Optical sensor for measuring salinity and visibility in sea water

Optical sensor for measuring salinity and visibility in seawater.

1. Field of the invention

The field of the invention is that of the measurement of physical characteristics of fluids (for example, liquids, gases, etc.).

More precisely, the invention relates in particular to the measurement of salinity as well as to the measurement of visibility in sea water in the context of oceanographic applications, for example, in the context of the equipment of submerged measurement probes. in coastal environment.

2. Prior art solutions

The determination of the salinity of water is carried out today mainly via a measurement of its electrical conductivity, knowing its temperature and pressure. However, conductivity sensors are unreliable because they are very sensitive to degradations linked to the marine environment (as explained by the document “Conductivity sensors in oceanography: state of the art and perspectives.” By M. Le Menn, published in RSTD Vol. 64, pp. 107-111, June 2004).

The use of other methods based on the measurement of the refractive index of seawater (as illustrated by the document "Remote refractive index difference meter for salinity sensor", by H. Minato, et al. Published in IEEE Trans on Instrumentation and Measurements, Vol. 38, N ° 2, pp. 608-612, 1989) where the use of fiber sensors is possible.

The measurement of the index makes it possible, knowing the temperature and the pressure, to go back to the value of the salinity either by using the relations of RC Millard and G. Seaver (RC Millard and G. Seaver, “An index of refraction algorithm for seawater over temperature, pressure, salinity, density and wavelength ”, DeepSea Research, vol. 37, n ° 12, pp 1909-1926, 1990) whose accuracy is 0.024 ppt in salinity, or by direct temperature calibration , pressure and salinity. This assumes that the refractive index of seawater can be measured with a resolution of at least ÎO⁶ in oceanographic ranges.

Given the mainly targeted application, which is that of measurements in a coastal environment, it is not necessary for the salinity measurement to have an accuracy of less than 1/100 psu.

Thus, the problem of controlling the drift of the measurement with respect to the temperature and pressure variations that the salinity measuring device must undergo when it is used in situ is not critical in the present case.

Conventionally, measurements of visibility or turbidity in a liquid, such as seawater, are used for estimations of the transparency of the water, to obtain indications on the transport of sediments in this water, and for many other studies requiring an estimate of the amount of particles in the water. Turbidity sensors make it possible to know the concentration of particles in the water and this generally thanks to optical backscattering measurements as indicated in German patent application No. DE3326739Al.

3. Objectives of the invention

The object of the invention is in particular to overcome these drawbacks of the prior art.

More precisely, an objective of the invention, in at least one of its embodiments, is to provide a technique making it possible to simultaneously measure the salinity of a liquid and the visibility in the liquid.

Another objective of the invention, in at least one of its embodiments, is to implement such a technique which has good measurement precision and in particular for the measurement of visibility.

The invention, in at least one of its embodiments, also aims to achieve such a technique which is particularly suitable for submerged measurements and which has great resistance to soiling or organic deposition of all kinds.

Another objective of the invention, in at least one of its embodiments, is to achieve such a technique which is simple, compact and low cost.

4. Disclosure of the invention

These objectives, as well as others which will appear later, are achieved with the aid of an optical sensor for the simultaneous measurement of the deflection and the attenuation of at least one light beam in a fluid.

According to the invention, such an optical sensor comprises: means for generating a main light beam, and first and second secondary light beams coming from the main light beam;

means for measuring deviation, making it possible to obtain a measurement of deviation by refraction, undergone by the first secondary beam due to the crossing of at least one layer of a determined material contiguous to a first layer of fluid;

attenuation measurement means, making it possible to obtain an attenuation measurement, undergone by the second secondary beam due to the crossing of at least one second fluid layer;

means for correcting said attenuation measurement taking into account at least one item of information obtained by virtue of said deviation measurement.

The general principle of the invention consists in performing both a refractive deviation measurement and an attenuation measurement in a fluid, from the same main light beam separated into a first secondary beam and a second secondary beam, in particular in order to simultaneously obtain salinity and visibility in the fluid.

