FR2773226A1 - Protective filter for use with optical detector - Google Patents

Abstract

A variable transmission material responds quickly to an applied voltage change to prevent transmission of damaging pulses. The protective system includes a first photoconducting material lying in contact with a material offering variable transmission. Beneath this there is a second photoconducting material layer. The two external layers form electrodes in order to apply a voltage to the variable transmission material, such that by varying the voltage the transmission is also varied. The variable layer may comprise a polymer within which are dispersed liquid crystal molecules or electrochromic materials, or in an alternative arrangement the material may contain anisotropic sub-micronic molecules.

Description

PROTECTION SYSTEM AND OPTICAL VISION DEVICE
INCLUDING THIS PROTECTION SYSTEM
The field of the invention is that of optical vision devices (cameras, binoculars, goggles ...) and more precisely devices equipped with a protection system against light aggression of power strong enough to damage the optical device and / or the detector (eye, optronic sensor).

 Currently1 optical vision devices must include protection systems to protect themselves from laser sources including wavelength agile laser sources wavelength, each transmitted pulse may have a wavelength different from the previous one.

 There are in fact wavelength-agile laser sources wavelength in the visible and near-infrared now accessible. The use of such sources, especially in optical laboratories and for optical countermeasures, renders ineffective the devices for protecting the eye, or, more generally, sensors, based on the use of fixed filters. having the required optical properties in narrow wavelength ranges.

 Therefore, the invention proposes a new protection system against light aggression having the advantage of being adaptable to different ranges of wavelengths depending on the intended applications.

 More specifically, the subject of the invention is a protection system comprising first and second faces, the first face being capable of receiving an external light radiation, characterized in that it comprises a first photoconductive structure adjacent to the first face and a second structure energized and comprising a material whose transmission varies as a function of the applied voltage, said second structure being adjacent to the second face.

 By using a photoconductive material having photoconductive properties in a targeted spectrum, it is possible to use the protection system effectively in different ranges of wavelengths.

 The first and second structures are judiciously chosen so that in the presence of low illumination the protection system is transparent while in the presence of a higher power illumination, the protection system becomes filtering by diffusion and / or absorption.

 Preferably, the first and second faces respectively comprise first and second electrodes.

 According to a first variant of the invention, the protection system is characterized in that the first photoconductive structure is in contact with the second structure, a potential difference being applied between the first and second electrodes, the second structure being transparent in the absence of potential difference applied to said structure, the second structure being scattering and / or absorbing in the presence of a potential difference applied to said structure.

 According to a second variant of the invention, the protection system is characterized in that the first photoconductive structure is separated from the second structure by a third electrode, the first and second electrodes being subjected to the same potential, the third electrode being subjected to at another potential, the second structure being scattering and / or absorbing in the absence of potential difference applied to said structure, the second structure being transparent in the presence of a potential difference applied to said second structure.

 Advantageously, the variable transmission material may be based on liquid crystal. The second structure can thus be a liquid crystal cell capable of passing from a transparent state to an absorbing state when a voltage is applied thereto.

 It may also be a polymer matrix in which liquid crystal molecules are dispersed. In this case, the protection system exploits the electro-optical diffusion effects of these composite materials and is therefore not critical with respect to the polarization of the light beam which is to be protected.

 The second variable transmission structure may also include dichroic dyes or reverse saturable absorption molecules.

 It can also include electrochromic materials. More specifically, the second structure has in this case a layer of electrochromic material coupled to a layer of electrolyte material.

 The second structure may also comprise anisotropic particles of submicron size in solution in a liquid or a gel sufficiently fluid to allow their orientation in the electric field.

 The invention also relates to an optical vision device comprising a detector and at least one protection system, characterized in that the protection system comprises a first and a second face, the first face being capable of receiving an external light radiation. a first photoconductive structure adjacent to the first face and a second energized structure having a material whose transmission varies as a function of the applied voltage, said second structure being adjacent to the second face, the detector being located on the side of said second face.

