WO2008040875A1 - Device and method for determining a light signal additive to sunlight - Google Patents

Abstract

The invention relates to a method and a device for determining a light signal additive to sunlight. The device according to the invention comprises: means (2) for collecting light coming from an object, the collected light comprising solar light (5) reflected by the object (4) and an additive light signal (6); means (10, 15, 17) for measuring, for at least first and second atmospheric absorption lines, a signal that depends on the intensity of the collected light at a wavelength of the line; means (11) for determining, from the measured signals, a signal that depends on an intensity of the additive signal at a wavelength of the first line. The invention also relates to a method employed in a device according to the invention. The device or the method according to the invention does not require measurement on reference sunlight without the additive signal close to the first line. Application more particularly to the measurement of a fluorescence signal emitted by vegetation.

Description

 "Apparatus and method for determining an additive light signal to sunlight"

TECHNICAL FIELD The present invention relates to a method for determining an additive light signal to sunlight. It also relates to a device implementing this method.

Such a method makes it possible to determine, from sunlight reflected by any surface, an additive signal to sunlight for certain wavelengths. This method can be used in very varied fields of application, such as, for example, the measurement of the fluorescence of chlorophyll on a sheet under natural operating conditions, or else to isolate the signal of a light-emitting diode in daylight. State of the art

There exists, in a solar light intensity spectrum as a function of the wavelength of sunlight, dark lines linked to the absorption of atmospheric compounds. Such a line has a width that defines a range of wavelengths of the line. Some of these dark lines have been identified by Fraunhofer (eg Fraunhofer bands A and F). In the remainder of this document, we will call the dark lines of the "atmospheric lines".

It is known from the article by Moya et al. entitled M new instrument for passive remote sensing 1. Measurements of sunlight-induced chlorophyll fluorescence ", published in the 2004 journal" Remote sensing of Environment "(volume 91, pages 186-197), a method for determining the fluorescence of chlorophyll of a plant illuminated by sunlight.The method includes a measurement of an intensity of the atmospheric oxygen line centered on 760 nanometers on solar light serving as a white reference and devoid of fluorescence signal, and on solar light reflected by the plant.

The object of the present invention is to propose a method and a device for determining an additive light signal to sunlight reflected by an object, and not requiring: reference measurement of the solar light intensity spectrum without additive signal, or measurement of the reflectance spectrum of the object.

Presentation of the invention

This object is achieved with a method for determining an additive light signal to sunlight having passed through an atmosphere, comprising:

a collection of light coming from an object, the light collected comprising sunlight reflected by the object, and an additive light signal, a measurement, for at least a first and a second line of atmospheric absorption, a signal that depends on an intensity of the light collected at a wavelength of the line,

a determination, based on the measured signals, of a signal which depends on an intensity of the additive signal at a wavelength of the first line.

Thus, a method according to the invention makes it possible to determine a signal that is additive to sunlight in the vicinity of an atmospheric line, without having to collect and to make measurements on sunlight devoid of an additive signal in the vicinity of said line. atmospheric. A measured signal which depends on an intensity at a wavelength of the light collected can be expressed in Watt, Watt per square centimeter, but also in Volt for example at the output of a photodiode.

One of the measured signals may, for example, depend proportionally on the intensity at a wavelength of the light collected. Similarly, the determined signal may be proportionally dependent on the intensity of the additive signal at a given wavelength.

Preferably, the dependence between one of the measured signals and the intensity on which it depends is substantially the same for all the measured signals. The dependence can for example follow the same quadratic or proportional law for all the measured signals.

Preferably, the intensity of the additive signal is not negligible compared to the intensity of the reflected sunlight for wavelengths in the vicinity of the first line, and the intensity of the signal additive is substantially negligible compared to the intensity of the reflected solar light for wavelengths located in the vicinity of the second line.

The measurements may comprise for each line i * - a measurement of a signal which depends on an intensity of the light collected at a substantially central wavelength of the line, - a measurement of a signal which depends on a intensity of light collected at a wavelength on one of the edges of the line.

The determination may comprise an implementation of an algorithm for determining the additive signal, said algorithm comprising as input variables the measured signals.

The algorithm can implement a formula in which a depth of the first ray on a sunlight intensity spectrum is expressed as a depth function of other lines on an intensity spectrum of the collected light.

