W represents a panchromatic filter, the pattern being defined at a permutation near R, G, B.

où Ye, Ma, Cy représentent respectivement des filtres de couleur primaire jaune, magenta et cyan, et W représente un filtre panchromatique, le motif étant défini à une permutation près de Ye, Ma, Cy.As a variant, the matrix of elementary filters can be generated by the two-dimensional periodic repetition of the following pattern: $$\mathrm{M}=\left\{\begin{array}{cccc}\mathit{Ye}& W& \mathit{My}& W\\ W& W& W& W\\ \mathit{My}& W& \mathit{Cy}& W\\ W& W& W& W\end{array}\right\}$$

where Ye, Ma, Cy respectively represent primary yellow, magenta and cyan primary color filters, and W represents a panchromatic filter, the pattern being defined at a permutation near Ye, Ma, Cy.

• déterminer que ladite zone est à faible éclairement, si la grandeur utile est inférieure à un premier seuil ; et
• déterminer que ladite zone est à fort éclairement, si la grandeur utile est supérieure à un second seuil, le second seuil étant supérieur au premier seuil.

Preferably, the treatment means are adapted to:

• determining that said zone is at low illumination, if the useful magnitude is less than a first threshold; and
• determining that said zone is at high illumination, if the useful magnitude is greater than a second threshold, the second threshold being greater than the first threshold.

If the useful magnitude is between the first and second thresholds, the processing means are advantageously adapted to combining a monochrome image and the color image of said zone, the monochrome image of said zone being obtained from the panchromatic pixels of this zone. zoned.

• former une image monochrome à partir de l'ensemble des pixels panchromatiques du capteur;
• segmenter cette image monochrome en régions homogènes ; et
• pour chaque zone du capteur associée à une région homogène, calculer indépendamment la grandeur utile correspondante pour déterminer si ladite zone est dans des conditions de faible ou de fort éclairement.

Preferably, the treatment means are adapted to:

• forming a monochrome image from all the panchromatic pixels of the sensor;
• segment this monochrome image into homogeneous regions; and
• for each zone of the sensor associated with a homogeneous region, independently calculating the corresponding useful magnitude to determine whether said zone is in low or high illumination conditions.

The matrix of elementary filters may furthermore comprise infrared filters that do not transmit the visible part of the spectrum, with each infrared filter being associated with at least one pixel of the so-called infrared pixel sensor.

• comparer un seuil infrarouge prédéterminé et une grandeur, dite grandeur secondaire, représentative d'un flux surfacique moyen de photons ou d'électrons détecté par les pixels infrarouges de cette zone ;
• lorsque ladite grandeur secondaire est supérieure au seuil infrarouge prédéterminé, superposer une image monochrome obtenue à partir des pixels panchromatiques de cette zone et une image en fausse couleur obtenue à partir des pixels infrarouges de cette zone.

When a zone is in conditions of low illumination, the processing means are advantageously adapted to:

• comparing a predetermined infrared threshold and a magnitude, called secondary magnitude, representative of an average surface flux of photons or electrons detected by the infrared pixels of this zone;
• when said secondary quantity is greater than the predetermined infrared threshold, superimpose a monochrome image obtained from the panchromatic pixels of this zone and a false-color image obtained from the infrared pixels of this zone.

• à partir des pixels infrarouges de cette zone, identifier des sous-zones de cette zone, détectant un flux surfacique moyen de photons ou d'électrons homogène dans le spectre infrarouge ;
• pour chaque sous-zone ainsi identifiée, comparer un seuil infrarouge prédéterminé et une grandeur, dite grandeur secondaire, représentative d'un flux surfacique moyen de photons ou d'électrons détecté par les pixels infrarouges de cette sous-zone ;
• lorsque ladite grandeur secondaire est supérieure au seuil infrarouge prédéterminé, superposer une image monochrome obtenue à partir des pixels panchromatiques de cette sous-zone et une image en fausse couleur obtenue à partir des pixels infrarouges de cette sous-zone.

In a variant, when a zone is in conditions of low illumination, the processing means are advantageously adapted to:

• from the infrared pixels of this zone, identify sub-zones of this zone, detecting an average surface flux of photons or electrons homogeneous in the infrared spectrum;
• for each sub-zone thus identified, comparing a predetermined infrared threshold and a quantity, called secondary quantity, representative of an average surface flux of photons or electrons detected by the infrared pixels of this sub-zone;
• when said secondary quantity is greater than the infrared threshold predetermined, superimpose a monochrome image obtained from the panchromatic pixels of this sub-area and a false-color image obtained from the infrared pixels of this sub-area.

The matrix of elementary filters may consist of an image projected by a projection optical system.

• filtrage d'un flux initial de photons, pour fournir ledit flux incident de photons, ce filtrage mettant en oeuvre une matrice de filtres élémentaires comprenant des filtres de couleur primaire, un filtre de couleur primaire ne transmettant pas une partie du spectre visible complémentaire de ladite couleur primaire, et des filtres transmettant l'intégralité du spectre visible, dits filtres panchromatiques ;
• calcul d'une grandeur, dite grandeur utile, pour déterminer si au moins une zone du capteur est dans des conditions de faible ou de fort éclairement, la grandeur utile étant représentative d'un flux surfacique moyen de photons ou d'électrons détecté sur un ensemble de pixels dits panchromatiques du capteur, chaque pixel panchromatique étant associé à un filtre panchromatique ;
• uniquement si ladite zone est dans des conditions de fort éclairement, formation d'une image couleur de ladite zone à partir des pixels de cette zone associés à des filtres de couleur primaire.

