FR2985028A1 - Device for measuring directional transmission of light in vegetation cover for determining index surface of cover, has sensor whose collecting solid angle covers zenith angle and azimuth angle ranging between specific degree - Google Patents

Abstract

The device (1) has a radiation sensor (3) arranged such that its aiming direction (3a) defines a zenith angle (A) included between 45 and 60 degree along a longitudinal axis (2') of a vertical support (2). Another radiation sensor (4) is arranged such that its aiming direction (4a) defines another zenith angle included between 15 and 45 degree. A third zenith angle (C) is defined in the aiming directions of the radiation sensors. A collecting solid angle of each sensor covers a fourth zenith angle ranging between 5 and 15 degree and an azimuth angle ranging between 10 and 45 degree. An independent claim is also included for a method for measuring directional transmission of light in a vegetation cover.

Description

FIELD OF THE INVENTION The present invention relates to a device for measuring the directional transmission of light in a plant canopy, for the determination of a surface index of said canopy.

PRIOR ART One of the important parameters to be taken into account for the evaluation of the yield of a growing vegetation is the quantity of dry matter that is produced by photosynthesis in the leaves of plants, itself depending on the quantity of radiation intercepted by leaf cover. In practice, two main surface indices are interested in the amount of radiation intercepted by the canopy: (i) the leaf area index (still referred to as the "Leaf Area Index" or "LAI"), or (ii) the Plant Area Index (PAI). In some cases, we can still evaluate the so-called green area index (GAI). In a given climatic and stationary context, the leaf area index is an essential factor to evaluate the ability of the vegetation cover to intercept the radiation (and therefore to estimate the amount of radiation intercepted). The leaf area index is a dimensionless size, which expresses the leaf area of a plant cover (eg a crop, a low natural cover or a forest) per unit of soil area. This foliar index represents the surface through which flows of carbon (photosynthetic exchange) and water (transpiration) flow. This parameter is the basis for many diagnostics for decision making, and is a fundamental variable for phenotyping. This data also makes it possible to inform models of the functioning of the vegetation. It will still allow the assessment of the productivity of a plant cover.

This leaf area index is, for example, described in Chen, J.M., & Black, T.A. (1992) "Defining leaf area index for non-flat leaves", Plant, Cell and Environment, 15, 421-429. The plant area index, or hereinafter "PAI", includes the surface of the leaves, but also the surface of all the plant organs above ground (trunk, branch, stem, flower, etc.). This parameter is described for example in the document Neumann, HH, Den Hartog, G., & Shaw, RH (1989) "Leaf area measurements based on hemispheric photographs and leaf litter collection in a deciduous forest during autumn leaf fall", Agricultural and Forest Meteorology, 45, 325-345. In order to determine the value of these surface indices (in particular LAI or PAI), there are in particular specific optical measuring devices, commonly called "cover analyzers", making it possible to make non-destructive, rapid and precise estimations of this parameter since floor. Such measuring devices evaluate the interception of radiation (direct or diffuse), by means of radiation sensors which are distributed over at least two stages: an upper stage, in which the radiation sensor or sensors are intended to be placed above the foliar cover of the said vegetative cover for the measurement of incident light radiation, and - at least one lower stage, in which the radiation sensor or sensors are intended to be placed beneath the foliar cover of the said canopy plant to measure the value of transmitted light radiation.

From the collected data, models for calculating the probability of contact of a light radiation with a foliage element, taking into account (i) the inclination of the leaves, (ii) the angle of incidence radiation, and possibly (iii) a factor taking into account the aggregation of the leaves, have been developed to calculate the aforementioned surface indices (in particular LAI and / or PAI). Such models are presented, for example, in Weiss, M., Baret, F., Smith, G. J., Jonckheered, I., & Coppin, P. (2004). "Review of methods for in situ leaf area index determination, Part II: Estimation of LAI, errors and sampling", Agricultural and Forest Meteorology, 121, 37-53.

There are thus for example instruments using the transmittance of the visible radiation in the direction of the sun (for example TRAC, DECAGON, CEPTOMETER, DEMON), in several directions (LAI 2000) or without particular consideration of directions (PICQHELIOS).

However, the current measuring devices are often either imprecise (notably because of the leaf angular distribution and the aggregation effect of the leaves), or depending on the lighting conditions, or the quality of the measurements (in particular for the use of hemispheric photography).

In addition, most current devices are relatively expensive and complex to implement (especially for the classification of images in the case of photography). A measuring device called "PASTIS-57" (for "PAI Autonomous System of Transmittance Instantaneous Sensors") is all the same relatively interesting for obtaining the PAI of a plant cover from the ground. This PASTIS-57 measuring device is equipped with radiation sensors for recording data relating to light radiation.

