FR2732850A1 - LOAD COUPLING CAMERA EQUIPPED WITH FAULT ELIMINATION MEANS AND PROCESS IMPLEMENTED BY SUCH A CAMERA - Google Patents

Abstract

A CCD camera comprising means for eliminating defects that reduce sensed image quality, and a method therefor, are disclosed. The defect elimination means are portable and built into the camera housing, and include electronic filtering means for filtering in real time each pixel of an image sensed at time <u>Ti</u>, by averaging it with the pixel at the same address in the image sensed at time <u>Ti-1</u>, as well as electronic means for comparing the image averaged in real time with a reference image taken at a predetermined time.

Description

CHARGE COUPLING CAMERA EQUIPPED WITH MEANS
FAULT ELIMINATION AND PROCESS IMPLEMENTED BY
SUCH A CAMERA
The present invention relates to a charge-coupled camera comprising means for eliminating defects affecting the quality of the captured image.

It also relates to a method of eliminating defects from the image obtained with a charge-coupled camera.

It finds a particularly important, although not exclusive, application in the field of real-time or quasi-real time imaging with very low light flux using television cameras.

These include, but are not limited to, the fields of optical microscopy in fluorescence at very low light level, electron microscopy, industrial and medical radiology, astronomy, particle detection and / or of the nuclear industry for the detection of low radiation.

The present invention is also applicable to night surveillance or in the field of infrared vision or even more generally to fields which relate to imaging in difficult optical conditions requiring information in real time for automatic processing.

Under such conditions several factors alter the quality and the representation of details on images from television cameras, especially when they have a very high sensitivity.

The defects affecting the quality of the image are first and foremost due to the sources of quantum noise related to the number of incident photons generating the image, to the sources of quantum noise related to the fluctuation of the number of electrons in the case of of an electron microscope, or to sources of noise of electronic origin due to the amplification factors necessary to optimize the sensitivity of the camera.

The noise also comes from the detector converting photons into electrons.

It can also be created by the light intensifier when one is used to increase the sensitivity of the camera and / or be generated by particle fluctuation in nuclear fields as well as in radiology.

The defects can then be due to an inhomogeneity of the structure of revolution, for example in the case of vignetting of the optics (for example in astronomy) in the case of inhomogeneity of the light sources, and this in particular when the absorption of light is not homogeneous as with optical fiber reducers where the differences in optical path lengths between the center and the edges of the reduction networks generate this type of defects.

Other defects are related to the inhomogeneity of the medium without this having a structure of revolution.

These defects are for example localized defects due to a loss of sensitivity, an abnormal absorption of light (in the case of optical spots in particular), or even structural defects linked to the technology of optical fiber networks (known as effect of "Chicken Wire" in English terminology)., defects generated during the coupling of said networks, for example because of a glue problem or because of a high gain used in electronic circuits, etc.

Finally, the defects affecting the quality of the captured image may be of the point type.

For example, these are black dots due to a punctual loss of sensitivity originating either from the intensifier of the charge-coupled camera ("CCD" in English terminology) or even black dots inherent in the assembly technology of the cameras. optical fiber.

I1 can also be white dots, for example coming from defective pixels of the charge-coupled camera.

Devices are already known which make it possible to limit and / or eliminate some of the defects of the type described above, which alter the quality of the images captured by a charge-coupled camera.

For example, these can be minimized by a drastic selection of the components of the camera in order to obtain an acceptable overall characteristic with regard to technological defects.

However, such devices have drawbacks, namely poor performance, a significant increase in costs and delivery times, the need to provide for large stocks with the obligation to create a market for products with reduced specification, which is not possible. not acceptable in the context of high-end products and does not really remedy the problems originating from physical phenomena.

Another known means consists in introducing a corrective circuit known as “shading” correction in English terminology.

However, such a solution only makes it possible to compensate for defects having a character of revolution, as is the case for the vignetting of optics or the non-conformity of electronic lenses in intensifier tubes.

