FR2610735A1 - Self-adapting optical dynamic compensator and its use in an optical radiological-imaging system - Google Patents

Abstract

The compensator comprises a plate 1 of a photoconductive material and a plate 2 of a single-crystal photoconductive material. The two plates are sandwiched between two electrodes 5, 6. The assembly is placed between crossed optical polariser 3 and analyser 4. Application: radiology.

Description

Autoadaptive optical dynamic compensator
and its use in an optical chain
radiological imaging
The present invention relates to an autoadaptive optical dynamic compensator and its use in an optical radiological imaging chain.

 In radiological imaging chains using luminance amplifiers, the visualization of the details of low light levels is made difficult when the objects to be examined have very large contrasts, this being due in particular to the electronic noise of the acquisition camera of the light. 'picture.

 However, the devices currently used in the acquisition chain to reduce the internal contrast of the images are heavy to implement because they require action on the object or on the X-ray beam.

et une expansion de la forme Ue = Uc2, et dans le deuxième cas, par exemple, un autre procédé connu sous la désignation DOLBY effectue une compression en Uc = U# et une expansion en Ue = Uc1/α. The problem to be solved is comparable to that encountered in the audio tape recording of audio-frequency analog signals where the dynamic limit comes mainly from the magnetic tape and for which several methods of compression at the recording and expanding at the reading are known. A distinction is often made in these methods according to whether they relate to the entire audio frequency spectrum or only parts of the spectrum, the latter being treated separately and then recombined. In the first case, for example, the known method under the designation DBX performs a compression of the form

and an expansion of the form Ue = Uc2, and in the second case, for example, another method known as DOLBY performs Uc = U # compression and Ue expansion = Uc1 / α.

In the case of an image chain, it is sufficient to define the optical dynamic compressor to be placed in front of the image sensor, since the expansion can be performed in the restitution chain by an electronic linearization circuit, provided that either known the law of compression. By working by analogy with the audiofrequency processes mentioned above, the optical compression can relate to the entire spectrum of spatial frequencies of the image (type
DBX), or only on low frequencies, which amounts to relatively increasing the level of high frequency (typical
DOLBY). In the latter case, the dynamic compressor provides the known function of autoadaptative fuzzy mask; at the limit, it behaves like an image differentiator, operating in parallel and in real time.

 In the case of animated images, or images appearing during illumination pulses, one of the problems to be solved is that of the temporal response of the compressor; indeed, as in the audio domain, the artifacts or distortions on the linearized image can be generated in the convolution product of the compressor's space-time transfer function by the spatiotemporal content of the image, in particular when the present response damped oscillations, more known in the Anglo-Saxon literature under the designation "Overshoot".

Also, the response time must be negligible in front of the time of existence of the zone to be corrected, on pain of an insufficient correction, and the time of reset must be too, under penalty of an alteration of the next picture.

It can be noted, however, that if the initial reset time is particularly long, that is to say the compensation image has been stored, it becomes possible to proceed with the compensation of successive images having part of their content in common, that is to say to highlight the differences between images. At the limit such a compensator, covering the entire spectrum of spatial frequencies, and endowed with memory, must be able to be used for the subtraction of images, in parallel and in real time.

 The object of the invention is to propose solutions to the problem posed.

 To this end, the subject of the invention is an autoadaptive optical dynamic compensator for an image transmitted by a radiation beam, characterized in that it comprises a light transmitting body consisting of a photoconductive electro-optical material sandwiched between two transparent polarization electrodes.

 The subject of the invention is also a use of the compensator in an optical radiological image chain.

 The optical compensator according to the invention has the advantage that it allows dynamic compressions as well as expansions, that is to say both attenuations and enhancements of contrasts, in a completely self-adaptive way, that is to say that is, without the action of other outside control means than that caused by the impact of the photons on the photoconductive electro-optical material. This results in extremely short response times of less than 40 milliseconds which is very sufficient to provide effective corrections to the image.

Other characteristics and advantages of the invention will become apparent hereinafter with the following description made with reference to the appended drawings which represent:
Figure 1 a curve to illustrate the compensation principle implemented by the invention
FIG. 2 is a diagram for illustrating the principle of an optical compensator according to the invention
FIG. 3, a graph schematizing the various stages of the compensation process according to the invention
FIG. 4, a curve of a transmission of an optical compensator according to the invention
FIG. 5, a first exemplary embodiment of an optical compensator according to the invention
FIG. 6 a polarimetric assembly made with an optical compensator of the type of FIG.
FIGS. 7, 8A and 8B of the second, third and fourth different structures of optical compensators according to the invention
FIG. 9, a transmission curve of a liquid crystal used as an electrotropic material
FIG. 10, a transmission curve of the device of FIG. 8 as a function of the voltage
FIGS. 11 and 12 are two diagrams of embodiment of radiological imaging optical chains each comprising an optical compensator according to the invention
The principle according to the invention consists in creating, at each point of a radiological image to be compensated, a variable optical density which is a function of the illumination of each of the points. As the digital image processing in radiology is carried out in logarithmic units, the transfer curve representing the intensity transmitted It as a function of the incident intensity I.