The radiation sensors are oriented so that their respective viewing directions define a zenith angle of 57.5 ° with the longitudinal axis of the vertical support, and if necessary advantageously in an azimuthal direction perpendicular to the crop rows. The radiation sensors are distributed on a horizontal plane, advantageously for sampling the spatial variability of the radiation within the canopy. This particular orientation of the sighting direction minimizes the aggregation effect of the leaves. It makes it possible to make the estimation of the PAI independent of the leaf inclination, which limits the important aggregation effects notably in row crops. In practice, this PASTIS-57 device is intended for the continuous monitoring of leaf development; it is not intended to be mobile. However, even this measuring device PASTIS-57 still has some points that can be improved.

In particular, this measuring device PASTIS-57 does not allow obtaining accurate values in the case of fairly dense cover. This device also provides no information on the orientation of the leaves and their possible aggregations.

In addition, a spatial representativeness requires a multiplication of the number of sensors, which can prove to be heavy and expensive when one does not seek to obtain a continuous temporal monitoring.

SUMMARY OF THE INVENTION In view of the above, the Applicant has developed a new device for measuring the directional transmission of light in a plant cover. The device according to the invention is therefore equipped with judiciously distributed radiation sensors, which thus significantly improves the sensitivity of the surface index (s) (in particular LAI and / or PAI) obtained from the data collected, in particular in the case fairly dense cover. The device according to the invention also makes it possible to obtain information on the orientation of the leaves, and on their possible aggregations. This device is also advantageously mobile, thus providing access to good spatial representativeness by repeating the measurement points, or by performing horizontal profiles (transects) in the canopy. This device for measuring the directional transmission of light in a plant cover, according to the invention, comprises a vertical support having a vertical longitudinal axis and on which are reported sensors for collecting light radiation. These radiation sensors are distributed over at least two stages: an upper stage, in which the radiation sensor or sensors are intended to be placed above the leaf cover of said plant cover to measure the value of a luminous radiation indicative of and at least one lower stage, in which the radiation sensor or sensors are intended to be placed below the leaf cover of said vegetation cover to measure the value of a transmitted light radiation. These radiation sensors each have a viewing direction with a solid sensing angle; and they are arranged so that their respective viewing directions extend in at least one vertical plane parallel to said longitudinal axis of said vertical support. The solid angles of capture and the directions of sight are defined by two angles: a zenith angle, according to an orthogonal projection in said vertical plane, and an azimuth angle, according to an orthogonal projection in a horizontal plane. And according to the invention, the lower and upper stages each comprise (at least a first side of said longitudinal axis of the vertical support) at least two lateral radiation sensors: a first radiation sensor, which is arranged so that its aiming direction defines a zenith angle between 45 ° and 60 ° (preferably between 57 ° and 58 °) with said longitudinal axis of the vertical support, - a second radiation sensor, which is arranged so that its direction of target defines a zenith angle between 15 ° and 45 ° (preferably between 34 ° and 36 °) with said longitudinal axis of the vertical support. The viewing directions of said first radiation sensor and said second radiation sensor together define a zenith vertical angle of at least 10 °. In addition, the solid angle of capture of each radiation sensor covers, on either side of its associated direction of view, the following angles: a zenith angle of between 5 ° and 15 °, preferably of order of 10 °, and an azimuth angle of between 10 ° and 45 °, preferably of the order of 20 °. Advantageous features, which can be taken independently or in combination with one another, are developed below. The lateral radiation sensors fitted to the lower stage are preferably arranged so that their viewing directions and the longitudinal axis of the vertical support together constitute intersecting straight lines. Advantageously, the point of concurance is in the immediate vicinity of the lateral radiation sensors, for example at a distance of between 0 and 3 cm.

This characteristic makes it possible to ensure that the transmission measurement is implemented at a defined height in the canopy. Advantageously, the stages each comprise two pairs of lateral radiation sensors which are arranged on either side of the longitudinal axis of the support, with their azimuth opposed viewing directions. The upper stage advantageously comprises a third radiation sensor whose aiming direction has a zero zenith angle with the longitudinal axis of the vertical support.

In addition, this measuring device advantageously comprises two lower stages of radiation sensors, with (i) an underlying lower stage whose lateral radiation sensors are intended to come close to the ground, and (ii) a lower stage above -jacent whose side radiation sensors are adjustable in height.

This characteristic allows a two-level calculation of the above-mentioned surface indices, namely (i) the total LAI / PAI of the canopy, as well as (ii) the LAI / PAI of the active vegetation layer by suitably adjusting the height of the lower floor above. In addition, the support advantageously consists of a pole (useful in particular for easy movement of the device). In this case, the pole having two ends, one lower and the other higher; the underlying lower stage of radiation sensors is then advantageously provided at said lower end, and the upper stage of radiation sensors is advantageously provided at said upper end. Again in this case, the pole advantageously has an adjustable length, advantageously between 2 and 3 m. The radiation sensors are advantageously chosen from radiation sensors that are sensitive to visible light, and preferably to blue light. The device is advantageously equipped with processing means for acquiring data relating to the amount of incident radiation and the amount of radiation transmitted, and for determining the value of a surface index of said vegetation cover from said data. acquired.