The algorithm can use parameters that depend on a concentration in the atmosphere of components at the origin of one of the atmospheric lines, and / or the molar extinction coefficient of one of these components to a length of wave close to one of these lines, and / or the transmittance of the atmosphere to a wavelength close to one of these lines.

The device according to the invention may further comprise a calibration or a parameter adjustment of the algorithm.

The algorithm can be implemented by substantially implementing the formula:

S _a (λ ₀ , t) being the determined signal which depends on the intensity of the additive signal at a wavelength I ₀ and at time t; the first and second lines being respectively centered on the wavelength λ ₀ and X ₁ ; X ₀ and / L ₁ ⁺ being wavelengths located on one edge respectively of the first and second line; At ₁ ^" and X ₀ being wavelengths located on the other edge - AT -

S[X₁ J), S[Xf , t), et S(KJ) étant des signaux mesurés qui dépendent de l'intensité de la lumière collectée au temps t et respectivement à la longueur d'onde X₀ , X⁺ , X₀ , X_x , Xf et K ; S^* (X_oj) étant défini par S' (X₀J) = S(X₀ ⁺J) ou S' (X₀J) = S(X₀ J) ourespectively the first and second line; S [X ₀ , t), S (X ₀ ⁺ JJ _r

S [X ₁ J), S [Xf, t), and S (KJ) being measured signals which depend on the intensity of the light collected at time t and respectively at the wavelength X ₀ , X ⁺ , X ₀ , X _x , Xf and K; S ^* (X _o j) being defined by S '(X ₀ J) = S (X ₀ ⁺ J) or S' (X ₀ J) = S (X ₀ J) or

S ^* β ₀ J) = S (X ₀ ⁺ , t) + ^ h. (S (XlJ) - S (X ₀ , t))

A ₀ - A ₀

Is (X ₀ J) being defined by Is (X ₀ J) = A Is (X _x Jf where A and B are parameters

(Xl-X ^) S (X _x J) variables, and Is (X ₁ J) is defined by Is (X _x J) = ^~ -

(K - X _X ) S (KJ) - (K - X _X ) S (KJ)

or _Is

The atmospheric lines can be due to the absorption of identical atmospheric compounds.

The first line may consist of the oxygen line centered on 760 nanometers. The second line may consist of the oxygen line centered on

1270 nanometers.

The additive signal may consist of a light signal emitted by the object. In particular, it may consist of a fluorescence signal emitted by the object in response to reflected sunlight.

According to yet another aspect of the invention, there is provided a device for determining an additive light signal to sunlight having passed through an atmosphere, comprising: means for collecting light from an object, the light collected comprising sunlight reflected by the object, and an additive light signal,

means for measuring, for at least a first and second atmospheric absorption line, a signal which depends on an intensity of the light collected at a wavelength of the line,

means for determining, from the measured signals, a signal which depends on an intensity of the additive signal at a wavelength of the first line.

A device according to the invention makes it possible to determine an additive signal to sunlight in the vicinity of an atmospheric line, without having to collect and to make measurements on sunlight devoid of additive signal in the vicinity of said atmospheric line. Preferably, the device according to the invention is arranged so that the dependence between one of the measured signals and the intensity on which it depends is substantially the same for all the measured signals.

Preferably, the determination means are provided for a significant additive signal intensity relative to the intensity of the reflected sunlight for wavelengths in the vicinity of the first line, and are provided for an intensity of additive signal substantially negligible compared to the intensity of sunlight reflected for wavelengths located in the vicinity of the second line. The collection means typically comprise an achromatic lens, or a Cassegrain-type assembly comprising two parabolic reflectors. The measuring means can comprise for each line:

means for measuring a signal which depends on an intensity of the light collected at a substantially central wavelength of the line,

means for measuring a signal which depends on an intensity of the light collected at a wavelength located on one of the edges of the line.