Thus, the sensor according to the invention makes it possible to simultaneously measure the salinity (thanks to the deviation measurement) and the visibility (thanks to the attenuation measurement) in the fluid.

Thus, due to the correction of the attenuation measurement by means of at least one item of information obtained thanks to the deviation measurement, there is a real synergy between the measurement of the deviation and the measurement of the attenuation in the sensor according to the invention.

These two measures are implemented in a cooperative manner to guarantee simultaneity of results.

Advantageously, the correction means comprise:

means for receiving the deviation measurement from the deviation measurement means;

means for obtaining the index of the fluid from the deviation measurement; and means for transmitting, to the attenuation measuring means, the index of the fluid obtained.

Thus, because the two measurements both depend on at least one physical characteristic of the fluid which is the refractive index of the fluid, in the context of the invention, a correction of the attenuation measurement can be carried out by virtue of at least one item of information obtained from the deviation measurement.

This gives more precise measurements due to the correction.

According to a first embodiment of the invention, the generation means comprise a light source and means for temporal separation of the main light beam into said first and second secondary light beams.

Thus, for example, the first and second secondary beams are obtained by temporal sampling of the main beam by means for example of a shutter operating in stroboscopic mode.

According to a second embodiment of the invention, the generation means comprise a light source and means for spectral separation of the main light beam into said first and second secondary light beams.

Thus, for example, the main beam has a spectrum comprising at least two wavelengths (the first corresponding to the first secondary beam and the second corresponding to the second secondary beam).

Thus, according to these first and second embodiments, the secondary beams correspond to temporal (first embodiment) and spectral (second embodiment) components of the main beam. These embodiments make it possible to implement only one detection means (position and intensity) of the first and second secondary beams.

According to a particular embodiment of the invention, the sensor comprises a prism delivering an intermediate secondary beam coming from the main beam and third reflecting means adapted to reflect the first secondary beam, coming from the intermediate beam, in the direction of the first sensor position and to transmit the second secondary beam, coming from the intermediate beam, to the second position sensor.

Thus, in this case, the third reflecting means must be partly transparent, for example they are a semi-transparent plate.

According to a third embodiment of the invention, the generation means comprise a light source and means for spatial separation of the main light beam into said first and second secondary light beams.

Thus, according to this third embodiment, it is necessary to implement a separate detection means (position, intensity) for each of the first and second secondary beams.

Preferably, said separation means comprise at least one optical prism and said determined layer of material is included in said at least one optical prism.

Thus, the function of the prism is simultaneously to perform the separation of the first and second secondary beams from the main beam and, when associated with the fluid, to give rise to the refraction at the origin of the deviation of the first secondary beam.

Moreover, the intensity of the second secondary beam depending in particular on the index of the fluid, the implementation of the prism means that the attenuation measure (making it possible to obtain visibility) can be corrected by obtaining the index within the framework of the measurement of the deviation (making it possible to obtain the salinity) of the first secondary beam.

Thus, a correction of the attenuation measurement can be made as a function of the measurement of the index within the framework of the measurement of the deviation and thus a more precise measurement of the attenuation can be obtained.

Thus, the sensor according to the invention makes it possible in particular to obtain good precision for measuring visibility.

In addition, because the prism shares several functions, the sensor according to the invention is simple to produce, compact and can be manufactured at low cost.

Advantageously, the optical prism is made from at least one of the materials belonging to the group comprising:

N-K5;

N-F2;

N-BAF51;

N-LF5;

N-SF5;

N-SK10;

N-BK7;

N-LLF1.

Advantageously, the deviation measuring means comprise at least a first position sensor.

Preferably, the attenuation measurement means comprise at least one second position sensor.

Thus, the use of a position sensor (which acts as a means of detecting the position and intensity of the first secondary beam) in the deviation measuring means makes it possible to measure both:

the position of the first secondary beam which is necessary for the measurement of the deflection of the first secondary beam and the intensity of the first secondary beam which is necessary for determining visibility in the fluid.

Furthermore, the use of a position sensor instead of the use, for example, of a photodiode makes it possible to improve the accuracy of the deviation or attenuation measurement, since the position sensor is less sensitive to the diffusion of secondary beams in the fluid than a photodiode.