Preferably, this device is equipped with means for acquiring its geolocation, its spatial orientation and a time stamp. In this case, the means for acquiring its spatial orientation advantageously comprise means for acquiring its orientation with respect to a reference direction, for example magnetic north, and means for acquiring the inclination of the longitudinal axis of the support relative to the vertical (for example a level). The present invention also relates to a method for measuring the directional transmission of light in a plant canopy, for the determination of a surface index of said vegetation cover (in particular LAI and / or PAI), by means of a measuring device such as as defined above. This method comprises the following steps: (a) a step of positioning said measuring device at a point of origin of a transect of a plot of vegetation cover to be characterized, and so that the longitudinal axis of its vertical support is oriented vertically, and (b) a step of acquiring data relating to the amount of incident radiation and the amount of transmitted radiation, during which said measuring device is moved from said point of origin of the transect to an end point of said transect. By "transect" is meant here a line that crosses the parcel, along which is moved the measuring device, so as to accurately represent the average characteristics of the parcel. In this case, the method preferably further comprises (c) a step of processing the data from the acquisition step (b), for measuring the value of at least one surface index of said canopy. If necessary, the acquisition step (b), and possibly the processing step (c), ensure obtaining a succession of radiation quantity values and surface index values that are geolocated and time stamped .

For the measurement of a surface index of a plant cover which consists of a plurality of rows defining paths each having a longitudinal axis, during the acquisition step (b), the measuring device is advantageously moved so that its vertical plane extends perpendicularly to said longitudinal axis of the associated aisle, the aiming direction of the lateral radiation sensors being thus oriented perpendicular to said longitudinal axis of said associated aisle and perpendicular to the direction of displacement. Moreover, the treatment step (c) generates successive groups of surface index values, and advantageously of average leaf angle, which groups comprise: a value derived from the lateral radiation sensors of the lower stage, or where appropriate of each lower stage, which are situated on the first side of the longitudinal axis of the support, and - a value originating from the lateral radiation sensors of the lower stage, or where appropriate of each lower stage, which located on the second side of the longitudinal axis of the support. DESCRIPTION OF THE FIGURES The invention will be further illustrated, without being in any way limited, by the following description of a particular embodiment in relation to the appended drawings in which: FIG. 1 represents, in a general and schematic manner, the device of FIG. measurement according to the invention, seen from the front and in position at the level of a plot of vegetation cover to be characterized; - Figure 2 shows schematically and isolated, one of the lower sensor stages of the measuring device according to Figure 1, to illustrate the positioning of its radiation sensors and their respective viewing directions and sensing angles; FIG. 3 represents, in a schematic and isolated manner, the upper sensor stage of the measuring device according to FIG. 1, so as to show the distribution of its radiation sensors, with their viewing directions and sensing angles. respective; - Figure 4 is a very schematic representation, seen from above, of the measuring device according to Figures 1 to 3 in a plot of the plant cover to be characterized when measuring the directional transmission of light in this plant cover. DETAILED DESCRIPTION OF THE INVENTION The measuring device 1 according to the invention, as shown in FIG. 1, is of the "cover analyzer" type. This measuring device 1 is intended to be positioned and moved within a plant cover C of a parcel P, for the measurement of the directional transmission of light in this vegetation cover C. In general, the "plant cover C "Means all the vegetation existing on a given surface (here a plot P); the leaf cover refers to all the leaves borne by this plant cover C.

This parcel P of plant cover C here consists of a row crop. Two rows of plants R are separated by a path A (Figures 1 and 4). The crop rows R and the aisles A each define a longitudinal axis R 'and A' respectively (FIG. 4). These longitudinal axes R 'and A' advantageously extend parallel, or at least approximately parallel, with respect to one another (FIG. 4). By "directional light transmission measurement" is meant the measurement of the amount of light passing through the canopy in a given direction.

The directional light transmission data obtained will allow a determination of at least one surface area index of the canopy C, for example its leaf area index (LAI) and / or its plant area index (PAI). By "foliar index" or LAI is meant a dimensionless size that expresses the leaf area of a plant cover per unit area of soil. The PAI consists of a dimensionless size, which expresses the surface of the leaves but also the surface of the other aboveground organs of the canopy plants (including trunks, stems, flowers, etc.), per unit of soil surface. For this purpose, as shown in FIG. 1, the measuring device 1 comprises a vertical support 2 on which are reported a set of sensors 3, 4 and 5 (described in more detail below with reference to FIGS. 3) which are intended to collect each the light radiation.