γ is expressed according to a law of the form It = Ii, where γ is
i, between 0.5 and 0.8 for example. A corresponding representation in logarithmic coordinates is given in FIG. 1 which shows two curves A and B obtained for two different coefficients g <. This result is obtained, according to the invention, by joining a plate of a photoconductive material 1 to a plate of an electro-optical material 2 as shown in FIG. 2 or again, using a material having both combined effects. that is, possessing the photorefractive effect. This structure is used between a polarizer 3 and an analyzer 4 to constitute a polarimetric mounting. The structure composed of the two materials 1 and 2 is electrically biased by an alternating or possibly continuous voltage and in such a way that the electric field obtained is collinear with the optical axis X'X of the system, this axis being itself perpendicular to the planes of the two plates 1 and 2.

 In FIG. 2, for example, the outer faces that are not common to the two plates 1 and 2 are covered by transparent electrodes 5 and 6 between which an electric field E produced by a voltage generator 7 is applied.

Under these conditions the electric field T is parallel to the optical axis XX 'of the device and each photon, energy ho passing through the electrode 5 parallel to the optical axis
XX ', gives rise to electric charges moving on the action of the field E in the optical direction XX'. In the above expression h is the Planck constant and 8 is the radiation frequency. The spatial distribution of the light intensity of the image to be compensated results in a non-uniform distribution of the electric potentials on the electro-optical material 2 which sees its transmission properties varied locally. The transmission law naturally depends on the characteristics of the photoconductive material and the transmission response of the electro-optical material 2 as a function of the electrical voltage applied to the electrodes 5 and 6. Corresponding curves are shown in FIGS. 3 and 4.

 According to the 4-quadrant diagram of FIG. 3, the first quadrant represents the photoconductivity characteristics 6 = f (i) of the selected material assumed to be linear in the diagram. The second quadrant gives the voltage transfer Vco for the electrooptical material as a function of the resistivity & of the photoconductive material, or more generally of its electrical characteristics. The third quadrant gives the variation of the transmission T co of the polarimetric assembly as a function of the electrical voltage VCO developed between the parallel faces of the electro-optical material 2.

 Finally, the transmission law ot = Tco x is shown in Figure 4.

A first embodiment of a compensator according to the invention is shown in FIG. 5. In this example, the compensator is made with a PROM structure which is the abbreviation of the Anglo-Saxon term (Pockels Read out
Optical Memory), this structure is composed of a plate 8 of monocrystalline materials, of the type belonging to the family Bil2 X 020 where X = Si, Ge or Ti or a crystal
= 1 suitably oriented liquid and placed between two blocking electrodes 9 and 10 consisting of tin oxide and indium (ITO) separated from the monocrystalline wafer 8 by two thin insulating layers 11 and 12 respectively.To work properly , the assembly is placed as shown in FIG. 6 between a polarizer 13 and an analyzer 14 crossed and biased by a DC voltage less than or equal to a half-wave voltage of the single crystal equal, for example, if the material is Bil2 Si 020, at VflT = 3.9 Kvolts. This device uses both the photoconductive and electro-optical properties of the single crystal.

 The clearing process takes place in two stages.

Under the effect of illumination the voltage across the single crystal decreases because it is photoconductive.

dans laquelle # o et # r représentent les permittivités diélectiques dans le vide et dans le monocristal et, simultanément une loi de variation de la transmission du montage électrooptique qui s'exprime alors suivant une relation de la forme

As we can consider that the law of variation of the conductivity as a function of the luminous intensity I is of the form
6 = o + BI (1), we obtain for exposure times much greater than the lifetime of the photocarriers generated in the single crystal, a law of temporal decay of the voltage at the terminals of the single crystal which verifies a relation of the form

in which # o and # r represent the dielectic permittivities in the vacuum and in the single crystal and, simultaneously, a law of variation of the transmission of the electro-optical assembly which is then expressed according to a relation of the form

 The relation (3) is true for an oriented crystal (100) for an incident light polarized at 450 of the main axes and for polarizers and analyzers 13 and 14 crossed.