According to a first embodiment, no reflecting means is implemented in the optical sensor. An advantage of such an optical sensor is that it has great resistance to soiling or to organic deposition of all kinds resulting from the fluid included in the sensor (in fact, the reflecting means are very sensitive to soiling). Thus, such an optical sensor is particularly suitable for submerged measurements.

According to a second embodiment of the invention, the optical sensor comprises first reflecting means suitable for reflecting the first secondary beam in the direction of the first position sensor.

Thus, in the context of this second mode of implementation, the sensor can be more compact than the sensor of the first mode of implementation.

According to a third embodiment of the invention, the optical sensor comprises second reflecting means adapted to reflect the second secondary beam in the direction of the second position sensor.

Thus, in the context of this third embodiment, the electronics in the sensor can be common to the light source as well as to the first and second position sensors.

Preferably, the fluid is sea water and in that it is able to operate when it is submerged.

Advantageously, said light source is a laser source.

Advantageously, the laser source cooperates with focusing means making it possible to focus the laser beam on at least one of said position sensors.

Preferably, the size of the spot of the laser beam focused on the position sensor (s) is between 200 μm and 1 mm.

Preferably, the focusing means are arranged near the laser source.

Advantageously, the laser source emits in the visible.

Preferably, the optical sensor according to an embodiment of the invention comprises means for circulating the fluid in the optical sensor.

5. List of Figures

Other characteristics and advantages of the invention will emerge more clearly on reading the following description of preferred embodiments, given by way of simple illustrative and non-limiting examples, and the appended drawings, among which:

FIG. 1 shows a diagram of an optical sensor according to a first embodiment of the invention;

FIG. 2 shows a diagram of the second electronic block, third electronic block and of the correction means according to the second embodiment of the invention;

FIG. 3 shows a diagram of an optical sensor according to a second embodiment of the invention;

FIG. 4 shows a diagram of an optical sensor according to a third embodiment of the invention;

FIG. 5 shows a diagram of an optical sensor according to a fourth embodiment of the invention.

6. Description of an embodiment of the invention

Various embodiments of an optical sensor according to the invention for measuring at least one physical characteristic of a fluid are described below.

For example, the fluid is seawater and the optical sensor is able to operate when submerged.

This optical sensor comprises in particular:

a light source emitting a main light beam;

means for measuring the deviation by refraction undergone by a first secondary beam, issuing from the main beam, due to the passage of a layer of a determined material contiguous to a first layer of sea water;

means for measuring the attenuation undergone by a second secondary beam, coming from the main beam, due to the crossing of a second layer of sea water; and means for correcting the attenuation measurement taking into account at least one item of information obtained by virtue of the deviation measurement.

For example :

the means for measuring the deviation include a first position sensor of the PSD (for "Position Sensor Device") type which acts as a means of detecting (position and intensity) of the first secondary beam, the first sensor being associated with a second electronic block described below; and the means for measuring the attenuation comprise a second position sensor of the PSD type which acts as a means of detecting (position and intensity) of the second secondary beam, the second sensor being associated with a third electronic unit described below. .

Thus, the use of a position sensor in the deviation measuring means makes it possible to measure both:

the position of the first secondary beam which is necessary for the measurement of the deflection of the first secondary beam and the intensity of the first secondary beam which is necessary for determining visibility in the fluid.

Furthermore, the use of a position sensor instead of the use, for example, of a photodiode makes it possible to improve the accuracy of the deviation or attenuation measurement, since the position sensor is less sensitive to the diffusion of secondary beams in the fluid than a photodiode.

Thus, this optical sensor according to the invention makes it possible to simultaneously measure the salinity (thanks to the deviation measurement) and the visibility (thanks to the attenuation measurement) in sea water.

In addition, the two measurements both depending on the refractive index of seawater, in the context of the invention, a correction of the attenuation measurement by means of at least one item of information obtained from the measurement deviation can be implemented.

This gives more precise measurements due to the correction.