The vertical support 2 here consists of a telescopic pole, having a longitudinal axis 2 'intended to be oriented vertically. This pole 2 is intended to be worn, for example by an operator.

The perch 2 preferably has an adjustable length, for example between 2 m and 3 m, to adapt to the height of the plant cover C to be analyzed. For this, this pole 2 is composed here of a main tubular segment "master" 2a, for example of a length of the order of 2 m, associated with an expandable upper segment "auxiliary" 2b. The maneuvering of the deployable upper segment 2b is advantageously effected by an ascending / descending translation movement, in a direction coaxial with the aforementioned longitudinal axis 2 '(or else by a rotation in a vertical plane with respect to a horizontal axis lying between at the upper end of the master segment). The length adjustment can be further obtained by adding the auxiliary segment 2b, and attachment to the upper end of the master segment 2a. The master segment 2a has two ends: - an upper 2a1, at which the auxiliary segment 2b opens, and - a lower end 2a2, forming the lower end of the pole 2 which is intended to extend close to the ground S, or even to rely on it (for example at a distance between 0 and 3 cm). The auxiliary segment 2b extends from the upper end 2a1 of the master segment 2a of the pole 2. This auxiliary segment 2b has an upper end 2b1 which defines the upper end of the pole 2, adjustable in height relative to the ground S. The radiation sensors 3, 4 and 5 of the measuring device 1 are distributed over the height of the pole 2 according to several sensor stages 7, 8 and 9, namely here: an upper sensor stage 7 implanted at the upper end 2b1 of the auxiliary segment 2b of the pole 2 (at the upper end of the pole 2), on which the radiation sensors 3, 4 and 5 are intended to extend above plant cover C for measuring the value of the incident light radiation, and - two lower sensor stages 8 and 9 carried here by the master segment 2a of the pole 2, in which the radiation sensors 3 and 4 are intended to be placed above dess or foliar cover of vegetation cover C to measure the value of transmitted light radiation. By "value of the light radiation" is advantageously meant a light radiation flux expressed in W.m-2. By "incident light radiation" is advantageously meant the surrounding light radiation from a source of natural light such as the Sun or the celestial vault. By "transmitted light radiation" is meant advantageously the light radiation reaching the ground S, which has not been captured by the leaf cover or the canopy C.

According to FIG. 1, the measuring device 1 therefore comprises two lower sensor stages 8 and 9 superimposed, namely: (i) a first lower underlying sensor stage 8, whose radiation sensors 3, 4 are arranged at the level of from the lower end 2a2 of the master segment 2a of the pole 2 (or in other words at the lower end of the pole 2), and (ii) a second lower, above-mentioned, lower sensor stage 9, whose sensors 3, 4 are carried by the master segment 2a of the pole 2 but at a distance from said lower end 2a2. As developed below, the use of these two lower sensor stages 8 and 9 superimposed has the advantage of allowing a separate measurement of the PAI (by means of the lower lower stage 8) and LAI (to average of the underlying lower layer 9). Within each sensor stage 7, 8 or 9, the radiation sensors 3, 4 and 5 are distributed and aligned on a support 10 adapted.

This support 10 consists for example of a plastic structure which is machined so as to allow the implantation and orientation of the radiation sensors 3, 4 (and possibly 5). These sensor stages 7, 8 or 9, as well as their radiation sensors 3, 4 and 5, are described in more detail below with reference to FIGS. 2 and 3.

Each sensor stage 7, 8 or 9 here comprises two pairs (or pairs) of lateral radiation sensors 3 and 4, which are arranged symmetrically on either side of the vertical longitudinal axis 2 'of the support 2. The stage Upper sensor 7 has, here again, a vertical radiation sensor 5. These different radiation sensors 3, 4 and 5 are advantageously chosen from radiation sensors sensitive to visible light. More preferably, these radiation sensors 3, 4 and 5 are sensitive to blue light, that is to say a wavelength of approximately 400 to 550 nanometers, spectral range where the radiative transfer in the canopy is simplified because of the very strong absorption of the leaves, making the multiple diffusion almost null. In addition, the illumination of the sky then provides a very good contrast (very strong diffusion in the blue).

Preferably, these radiation sensors 3, 4 and 5 still have good sensitivity over a wide range of illumination, to measure very low transmission levels, for example of the order of 0.01 W. m-2. Such radiation sensors 3, 4 and 5 consist for example of light emitting diodes (abbreviated still under the acronym LED or LED), operating in light receiving mode. Each radiation sensor 3, 4 or 5 has a sighting direction, designated respectively by the marks 3a, 4a and 5a (FIGS. 1 to 4). By "direction of sight" is meant the direction of the optical axis of the radiation sensor.