The measuring means may comprise a photodiode, _^ a photodiode array or a CCD sensor. The photodiodes are preferably indium gallium arsenide photodiodes (InGaAs) or silicon. This type ^"photodiode has the advantage of covering a large wavelength range. For photodiodes made of silicon, this interval 'E, typically tends 300 to 1100 nanometers. For the InGaAs photodiodes, this range extends typically from 900 to 1700 nanometers, and even up to 2500 nanometers if they are cooled. Thus, the same type of photodiode can measure signals that depend on intensities of the light collected at atmospheric line wavelengths spaced several hundred nanometers apart. In an embodiment in which the measuring means generates for each measurement an electrical signal which depends on an intensity of the light collected, the device according to the invention may furthermore comprise means for shaping each electrical signal, in particular means for amplifying and filtering each electrical signal, and means for digitizing each of the electrical signals.

The device according to the invention may furthermore comprise means for separating wavelengths from the collected light. The device according to the invention may comprise at least one filter allowing substantially only one wavelength of the light collected, and / or at least one diffraction grating to separate the wavelengths in the vicinity of one of the atmospheric lines. The device according to the invention may further comprise an optical fiber strand, the collection means directing the collected light towards a first end of the strand, the other end of the strand being subdivided into several subsets of fibers each optically connected to each other. a light sensor.

The determination means may comprise means for implementing an algorithm for determining the additive signal, said algorithm comprising as input variables the measured signals. The device according to the invention may further comprise means for calibrating or adjusting parameters of the algorithm. To implement the determination algorithm, the determination means may comprise a microprocessor, a microcomputer, or means for implementing a calculation table.

The measuring means may be arranged to measure, for more than one line in the vicinity of which an intensity of an additive signal is not negligible compared to the intensity of the reflected sunlight, a signal which depends on the intensity light collected at a wavelength of the line. A device according to the invention can then furthermore comprise means for determining a plurality of signals which each depend on a intensity of an additive signal at a different wavelength. In an exemplary device according to the invention applied to fluorescence measurements, the device according to the invention may comprise means for determining signals which each depend either on the intensity of the fluorescence of a different molecule such as chlorophyll or a polyphenol, or the intensity of the fluorescence of the same molecule at a different wavelength. Relationships between specific signals may, for example, make it possible to characterize the photosynthetic activity or the water stress of a plant system. For example, the determination of the fluorescence of chlorophyll at several wavelengths (in particular in the blue / green and the near infrared) makes it possible to draw up a remote map of the photosynthetic activity of plants. The unity of the signals determined is of little importance, since the characterizations generally depend on signal ratios. The device according to the invention may comprise a unit for processing specific signals and for performing such characterizations.

A device according to the invention can be arranged to measure signals by being in contact with the object. It can for example include means to pinch the object. The device according to the invention can also be arranged to measure signals remotely. It can for example be in the form of a portable device to point a few centimeters from the object, in the form of a device embedded on an agricultural tool or even in the form of a device embedded in an airplane or a remote sensing satellite. The field of the invention is not limited to the determination of fluorescence signals. For example, a device or method according to the invention may be used to evaluate the brightness of a display or a diode in direct sunlight.

DESCRIPTION OF THE FIGURES AND EMBODIMENTS Other advantages and particularities of the invention will appear on reading the detailed description of implementations and non-limiting embodiments, and the following appended drawings:

FIG. 1 illustrates a schematic side view of a first device embodiment according to the invention, oriented towards a sheet, FIG. 2 illustrates, over a wavelength interval comprising a first atmospheric line, a solar light intensity spectrum which has passed through the terrestrial atmosphere but has not been reflected by the sheet, FIG. over a wavelength range comprising the first atmospheric line, a light intensity spectrum collected by the device according to the invention,

FIG. 4 illustrates, over a wavelength interval comprising a second atmospheric line, an intensity spectrum of the light collected by the device according to the invention,

FIG. 5 illustrates a schematic side view of a second device embodiment according to the invention;

With reference to FIGS. 1 to 4, a first embodiment of device 1 according to the invention embodying a method according to the invention will be described. The device 1 according to the invention comprises an optical system 2 for collecting light 5, 6. The optical system comprises in particular an achromatic lens 3.

To illustrate the operation of the device according to the invention 1, one places oneself in the case where the device 1 collects light coming from a sheet 4. The light collected comprises sunlight 5 reflected by the surface of the sheet 4, and an additive fluorescence signal 6 emitted by the sheet following absorption of a portion of the sunlight 13 incident on the sheet.