For example, the light source is a laser diode emitting in the visible (the laser diode can also emit in the near infrared, in the near ultra-violet or even at any other wavelength).

According to a first embodiment of the invention, the first and second secondary beams correspond to two temporal components of the main beam.

Thus, for example, the first and second secondary beams are obtained by temporal sampling of the main beam by means for example of a shutter operating in stroboscopic mode.

According to a second embodiment of the invention, the first and second secondary beams correspond to spectral components of the main beam.

Thus, for example, the main beam has a spectrum comprising at least two wavelengths (the first corresponding to the first secondary beam and the second corresponding to the second secondary beam).

Thus, according to these first and second embodiments, the secondary beams correspond to temporal (first embodiment) and spectral (second embodiment) components of the main beam. These embodiments make it possible to implement only one detection means (position and intensity) of the first and second secondary beams.

According to a third embodiment of the invention, according to which the optical sensor comprises means for separating the main light beam into a first secondary beam and a second secondary beam.

Thus, according to this first embodiment, it is necessary to implement a separate detection means (position, intensity) for each of the first and second secondary beams.

In the following, we will place ourselves within the framework of this third embodiment of the invention.

In relation to FIG. 1, an optical sensor 1000 is illustrated according to a first embodiment of the invention.

Optical sensor 1000 includes first 1100, second 1200, and third 1300 compartments.

In the context of this first embodiment, the second compartment 1200 is filled with sea water and the optical sensor comprises means for circulating sea water in this second compartment. Sea water enters the second compartment 1200 via a first opening 1207 acting as an inlet and leaves the second compartment 1200 via a second opening 1208 playing the role of exit. Of course, the role of the openings 1207, 1208 can be reversed.

In the context of this first mode of implementation, the first 1100 and third 1300 compartments are watertight.

The first compartment 1100 comprises a laser diode 1101 as well as a first electronic block (not shown) comprising an electronic circuit and a power supply making it possible to provide the power supply necessary for the operation of the diode.

The second compartment 1200 comprises an optical prism 1201 attached in front of the output of the laser diode 1101 from which a main light beam 1202. This main light beam 1202 is separated into a first secondary beam 1203 and a second secondary beam 1204 by the prism. optic 1201.

The prism is made of a material transparent to the wavelengths of the laser diode.

The second compartment 1200 also comprises a first position sensor 1205 of the PSD type which makes it possible to measure the deviation by refraction undergone by the first secondary beam 1203 due to the crossing of the prism (constituting a layer of a determined material) and of a first layer of seawater (between the prism 1201 and the first position sensor 1205) contained in the second compartment 1200.

The association of the laser diode 1101, the optical prism 1201, the first layer of seawater contained in the second compartment 1200 of the optical sensor 1000 (between the prism and the first position sensor 1205) as well as the first sensor of position 1205 constitutes a first optical refractometer which makes it possible to determine the refractive index of the sea water in the optical sensor 1000 and therefore to deduce therefrom the salinity of this sea water.

Indeed, the first secondary beam 1203 is deflected by refraction at the optical prism 1201 / seawater interface of the optical sensor 1000. The position of the first beam 1203 is then detected and measured by the first position sensor 1205 such as 'indicated in French patent application No. FR0507996 filed on July 26, 2005.

A second electronic unit 52 (illustrated below in relation to FIG. 2), for example included in a first sealed sub-compartment of the second compartment 1200, makes it possible to supply the first position sensor and to process the signals from the first sensor of position 1205 in order to deduce the refractive index of sea water as well as the salinity of sea water.

The first position sensor 1205 has a useful length of approximately mm.

Preferably, a collimation optic (not shown in this FIG. 1), for example a converging lens, makes it possible to focus the first secondary beam 1203 of the laser diode 1101 on the first position sensor 1205 so as to obtain a first focused secondary beam of about 200μηι spot diameter at the first position sensor 1205. For example, the collimating optic is disposed near the output of the laser diode 1101 between the laser diode and the optical prism 1201.

Of course, one can choose other spot diameters (preferably a diameter between 200 μπι and 1 mm) for the first secondary beam focused at the first position sensor 1205 without departing from the scope of the present invention.