Within each sensor stage 7, 8 and 9, each pair of lateral radiation sensors 3, 4 has viewing directions 3a, 4a which extend in the same vertical plane, parallel to the vertical longitudinal axis 2 'of the vertical support 2 (or passing through this vertical longitudinal axis 2 '). Similarly, the different sensor stages 7, 8 and 9 comprise a pair of lateral radiation sensors 3, 4 whose sighting directions 3a, 4a extend in a common vertical plane, parallel to the vertical longitudinal axis 2 'of the vertical support 2 (or passing through this vertical longitudinal axis 2 '); these different lateral radiation sensors 3, 4 thus have the same orientation in azimuth.

In this case, the viewing directions 3a, 4a and 5a of the radiation sensors 3, 4 and 5 of the measuring device extend in a single vertical plane 11 (FIG. 4) which is parallel to the vertical longitudinal axis 2 of the vertical support 2 (even passing through this vertical longitudinal axis 2 ').

Each radiation sensor 3, 4 and 5 is also capable of collecting the light radiation in a small solid angle shown diagrammatically in FIGS. 2 to 4, and designated respectively by the marks 3b, 4b and 5b. In the remainder of the description, these parameters will be defined by two angles or angular sectors: a so-called "zenith angle", according to an orthogonal projection in the aforementioned vertical plane 11 (FIGS. 2 and 3), and an angle called "azimuth" ", According to an orthogonal projection in a horizontal plane 12 which is perpendicular to the vertical longitudinal axis 2 'of the vertical support 2 (Figure 4). The lateral radiation sensors 3 and 4 will each be used to record radiation at different zenith angles. In other words, for each of the sensor stages 7, 8 or 9, two pairs of lateral radiation sensors 3, 4 are oriented so that their viewing directions 3a, 4a are opposite in azimuth, with zenith exposures symmetrically both sides of the longitudinal axis 2 'of the pole 2. For this, within each sensor stage 7, 8 or 9, each pair of lateral radiation sensors 3, 4, formed on one side of the longitudinal axis 2 'of the vertical support 2, is implanted so that: - a first lateral radiation sensor 3 is arranged so that its viewing direction 3a defines a zenith angle A between 45 ° and 60 °, more preferably between 57 and 58 °, and more preferably 57.5 ° (Figures 2 and 3), with respect to the longitudinal axis 2 'of the pole 2, and - a second lateral radiation sensor 4 is positioned so that its direction of sight 4a defines a zenith angle B between 15 ° and 45 °, preferably still between 34 and 36 °, and more preferably 35 °, with respect to the longitudinal axis 2 'of the pole 2 (Figures 2 and 3). The aiming direction 3a and 4a of each lateral radiation sensor 3, 4 is inclined upwards, ascending from the pole 2.

The viewing directions 3a and 4a of each pair of lateral radiation sensors 3, 4 (located on the same side of the longitudinal axis 2 'of the pole 2) also together define a vertical zenith angle C in the vertical plane 11 .

This vertical angle C is advantageously at least 10 °, more preferably at least 20 °. According to the present embodiment, this angle C is advantageously of the order of 20 ° to 30 °. Such a pair of lateral radiation sensors 3, 4 makes it possible to increase the sensitivity of the transmission to the PAI and / or LAI; it also makes it possible to characterize the angular distribution of the plant elements from the angular variation associated with their cross section, as described in particular in the document Weiss, M., Baret, F., Smith, GJ, Jonckheered, I., & Coppin , P. (2004). "Review of methods for in situ leaf area index determination, Part II: Estimation of LAI, errors and sampling", Agricultural and Forest Meteorology, 121, 37-53. Thus, the viewing directions 3a of the first two lateral radiation sensors 3 for each sensor stage 7, 8 or 9 are symmetrical relative to each other, on either side of the longitudinal axis 2 'of the pole 2.

Similarly, the viewing directions 4a of the two second lateral radiation sensors 4 of each stage 7, 8 or 9 are symmetrical with respect to each other, on either side of the longitudinal axis 2 'of the pole 2. In FIG. 2, it can still be seen that the lateral radiation sensors 3 and 4 fitted to each lower stage 8, 9 are arranged so that their viewing directions 3a, 4a and the longitudinal axis 2 'of the support vertical 2, constitute concurrent lines at the same point 13. This concurance point 13 is advantageously in the immediate vicinity of the side radiation sensors 3 and 4, for example at a distance between 0 and 3 cm.

To be complete, as represented in FIG. 3, the vertical radiation sensor 5 of the upper sensor stage 7 has a viewing direction 5a defining a zero zenith angle with respect to the longitudinal axis 2 'of the pole 2, to aim vertically (upwards).