The invention also relates to a method for forming an image, implemented in a device comprising a photocathode adapted to convert an incident flux of photons into an electron flow, and a sensor constituted by a matrix of elements, said pixels, the method comprising the following steps:

• filtering an initial photon flux, to provide said incident photon flux, this filtering implementing a matrix of elementary filters comprising primary color filters, a primary color filter not transmitting a part of the complementary visible spectrum of said primary color, and filters transmitting the entire visible spectrum, called panchromatic filters;
• calculating a quantity, called useful quantity, to determine if at least one zone of the sensor is in conditions of low or high illumination, the useful quantity being representative of an average surface flux of photons or electrons detected on a set of so-called panchromatic pixels of the sensor, each panchromatic pixel being associated with a panchromatic filter;
• only if said zone is in conditions of high illumination, forming a color image of said zone from the pixels of this zone associated with primary color filters.

BRIEF DESCRIPTION OF THE DRAWINGS

• la figure 1 illustre de manière schématique le principe d'un dispositif selon l'invention ;
• la figure 2 illustre de manière schématique un premier mode de réalisation d'un traitement mis en oeuvre par les moyens de traitement selon l'invention ;
• les figures 3A et 3B illustrent de manière schématique deux variantes d'un premier mode de réalisation d'une matrice de filtres élémentaires selon l'invention ;
• la figure 4 illustre de manière schématique un premier mode de réalisation d'un dispositif selon l'invention ;
• les figures 5A et 5B illustrent de manière schématique deux variantes d'un deuxième mode de réalisation d'un dispositif selon l'invention ;
• la figure 6 illustre de manière schématique un deuxième mode de réalisation d'une matrice de filtres élémentaires selon l'invention ; et
• la figure 7 illustre de manière schématique un deuxième mode de réalisation d'un traitement mis en oeuvre par les moyens de traitement selon l'invention.

The present invention will be better understood on reading the description of exemplary embodiments given purely by way of indication and in no way limiting, with reference to the appended drawings in which:

• the figure 1 schematically illustrates the principle of a device according to the invention;
• the figure 2 schematically illustrates a first embodiment of a treatment implemented by the processing means according to the invention;
• the Figures 3A and 3B illustrate schematically two variants of a first embodiment of a matrix of elementary filters according to the invention;
• the figure 4 schematically illustrates a first embodiment of a device according to the invention;
• the Figures 5A and 5B illustrate schematically two variants of a second embodiment of a device according to the invention;
• the figure 6 schematically illustrates a second embodiment of a matrix of elementary filters according to the invention; and
• the figure 7 schematically illustrates a second embodiment of a treatment implemented by the processing means according to the invention.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

The figure 1 schematically illustrates the principle of an image acquisition device 100 according to the invention.

The device 100 comprises a photocathode 120, operating as described in the introduction, and a matrix 110 of elementary filters 111 located upstream of the photocathode. For example, a GaAs photocathode is used. (gallium arsenide). Any other type of photocathode, in particular sensitive photocathodes, can be used in a widest wavelength spectrum, including the visible (about 400 to 800 nm), and possibly the near infra-red or even the same. infra-red, and / or the near UV (ultraviolet), or even the UV.

Each elementary filter 111 filters the incident light at a location of the photocathode 120. Each elementary filter 111 thus defines a pixel on the photocathode 120.

The elementary filters 111 are transmission filters of at least two different categories: primary color filters, and transparent (or panchromatic) filters.

A primary color elementary filter is defined in the introduction. The elementary filters of the matrix 110 include three types of primary color filters, i.e. filters of three primary colors. This allows an additive or subtractive synthesis of all the colors of the visible spectrum. In particular, each type of primary color filter transmits only a portion of the visible spectrum, i.e., a band of the 400-700 nm wavelength range, and the different types of primary color pixels. cover all this gap. In addition to a portion of the visible spectrum, each primary color filter can transmit a portion of the near infra-red or even infra-red spectrum and / or a portion of the near UV or UV spectrum. The color filters can be red, green, blue filters, in the case of an additive synthesis, or yellow, magenta, cyan filters, in the case of a subtractive synthesis. Other sets of primary colors may be contemplated by those skilled in the art without departing from the scope of the present invention.

The panchromatic elementary filters let pass the whole visible spectrum. Where appropriate, they may also transmit at least a portion of the near-infrared and even infrared spectrum and / or at least a portion of the near UV and even UV spectrum. Elementary filters panchromatic can be transparent elements in the visible, or openings (or savings) in the matrix 110. In this second case, the pixels of the photocathode located under these panchromatic elementary filters receive unfiltered light.

The different types of primary color filters, and the panchromatic filters, are scattered on the elementary filter matrix.

The elementary filters are advantageously arranged in the form of a periodic repeating pattern in two distinct, generally orthogonal directions in the plane of the photocathode 120. Each pattern preferably comprises at least one primary color filter of each type. , and panchromatic filters.

Although elementary filters of square shape have been illustrated, these may have any other geometrical shape, for example a hexagon, a disk, or a surface defined according to constraints relating to the transfer function of the device 100 according to the invention.

The matrix of elementary filters according to the invention can be real, or virtual.

The matrix of elementary filters is said to be real when it comprises elementary filters having a certain thickness, for example elementary filters made of polymer material or interference filters.

The matrix of elementary filters is called virtual when it consists of an image of a second matrix of elementary filters, projected upstream of the photocathode. In this case, the second matrix of elementary filters consists of a real matrix of elementary filters. It is located in the object plane of a projection optical system. The image formed in the image plane of this projection optical system corresponds to said matrix of virtual elementary filters. An advantage of this variant is that we get rid of possible difficulties in positioning an actual matrix at the desired location.