For example, the first secondary beam 1203 forms an angle of incidence at the prism / seawater interface which is 62 ° and which is less than the total reflection angle which is substantially 88 ° (for an index of prism of 1.50854 and for a seawater index of 1.333).

The second compartment 1200 also includes a second position sensor 1206 of the PSD type which makes it possible to measure the attenuation undergone by the second secondary beam 1204 due to the crossing of a second layer of sea water (included between the prism 1201 and the second position sensor 1206) contained in the second compartment 1200.

Furthermore, the first position sensor 1205 makes it possible to measure the attenuation undergone by the first secondary beam 1203 due to the crossing of the first layer of sea water (between the prism 1201 and the first position sensor 1205) contained in the second compartment 1200.

As explained below, the measurement of the visibility in sea water contained in the second compartment 1200 of the optical sensor 1000 is obtained by comparison:

of the light intensity of the first secondary beam 1203 detected on the first position sensor 1205 (after having undergone an attenuation due to the crossing of the first layer of sea water) and of the light intensity of the second secondary beam 1204 detected on the second position sensor 1206 (after having undergone an attenuation due to the crossing of the second layer of sea water).

Preferably, the first layer of water (between the prism 1201 and the first position sensor 1205) and the second layer of seawater (between the prism 1201 and the second position sensor 1206) have different thicknesses.

Thus, the optical paths traveled by the first 1203 and second 1204 secondary beams before reaching each of the first 1205 and second 1206 position sensors are different. This path difference determines the accuracy of the measurement of visibility in seawater (the index being calculated elsewhere by the refractometer).

Consequently, the association of the laser diode 1101, of the optical prism 1201, of the first layer of water contained in the second compartment 1200, of the first position sensor 1205, of the second layer of sea water contained in the second compartment 1200 as well as the second position sensor 1206 make it possible to determine the visibility in sea water in the optical sensor 1000.

Indeed, the intensity of the first secondary beam 1203 is detected and measured by the first position sensor 1205 and the intensity of the second secondary beam 1204 is detected and measured by the second position sensor 1206.

A third electronic unit 53 (illustrated below with reference to FIG. 5), for example included in a second sealed sub-compartment of the second compartment 1200 or in the third compartment 1300, makes it possible to supply the second position sensor 1206.

Furthermore, the second 52 and the third 53 electronic unit make it possible to process the value of the intensity of the first secondary beam 1203 measured by the first position sensor 1205 the value of the intensity of the second secondary beam 1204 measured by the second sensor position 1206 in order to deduce the visibility in seawater.

To do this, we first measure the attenuation (due in particular to the absorption and the propagative and non-counter-propagative diffusion as in the case of the reflexivity measurement implemented in the context of the determination of the turbidity) of seawater (turbid environment). It can be noted that the measurement of seawater attenuation provides a satisfactory estimate of seawater turbidity. The measurement of the attenuation (or contrast) hereinafter referenced "C" is given in this case by the following formula:

C = ln (Ιρ »ι12Οό / Ιρκ112Ο5) / 012Ο5 ·· 1ΐ2Ο6) ^0Uln is the natural logarithm;

I_psdi206 ^is the intensity of the second secondary beam 1204 measured by the second position sensor 1206;

I_Psdi205 is the intensity of the first secondary beam 1203 measured by the first position sensor 1205;

1₁₂₀₅ is the optical path associated with the first water layer (between the prism 1201 and the first position sensor 1205)

1₁₂₀₆ is the optical path associated with the second layer of seawater (between the prism 1201 and the second position sensor 1206).

Preferably, the values ​​of the measured intensities I_psdl205 and I_psdl206 are corrected to take into account the reflection at the seawater / position sensor interface 1205, 1206.

Thus, we can obtain the visibility V which is given by the following formula:

V = - ln (norm C_L) / C where C_L is the human limit contrast (or “inherent contrast of the black target”) which in this case is equal to: -1.