In addition, the sighting direction 3a, 4a and 5a of each of the radiation sensors 3, 4 or 5 defines a zero azimuth angle, or at least approximately zero, relative to the vertical plane 11 (Figure 4). On the other hand, the solid capture angle 3b, 4b and 5b of each radiation sensor 3, 4 and 5 advantageously defines a volume whose section in a plane perpendicular to its direction of sight 3a, 4a and 5a is of shape. rectangular. The solid capture angle 3b, 4b and 5b of each radiation sensor 3, 4 and 5 covers, on either side of their respective viewing directions 3a, 4a and 5a, two angular sectors presenting the values of angles following: - a zenith angle D between 5 ° and 15 °, preferably of the order of 10 ° (Figure 2), and - an azimuth angle E between 10 ° and 45 °, preferably of the order of 20 ° (Figure 4). This particular sounding angle of capture 3b, 4b and 5b aims to restrict the zenith angle D to be closer to the aiming direction 3a, 4a and 5a, and to improve the spatial representativeness of the canopy by a larger azimuth angle E .

The measuring device 1 is further equipped with control means 15 for its operation (FIG. 1). These control means 15 include in particular processing means (not shown) comprising two functionalities: (i) the acquisition of the data transmitted by the different radiation sensors 3, 4 and 5 (corresponding to the amount of incident radiation and to the quantity of transmitted radiation), and (ii) the determination of the value of at least one surface index (LAI and / or PAI) of the vegetation cover C from the acquired data. For this, these processing means 15 here consist of an electronic / computer system, integrated in a housing 16 which is fixed on the pole 2, for the implementation of an appropriate algorithm in the form of a computer application (software application) or a computer program. It is quite possible to develop such a custom computer application, in particular for determining the LAI and / or the PAI from the data acquired by each pair of lateral radiation sensors 3, 4. For example, the estimation of the PAI from the transmittance measurements T (Theta), made by a pair of lateral radiation sensors 3, 4 with the viewing directions 3a (Theta_3a) and 4a (Theta_4a), is based on the following equation (I): T (Theta) = exp (-PAI x G (Theta, AIF) / cos (Theta)) (I) wherein: - G (Theta, AIF) is the projected surface in the Theta direction of a plant surface unit , and - AIF is the Angle of Inclination of Leaves. We thus have a system with two equations (one for Theta_3a and the other for Theta_4a) with 2 unknowns (PAI and AIF) analytically or numerically soluble.

For this purpose, reference may be made, for example, to the following articles: (a) Weiss, M., Baret, F., Smith, G. J., Jonckheered, I., & Coppin, P. (2004). "Review of methods for in situ leaf area index determination, Part II: Estimation of LAI, errors and sampling", Agricultural and Forest Meteorology, 121, 37-53, and (b) Baret, F., De Solan, B., Lopez-Lozano, R., Ma, K., & Weiss, M. (2010). "GAI estimates of a row of photographs taken perpendicular to rows at 57.5 ° zenith angle. Theoretical considerations based on 3D architecture and application to wheat crops, "Agricultural and Forest Meteorology, 150, 1393-1401. On the other hand, the housing 16 advantageously comprises a user interface (for example a liquid crystal display) making it possible to display information, as well as a numerical keyboard (see an alphanumeric keyboard) for entering elements of identification of the data acquired. directly on the acquisition site. This housing 16 may also include wired computer connectors (for example a USB or "Universal Serial Bus") or non-wired, allowing its connection to a computer for advanced control or unloading data acquired. The housing 16 may further incorporate an accumulator device for electrical energy (for example a battery) to ensure autonomy of the measuring device 1 during the acquisition of measurements.

The housing 16 may further integrate a geolocation system (for example GPS type for "Global Positioning System"), and possibly a compass for the determination of magnetic north. The data acquired by the various radiation sensors 3, 4 and 5 will thus be associated with (i) the geographical coordinates (or geolocation) of the device 1, (ii) the orientation of the measuring device 1 with respect to a reference direction (calculation of the azimuth by processing the GPS positions during a trip or by means of the compass), as well as (iii) the time of acquisition.

The pole 2 can also be provided with means for verifying its verticality during the implementation of the device 1 (for example in the form of a round bubble level). In general, the measuring device 1 can be implemented according to three acquisition modes: a calibration mode, to be used before the measurement recording in the field; - a continuous recording mode, where the operator triggers the beginning and the end of measurement for transects of variable sizes, - a specific recording mode (or "shot by blow"), where the operator defines a number of individual measurements that he wishes to average and which are triggered manually one by one.