 According to a variant of the invention, the optical vision device comprises several protection systems, comprising photoconductive structures sensitive to different wavelength ranges.

 Advantageously, the optical sighting device of the invention may comprise focusing means comprising at least a first lens, the protection system being located between said first lens and the detector.

 According to a variant of the invention, the focusing means comprise a second lens and a field lens, said field lens being placed in a focal plane intermediate between the first lens and the second lens.

 The protection system may advantageously be placed in the vicinity of one of the three lenses.

The invention will be better understood and other advantages will become apparent on reading the description which follows, given by way of non-limiting example and thanks to the appended figures among which:
FIG. 1 illustrates a first example of a protection system
according to the invention;
FIGS. 2a and 2b illustrate the operation of a cell with
liquid crystal dispersed in a polymer (without field figure
2a and with electric field FIG. 2b)
FIG. 3 illustrates a second example of a protection system
according to the invention;
FIG. 4 illustrates a first example of an optical device
vision equipped with a protection system located in the vicinity
a detector;
FIG. 5 illustrates a second example of an optical device
vision equipped with a protection system located in the vicinity
an intermediate focal plane of the device;
FIGS. 6a and 6b illustrate optical vision devices
equipped with protection system placed out of the focal plane
intermediate of the device.

 The protection system of the invention is a photoactivated device of the liquid crystal optical valve type. It consists of the association of a photoconductive structure PC exposed to an incident beam I at its face F1, and a variable transmission structure denoted T (v) by which the beam I is transmitted or not at the level of its outer face F2 as illustrated in Figure 1 which shows a first example of a protection system according to the invention. According to this variant, a layer of photoconductive material is in direct contact with a layer of variable transmission material T (v). The set of two layers is inserted between two electrodes El and E2. The set of layers is biased by a low DC or AC voltage. Thus, under the action of an illumination beam, the photoconductor which is chosen to be sensitive to the spectral range of the illumination beam, locally transfers the voltage. applied to the structure T (v). Under these conditions, this reported voltage can modify the transmission of the structure T (v) and thus modify the transparency of the protection system, since the variable transmission material is chosen to be transparent to zero voltage and absorbing and / or diffusing under pressure.

 In order for the limiting effect to be independent of the polarization of the incident beam, a liquid crystal structure which is dispersed in a polymer matrix (PDLC material) is advantageously used. More specifically it is liquid crystal beads, dispersed in a polymer as shown in Figures 2a and 2b which show the operation of a conventional PDLC cell. In general, the exact structure of the PDLC depends on the ratio between the liquid crystal phase and the polymer phase, as well as the technology used to carry out the mixing. The birefringent liquid crystal molecules have two indices n1 and n2 along two orthogonal axes X and Y judiciously chosen so that one of these two indices is equal to the refractive index np of the polymer. In the absence of electric field (E = O) liquid crystal molecules are not oriented relative to each other, the average index of liquid crystal beads ncL is different from the index of the polymer np. The cell is in a diffusing state as shown in Figure 2a. The diffusion pattern is determined by the diameter of the liquid crystal beads as well as by the thickness of the PDLC layer.

 In the presence of an electric field, the liquid crystal molecules are oriented relative to each other as shown in Figure 2b.

The average index of the liquid crystal beads ncL becomes equal to the index n1 equal to the index of the polymer np. The cell has a homogeneous index and is no longer diffusing.

 In the protection system of the invention and in particular in the example illustrated in FIG. 1, it may be particularly suitable to use an inverse PDLC cell, ie a transparent liquid crystal structure when no electric field is applied and a liquid crystal structure that becomes diffusive in the presence of an electric field. This is done using a liquid crystal whose dielectric anisotropy changes sign with frequency. During manufacture, the liquid crystal is subjected to an electric field at the frequency ol, which directs the liquid crystal molecules parallel to the field. This position will become the memory position at rest. The composite is then transparent in the absence of the electric field. The application of an electric field of so2 frequency such as the dielectric anisotropy of the liquid crystal can change sign, switches the molecules perpendicular to the electric field so that the material becomes diffusive. Thus, when the protection system is subjected to a + V voltage, under low illumination the photoconductor is insulating, no voltage is applied to the liquid crystal structure which remains transparent. When the protection system is subjected to illumination sufficiently intense to make the photoconductor conductive, the applied voltage is transferred to the liquid crystal structure which becomes diffusing.