On the other hand, it does not resolve localized faults or those of physical origin.

Another known way of minimizing the defects affecting the quality of the image consists in carrying out image processing with a correction function corresponding to the formula
SC = S - N / B - N with
SC: Video signal after Correction
S: Useful signal to detect
N: Black signal
B: Signal delivered by the camera when it observes an intense homogeneous image (White Signal)
This latter solution, only implemented with a complex image processing system, does not however, on its own, allow the defects of the charge-coupled camera originating from noise sources, and this in particular when it is used. in very low light flux imaging.

In fact, the only solution known to date ensuring a really effective correction of the defects of the camera implements a device comprising an external image processing system with associated computer.

This solution again presents drawbacks. In particular, it does not make it possible to operate in real time, it is expensive and requires specialized operators. It is more complex and cumbersome to use.

The present invention aims to provide a charge-coupled camera comprising means for eliminating defects that alter the quality of the captured image, and a method implementing such a camera, meeting the requirements of practice better than those previously known. in particular in that it allows effective correction in real or near real time of all the faults mentioned above, in a very low light environment, and in that it is simple to use, portable, compact and inexpensive .

The camera according to the invention can also operate in two or three modes that can be used simultaneously, which makes it possible to increase its sensitivity performance by a factor greater than ten, while maintaining, thanks to the elimination of defects, a quality of. exceptional image.

The three functions which can be used simultaneously are those of integration on the "CCD", a conventional function making it possible to increase the quantity of light obtained per image, of averaging of images after integration making it possible to improve the signal-to-noise ratio characteristic. by erosion, and elimination of point type defects.

To this end, the present invention proposes in particular a charge-coupled camera comprising means for eliminating defects affecting the quality of the captured image, characterized in that said elimination means are portable, integrated into the camera housing and comprise - electronic filtering means arranged to perform real-time filtering on each pixel of the image captured at time Ti, by averaging with the pixel located at the same address of the image captured at time Ti, - And electronic means of comparison between the image averaged in real time and a reference image taken at a determined instant.

In advantageous embodiments, recourse is also had to one and / or other of the following arrangements - The camera comprises means for accumulating the video image for a determined sequenced time t before restitution of the video image. image, said accumulation means being arranged to operate simultaneously with the filtering and comparison means.

This makes it possible to obtain, with a camera of given sensitivity, higher performance than those obtained with a camera equipped with an additional intensification stage, without however introducing the additional constraints inherent in such a device and in particular, in addition to the cost, decrease in resolution, non-uniformity and increase in volume of the camera head.

- The filtering means include
means for acquiring the digital luminosity value Ii-1 of each pixel of the image captured at an instant Ti-i,
means of storing these digital values
Ii-1 at determined addresses corresponding to each pixel,
means for acquiring the digital luminosity value Ii of the pixels of the following image located at the same address, at the instant Ti,
means for calculating an average value of the digital values of each of said pixels according to the formula:
Mi = K x (Ii / d + Mi-1) with
K: averaging time constant d: division factor Mi-1: average value at time Ti-1
means of video display of the value Mi, for each of said pixels,
and means for storing said value Mi, at the address of each pixel concerned, for constitution of the next image; the camera has a control clock with a frequency greater than or equal to 14 MHz; the means for acquiring the digital brightness value are arranged to digitize the information on eight bits ;; the filtering and comparison means comprise two memory planes, namely a first memory plane P1 arranged to calculate and store the averaged image information in real time, called
Signal, for the elimination or reduction of the effect of noise sources, and a second reference memory plane P2, arranged to store at a determined instant the image information also averaged, called reference or reference information.
Signal White, delivered by the camera during the observation of a homogeneous scene, said filtering and comparison means being arranged to calculate the ratio, called the Signal / White ratio, by comparison between the Signal information extracted from the memory P1 and the information of
Signal White extracted from the memory P2 - the comparison means comprise means for calculating the Signal / White ratio arranged so that the value of the signal supplied by the plane P1, for a given address pixel, addresses the most significant bits of the address bus of an EPROM memory, and the value of the signal supplied by the memory P2, for the pixel situated at the same address of the reference plane, addresses the least significant bits of the address bus of said EPROM memory.