In the set of examples developed with reference to the figures, the example of a real elementary filter matrix has been developed. Many variants can be envisaged, by replacing the matrix of real elementary filters by a matrix of virtual elementary filters. Preferably, the device according to the invention will then comprise the second matrix of elementary filters and the projection optical system, as mentioned above.

Preferably, but in a nonlimiting manner, the proportion of panchromatic elementary filters in the matrix 110 is greater than or equal to 50%. Advantageously, the proportion of panchromatic elementary filters is equal to 75%. The elementary filters of primary color can be divided in equal proportions. As a variant, the elementary filters of primary color are distributed in unequal proportions. Preferably, the proportion of a first type of primary color filter does not exceed twice the proportion of the other types of primary color filters. For example, the proportion of panchromatic elementary filters is equal to 75%, the proportion of filters of a first primary color is equal to 12.5%, and the proportion of filters of a second and a third primary color is equal to at 6.25% and 6.25%.

Matrix 120 receives an initial photon flux. For illustrative purposes, initial elementary fluxes of photons 101, each associated with an elementary filter 111, are represented. The initial elementary fluxes of photons 101 together form a poly-chromatic image, and may comprise photons located in the visible spectrum, which are close to one another. infrared and even infrared.

An elementary filter 111 transmits a filtered elementary flux 102, the filtered elementary streams together forming a flux of photons incident on the photocathode. In response to this incident flux of photons, the photocathode 120 emits a stream of electrons. Each filtered elementary flux 102 corresponds to an elementary electron flux 103. An elementary electron flux 103 is all the more important that the corresponding filtered elementary flux 102 comprises photons. The elementary electron fluxes 103 do not directly convey chromatic information, but depend directly on a number of photons transmitted by a corresponding elementary filter 111. The elementary electron fluxes 103 together form a stream of electrons emitted by the photocathode 120.

The device 100 according to the invention further comprises a digital sensor 130. As detailed below, the sensor 130 can directly receive the stream of electrons emitted by the photocathode 120. In a variant, this stream of electrons emitted by the photocathode 120 can be converted into a photon flux so that the sensor 130 finally receives a photon flux. The figure 1 being a simple illustration of principle, the sensor 130 is shown directly following the photocathode 120. The sensor 130 may be a photon-sensitive or electron-sensitive sensor, and other elements may be interposed between the photocathode 120 and the sensor 130.

The sensor is sensitive to electrons as emitted by the photocathode, or to photons obtained from these electrons.

• aux photons situés dans la bande 400-900 nm, voire 400-1100 nm, voire une bande spectrale allant de l'UV au proche infrarouge, par exemple 200-1100 nm ; ou
• aux électrons provenant de photons situés dans cette bande. Le capteur est formé par une matrice d'éléments, dits pixels 131, sensibles aux photons ou aux électrons.

Preferably, the sensor is sensitive:

• with photons situated in the band 400-900 nm, or even 400-1100 nm, or even a spectral band going from the UV to the near infrared, for example 200-1100 nm; or
• electrons from photons in this band. The sensor is formed by a matrix of elements, said pixels 131, sensitive to photons or electrons.

Each elementary filter 111 is associated with at least one pixel 131 of the sensor. In other words, each elementary filter 111 is aligned with at least one pixel 131 of the sensor, so that a major part of a stream of electrons or photons, resulting from the photons transmitted by this elementary filter 111, reaches this at least one pixel 131. Preferably, each elementary filter 111 is associated with exactly one pixel 131 of the sensor. Preferably, the surface of an elementary filter 111 corresponds to the surface of a pixel 131 of the sensor or to a surface corresponding to the juxtaposition of an integer number of pixels 131 of the sensor.

Since each elementary filter 111 is associated with at least one pixel 131 of the sensor, the term "panchromatic pixel" can be called a pixel of the sensor associated with a panchromatic elementary filter, and "primary color pixel" a pixel of the sensor associated with an elementary filter. of primary color. The panchromatic pixels detect electrons or photons associated with the spectral band transmitted by the panchromatic filters. Each type of primary color pixel detects electrons or photons associated with the spectral band transmitted by the corresponding primary color filter type.

The sensor 130 is connected to processing means 140, that is to say calculation means including a processor or a microprocessor. The processing means 140 receive as input electrical signals supplied by the sensor 130, and corresponding, for each pixel 131, to the stream of photons received and detected by this pixel when the sensor is sensitive to photons, or to the electron flow received. and detected by this pixel when the sensor is sensitive to electrons. The processing means 140 output an image, corresponding to the initial flux of incident photons on the matrix of elementary filters, this flux having been intensified.

The processing means 140 are adapted to assign, to each pixel of the sensor, information on a type of elementary filter associated with this pixel. For this purpose, they store information making it possible to connect each pixel of the sensor and a type of elementary filter. This information can be in the form of a deconvolution matrix. Thus, the spectral information that is lost during the passage through the photocathode, is restored by the processing means 140.

The processing means 140 are adapted to implement a treatment, as illustrated in FIG. figure 2 .

According to the first embodiment as detailed below, the processing means realize a monochrome image by interpolation of all the panchromatic pixels of the sensor. This image is called "monochrome image of the sensor". They then implement a segmentation of the sensor in several zones, each zone being homogeneous in terms of the flux of photons or electrons detected by the corresponding panchromatic pixels.

Such a segmentation is for example described in the article of S. Tripathi et al. entitled "Image Segmentation: A Review" published in International Journal of Computer Science and Management Research, Vol. 1, No. 4, Nov. 2012, pp. 838-843 .