The continuous measurement mode can be interesting: - to acquire fixed data, in the context of a post-processing of measurements acquired in forest or in calibration (one defines the frequency of acquisition and a maximum measurement); - to acquire data along a transect, with a definition of the acquisition frequency. The piecemeal measure may be useful for measurements in crops (low heterogeneity). In this case, typically twenty measurements are recorded per site. In practice, the measuring device 1 is first calibrated very simply, and infrequently, by a measurement made outside the plant cover. The compass can also be calibrated by pointing the pole 2 to the north. Then, before starting the measurements, the operator enters at the level of the housing 16 (or the computer connected to said housing 16), the information necessary for the identification of the measurement site and the information necessary for calculating the LAI and / or PAI (in particular the dominant sampled species whose typical range of AIF will have been characterized).

In addition, the operator first determines the transect T adapted according to the parcel P to be analyzed. In this case, this transect T advantageously corresponds to the virtual line constituted by the longitudinal axis A 'of each aisle A of the parcel P to be studied.

The operator then positions the pole 2 so that its longitudinal axis 2 'extends vertically (FIG. 1), at the point of origin T1 of the aforementioned transect T (FIG. 4). In this case, for the measurement of a plant cover area index C of the parcel P which consists of a plurality of ranks R, the measuring device 1 is positioned at the beginning of one of its aisles A At this point, the measuring device 1 is oriented so that its vertical plane 11 (passing through the viewing directions 3a, 4a and 5a of its radiation sensors 3, 4, 5) extends perpendicularly to the axis longitudinal A 'of the aisle A associated.

In azimuth, the sighting direction 3a, 4a of the lateral radiation sensors 3, 4 is thus oriented also perpendicularly with respect to the longitudinal axis R 'of the two associated rows R (illustrated in FIG. 4). In addition, if necessary, the upper segment 2b of the pole 2 is adjusted in deployment (elongated or shortened) according to the height of the canopy C, so that the upper sensor stage 7 extends beyond C. The lower sensor stage 8 is intended to come to a height between 0 and 3cm from the ground S. The second lower sensor stage 9 is advantageously adjustable in height. Its distance from the first lower sensor stage 8 is advantageously adjustable between 5 cm and 150 cm. In practice, this distance is adjusted so as to position this second lower sensor stage 9 at the boundary between (i) the non-green part (senescent) of the lower part of the canopy, and (ii) the upper green part of this canopy. .

Once the measuring device 1 is properly configured, oriented and positioned, the operator triggers the measuring operations. This acquisition step will consist of collecting data relating to the amount of incident radiation (from the sensors 3, 4 and 5 of the upper sensor stage 7) and to the quantity of transmitted radiation (from the side sensors 3, 4 lower sensor stages 8, 9), at least part of the plant cover C constituting the plot P. For this, the operator moves the measuring device 1 along the transect T, at a roughly regular speed from its point of origin T1 to its end point T2 (FIG. 4, for example the end of aisle A). During this journey, the measuring device 1 is advantageously maintained so that its vertical plane 11 extends perpendicular to the longitudinal axis A 'of the associated aisle (that is to say, still perpendicular to the the longitudinal axis R 'of the two rows R associated and relative to the direction of movement) (Figure 4). The measuring device 1 must allow the acquisition of data with a relatively high frequency, for example between 10 Hz and 30 Hz, and more preferably of the order of 20 Hz. This frequency will make it possible to perform statistical calculations on the transmittance distributions. The measuring device 1 is thus moved to an end point T2 of the transect T where the end of data acquisition takes place. The processing means 15 integrated on the measuring device 1 make it possible to manage the acquisition of the data in real time, and can be used for the processing of the recorded measurements. All the acquired data can then be processed, and the main results can be displayed in real time on the interface. The processing of the information is relatively simple, with an extraction of the average transmittance values, and then their transformations into at least one surface index of the LAI and / or PAI type. The ratio between the data acquired by the sensors 3, 4 and 5 located above the canopy (upper sensor stage 7) on the one hand, and the sensors 3 and 4 located under the vegetation (lower sensor stages 8 and 9) On the other hand, provides the transmittance of the canopy C.

In other words, the transmittance advantageously corresponds to the ratio between the transmitted light flux (numerator) and the luminous flux index (denominator). Hole fraction (transmittance) models, which also take into account the inclination of the leaves and the angle of incidence of the radiation, make it possible to calculate in particular the PAI of the vegetation cover C. The use of the second radiation sensors Lateral 4 allows to increase the sensitivity to the PAI for dense cover, and to estimate the leaf inclination.

This treatment step advantageously generates successive groups of surface index values (LAI and / or PAI), and advantageously of average leaf angle (ie the average of the distribution of the zenith angles between the perpendicular to the sheet and vertical, in the case of non-flat sheets, the sheet will be considered as consisting of approximately flat facets), for each pair of lateral radiation sensors 3, 4 of the lower sensor stages 8 and 9. These groups values then advantageously comprise: a first value derived from the pair of lateral radiation sensors 3, 4 of the overlying lower sensor stage 9 situated on a first side of the longitudinal axis 2 'of the pole 2, a second value derived from the pair of lateral radiation sensors 3, 4 of the overlying lower sensor stage 9, located on a second side of the longitudinal axis 2 'of the pole 2, a third value derived from the couple of e side radiation sensors 3, 4 of the lower underlying sensor stage 8, located on a first side of the longitudinal axis 2 'of the pole 2, and - a fourth value derived from the pair of side radiation sensors 3, 4 of the lower underlying sensor stage 8, located on a second side of the longitudinal axis 2 'of the pole 2.