 According to another example of a protection system of the invention, the photoconductive structure is separated from the liquid crystal structure via an electrode E3 as illustrated in FIG. 3. The variable transmission material is chosen so as to be absorbent. and / or diffusing at zero voltage, and transparent under voltage. The faces F1 and F2 comprise the electrodes E1 and E2 subjected to the same potential V, the intermediate electrode E3 can typically be connected to ground. In this configuration, it is advantageous to use a PDLC material as described in the case of a conventional PDLC type cell in which the material is diffusing in the absence of an applied and transparent electric field in the presence of an applied electric field. Indeed, when the illumination is low intensity, the photoconductor remains insulating, a potential difference is maintained at the terminals of the variable transmission structure and the structure remains transparent.

 In the presence of sufficiently intense illumination, the photoconductive structure becomes conductive, the voltage V is applied at the electrodes E3 and E2; the potential difference across the variable transmission structure vanishes and the variable transmission structure becomes absorbent and / or diffusing.

 In general, the response time of the protection system is controlled by the variable transmission layer, which corresponds for example to values of 5 to 100 ms for the orientation of liquid crystal molecules.

 Given the current photoconductive materials, the protection system may advantageously have a range of spectral sensitivity extended from the ultraviolet to the visible and the infrared in band 2 (3.4 to 4.2 μm) and in band 3 (8 to 12 μm), the liquid crystals remaining transparent in this area.

 More specifically, the photoconductor of bismuth oxide and silicon (BSO) is particularly suitable for the blue-green spectral domain.

For the visible range, other semiconductor materials deposited in a thin layer can be used, in particular CdS, As2S3, CdSe, Si arrTorphe.

For the infrared domain semiconductor materials such as GaAs,
CdTe or even HgCdTe are very interesting.

Activation thresholds can typically be less than 1 mW.cm
The use of thin film technologies for the photoconductor and for the aforementioned variable transmission materials makes it possible to produce devices of suitable dimensions for protection systems, typically 10 cm × 10 cm.

The different materials used in the variable transmission structure can be:
1) Liquid crystal molecules.

The variable transmission structure is a crystal cell
liquid, operating between polarizer and cross analyzer.

 2) Polymeric matrices comprising liquid crystal.

The variable transmission structure works
regardless of the polarization.

 Furthermore, the liquid crystal used in the liquid crystal structure can be doped with dichroic dyes which orient with the liquid crystal molecules. Under electrical tension, the dye absorbs very little (the dipolar axis of the dye molecules is perpendicular to the light polarization). In the absence of electrical voltage, the liquid crystal and dye molecules disorient and dye uptake is strong in the blocking state. Such doping makes it possible to increase the protection efficiency by combining photoinduced absorption and diffusion, while maintaining a strong transmission in the on state.

 Advantageously, the liquid crystal structure may also be doped with inverse saturable absorbent dyes. These phthalocyanine, metal naphthalocyanine and C60 fullerene molecules exhibit an absorption which increases with the light energy density they receive in a wide spectral range (200 to 300 nm in the visible). The response time of this non-linear effect is of the order of the sub-nanosecond. The protection system using such dyes then provides protection against laser threats whose duration varies between the picosecond and the continuous domain.

3) Electrochromic materials
The structure comprises the coupling of a layer of material
electrochromic and a layer of electrolyte material.

Indeed, an electrochromic material can become absorbing by insertions of cations such as oxides WO3, V2O5 or Nb2O5 or else
TiO2, or anions such as oxide 1202 or protonated nickel oxide NlQ2,
H. In all cases, the ion exchange is ensured by the presence of a layer of electrolyte material in contact with the layer of electrochromic material. The electrolyte employed may be an organic complex formed of polyether type polymers and alkali metal salts.