The processing means then implement the following steps.

In a first step 280, a magnitude is estimated F , representative of an average surface flux of photons or electrons received and detected by the panchromatic pixels of an area of the sensor, sensitive respectively to photons or electrons.

This quantity is called "useful size". The useful magnitude may be equal to the average surface flux of photons or electrons. If the sensor 130 is sensitive to photons, the useful magnitude may be an average luminance on the panchromatic pixels of the sensor area. Thus, the useful magnitude may be an average surface flux of photons or electrons detected on a set of so-called panchromatic pixels of the sensor.

It can therefore be considered that the useful quantity provides a measurement of the illumination on said sensor zone.

Low light conditions are associated with a low value of the useful size (in absolute value). Conditions of high illumination are associated with a high value of the useful size (in absolute value).

Conditions of high illumination are associated for example with a higher illuminance at a first threshold between 450 and 550 μLux. Conditions of low illumination are associated for example with a light illumination less than a second threshold of between 400 and 550 μLux, the first and the second threshold being equal. If the first and second thresholds are not equal, the first threshold is strictly greater than the second threshold.

In a second step 281, the useful magnitude is compared F and a threshold value F _th . If the usefulness F is greater than the threshold value F _th , the sensor zone is in conditions of high illumination. If the usefulness F is below the threshold value F _th , the sensor area is in low light conditions.

Steps 280 and 281 together form a step to determine whether the sensor zone 130 is in low or high light conditions.

A strong illumination corresponds for example to the acquisition of an image of a night scene, illuminated by the moon (night level 1 to 3). A low illumination corresponds for example to the acquisition of an image of a night scene, not illuminated by the moon (night level 4 to 5, or a luminous illumination less than 500 μLux).

If the area is under high illumination conditions, a color image of this area is formed using the primary color pixels of this area (step 282A). It is said that the device operates in high illumination mode.

In particular, an image is formed of each primary color, and the images of each primary color are combined with each other. An image of a primary color is formed by interpolating the pixels of this area associated with said primary color. The interpolation makes it possible to compensate for the small proportion of pixels of the sensor of a given primary color. The interpolation of the pixels of a primary color is to use the values taken by these pixels to estimate the values that would be taken by the neighboring pixels if these were also pixels of this primary color.

Primary color images can be optionally processed to sharpen image sharpening . For example, one can obtain a monochrome image of the area by interpolating the panchromatic pixels of this area, and combine this monochrome image, where appropriate after high-pass filtering, with each primary color image of the same area. As the proportion of panchromatic pixels in the matrix is higher than that of the primary color pixels, the resolution of the primary color images is thus improved.

If the zone is in low illumination conditions, a monochrome image of said zone is formed from the panchromatic pixels of this zone. In particular, a monochrome image is formed using the panchromatic pixels of this area (step 282B), and without using the primary color pixels of this area. Here again, the monochrome image can be obtained by interpolation of the panchromatic pixels of this zone. The device is said to operate in low illumination mode.

It is important to note that the distinction between low illumination and high illumination is based on a measurement obtained from the panchromatic pixels of the sensor, therefore for the entire spectrum detected by such a sensor that is to say for at least all visible spectrum.

These steps are performed for each zone of the previously identified sensor.

Then, the color or monochrome images of the different areas of the sensor are combined to obtain an image of the entire sensor. The image of the entire sensor can be displayed, or stored in memory for further processing.

Alternatively, a color image of each zone of strong is formed illumination, then, in the monochrome image of the sensor used for segmentation, the areas corresponding to these zones of high illumination are replaced by the color images of these zones.

According to another variant, a linear combination of the monochrome image of the sensor and these color images is performed. Thus, in high-light regions, the color image and the monochrome image are superimposed.

In the example just described, the zones of the sensor are treated separately. Alternatively, it is determined whether the entire sensor is in conditions of low or high illumination, and is treated in the same way the entire sensor. In this case, there is no segmentation of the monochrome image of the sensor, or combination of the images obtained. Steps 280, 281 and 282A or 282B are implemented over the entire surface of the sensor. In other words, the sensor zone as mentioned above corresponds to the entire sensor.

Thus, the processing means 140 receive as input signals from the sensor, store information for associating each pixel of the sensor with a type of elementary filter, and output a color image, or a monochrome image or a combination of a color image and a monochrome image.

The invention thus provides an image acquisition device for acquiring a color image of an area of the sensor, when the illumination of the scene detected on this area allows it. When this illumination becomes insufficient, the device provides an image of the area obtained from the panchromatic elementary filters, thus with a minimal energy loss. The device automatically selects one or the other mode of operation.

Note that no second matrix of elementary filters is present on the sensor 130, since it suffices to take into account, during processing, the fact that a particular sensor pixel is associated with such or such elementary filter located upstream of the photocathode. A device is thus produced image acquisition with high energy efficiency.

According to a first variant of this first embodiment, switching from one mode to another operates with hysteresis so as to avoid any switching noise ( chattering ). To do this, a first threshold for the useful magnitude is provided for the transition from the high illumination mode to the low illumination mode and a second threshold for the useful magnitude is provided for the inverse transition, the first threshold being chosen lower than the second threshold.

According to a second variant of the first embodiment, the switching from one mode to the other is done progressively through a transition phase. Thus, the image acquisition device operates in low illumination mode when the useful magnitude is less than a first threshold and in high illumination mode when it is greater than a second threshold, the second threshold being chosen greater than the first threshold. When the useful magnitude is between the first and second thresholds, the image acquisition device performs a linear combination of the image obtained by the treatment in high illumination mode and that obtained by the low light mode treatment, the weighting coefficients being given by the deviations of the useful magnitude with the first and second thresholds respectively.