The device 1 according to the invention furthermore comprises means for separating wavelengths from the light collected, comprising a strand 7 of optical fibers and interference filters 9. The lens 3 directs the light collected at the input of the strand of light. At the other end of the fibers, the fiber strand 7 is subdivided into a plurality of fiber subsets 8. The output of each subset of fibers is coupled to a filter 9 which substantially selects a length of fiber. The light is collected by the optical system and transported along the subset of fibers.

The device according to the invention further comprises means for measuring a plurality of signals which each depend on an intensity of the light collected at a different wavelength. Each interference filter is coupled to a photodiode 10 which captures the light selected by the filter. The light received by each of the photodiodes is converted into an electrical signal which is proportionally dependent on the intensity of the light collected at the wavelength selected by the filter coupled to the photodiode.

The electrical signals are generally of low level at the output of the photodiodes. The signals are processed by an electronic card 11 which comprises: a signal shaping unit, which amplifies and filters each electrical signal,

an analog / digital converter, which converts each electrical signal into a digital signal.

The device according to the invention can thus measure on the collected light signals which depend on the intensity of the light collected near a wavelength of a first atmospheric line or in the vicinity of a wavelength. a second atmospheric line. The subdivision of the light collected in the subsets of fibers, the transmission coefficients of the optical fibers and the filters, and the responses of the photodiodes are such that the measured signals all depend on an intensity of the light collected according to substantially the same coefficient of proportionality. In particular, the first embodiment comprises enough photodiodes to measure:

a signal S [X ₀ J] which depends on the intensity of the light collected substantially at a central wavelength λ ₀ of a first atmospheric line,

qui dépend de l'intensité de la lumière collectée à une longueur d'onde respectivement AJ^" ou A₀ ^" située sur un des bords de la première raie, ' - un signal S[X₁ Jj qui dépend de l'intensité de la lumière collectée sensiblement à une longueur d'onde A₁ centrale d'une deuxième raie atmosphérique, et - Un signal S[λf,t) ou S[λ[,t) qui dépend de l'intensité de la lumière collectée à une longueur d'onde respectivement /L₁ ⁺ ou /I₁ ^" située sur un des bords de la deuxième raie.- a signal

which depends on the intensity of the light collected at a wavelength respectively AJ ^" or A ₀ ^" located on one of the edges of the first line, - a signal S [X ₁ Jj which depends on the intensity of the light collected substantially at a central wavelength A ₁ of a second atmospheric line, and A signal S [λf, t) or S [λ [, t) which depends on the intensity of the light collected at a wavelength respectively / L ₁ ⁺ or / I ₁ ^" located on one of the edges of the second line.

The device according to the invention further comprises means for determining the additive signal 6 in the vicinity of the first line, in the event that in the vicinity of the first line the intensity of the additive fluorescence signal is not negligible compared to to the intensity of the reflected sunlight, and in the event that in the vicinity of the second line the intensity of the additive fluorescence signal is negligible compared to the intensity of the reflected sunlight. Signal measurements in the vicinity of the second line serve as reference measurements, and replace reference measurements on collected solar light without an additive signal in the vicinity of the first line. The electronic card 11 further comprises a digital signal processing unit which implements an algorithm for determining the additive signal 6 from the measured signals.

The additive signal can be determined punctually in time, or continuously. A display screen 12 displays the determined additive signal, or the time evolution of the additive signal determined. We will now explain the determination algorithm implemented by the device according to the invention. Notations We pose:

- At ₀ the central wavelength 19 of the first atmospheric line, in the vicinity of which it is proposed to determine a signal S ₀ (A ₀ J) which - depends on the intensity of the additive signal 6 to the length of wave? ^,

- / I ⁺ a wavelength greater than) ^ and located on one of the edges 22 of the first line,

- λ ^~ a wavelength less than / I ₀ and located on the other edge 21 of the first line, - λ _i , λ ₂ , ..., X _n the central wavelengths of n atmospheric lines (among which the second line, X ₁ being the central wavelength 20 of the second line), free of any signal additive on the spectrum of the light collected, ie affected only by multiplicative absorption phenomena,

Xf a wavelength greater than X ₁ and located on one of the edges 24 of the first line, and