4) Materials comprising anisotropic particles
These anisotropic particles of submicron size do not diffuse light. They exhibit a dielectric anisotropy As and optics
Aa which allows to orient them in an electric field and to obtain variations of absorption. These particles are in solution in a liquid or in a gel sufficiently fluid to allow the orientation of the particles.

The electrically controlled absorption cell therefore consists of two substrates (glass or flexible plastic) with conductive electrodes enclosing a fluid or gel containing anisotropic particles. This material may for example be that developed by Research Frontier Inc.

(RFI) in the United States.

 The protection system described above can advantageously be used in an optical vision device, seeking to image a scene at a photodetector, via a lens or not, and which is to be protected from an attack of type laser attack.

 It should be noted that in the case where the detector is the eye using an absorbent variable transmission rather than diffusing structure, it is possible to place the protection system directly in front of the eye, by choosing a photoconductor with a threshold of sufficiently low activation (typically less than mW / cm 2), a variable transmission structure with a sufficiently short response time (typically less than one second).

According to a first example of a protected vision optical device, the protection system SP is placed upstream of the focal plane of a lens
L1, focal plane at which the detector D is placed as illustrated in FIG. 4.

 When the protection system SP is activated by laser aggression, the photodetector results in a spreading of the laser beam due to the diffusion of the material of the liquid crystal structure. This spreading of the laser beam reduces the localized illumination on the photodetector to a value below the destruction threshold.

According to another variant of optical vision device, the focusing means may comprise an intermediate focal plane. The scene thus seen by the lens L1 is reimaged by the lens L2 in the plane of the photodetector. A lens of field Lc placed at the intermediate focal plane, makes it possible to collect in the plane of the photodetector,
The set of rays arriving in the angular field of the optical viewing device.

 It should be noted that in the case of eye protection, to which the device of the invention can be applied, the lens L2 then symbolizes an eyepiece returning the beam to infinity, the eye of the observer being located behind L2.

 Given the low activation thresholds of the protection systems proposed in the invention, these systems can be placed at different positions Zv along the optical axis z of the optical device (see FIG. 5).

Protection system placed in the vicinity of the intermediate focal plane (FIG. 5):
The protection system is in this case placed behind the field lens Lc: Zv = Zc + s with small E. In this configuration, the laser impact is of small area (focused beam) and only a field area (of the same order of magnitude as the irradiated surface SL) is blocked during the attack. This configuration has the advantage of maintaining a view of the low flow scene over almost the entire field for the duration of the attack. However, the extinction rate in the case of diffusion can be limited because of the large solid angle under which the protection system of the L2 lens is seen. This is typically the case for PDLCs.

Protection system placed near lenses L1 or L2 (FIGS. 6a or 6b):
In these configurations with zv = z1 + s (figure 6a) or zv = z2 + s (figure 6b) and E> 0, the surface SL of the protection system, irradiated by laser aggression, is large. During the aggression, the quasi-totality of the field of the instrument is affected by the presence of the protection system: in the blocking state, the low vision flux of the scene is thus attenuated during all the duration of the aggression. However, the extinction rate is generally higher than in the previous case because the scattered flux is collected on a lower solid angle.

 More generally, the positioning of the valve between the lens L1 and the detector is zl <zv <Zd, makes it possible to achieve, according to the geometry of the imaging system and the choice of the manufacturing technology of the optical system, a compromise between maintaining a low-flown vision of the scene during the aggression and a high extinction rate.

Exemplary embodiment of an optical vision device using a protection system
Realization of the protection system
1) Photoconductive structure: a thin layer of BSO
monocrystal 100 pm -lmm or AS2S3 of a thickness
about 10 pm;
2) Variable transmission structure: a layer of a thickness
between 50 and 100 μm, of liquid crystal material PDLC
and can be doped with dichroic dyes or
reverse saturable absorbent dyes and prepared by
conventional means of photopolymerization, with
liquid crystal with a diameter of between 0.1 and 1 μm.