Ideally, each elementary filter 111 is aligned with at least one pixel 131 of the sensor, so that each pixel of the sensor associated with an elementary filter receives only photons or electrons corresponding to this elementary filter. However, there may be a spatial spreading through the device according to the invention, including a spatial spread of the electron flow emitted by the photocathode. This disadvantage can be countered by an initial calibration step making it possible to compensate for the misalignment between an elementary filter and a sensor pixel. This calibration aims to compensate for the slight degradation due to the transfer function of the optical elements of the device according to the invention (photocathode and, if appropriate, multiplication means and phosphor screen). During this calibration, we illuminate the matrix of elementary filters in turn with different monochromatic light beams (each corresponding to one of the primary colors of the primary color filters), and the signal received by the sensor 130 is measured. A deconvolution matrix, which is stored by the processing means 140. In operation, the processing means 140 multiply the signals transmitted by the sensor by this deconvolution matrix. Thus, after multiplication by the deconvolution matrix, the signals were reconstructed as they would be transmitted by the sensor under ideal conditions, without spatial spreading. Each primary color filter (and optionally each infra-red filter, see below) is preferably entirely surrounded by panchromatic filters. Thus, in the case of spatial spreading of the electron flux emitted by the photocathode, the calibration is simplified.

Alternatively or additionally, the geometric shape of the filters composing the matrix of elementary filters is calibrated so as to compensate for the effect of said spatial spread. After deformation by the optical elements of the device according to the invention (photocathode and, if appropriate, multiplication means and phosphor screen), the image of an elementary filter is then superimposed perfectly on one or more pixels of the sensor.

Interstices between adjacent elementary filters are advantageously opaque, in order to block any radiation likely to reach the photocathode without having passed through an elementary filter.

The Figures 3A and 3B schematically illustrate two variants of a first embodiment of a matrix 110 of elementary filters according to the invention.

On the figure 3A Primary elementary filters are red (R), green (G) or blue (B) filters. The matrix has 75% panchromatic filters (W).

The matrix 110 is generated by a two-dimensional periodic repetition of the 4x4 basic pattern: $$\left\{\begin{array}{cccc}R& W& \mathrm{BOY\; WUT}& W\\ W& W& W& W\\ \mathrm{BOY\; WUT}& W& B& W\\ W& W& W& W\end{array}\right\}$$

Variations of this matrix can be obtained by permutation of the filters R, G, B in the pattern (1). The green pixels are twice as many as the red pixels, respectively blue. This imbalance can be corrected by weighting coefficients adapted when combining three primary color images to form a color image.

The matrix of the figure 3B corresponds to the matrix of the figure 3A , in which the primary color elementary filters R, G, B are replaced respectively by elementary primary color filters yellow (Ye), magentas (Ma), cyans (Cy). Again, the filters Ye, Ma, Cy can be switched.

avec X=R, G ou B, Y=R, G ou B, et Y≠X.According to a non-represented variant of the matrix represented in figure 3A , the panchromatic filters representing 50% of the elementary filters, and the elementary pattern is as follows: $$\left\{\begin{array}{cccc}W& R& W& \mathrm{BOY\; WUT}\\ R& W& X& W\\ W& \mathrm{BOY\; WUT}& W& B\\ Y& W& B& W\end{array}\right\}$$

with X = R, G or B, Y = R, G or B, and Y ≠ X.

Again, the filters R, G, B can be switched.

Alternatively, the filters R, G, B of the pattern (2) are replaced by filters Ye, Ma, Cy.

The figure 4 schematically illustrates a first embodiment of a device 400 according to the invention. The figure 4 will only be described for its differences in the figure 1 . The use of a calibration step such that detailed above, is particularly advantageous in this embodiment.

The device 400 is based on the so-called intensified CMOS or intensified CCD technology (ICMOS or ICCD, for the English " Intensified CMOS" or " Intensified CCD").

The photocathode 420 is disposed inside a vacuum tube 450 of the vacuum tube type of an image intensifier tube according to the prior art and as described in the introduction. A vacuum tube designates a vacuum chamber having more particularly a tube shape.

The vacuum tube 450 has an inlet window 451, transparent in particular in the visible, and optionally in the near infrared or even the infrared. The input window allows to let enter, inside the vacuum tube, the flux of photons incident on the photocathode. The entrance window is in particular glass. The input window is preferably a simple plate. The matrix of elementary filters 410 is glued on one face of the inlet window 451, preferably on the inside of the vacuum tube. The photocathode is pressed against the matrix of elementary filters 410. A metal layer (not shown) may be deposited on the input window, around the matrix of elementary filters 410, to form an electrical contact point for the application. an electrostatic field.

Downstream of the photocathode 420 are multiplication means 461 and a phosphor screen 462 as described in the introduction.

The phosphor screen emits a stream of photons, called useful flux, which is received by the sensor 430. The sensor 430 is photosensitive. It is in particular a CCD ( Charge-Coupled Device ) sensor, or a CMOS sensor ( Complementary Metal Oxide Semiconductor ). On the figure 4 , the sensor 430 is shown inside the vacuum tube, the tube being traversed by electrical connections between the sensor 430 and the processing means 440.

The processing means 440 operates as described with reference to the figure 2 , the useful quantity being representative of the surface flux of photons detected by the panchromatic pixels of the sensor 430.