According to a variant of the present embodiment, the second and third electronic blocks are produced in the form of a single overall electronic block, for example included in a sealed sub-compartment of the second compartment 1200.

The second position sensor 1206 has a useful length of approximately mm.

Preferably, the aforementioned collimation optics (not shown in this figure 1) makes it possible to focus the second secondary beam 1204 of the laser diode 1101 on the second position sensor 1206 so as to obtain a second focused secondary beam of about 200μηι of spot diameter at the second position sensor 1206.

Of course, one can choose other spot diameters (preferably a diameter between 200 μπι and 1 mm) for the second secondary beam focused at the second position sensor 1206 without departing from the scope of the present invention.

Consequently, the optical prism 1201 simultaneously has the function of effecting the separation of the first and second secondary beams from the main beam and, when associated with the fluid, of giving rise to the refraction causing the deflection of the first secondary beam. .

Furthermore, the proportion in intensity of the main beam 1202 which is transmitted in the second secondary beam 1204 is calculated from the reflection coefficient at the level of the prism 1201 / seawater interface which depends in particular on the index of the seawater contained in the second compartment 1200 of the optical sensor 1000.

Thus, the proportion in intensity of the main beam 1202 which is transmitted in the second secondary beam 1204 depends in particular on the index of the sea water contained in the second compartment 1200.

As a result, the implementation of the prism means that the measurement of the attenuation (making it possible to obtain visibility) undergone by the second secondary beam 1204 can be corrected by knowing the exact proportion of intensity coming from the main beam 1202 in the second secondary beam 1204, proportion which depends on the value of the refractive index of the sea water obtained within the framework of the measurement of the deviation (making it possible to obtain the salinity) of the first secondary beam.

In relation to FIG. 2, a diagram of the second electronic block 52, third electronic block 53 and of the aforementioned correction means 51 is presented.

The correction means 51 which are for example a computer, comprise:

means for receiving the deviation measurement coming from the deviation measurement means (first position sensor 1205 associated with the second optical unit 52);

means for obtaining the index of the fluid from the deviation measurement; and transmission means, to the attenuation measuring means (second position sensor 1206 associated with the second optical unit 53), of the index of the fluid obtained.

Thus, it is possible to correct the attenuation measurement undergone by the second secondary beam 1204 as a function of the measurement of the index within the framework of the measurement of the deviation undergone by the first secondary beam 1203 and thus obtain a measurement attenuation and therefore more precise visibility.

In addition, because the prism shares several functions, the sensor according to the invention is simple to produce, compact and can be manufactured at low cost.

Consequently, no reflecting means is implemented in the optical sensor 1000 according to the aforementioned first mode of implementation. An advantage of such an optical sensor is that it has great resistance to soiling or to organic deposition of all kinds resulting from the fluid included in the sensor (in fact, the reflecting means are very sensitive to soiling). Thus, such an optical sensor is particularly suitable for submerged measurements.

In relation to FIG. 3, an optical sensor 2000 is illustrated according to a second embodiment of the invention.

The optical sensor 2000 according to the second mode of implementation, comprising a first compartment 2100 and a second compartment 2200, is identical to the optical sensor 1000 according to the first mode of implementation except:

that it does not include a third compartment; and that it further comprises first reflecting means 2209 (for example a first mirror) adapted to reflect the first secondary beam 1203 towards the first position sensor 1205.

Furthermore, in the context of this second mode of implementation, the first layer of seawater is between the prism 1201 and the first reflecting means 2209.

The association of the laser diode 1101, of the optical prism 1201, of the aforementioned first layer of seawater, of the first reflecting means as well as of the first position sensor 1205 constitutes an optical refractometer which makes it possible to determine the refractive index sea ​​water in the optical sensor 2000 and therefore deduce the salinity of this sea water.

In addition, in the context of this second mode of implementation, the measurement of the visibility of the sea water contained in the second compartment 2200 of the optical sensor 2000 is obtained by comparison:

of the light intensity of the first secondary beam 1203 detected on the first position sensor 1205 (after having undergone an attenuation due to the crossing of the first layer of sea water as well as a layer of sea water between the first reflecting means 2209 and the first position sensor) and the light intensity of the second secondary beam 1204 detected on the second position sensor 1206 (after having undergone an attenuation due to the crossing of the second layer of seawater).