Thus for a moment to be between 0 and to (exposure time) the transmission of the device is obtained by combining the relations (2) and (3), which gives the relation

 The compensation law obtained defined by the relation (4) is strictly decreasing with I. However, it has the disadvantage of rendering the phenomenon of transient compensation, since this depends on the exposure time t and requires an acquisition of very fast image.

 Another embodiment of the invention is shown in FIG. 7. It associates a photoconductive crystal 15 with an electropopic crystal 16 of lithium niobates polarized between two electrodes 17 and 18. In this case, the two functions of photoconduction and electro-optical effect lithium niobate are separated. To have an electro-optical effect with the lowest possible half-wave voltage (2.5 to 3 KVolts) the large face of the lithium niobate crystal is oriented at 550 of the Z axis in the quadrant -Y + Z, but the natural birefringence phenomenon that is obtained must be compensated. In another variant, the lithium niobate may be substituted for a KDP crystal.

Finally, according to a third possible embodiment of the invention, the principle of compensation can also be implemented by using an electro-optical device a liquid crystal valve. A corresponding device implementing a monocrystalline photoconductor is shown in FIG. 8A. It consists of a liquid crystal 19 enclosed in a sealed enclosure by two parallel gripping layers 20 and 21 spaced apart by two thickness shims 22 and 23. The layer 20 is covered on its outer part with the enclosure by a photoconductive layer 24 of photoconductive monocrystalline material and the plate 21 is covered on its outer face with the enclosure by a transparent electrode 25.An electrode 26 is also deposited on the outer surface of the photoconductive layer 24 parallel to the surface of the layer 24 in contact with the attachment layer 20. Also a glass plate 27 covers the electrode 25. The assembly is held inside a ring or a frame 28. The electrodes 25 and 26 are polarized by an alternating voltage to prevent the degradation of the liquid crystal 17 by electrolysis. According to another variant embodiment which is shown in FIG. 8B, where the same elements as those of FIG. 8A are represented with the same references, the photoconductive element 24 is made in a thin layer and the electrode 20 which covers it is It is covered by a glass substrate 29. In the two representations of FIGS. 8A and 8B, the liquid crystal 19, the thickness of which does not exceed ten microns, constitutes the electro-optical element. It is optionally constituted by a working liquid crystal, either in helical or twisted nematic mode, or electrically controlled birefringence, or by a dicylic liquid crystal. In the nematic operation mode in helix or twisted the two alignment directions of the molecules on the opposite substrates are perpendicular. The molecular structure is a quarter helix with a pitch P = 4. e where e is the thickness of the liquid crystal. This helicoidal structure rotates the polarization direction of an incident light polarized rectilinearly parallel to the two alignment directions of the molecules under conditions, although the relationship
We p
, A>> 1 (5) is satisfied. In relation (5) A n denotes the birefringence of the liquid crystal and the wavelength of the light. When energized, the liquid crystal used a. a positive dielectric anisotropy and the molecules are oriented parallel to the electric field which destroys the helicoidal structure. The optical axis of the liquid crystal is then parallel to the electric field and the polarization of the light is not changed by crossing the liquid crystal polarized by an electric field. In this case, by placing the valve between polarizer and analyzer parallel to the two alignment directions of the molecules, the light is transmitted when the voltage on the liquid crystal is lower than a threshold voltage VS below which no variation effect of the transmission is observable, that is to say say when the light intensity is low. On the other hand, it absorbs light when the voltage becomes higher than the voltage threshold VS. Under these conditions, liquid crystal transmission curves having the shapes shown in FIG. 9 are obtained. In practice, it will be necessary in certain cases to take account of the important rotational power of the photoconductor 24 for the orientation of the polarizer. especially when the photoconductor will have the structure of a BSO type crystal. As the rotational power strongly depends on the wavelength and the light spectrum has a broad spectrum, the extinction rate of the system will be strongly affected, but this can be compensated for by the choice of polarizers. In particular, for the adjustment of the device, it will be possible to search, with zero polarization, the indication using the polarizer and the analyzer, then the maximum transmission by turning the analyzer to 900.

où Ii est l'intensité lumineuse incidente ; 6 (V) le retard optique induit par l'épaisseur du cristal liquide, ce retard étant fonction de la tension appliquée aux bornes du cristal.In the electrically controlled birefringence operating mode, the liquid crystal cell is optically equivalent to a parallel face eristaline plate, a neutral line of which is the projection on the input face of the cell of the alignment direction of the molecules. . In this case polarizer and analyzer are crossed and oriented at 450 neutral lines of the liquid crystal and the transmitted light intensity It follows a law of the form

where Ii is the incident light intensity; 6 (V) the optical delay induced by the thickness of the liquid crystal, this delay being a function of the voltage applied across the crystal.