Such an arrangement allows particularly efficient operation.

The invention also proposes a method for eliminating defects affecting the quality of the image captured by a charge-coupled camera, characterized in that each point of the image captured at time T is filtered in real time; , by averaging with the image captured at the instant Ti-i and comparing the image information averaged in real time with reference information to eliminate the black points.

In an advantageous embodiment, the video image is also integrated for a time sequenced simultaneously with the averaging.

Advantageously, the method comprises the following steps - the digital luminosity value Ii of each pixel of the image captured at an instant Ti-1 is acquired, - each of these digital values Ii-i is stored at determined addresses corresponding to each pixel, - one acquires the numerical value of luminosity Ii of the pixels of the following image located at the same address, at the instant Ti, - one calculates an average value of the numerical values of each of said pixels according to the formula:
M = K x (Ii / d + Mi-1) with
K: time constant of averaging d: division factor Mi-l: value averaged at instant Ti- - the value Mi is displayed in video, for each of said pixels, - and said value Mi is stored at the address of each pixel concerned, for constitution of the next image.

Also advantageously - the averaged image is permanently calculated and stored in real time, known as Signal information on a first memory plane P1 in order to eliminate or reduce the effect of the noise sources, - the averaged image delivered by the camera is stored. during the observation of a homogeneous scene, called White Signal information on a second reference memory plane P2, - and the Signal / White ratio is calculated by comparison between the Signal information extracted from the memory P1 and the information from so-called White Signal reference extracted from memory P2.

The invention will be better understood in view of the following description of a particular embodiment, given by way of non-limiting example.

It refers to the accompanying drawings in which - Figure 1 shows a television device usable with a camera according to the invention.

FIG. 2 is a graphic representation illustrating the averaging used in the method according to the invention.

- Figure 3 is a block diagram of the filtering and integration means of a camera according to the embodiment of the invention more particularly written here.

FIG. 4 is a time diagram showing the normal operation and with the integration function of a camera according to the invention.

Figure 1 shows a charge coupled camera 1 connected via a connection circuit 2 to a television monitor 3.

The camera 1 comprises a housing 4 provided with means 5 for eliminating defects affecting the quality of the captured image 6.

The fault elimination means 5 will be detailed with reference to FIG. 3.

They implement a filter, the principle of which is detailed below with reference to FIG. 2.

This filter makes it possible to carry out an average weighted by an exponential law of the successive images.

This results, for still images, in an improvement in the signal-to-noise ratio.

The calculation performed by the digital filter is as follows
Each point of the image is the sum multiplied by the two constants of a fraction of the point or pixel provided by the video source and of the point or pixel of the previous image, calculated by the same rule.

Let Ii be the brightness of a point provided by the video source, Ii1 that of the same point in the previous image, 1il2, etc.

Let d be the division factor of Ii.

Only the bits of the whole part are affected in this regard.

Let k be the multiplicative factor.

Let Mil be the brightness of the pixel contained in memory and calculated during the previous image.

This luminosity is therefore replaced by Mi after the calculation represented by the block diagram of FIG. 2 commented on below.

In the embodiment more particularly described here, Ii and Mi are also digitized over eight bits.

The input 7 of the signal Ii attacks the divider 8 connected to the Sum block 9, which in turn attacks the operator 10 (multiplication by K) before being stored in it.

The signal is picked up via 12 to attack the block again
Sum 9.