In the context of this second mode of implementation, the sensor can be more compact than the sensor of the first mode of implementation, in particular because of the arrangement of the position sensors. Furthermore, in the context of this second mode of implementation, the first and second electronic blocks (possibly combined) can both be included in the first compartment 2100.

In relation to FIG. 4, an optical sensor 3000 is illustrated according to a third embodiment of the invention.

The optical sensor 3000 according to the third embodiment, comprising a first compartment 3100 and a second compartment 3200, is identical to the optical sensor 2000 according to the second embodiment except that it further comprises second reflecting means 3210 (for example a second mirror) adapted to reflect the second secondary beam 1204 in the direction of the first position sensor 1205.

Furthermore, in the context of this third mode of implementation, the second layer of sea water consists of a layer of sea water between the prism 1201 and the second reflecting means 3210 and of a layer of sea water between the second reflecting means 3210 and the second position sensor 1206.

Thus, in the context of this third mode of implementation, the sensor can be more compact than the sensor of the second mode of implementation because, the first, second and third electronic blocks (possibly combined) can both be included. in the first compartment 3100.

In relation to FIG. 5, an optical sensor 4000 is illustrated according to a fourth embodiment of the invention.

The optical sensor 4000 according to the fourth mode of implementation, comprising a first compartment 4100 and a second compartment 4200, is identical to the optical sensor 1000 according to the first mode of implementation except:

that it does not include a third compartment; and that it further comprises third reflecting means 4211 (for example a semi-transparent plate).

As part of this fourth mode of implementation, the prism 1201 delivers an intermediate secondary beam 4203 coming from the main beam 1202 and the third reflecting means 4211 are adapted to reflect a part (hereinafter called the first secondary beam) of the intermediate beam 4203 in the direction of the first position sensor 1205 and to transmit a part (hereinafter referred to as the second secondary beam) of the intermediate beam 4203 to the second position sensor 1206.

Thus, within the framework of this fourth mode of implementation, the first position sensor 1205 makes it possible to measure the deviation by refraction undergone by the first secondary beam due to the crossing of the prism (constituting a layer of a determined material) and a first layer of seawater (between the prism 1201 and the third reflecting means 4211) contained in the second compartment 1200.

The association of the laser diode 1101, of the optical prism 1201, of the aforementioned first layer of seawater, of the third reflecting means as well as of the first position sensor 1205 constitutes an optical refractometer which makes it possible to determine the refractive index seawater in the optical sensor 4000 and therefore deduce the salinity of this seawater.

In addition, in the context of this fourth embodiment, the measurement of the visibility of the sea water contained in the second compartment 4200 of the optical sensor 4000 is obtained by comparison (as indicated above in relation with figure 1):

the light intensity of the first secondary beam (coming from the intermediate beam 1203) detected on the first position sensor 1205 (after having undergone: attenuations due to the crossing of the first layer of sea water and due to the crossing of a layer of sea water between the third reflecting means 4211 and the first position sensor) and the light intensity of the second secondary beam (from the intermediate beam 1203) detected on the second position sensor 1206 (after having undergone an attenuation due to the crossing of the first layer of sea water.

In the context of this fourth embodiment, the first and second electronic blocks (possibly combined) can both be included in the first compartment 4100.

The third reflecting means 4211 must be partly transparent, for example they are a semi-transparent plate.

Furthermore, the proportion in intensity of the main beam 1202 which is transmitted in the intermediate beam is calculated from the reflection coefficient at the prism 1201 / seawater interface which depends in particular on the water index of sea ​​contained in the second compartment 4200 of the optical sensor 4000.

The proportion in intensity of the intermediate beam 4203 which is transmitted in the first and second secondary beams depends on the reflection coefficient of the semi-transparent plate 4211.

Thus, the proportion in intensity of the main beam 1202 which is transmitted in the second secondary beam depends in particular on the index of the sea water contained in the second compartment 1200.