At ₁ ^" a wavelength less than X ₁ and located on the other edge 23 of the first line; - s {x, t) a signal measured by the device according to the invention and which depends on the intensity light collected at a wavelength λ and at time t,

- S [X, t) the signal s [x, t) that depends on the intensity of the light collected at a wavelength λ and at time t and which would be measured by the device according to the invention in the absence of atmospheric line, b the intensity of the sunlight 13 at the wavelength λ ₀ , before the sunlight is reflected by the sheet 4,

at the theoretical intensity of the sunlight 13 at the wavelength in the absence of an atmospheric line, before the sunlight is reflected by the sheet 4, and - SJX, t) a signal which depends on the intensity of the additive signal 6 at the wavelength X and at time t.

Sunshine index

The insolation index Is (X ₁ , t) of an atmospheric line centered on a wavelength λ, such as the intensity at the wavelength λ, of a solar light signal devoid of additive signal at the wavelength λ ,, divided by the theoretical intensity of the signal at the wavelength λ, in the absence of an atmospheric line.

Fig. 2 illustrates the intensity spectrum of sunlight before it is reflected by sheet 4 over a wavelength range comprising the first atmospheric line. As illustrated in FIG. 2, the insolation index of the first line is equal to:

Is (J X ₀₎ = - a _t

FIG. 4 illustrates the intensity spectrum S of the light collected, over a wavelength interval comprising the second atmospheric line centered on the wavelength λ _x . The additive signal 6 is assumed to be negligible with respect to sunlight 5 in the vicinity of the second line. As illustrated in FIG. 4, the insolation index of the second line is equal to: is _M = WI. and, similarly, Is (X ₂ J) = L, ..., i _s (X _nit) = ^ β ^ L

^ S- (X ₁ J) ^{Ï> J} S- (X ₂ J) '^{[n' J} S '(X _n , t)

As illustrated in FIGS. 2 and 4, the insolation index of a line devoid of an additive signal is representative of the depth 25, 26 of this line.

The insolation index Is (X ₁ J) can be estimated as the transmittance of the atmosphere at the wavelength λ 1.

avec Q(t) la concentration du composé j de l'atmosphère au temps t, ε^λ) le coefficient d'extinction molaire du composé j de l'atmosphère à la longueur d'onde λ, et L(t) la longueur du chemin parcouru dans l'atmosphère par le signal lumineux au temps t. Donc :

On peut tenir compte d'éventuels effets d'absorption atmosphérique non linéaires en longueur d'onde par le composé j en introduisant un paramètre α._j(λ_ι) variant avec λ, et tel que :

Is (X ₁ J) = T ₀ (X ₁ J) The Beer-Lambert law on transmittances gives:

with Q (t) the concentration of the compound j of the atmosphere at time t, ε ^ λ) the molar extinction coefficient of the compound j of the atmosphere at the wavelength λ, and L (t) the length the path traveled in the atmosphere by the light signal at time t. Therefore :

Non-linear wavelength atmospheric absorption effects can be taken into account by the compound j by introducing a parameter α. _j (λ _ι ) varying with λ, and such that:

Link between the insolation indices of the different lines

The insolation index Is (X ₀ J) can be estimated as a function F of the insolation indices of the centered lines SUrA ₁ , A ₂ , .., A ₁₁ : Is (X ₀ , t) = F (Is (X ₁ , t), Is (X ₂ , t), ..., Is (X _n , t))

For example, in the case where the first line centered on λ ₀ is due to the absorption of a first atmospheric compound X, and the second line centered on λ i is due to the absorption of a second atmospheric compound Y:

Is (I ₀ , t) ≈ Is (I _x J) ^? M ^2? ( ')

Or :

Estimate of S ^* U, j)

We pose, for a line centered on A ₁ :

- A _; ⁺ a wavelength greater than A ₁ and located on one of the edges of this line,

- λ ^~ a wavelength less than A ₁ and located on the other edge of this line.