For each image, a value is thus memorized (output 13)
Mi = K (Ii / d + Mi-1 either by setting
ii = Ii / d Mi = K x ji + K x ji-1 + K3><X ji-2 +
The step response of the filter is therefore a discretized exponential, the filtering time constant being given (in number of images) by the formula
r = 1/1-K
The index j corresponds to the number of images used, i.e.
j = - ln (255 / d) / ln K
The improvement in the signal / noise ratio is given by the formula

The values used in the invention are for example the following
d K rj AA (dB)
Position 1 1 0.5 2 8 2i 5
Position 2 2 0.666 3 12
Position 3 4 0.8 5 19 W 9.5
The elimination means 5 are represented more precisely in FIG. 3 and revolve around a double image memory 14.

The addressing of the memory 14 is generated by chains of digital counters 15 driven by the HD, VD and pixel clock signals from the camera (not shown).

More precisely, the horizontal addressing counters receive the pixel clock signal as a clock signal, these counters being reset to zero by the horizontal synchronization signal HD coming from the camera.

The vertical addressing counters for their part receive the signal HD as a clock signal, the resetting of these counters being effected on the other hand by the frame parity signal generated from the signal VD coming from the camera.

At the output of these counters, a twenty-bit bus 16 simultaneously addresses each of the two memory planes P1 and P2 by the same signals A0 to A19.

The digitalization of the video signal 17 is carried out by the analog to digital converter 18.

In the embodiment more particularly described here, the video signal is digitized on eight bits.

This digitization depth does not constitute a limit, the embodiment being able for example to operate with a digitization on twelve, sixteen or twenty-four bits.

However, an eight-bit implementation is particularly well suited to vision at very low light levels.

The signal, thus digitized is structured in two identical data buses 19 called input bus P1 DE-O to P1 DE-7 for the one corresponding to the memory plane P1, and P2 DE-8 to P2 DE-15 for the memory plane P2.

The data P1 DE-O to P1 DE-7 are identical to those present on the P2 DE-8 to P2 DE-15 bus.

The two memory planes P1 and P2 are moreover organized in a similar manner, although having a different role.

The plane P1 calculates and stores the averaged signal in real time for the elimination (or reduction) of the effect of noise sources.

The plane P2 memorizes the averaged image, delivered by the camera when the latter observes a homogeneous scene.

This sequence, called "reference taking", does not need to be repeated frequently but for example only after each power cut to the camera.

A battery can also be provided to save the data of the memory plane P2 to avoid regular taking of the reference plane.

The plane P1 is therefore used for the "real time of the image" operation, that is to say that it operates continuously according to the process below
Acquisition of the digital value of a pixel at a given instant TO by its input data bus (P1 DE-O to P1 DE-7).

Search, in its own memory plane, for the value of this pixel stored during the construction of the previous image (brightness Mi1 at time Ti1, ie forty ms earlier in the CCIR standard). The corresponding value is then stored in a location characterized by its address obtained by reading the address bus (AO to A19)
Calculation of averaging
Mi = K (Ii / d + Mi ~ 1).

Display of the result Mi thus obtained.

Storage of this new value Mi, at the address of the pixel concerned, so that it becomes the averaged value intended to carry out the next averaging, which will take place during the production of the following image, i.e. here again by way of example for a CCIR standard camera, forty ms later.

The plane P2 is only used in acquisition when the reference image is taken. During normal operation of the device, only its output data bus is therefore active, the input data bus being inhibited.

The size of the memory planes P1 and P2 is for example 756 × 576 × 8 bits, this configuration corresponding to the size of the CCD matrix used.

The portion of circuit 20 eliminating the other types of defects ensures the calculation in real time, pixel after pixel, at the rate of reading thereof, of the Signal / White ratio.

To do this, the Signal information is extracted from the memory plane P1 at 21, while the White Signal information (corresponding to the white image taken as a reference) is extracted at 22 from the corresponding pixel, located at the same address of the memory plane. P2.

A module 23 then calculates the ratio between the two values thus obtained and then directs it to the output stages of the device.