As a result, the implementation of the prism means that the measurement of the attenuation (allowing visibility to be obtained) experienced by the second secondary beam can be corrected by knowing the exact proportion of intensity coming from the main beam 1202 in the second secondary beam, a proportion which depends on the value of the refractive index of seawater obtained within the framework of the measurement of the deviation (making it possible to obtain the salinity) of the first secondary beam.

Thus, it is possible to correct the attenuation measurement undergone by the second secondary beam as a function of the measurement of the index within the framework of the measurement of the deviation undergone by the first secondary beam and thus obtain a measurement of l attenuation and therefore more precise visibility.

For example, in optical sensors 1000, 2000, 3000 and 4000, the means for circulating sea water in the second compartment include:

a first duct (not shown), for example of elliptical section, which is connected to the first opening 1207 (inlet) and which allows sea water to enter the second compartment 1200;

a second duct (not shown), for example of elliptical section, which is connected to the second opening 1208 (outlet) and which allows sea water to exit from the second compartment 1200.

Each of the optical sensors 1000, 2000, 3000 and 4000 is therefore not only suitable for taking in situ measurements, and measurements at different depths or geographical situations (in particular in an ocean, a sea, a lake, etc.), but it also makes it possible to limit the effects of "fouling" of marine origin thanks to its particular geometry. In fact, for example, a duct with a section of approximately 1cm is chosen, which makes it possible to avoid the effects of turbulence and which makes it possible, for example, to fix TBT (Tri-Buthyltin) pellets in the duct which have the effect of limit the effects of "fouling" of marine origin.

Indeed, the response time depends on the length / diameter ratio of the heat exchange surface between the seawater and the pipe as well as on the liquid circulation speed. The use of a pipe makes it possible to set the value of these two variables at will, the value of the speed of circulation of the sea water also being able to be made constant by the use of a pump.

The choice of a duct section of 1 cm corresponds to a compromise between a section sufficient to allow good water circulation (between seawater and the optical sensor) but not too large to avoid turbulence regimes. .

For example, the optical prism is made from at least one of the materials belonging to the group comprising:

N-K5;

N-F2;

N-BAF51;

N-LF5;

N-SF5;

N-SK10;

N-BK7;

N-LLF1.

Of course, the optical prism can be in one piece or be made up of several sub-blocks.

The position sensors are, for example, PSDs sold by the company HAMAMATSU under the reference S4584-06. They are sensitive in the red for wavelengths around 635 nm, they are also sensitive for wavelengths greater than 800nm. They have a maximum resolution of 0.1 μm (which is negligible when compared to the diameter of the laser spot), a useful length of 2.5 mm and cooperate with an electronic circuit referenced C3683-01.

These position sensors have good resolution, of the order of 0.3 μm, with a very large positioning tolerance. These characteristics of the position sensor make it possible to envisage the use of an effective spot size of between 200 μm and 1 mm. It is shown that the quality of the beam has little effect on the result of the measurement.

On the other hand, for example, in optical sensors 1000, 2000, 3000 and 4000, the laser diode cooperates with focusing means (for example a converging lens) making it possible to focus the main light beam (coming from the laser diode) on at least position sensors.

Preferably, the size of the spot of the focused laser beam is between 200 μm and 1 mm. For example, the focusing means are arranged near the laser source between the laser source and the prism.

The laser diodes are for example collimated laser diodes sold by the company PHOTONIC under the reference 301-P. They emit a laser beam with a wavelength of 635 nm, with a power approximately equal to 0.9 mW, with a beam size of approximately 1.8 mm x 1.8 mm, with a maximum divergence in air of 6 mrd and collimated beam diameter of 8 mm.

The embodiments as well as the aforementioned embodiments can be combined.

Of course, the invention is not limited to the embodiments mentioned above.

In particular, a person skilled in the art can make any variation in the choice of materials constituting the prism, the duct, or any other part of the optical sensors.

The invention applies of course, also in the context of other types of duct section, moreover, it is possible to implement in the optical sensors according to the present invention ducts having variable sections depending on the position on the conduit.