The sensor 430 may be in direct contact with the phosphor screen, to limit any spatial spread of the photon beam emitted by the phosphor screen. In this case, the sensor 430 may be inside the vacuum tube, or outside and against an outlet face of the vacuum tube, formed by the phosphor screen.

The sensor 430 may be offset outside the vacuum tube 450.

In particular, a bundle of optical fibers can connect the phosphor screen and the pixels of the sensor 430, the bundle of optical fibers forming an exit window of the vacuum tube. Such an optical fiber bundle is particularly suitable in the case where the surface of the sensor 430 is smaller than the inside diameter of the vacuum tube. In this case, each fiber has a diameter on the phosphor screen side greater than its diameter on the sensor side. The bundle of optical fibers is said to thin, and performs a reduction of the image provided by the phosphor screen.

The Figures 5A and 5B illustrate schematically two variants of a second embodiment of a device 500 according to the invention.

The Figure 5A will only be described for its differences in the figure 1 .

The device 500 is based on the electro-bombarded CMOS technology, or EBCMOS for the English " Electron Bombarded CMOS".

The photocathode 520 is disposed inside a vacuum tube 550.

The vacuum tube 550 has an inlet window 551, transparent in particular in the visible, and if necessary in the near infrared or even the infrared.

The elementary filter matrix 510 is adhered to one face of the input window 551, preferably on the inside of the vacuum tube.

The sensor 530 is disposed inside the vacuum tube 550, and directly receives the stream of electrons emitted by the photocathode.

The photocathode 520 and the sensor 530 are within a few millimeters of each other, and subjected to a potential difference to create an electrostatic field in the interstice between them. This electrostatic field accelerates the electrons emitted by the photocathode 520 towards the sensor 530.

The sensor 530 is sensitive to electrons. It is typically a CMOS sensor, adapted to make it sensitive to electrons.

According to a first variant, the electron-sensitive sensor is illuminated on the back side (" back side illuminated "). For this, one can use a CMOS sensor whose substrate is thinned and passivated (in English, " back - thinned "). The sensor may include a passivation layer, forming an outer layer on the side of the photocathode. The passivation layer is deposited on the thinned substrate. The substrate receives detection diodes, each associated with a pixel of the sensor.

According to a second variant, the electron-sensitive sensor is illuminated on the front face. For this purpose, it is possible to use a CMOS sensor whose front face is treated so as to remove the protective layers covering the diodes. The front face of a standard CMOS sensor is thus made sensitive to electrons. The processing means 540 operate as described with reference to the figure 2 , the useful quantity being representative of the surface flux of electrons detected by the panchromatic pixels of the sensor 530.

The Figure 5B illustrates a variant of the device 500 of the Figure 5A , in which the vacuum tube 550 is closed by a beam 552 of optical fibers receiving the matrix of elementary filters.

According to this variant, the beam 552 of optical fibers is traversed by photons coming from the scene to be imaged. A first end of the fiber optic bundle 552 closes the vacuum tube. A second end of the beam 552 of optical fibers is in front of the scene to be imaged. The vacuum tube 550 no longer has the input window 551, which is replaced by the optical fiber bundle which allows the vacuum tube of the scene to be imaged to be displaced.

Each elementary filter of the matrix 510 is associated with an optical fiber of the beam 552. In particular, each elementary filter is directly attached to an optical fiber end, advantageously on the opposite side to the vacuum tube. In this case, the matrix of elementary filters 510 is outside the vacuum tube, which simplifies its assembly.

Alternatively, each elementary filter is directly attached to an end of optical fiber, the side of the vacuum tube. In the same way, a variant of the device described with reference to FIG. figure 4 .

The figure 6 schematically illustrates a second embodiment of a matrix of elementary filters according to the invention. The matrix of elementary filters of the figure 6 differs from the matrices described above, in that it comprises infrared (IR) filters, not transmitting the visible part of the spectrum and allowing the near infrared to pass. Infrared filters let the wavelengths pass in the near infrared, or even in the infrared (wavelengths greater than 700 nm). Infrared filters transmit in particular the spectral band between 700 and 900 nm, or even between 700 and 1100 nm, and even between 700 and 1700 nm.

The filter matrix of the figure 6 differs from the matrix of the figure 3A in that in the elementary pattern, one of the two green pixels (G) is replaced by an infrared pixel (IR).

In the same way, different variants of the matrix of the figure 6 , for example from the matrix of the figure 3B and replacing by an infrared pixel one of the two magenta pixels of the elementary pattern.

According to other variants, the elementary pattern (2) as defined above is taken up again, defining X = Y = IR.

The figure 7 schematically illustrates a processing implemented by the processing means according to the invention, when the matrix of elementary filters comprises infrared pixels.

Steps 780, 781 and 782B correspond to steps 280, 281 and 282B, respectively, as described with reference to FIG. figure 2 .

When a zone of the sensor is in conditions of low illumination, the processing means measure a quantity, called secondary quantity, representative of the average surface flux of photons or electrons F _IR detected by the infrared pixels of this area (step 783). In particular, this average surface flux is an average surface flux of photons if the sensor is photosensitive, or an average surface flux of electrons if the sensor is sensitive to electrons.

The processing means then make a comparison between this secondary quantity and an infrared threshold F _{IR th} (step 784).

If the secondary size F _IR is below the infrared threshold F _{IR th} , a color image of the zone is constructed, as described with reference to the figure 2 about step 282A (step 782A).

If the secondary size F _IR is greater than the infrared threshold F _{IR th} , a false-color image of the zone is created, that is to say an image in which a given color is attributed to the infrared pixels of this zone. The false color image can be constructed by interpolating the infrared pixels of the considered area. The false color image is therefore a monochrome image, of a different color from the monochrome image associated with the panchromatic pixels. Then, we superimpose this image in false color to the monochrome image obtained using the panchromatic pixels of the same area of the sensor.