 The set of structures is subjected to a voltage V of between 50 and 100 volts, typically 50 Hz for a PDLC layer with a thickness of 50 μm.

The performance of the protection system is as follows:
- a response time of less than 10 ms;
- a typical 11100 contrast with an optical system open at f / 5
in a configuration where the protection system is placed
in the vicinity of the intermediate focal plane of a
optical vision as shown in Figure 5. The contrast
represents the ratio between the luminous flux coming out of the system
in the presence of the activated protection system (1) and
luminous flux coming out of the optical system without
protection (100).

 The protection system described above is particularly well suited to protect optical vision devices, in the case of aggression at wavelengths between 450 and 550 nm.

The average power incident for the activation of the system of
-2 protection is typically between 1 and 10 mW.cm
In the case of an intermediate focal plane, at which a spot with a diameter of 100 μm is produced, it is possible, given the sensitivity threshold of the protection system, to be about 10 mW.cm, a limiting function for power incident laser of the order of 1 pJ on the entrance pupil of the lens of the device (lens L1).

Claims (16)

 1. protection system comprising a first and a second face, the first face being capable of receiving an external light radiation characterized in that it comprises a photoconductive structure adjacent to the first face and a second structure energized and comprising a material whose transmission varies according to the applied voltage, said second structure being adjacent to the second face.  2. Protection system according to claim 1, characterized in that the first and second faces respectively comprise a first and a second electrode.  3. Protection system according to claim 2, characterized in that the photoconductive structure is in contact with the second structure, a potential difference being applied between the first and the second electrode, the second structure being transparent in the absence of difference potential of applied to said structure, the second structure being diffusing and / or absorbent in the presence of a potential difference applied to said structure.  4. Protection system according to claim 2, characterized in that the photoconductive structure is separated from the second structure by a third electrode, the first and second electrodes being subjected to the same potential, the third electrode being subjected to another potential, the second structure being scattering and / or absorbing in the absence of potential difference applied to said structure, the second structure being transparent in the presence of a potential difference applied to said structure.  5. Protection system according to one of claims 1 to 4, characterized in that the second structure comprises a liquid crystal cell.  6. Protection system according to one of claims 1 to 4, characterized in that the second structure comprises a polymer matrix in which are dispersed liquid crystal molecules.  7. Protection system according to one of claims 1 to 6, characterized in that the second structure further comprises dichroic dyes.  8. Protection system according to one of claims ~ 1 to 6 characterized in that the second structure further comprises inverse saturable dyes.  9. Protection system according to one of claims 1 to 3 characterized in that the second structure comprises a layer of electrochromic material coupled to a layer of electrolyte material.  10. Protection system according to one of claims 1 to 4, characterized in that the second structure comprises submicron particles dispersed in a fluid medium, having a dielectric anisotropy and optical anisotropy.  11. Protection system according to one of claims 1 to 10, characterized in that the photoconductive structure comprises a material of typeBSO.  12. Optical vision device comprising a detector and at least one protection system, characterized in that the protection system comprises a first and a second face, the first face being capable of receiving an external light radiation, a first adjacent photoconductive structure at the first face and a second structure energized and comprising a material whose transmission varies as a function of the applied voltage, said second structure being adjacent to the second face, the detector being located on the side of said second face.  13. Optical vision device according to claim 12 characterized in that it comprises focusing means comprising at least a first lens (L1), the protection system being located between said first lens and the detector.  14. Optical vision device according to claim 13, characterized in that the protection system is located in the vicinity of the detector and the focal plane of the focusing means.  15. Optical vision device according to claim 13, characterized in that the focusing means comprise a second lens (L2) and a field lens (Lc), said field lens being placed in a focal plane intermediate between the first and second lenses. lenses, the protection system being located between the first and second lenses.  16. Optical vision device according to one of claims 12 to 15, characterized in that it comprises several protection systems, comprising photoconductive structures sensitive to different wavelength ranges.