S ^* (X j) can be estimated in different ways here are three examples that rely on measurements available: a / assimilating the spectrum between A ₁ ^"and A ⁺ ₁ to a straight, giving ^'

s ^* (x _ι , t) = s (x;, t) + ^ f ^ / s (x;, t) -s (x; j))

A ₁ - A ₁ is, finally: (λ; -λ;). S (λ ,, t)

IsM = - a; -x _t ) .s (x;, t) - (x; -λ,). s (λ;, t)

soit, finalement :This estimation example of S "{X ,, i) is illustrated in Figures 3 and 4 respectively for X ₁ = A ₀ and X ₁ = A _1, b / by estimating S ^* (X _lt t) equal to the signal measured in X ^* , which gives:

either, finally:

_{Is (λ t) =} ^ M.

soit, finalement : ¹ S (X, t) - c / estimating S'faj) equal to the signal measured in λ ^~ , which gives:

either, finally:

S (X ;, t)

Calculation of the additive signal

The light collected by the device according to the invention is the sum of the reflection of sunlight 5 and the additive signal 6. In other words: ls ^* (X ₀ , t) = R (X ₀ , t) .a + S _a (X ₀ , t) [S (X ₀ , t) = R (X ₀ , t) .b + S _a β ₀ , t)

Where R (X ₀ J) is the reflectance of the object at the wavelength X ₀ and at time t. From this follows:

Or :

Is (X ₀ , t) = - a is the insolation index of the first line.

Algorithm implemented The device according to the invention determines at time t a signal S _a (X ₀ , t) which depends on the intensity of the additive signal 6 in the vicinity of the first line. implementing a calculation algorithm whose input variables are:

a measured signal S (λ _o , t) which depends on the intensity of the light collected by the device according to the invention substantially at the central wavelength A ₀ of the first line and at time t,

a measured signal S [X _Q , t) or 5 (/ I ₀ ^" , r) which depends on the intensity of the light collected at a wavelength located on one of the edges of the first line,

the insolation index of the first line, expressed as a function F, of other measured signals which depend on intensities of the light collected at wavelengths of other atmospheric lines, that is to say expressed as a function F of the insolation indices of these other lines.

The device comprises means for calibrating or adjusting parameters of the algorithm, and in particular parameters of the function F.

The calibration can be performed by manual adjustment means, or by calibration means by learning. The algorithm may also use calibration parameters to determine the value of the intensity of the additive signal in the vicinity of the first line. These calibration parameters may for example take into account the distribution of the light collected in the optical fibers, the transmission coefficients of the optical fibers and filters, and the responses of the photodiodes.

SyX₁ ,t),

La première raie centrée sur 760 nanomètres et la deuxième raie centrée sur 1270 nanomètres sont des raies d'absorption de l'oxygène de l'atmosphère. La mise en oeuvre de l'algorithme pour déterminer le signal S₀(A₀J) au voisinage de la première raie comprend une implémentation de la formule :A first variant of the first embodiment comprises six filters transmitting substantially only the wavelength respectively A ₀ = 760 nanometers, A ₀ ⁺ = 770 nanometers, A ₀ = 750 nanometers, A ₁ = 1267 nano meters, A ₁ ⁺ = 1276 nanometers, at ₁ ^" = 1245 nanometers and each coupled to a photodiode to measure S [X ₀ , t),

SyX ₁ , t),

The first line centered on 760 nanometers and the second line centered on 1270 nanometers are lines of oxygen absorption of the atmosphere. The implementation of the algorithm for determining the signal S ₀ (A ₀ J) in the vicinity of the first line comprises an implementation of the formula:

^ _ S (X ₀ J) -S ^* (X ₀ J) Js (X ₀ J) 1 -Is (X ₀ J)

S ^* (X _o j) being defined by:

s' (X _o j) = S (X ⁺ _0, t) + ^^ (S (X ⁺ _0, t) - S (χ- J)).

X ₀ - X ₀

Is (X ₀ J) being defined by the following formula connecting the depth of the first line on an intensity spectrum of sunlight to the depth 26 of the second line on an intensity spectrum of the collected light:

où A est un paramètre réglable pour tenir compte d'éventuels effets d'absorption atmosphérique non linéaires en longueur d'onde par l'oxygène, ε_ox(X₀) et B_0x(X₁) sont les coefficients d'extinction molaire de l'oxygène à la longueur d'onde respectivement λ_o et λi, et Is(X₁J) est défini par :

Une deuxième variante du premier mode de réalisation comprend quatre filtres ne transmettant sensiblement que la longueur d'onde respectivement X₀ = 760 nanomètres, ^ = 1267 nanomètres, une