 As in the case of the operating mode with a helical nematic liquid crystal, the transmission depends on the angle at which the incident flux arrives. To obtain the most homogeneous effect possible on the illuminated surface, it is necessary to use projection optics of great numerical aperture. The transmission curve of the valve as a function of the voltage across the liquid crystal follows in this case the typical curves of the electro-optical crystal blades of the type shown in FIG. of maximum and minimum corresponding to the values of the optical delay multiple of / 2. It can be seen that the slopes of the two decreasing parts of the curves are not the same.

As a result, it is possible to select the operating point by the applied voltage to modify the compensation. The optical adjustment is obtained by seeking the extinction of the transmission at zero voltage. In this case, polarizer and analyzer are crossed and parallel to a neutral line of the electro-optical system. To obtain the device in its configuration of use it is enough to turn polarizer and analyzer of 450 in-the same direction.

 In the case where the liquid crystal used is a dieroïque liquid crystal, a light absorption operation is obtained and not electro-optical effect so that the presence of polarizing elements (polarizer and analyzer is no longer necessary.

 Indeed, if the voltage on the liquid crystal is zero, the tranmission is maximum, that is to say that the orientation of the molecules is such that the absorption is minimal and when the voltage on the liquid crystal increases, the liquid crystal molecules are orienting themselves. so that the obsorption of light increases.

 A use of the optical compensators according to the invention previously described in a radiological imaging chain is shown in FIG. 11. In this use a compensator 30 according to the invention is placed between the output window 31 of an image intensifier radiological 32 and a video shooting tube 33. Optics 34, 35 on the one hand, and 36 on the other hand, ensure the collimation of the images on the one hand, on the compensator 30 and on the other hand, on the screen of the shooting tube 33. The set of optics and the compensator are fixed rigidly inside an envelope 37.

 However, instead of making the compensator 30 work in transmission as is the case with the mode of use shown in FIG. 11, it may be envisaged to have it work in reflection mode as shown in FIG. 12, where the elements homologous to FIG. those of Figure 11 are shown with the same references. In this example, the compensator 30 is no longer placed between the optics 35 and 36 but between the optics 35 and the mirror 37, and a semi-transparent plate 38 is placed in the optical path of the light rays transmitted by the optics 34 to ensure the collimation of the images on the mirror 37. The semitransparent blade 38 is also placed between the optics 35 and 36 to retransmit by transparency the images reflected by the mirror 37 in the optical direction 36 and the camera 33.

Claims (11)

CLAIMS S  1. An autoadaptive optical dynamic compensator for an image transmitted by a radiation beam, characterized in that it comprises a light transmitting body (1, 2 8; 15, 16; 19, 24) made of a photoconductive electro-optical material. sandwiched between two transparent polarization electrodes (5, 6, 9, 10, 25, 26) and placed between a polarizer.  2. Compensator according to claim 1, characterized in that the photoconductive electro-optical material is a monocrystalline material belonging to the family Bi12. X. 020 Where X = Si, G or T  e i  3. Compensator according to claim 1, characterized in that the light-transmitting body is constituted by a plate (1; 15; 24) of a monocrystalline photoconductive material resting on a second plate (2) formed in an electrooptical material ( 2; 16; 19)  4. Compensator according to claim 3, characterized in that the photoconductive material (1; 15; 24) belongs to the family Bi12.X.020 where X is silicon, germanium or titanium.  5. Compensator according to claim 3, characterized in that the photoconductive material (1; 15; 24) is formed by a thin layer consisting of bodies taken from columns II and VI of the periodic table of elements.  6. Compensator according to any one of claims 3 to 5, characterized in that the electro-optical material (2; 16; 19) is a KDP crystal or lithium Niobate.  7. Compensator according to any one of claims 3 to 5, characterized in that the electro-optical material (19) is a nematic liquid crystal helix.  8. Compensator according to any one of claims 3 to 5, characterized in that the electro-optical material (19) is an electrically controlled liquid crystal birefringence.  9. Compensator according to any one of claims 1 to 9, characterized in that the light transmitting body (1, 2; 8; 15, 16; 19, 24) is placed between a polarizer (3) and an analyzer ( 4) Cross optics.  10. Compensator according to any one of claims 3 to 5, characterized in that the electro-optical material (19) is a dichroic liquid crystal.  11. Use of the compensator according to any one of claims 1 to 9 in a radiological imaging chain (30, 36, 32).