These steps of constructing a false color image and superposition with the monochrome image together form a step 782C.

Thus, for an area located in low light conditions, one obtains either a monochrome image or the image overlay as defined above.

In summary, when a zone is in low illumination conditions, it is tested whether the infrared pixels belonging to this zone have an intensity greater than a predetermined infrared threshold and, if so, it is superimposed on the monochrome image of this zone. zone infrared pixels shown in false color. This embodiment is particularly advantageous for laser detection applications.

According to a first variant, a single secondary quantity for the same zone is not calculated, but a secondary quantity per infrared pixel of the zone is calculated separately. Only the infrared pixels, for which the corresponding secondary quantity is greater than the infrared threshold, are superimposed on the monochrome image obtained from the panchromatic pixels. Thus, if a sensor zone has a high intensity in the infrared range, it will be easily identifiable in the resulting image.

According to another variant, sub-zones of said sensor zone are identified, detecting an average surface flux of pixels or electrons homogeneous in the infrared spectrum, and each sub-zone is then treated separately as detailed above. In other words, the comparison with the infrared threshold is done by homogeneous sub-areas of the sensor. For each sub-area of the sensor for which the secondary magnitude is greater than the infrared threshold, a false color image is obtained by interpolation of the infrared pixels of said sub-zone. These false color images are then superimposed on the corresponding locations on the monochrome image of the sensor area. To identify such sub-areas, a segmentation is performed on the basis of an image made by interpolation of the infrared pixels. In summary, when a zone is in low illumination conditions, sub-zones of this zone are identified, having a uniform intensity in the infrared spectrum, and it is determined, for each sub-zone thus identified, whether the The average of the infrared intensity in this sub-area is greater than a predetermined infrared threshold and, if so, this sub-area is represented by a false-color image based on the infrared pixels of that sub-area. false-color image of said sub-area then being superimposed with the monochrome image of the area to which it belongs.

• calculer la grandeur utile, pour déterminer si au moins une zone du capteur est dans des conditions de faible ou de fort éclairement ;
• uniquement si ladite zone est dans des conditions de fort éclairement, former une image couleur de ladite zone à partir des pixels de cette zone associés à des filtres de couleur primaire, et en retrancher une image infrarouge de ladite zone obtenue à partir des pixels infrarouges de cette zone (par exemple par interpolation desdits pixels infrarouges).

The infrared pixels of the sensor can also be used to improve a signal-to-noise ratio on a final color image. For this, when a zone of the sensor is in conditions of high illumination, an infrared image of this zone is produced by interpolation of the infrared pixels of the sensor. This infrared image is then subtracted from the color image of this area, obtained as detailed with reference to the figure 2 . The subtraction of the infrared image makes it possible to improve the signal-to-noise ratio. To avoid saturation problems, a weighted infrared image can be subtracted from each of the primary color images. The weighting coefficients assigned to the infrared image may be identical or different, for each primary color image. Fragmented primary color images are obtained which are combined to form a denuded color image. Thus, the processing means are adapted to implement the following steps:

• calculating the useful magnitude, to determine if at least one zone of the sensor is in conditions of low or high illumination;
• only if said zone is in conditions of high illumination, forming a color image of said zone from the pixels of this zone associated with primary color filters, and subtracting an infrared image from said zone obtained from the infrared pixels of this area (for example by interpolation of said infrared pixels).

Claims (16)

1. Image acquisition device (100; 400; 500) comprising:

   - a photocathode (120; 420; 520), configured to convert an incident flux of photons into a flux of electrons;

   - a sensor (130; 430; 530) composed of a matrix of elements named pixels; and

   - processing means (140; 440; 540);

   - a matrix (110; 410; 510) of elementary filters, each associated with at least one pixel of the sensor, said matrix being located upstream from the photocathode, such that an initial flux of photons passes through said matrix before reaching the photocathode,

   - the matrix comprising primary colour filters (R, G, B; Ye, Ma, Cy), a primary colour filter not transmitting a part of the visible spectrum complementary to said primary colour, and filters transmitting the entire visible spectrum, named panchromatic filters (W);

   characterized in that the processing means (140; 440; 540) are configured to:

   - calculate a quantity, termed a useful quantity( F ), for determining whether at least one zone of the sensor is under conditions of weak or strong illumination, the useful quantity being representative of a mean surface flux of photons or electrons detected on a set of pixels named panchromatic pixels of the sensor, each panchromatic pixel being associated with a panchromatic filter (W);

   - forming, only if said zone is under conditions of strong illumination, a colour image of said zone on the basis of the pixels in this zone associated with primary colour pixels.
2. Device (400; 500) according to claim 1, characterised in that the photocathode (420; 520) is located inside a vacuum chamber (450; 550), and in that the matrix of elementary filters (410; 510) is located on an input window (451, 551) of said vacuum chamber.
3. Device (500) according to claim 1, characterised in that the photocathode (520) is located inside a vacuum chamber (550) closed by a bundle of optical fibres (552), and in that each elementary filter of the matrix of elementary filters (510) is deposited on one end of an optical fibre of said bundle (552).
4. Device (400) according to any one of claims 1 to 3, characterised in that the sensor (430) is a photosensitive sensor, in that the processing means (440) are configured to calculate a quantity representative of a mean surface flux of photons, and in that the device also comprises:

   - multiplication means (461) configured to receive the flux of electrons emitted by the photocathode, and supply a secondary flux of electrons in response; and