The principle used to perform this calculation is as follows
The value of the signal supplied by the memory plane P1 (for the record, this value corresponds to the Signal information), on its data bus, and for a given address pixel, is routed to the most significant bits of the bus d address of a 512 kilo Bits 27512 EPROM 24 memory belonging to module 23.

The value of the signal supplied by the memory plane P2 (for the record, this value corresponds to the information
Signal Blanc also known as Reference), on its data bus, and for the pixel located at the same address of the “Reference” plane, is routed to the least significant bits of the address bus of the same memory 24.

Thus, the memory 24 is addressed by a pair of data corresponding to a single value which has been previously programmed as being the result of the Signal / White calculation.

This technique makes it possible to ensure the operation of the assembly in real time.

In order to obtain maximum performance, the white reference can be taken to a level close to saturation so as to make the best use of the dynamics of the Reference memory.

To do this, a circuit 25 is provided which is connected to the output of the memory plane P2.

The circuit is active during the acquisition phase of the Reference signal.

During this, the averaged image is displayed and available in real time in the P2 plane of
Reference. A saturation detector is also provided, which has the role of presenting in black the pixels reaching saturation (supposed to be in white) in order to allow the operator to memorize this reference under optimal conditions.

This module is inhibited during normal camera operation.

The output stages of the means 5 for their part comprise a switch 26 for displaying the saturation as well as an output Digital / Analog converter 27, which delivers on its video output 28, a signal identical to that delivered by the camera. , but free from all the defects thanks to the invention.

The output stages also include the stage 29 for mixing the synchronization signals 30 as well as an impedance adapter stage (not shown) making it possible to drive, at low impedance, the coaxial cable 31 intended to route the image to d. '' other devices (monitor etc.)
In the embodiment described more particularly, the available signal is a composite video signal, to the CCIR standard, compatible with direct viewing on a television monitor.

The assembly using digital techniques, it is also possible to provide a video signal in digital form, by means of an interface circuit suitable for the application.

According to the embodiment of the invention more particularly described here, the camera also comprises means 32 for integrating the video signal, comprising a sequencer 33.

The sequencer 33 consists of a counting module 34 which generates a signal corresponding to a number of images and defined by the decoding of the value which is provided for example by a group of encoder wheels 35. A monostable circuit could also have been possible. be used to generate this signal which corresponds to the accumulation on the CCD.

By also referring to Figure 4, it is observed that during the time corresponding to the "accumulation" part (upper part of the figure) and whose duration is a function of the available light flux, the CCD sensor (not shown) is blocked and therefore does not deliver a video signal 17 at the output.

This phase 35 in fact corresponds to the accumulation of photoelectrons in the image zone of the sensor.
CCD.

The accumulation signal being the complement of the reading signal from the CCD sensor, this signal is delivered to the chain of counters 15 when the sequencer 33 triggers said reading.

Which channel 15 is then authorized to address the memory P1.

Thus the process of averaging the image being read (instant Ti) with that stored in P1 is carried out according to the normal process described above.

This is again interrupted as soon as a new accumulation cycle is triggered, again thanks to the action of the authorization signal from the chain of meters 15.

During reading, the CCD sensor supplies a video signal similar to that which it delivers in normal mode, the essential difference being that the duration of its presence is strictly limited to the reading time of the image zone of the CCD sensor.

The cyclical sequence of these phases is carried out by the sequencer 33.

Claims (13)