= 1276 nanomètres et

1245 nanomètres. Chaque filtre est couplé à une photodiode pour mesurer respectivement S[X₀ J), S[X₁ j), un parmi S[X₀ j)Is (X ₀ J)

where A is an adjustable parameter to take into account possible wavelength non-linear atmospheric absorption effects by oxygen, ε _ox (X ₀ ) and B _0x (X ₁ ) are the molar extinction coefficients of oxygen at the wavelength λ _o and λ i respectively, and Is (X ₁ J) is defined by:

A second variant of the first embodiment comprises four filters transmitting substantially only the wavelength respectively X ₀ = 760 nanometers, λ = 1267 nanometers, a

= 1276 nanometers and

1245 nanometers. Each filter is coupled to a photodiode to respectively measure S [X ₀ J], S [X ₁ j), one of S [X ₀ j)

> and SyJ ₀ , t), and one of S [K j) and S [K j). The implementation of the algorithm for determining the signal SJX ₀ J) comprises an implementation of the formula:

S (X ₀ J) - S '(J) Js (X ₀ J) X -Is (X ₀ J) being defined by: S '(λ _o , t) = S (λ _Q ⁺ , t) or S' (λ _o , t) = S (λ-, t) Is (X ₀ J) being defined by: εox (^ o) Is (X _0, t) = a is (X ₁ J) ^ ^{V ox} where a is an adjustable parameter to account for possible non-linear effects of atmospheric absorption wavelength by oxygen, ε _ox ( X _Q ) and S _0x (X ₁ ) are the molar extinction coefficients of oxygen at the wavelength λ ₀ and λ _{1, respectively;} and Is (X ₁ J) is defined by:

A second embodiment of the device 18 according to the invention will now be described with reference to FIG. This second embodiment will only be described for its differences with respect to the first embodiment illustrated in FIGS. 1 to 4.

In the second embodiment, the photodiodes are replaced by sensors 15, 17 comprising a plurality of sensitive cells, such as CCD cameras or lines of photodiodes. The interference filters are replaced by components 14, 16, each component being capable of separating the wavelengths of the light collected in the vicinity of an atmospheric line, and being associated with one of the sensors. A first diffraction grating 14 separates the wavelengths in the vicinity of the first atmospheric line centered on λ _o = 76O nanometers. A second diffraction grating 16 separates the wavelengths in the vicinity of the second atmospheric line centered on λ ₁ = 1270 nanometers. The output of each subset of fibers is coupled with one of the diffraction gratings. The light diffracted respectively by the first 14 and second network is respectively captured by a first 15 and second 17 sensor.

In a first variant ', the first sensor measures S [I ₀ , t), s {λ _o , t), and sfaj), and the second sensor measures s (? _Λ , t), s (Kj), and S [KJ] - The algorithm implemented to determine the additive signal 6 is similar to that implemented by the first variant of the first embodiment of the device according to the invention.

In a second variant, the first sensor measures S [X ₀ , t) and one of S [X ₀ ⁺ J) and S [X _Q , t). The second sensor measures S [X ₁ J) and one of ^" S [Xf, t) and s {λ [, t) .The algorithm used to determine the additive signal 6 is similar to that implemented by the second variant of the first embodiment of the device according to the invention.

Of course, the invention is not limited to the examples that have just been described and many adjustments can be made to these examples without departing from the scope of the invention.

For example, a device according to the invention may comprise means for determining additive signals at wavelengths of different atmospheric lines. In addition, a device according to the invention may comprise means for measuring, for more than two atmospheric lines, a signal which depends on an intensity of the light collected at a wavelength of the line, and / or means to measure more than three signals per line. The greater the number of measured signals, the more precise the algorithm implemented to determine an additive signal.