   - a phosphor screen (462), configured to receive the secondary flux of electrons and supply a flux of photons in response, named the useful flux of photons, the sensor (430) being arranged to receive said useful flux of photons.
5. Device (400) according to any one of claims 1 to 3, characterised in that the sensor (530) is a sensor sensitive to electrons, configured to receive the flux of electrons emitted by the photocathode, and in that the processing means (540) are configured to calculate a quantity representative of a mean surface flux of electrons.
6. Device (100; 400; 500) according to any one of claims 1 to 5, characterised in that the panchromatic filters represent 75% of the elementary filters.
7. Device (100; 400; 500) according to claim 6, characterised in that the matrix of elementary filters (110; 410; 510) is generated by the periodic two-dimensional repetition of the following pattern: $$\mathrm{M}=\left\{\begin{array}{cccc}R& W& G& W\\ W& W& W& W\\ G& W& B& W\\ W& W& W& W\end{array}\right\}$$

   

   in which R, G, B represent the primary colour filters red, green and blue respectively and W represents a panchromatic filter, the pattern being defined except for an R, G, B permutation.
8. Device (100; 400; 500) according to claim 6, characterised in that the matrix of elementary filters (110; 410; 510) is generated by the periodic two-dimensional repetition of the following pattern: $$\mathrm{M}=\left\{\begin{array}{cccc}\mathit{Ye}& W& \mathit{Ma}& W\\ W& W& W& W\\ \mathit{Ma}& W& \mathit{Cy}& W\\ W& W& W& W\end{array}\right\}$$

   

   in which Ye, Ma, Cy represent the primary colour filters yellow, magenta and cyan respectively, and W represents a panchromatic filter, the pattern being defined except for a Ye, Ma, Cy permutation.
9. Image acquisition device (100; 400; 500) according to one of claims 1 to 8, characterised in that the processing means are configured to:

   - determine that said zone has weak illumination, if the useful quantity ( F ) is less than a first threshold, and

   - determine that said zone has strong illumination, if the useful quantity ( F ) is more than a second threshold, the second threshold being higher than the first threshold.
10. Device (100; 400; 500) according to claim 9, characterised in that if the useful quantity ( F ) is between the first and second thresholds, the processing means are configured to combine a monochrome image and the colour image of said zone, the monochrome image of said zone being obtained from the panchromatic pixels of this zone.
11. Image acquisition device (100; 400; 500) according to one of claims 1 to 10, characterised in that the processing means are configured to:

    - form a monochrome image from the complete set of panchromatic pixels of the sensor;

    - segment this monochrome image into homogeneous regions; and

    - for each zone of the sensor associated with a homogeneous region, calculate the corresponding useful quantity independently to determine if said zone is under weak or strong illumination conditions.
12. Image acquisition device (100; 400; 500) according to one of claims 1 to 11, characterised in that the matrix of elementary filters (110; 410; 510) also includes infrared (IR) filters that do not transmit the visible part of the spectrum, wherein each infrared filter is associated with least one sensor pixel named infrared pixel.
13. Image acquisition device (100; 400; 500) according to claim 12, characterised in that, when a zone is under weak illumination conditions, the processing means are configured to:

    - compare a predetermined infrared threshold (F_{IR th} ) and a quantity named the secondary quantity ( F_IR ), representative of a mean surface flux of photons or electrons detected by the infrared pixels of this zone;

    - when said secondary quantity is higher than the predetermined infrared threshold, superpose a monochrome image obtained from the panchromatic pixels of this zone and a false colour image obtained from the infrared pixels of this zone.
14. Image acquisition device (100; 400; 500) according to claim 12, characterised in that, when a zone is under weak illumination conditions, the processing means are configured to:

    - using the infrared pixels in this zone, identify sub-zones of this zone, which detect a mean surface flux of photons or electrons homogeneous in the infrared spectrum;

    - for each sub-zone thus identified, compare a predetermined infrared threshold (F_{IR th} ) and a quantity named the secondary quantity ( F_IR ), representative of a mean surface flux of photons or electrons detected by the infrared pixels of this sub-zone;

    - when said secondary quantity is higher than the predetermined infrared threshold, superpose a monochrome image obtained from the panchromatic pixels of this sub-zone and a false colour image obtained from the infrared pixels in this sub-zone.
15. Image acquisition device (100; 400; 500) according to one of the previous claims, characterised in that the matrix (110; 410; 510) of elementary filters consists of an image projected by an optical projection system.
16. Image formation method, implemented in a device (100; 400; 500) comprising a photocathode (120; 420; 520) configured to convert an incident flux of photons into an flux of electrons, and a sensor (130; 430; 530) composed of a matrix of elements named pixels, comprising the following steps:

    - filter an initial flux of photons to supply said incident flux of photons, this filtering making use of a matrix of elementary filters (110; 410; 510) including primary colour filters (R, G, B; Ye, Ma, Cy), a primary colour filter not transmitting a part of the visible spectrum complementary to said primary colour, and filters transmitting the entire visible spectrum, named panchromatic filters (W);

    characterized in that it comprises the following steps:

    - calculate a quantity, termed a useful quantity( F ), for determining whether at least one zone of the sensor is under conditions of weak or strong illumination, the useful quantity being representative of a mean surface flux of photons or electrons detected on a set of pixels named panchromatic pixels of the sensor, each panchromatic pixel being associated with a panchromatic filter (W);

    - form, only if said zone is under conditions of strong illumination, a colour image of said zone on the basis of the pixels in this zone associated with primary colour filters (R, G, B; Ye, Ma, Cy).

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