1. Charge coupled camera (1) comprising means (5) for eliminating defects affecting the quality of the captured image, characterized in that said elimination means are portable, integrated into the housing (4) of the camera and include - electronic filtering means arranged to perform real-time filtering on each pixel of the image captured at the instant Ti, by averaging with the pixel located at the same address of the image captured at the instant Ti-i, - And electronic means of comparison between the image averaged in real time and a reference image taken at a determined instant. 2. camera (1) according to claim 1, characterized in that it comprises means for accumulating the video image for a determined sequenced time t before restitution of the image, said accumulating means being arranged to operate simultaneously. with the means of filtering and comparison. 3. Camera according to any one of claims 1 and 2, characterized in that the filtering means comprise: - means for acquiring the digital luminosity value Ii-1 of each pixel of the image captured at an instant. Ti-i, - means for storing these digital values Ii-i at determined addresses corresponding to each pixel, - means for acquiring the digital luminosity value Il of the pixels of the following image located at the same address , at the instant Ti, - means for calculating an average value of the digital values of each of said pixels according to the formula: Ni = K x (Il / d + Mi-1) with K: averaging time constant d: division factor Mi-1: average value at instant Ti-1 - means for video display of the value Mi, for each of said pixels, - and means for storing said value Mi, at the address of each pixel concerned, for constitution of the following image. 4. Camera according to claim 3, characterized in that it comprises a control clock with a frequency greater than or equal to 14 MHz. 5. Camera according to any one of the preceding claims, characterized in that the means for acquiring the digital brightness value are arranged to digitize the information on eight bits. 6. Camera according to any one of the preceding claims, characterized in that the filtering and comparison means comprise two memory planes, namely a first memory plane P1 arranged to calculate and store the averaged image information in real time, tell information about Signal, for the elimination or reduction of the effect of noise sources, and a second reference memory plane P2, arranged to store at a determined instant the image information also averaged, called reference or reference information. Signal White, delivered by the camera during the observation of a homogeneous scene, said filtering and comparison means being arranged to calculate the ratio, called the Signal / White ratio, by comparison between the Signal information extracted from the memory P1 and the reference information, extracted from the memory P2. 7. Camera according to any one of the preceding claims, characterized in that the comparison means comprise means for calculating the Signal / White ratio arranged so that the value of the signal supplied by the plane P1, for a given address pixel , either routed to the most significant bits of the address bus of an EPROM memory, and the value of the signal supplied by the memory P2, for the pixel located at the same address of the reference plane, or routed to the bits of low order of the address bus of said EPROM memory. 8. A method of eliminating defects altering the quality of the image captured by a charge-coupled camera, characterized in that each point of the image captured at the instant Ti is filtered in real time, by averaging with l image captured at instant Ti-i. and comparing the real-time averaged image information with reference information to eliminate black spots. 9. The method of claim 8, characterized in that the video image is further integrated for a time sequenced simultaneously with the averaging. 10. Method according to any one of claims 8 and 9, characterized in that in order to filter: - the digital luminosity value Ii- of each pixel of the image captured at an instant Ti-1 is acquired, - each is stored. of these digital values II at determined addresses corresponding to each pixel, - the digital luminosity value Ii of the pixels of the following image located at the same address, at the instant Ti, is acquired, - an average value of the values is calculated numerical values of each of said pixels according to the formula: Mi = K x (Il / d + Ni-i) with K: time constant of averaging d: division factor Mi-1: value averaged at instant Ti-1 - the value Mi is displayed in video, for each of said pixels, - and said value Mi is stored at the address of each pixel concerned, for constitution of the next image. 11. Method according to any one of claims 8 to 10, characterized in that the digital value of brightness on eight bits is digitized. 12. Method according to any one of claims 8 to 11, characterized in that - the averaged image is calculated and permanently stored in real time on a first memory plane P1 in order to eliminate or reduce the effect of the noise sources. , - the averaged image delivered by the camera is stored during the observation of a homogeneous scene, called a white signal on a second reference memory plane P2, - and the Signal / White ratio is calculated by comparison between the information image extracted from memory P1 and the so-called White Signal reference information extracted from memory P2. 13. The method of claim 12, characterized in that the ratio is calculated Signal / White by addressing the most significant bits of the address bus of an EPROM by the value of the signal supplied by the plane P1, for a given address pixel, and by addressing the least significant bits of the bus d the address of said EPROM memory by the value of the signal supplied by the memory P2, for the pixel situated at the same address of the reference plane.