Claims

A method for determining an additive light signal to sunlight having passed through an atmosphere, comprising: a collection of light (5, 6) from an object (4), the collected light comprising reflected sunlight ( 5) by the object, and an additive light signal (6), a measurement, for at least a first and a second atmospheric absorption line, of a signal which depends on an intensity of the light collected at a wavelength of the line, and a determination, based on the measured signals, of a signal which depends on an intensity of the additive signal at a wavelength of the first line. 2. Method according to claim 1, characterized in that the intensity of the additive signal (6) is not negligible compared to the intensity of the reflected sunlight (5) for wavelengths in the vicinity of the first ray, and in that the intensity of the additive signal (6) is substantially negligible compared to the intensity of the reflected sunlight (5) for wavelengths in the vicinity of the second line. 3. Method according to claim 1 or 2, characterized in that the measurements comprise for each line: a measurement of a signal which depends on an intensity of the light collected at a substantially central wavelength (19, 20) of the line, and a measurement of a signal which depends on an intensity of the light collected at a wavelength located on one of the edges (21, 22, 23, 24) of the line. 4. Method according to one of claims 1 to 3, characterized in that the determination comprises an implementation of an algorithm for determining the additive signal, said algorithm comprising as input variables the measured signals. 5. Method according to claim 4, characterized in that the algorithm implements a formula in which a depth (25) of the first line on an intensity spectrum of sunlight is expressed as a depth function (26). other lines on a spectrum of intensity of the light collected. 6. Method according to claim 4 or 5, characterized in that the algorithm uses parameters which depend on a concentration in the atmosphere of components at the origin of one of the atmospheric lines, and / or the coefficient of molar extinction of one of these components at a wavelength close to one of these lines, and / or the transmittance of the atmosphere at a wavelength close to one of these lines. 7. Method according to one of claims 4 to 6, characterized in that it further comprises a calibration or a parameter adjustment of the algorithm. 8. Method according to one of the preceding claims, characterized in that the two lines are due to the absorption of identical atmospheric compounds. 9. Method according to one of the preceding claims, characterized in that the first line consists of [a line of oxygen centered on 760 nanometers. 10. Method according to one of the preceding claims, characterized in that the second line consists of the line of oxygen centered on 1270 nanometers. 11. Method according to one of the preceding claims, characterized in that the additive signal consists of a light signal emitted by the object (4). 12. The method of claim 11, characterized in that the additive signal consists of a fluorescence signal emitted by the object in response to reflected sunlight. Apparatus for determining an additive light signal to sunlight having passed through an atmosphere, comprising: - means (2, 3) for collecting light (5, 6) from an object (4), the collected light comprising sunlight reflected (5) by the object, and an additive light signal (6), - means for measuring (10, 15, 17), for at least a first and a second atmospheric absorption line, a signal which depends on an intensity of the light collected at a wavelength of the line, and means for determining (11), from the measured signals, a signal which depends on an intensity of the additive signal at a wavelength of the first line. 14. Device according to claim 13, characterized in that the determining means are provided for a significant additive signal intensity compared to the intensity of sunlight reflected for wavelengths in the vicinity of the first ray. , and are provided for an additive signal intensity substantially negligible with respect to the intensity of reflected sunlight for wavelengths in the vicinity of the second line. 15. Device according to claim 13 or 14, characterized in that the measuring means comprise for each line: means for measuring a signal which depends on an intensity of the light collected at a substantially central wavelength (19, 20) of the line, and means for measuring a signal which depends on an intensity of the light collected at a wavelength located on one of the edges (21, 22, 23, 24) of the line. 16. Device according to one of claims 13 to 15, characterized in that the measuring means (10, 15, 17) comprise a photodiode, a photodiode array, or a CCD sensor. 17. Device according to one of claims 13 to 16, characterized in that it further comprises means (7, 8, 9, 14, 16) for separating wavelengths of the collected light. 18. Device according to claim 17, characterized in that it further comprises at least one filter (9) passing substantially only one wavelength of the collected light. 19. Device according to claim 17 or 18, characterized in that it further comprises at least one diffraction grating (14, 16) for separating the wavelengths in the vicinity of one of the atmospheric lines. 20. Device according to one of claims 17 to 19, characterized in that it further comprises a strand (7) of optical fibers, the collection means directing to a first end of the strand light collected, the other end the strand being subdivided into several subsets (8) of fibers each optically connected to a light sensor. 21. Device according to one of claims 13 to 20, characterized in that the determining means comprises means (11) for implementing an algorithm for determining the additive signal, said algorithm comprising as input variables the measured signals. . 22. Device according to one of claims 13 to 21, characterized in that it further comprises means for calibrating or adjusting parameters of the algorithm.