These values are therefore calculated here in two opposite azimuth directions. The calculated information is optionally stored in a memory module.

If necessary, the operator can enter additional information (identification of the parcel, visual notification for example). The whole procedure is therefore very fast, mainly determined by the speed of travel along the transect, and possibly by the time taken to enter additional information. The data acquired by the various radiation sensors 3, 4 and 5 make it possible to measure under fluctuating lighting conditions. However, taking into account information on the orientation by the compass and the position of the Sun by the geolocation means, a warning will be advantageously issued when the direction of sight of the lateral radiation sensors 3, 4 is close to that of the Sun. making the measurement unstable. These acquired data can be post-processed using geolocation coordinates to make positions more accurate. The coordinates should advantageously be available in several projection systems (depending on the user).

A two-dimensional coordinate system, managed by the user, is also advantageously available to allow easy linkage with the parcel numbering system used in the field experimentation platforms. The post-processing can be implemented from the data obtained on a single measuring device, but also from the data obtained simultaneously from two measuring devices. In general, the measuring device 1 according to the invention could be implemented in any other crop, low natural cover (savannah, natural meadow) or forest.

The measurement transect (corresponding to the path of the measuring device 1 within the parcel) is suitably adapted. In general, the measuring device according to the invention, with its pairs of radiation sensors, has the following advantages: - spatial sampling possible thanks to the "mobile" nature of the measuring device, which is not the case PASTIS 57 systems; - geolocation of each measurement and automatic time stamping; - independence of lighting conditions due to (i) simultaneity between incident and transmitted flux measurements, and (ii) illumination of the celestial vault is taken into account, making the measurement independent of the position of the sun; - Real-time processing and visualization of measures to assess their relevance; - possibility of adjusting the transmittance measurement height under the cover by the underlying lower sensor stage, thus allowing only the upper green parts to be considered. According to an alternative embodiment, the measuring device 1 according to the invention may comprise sensor stages each provided with a single pair of lateral radiation sensors 3, 4; the pairs of lateral sensors 3, 4 are located on the same side of the longitudinal axis 2 'of the pole 2 for acquiring the transmittance data in a single azimuthal direction. Such a structure would be useful for example in the case where it is desired to sample a parcel from its border. Also according to another variant embodiment, the measuring device 1 according to the invention may comprise sensor stages each provided with more than two pairs of lateral radiation sensors 3, 4 which are distributed for the acquisition of the transmittance data according to different azimuthal directions (for example four pairs of sensors 3, 4 oriented in a cross, in two vertical planes perpendicular to each other). Such a structure would be useful for example in the case where it is desired to obtain a spatial representativeness of the plant cover (for example in the case of a forest cover).

Claims (18)

CLAIMS1.- Device for measuring the directional transmission of light in a plant canopy (C), for the determination of a surface index of said canopy (C), which measuring device (1) comprises a vertical support (2) having a vertical longitudinal axis (2 ') and to which sensors (3, 4, 5) for collecting light radiation are connected, which radiation sensors (3, 4, 5) are distributed over at least two stages: an upper stage (7), wherein the one or more radiation sensors (3, 4, 5) are intended to be placed above said plant cover (C) for measuring the value of an indicative light radiation, and at least one lower stage (8, 9), wherein the one or more radiation sensors (3, 4) are intended to be placed beneath the leaf cover of said vegetation cover (C) for measuring the value of a radiation transmitted light, which radiation sensors (3, 4, 5) p each comprises a sighting direction (3a, 4a, 5a) and a solid sensing angle (3b, 4b, 5b), which radiation sensors (3, 4, 5) are arranged so that their respective sighting directions (3a , 4a, 5a) extend in at least one vertical plane (11) parallel to said longitudinal axis (2 ') of said vertical support (2), which solid sensing angles (3b, 4b, 5b) and which viewing directions ( 3a, 4a, 5a) are defined by two angles: - a zenith angle, according to an orthogonal projection in said vertical plane (11), and - an azimuth angle, according to an orthogonal projection in a horizontal plane (12), characterized in that said lower (8, 9) and upper (7) stages each comprise at least one pair of lateral radiation sensors (3, 4): - a first radiation sensor (3), which is arranged so that its direction of target (3a) defines a zenith angle (A) between 45 ° and 60 ° with said longitudinal axis (2 ') of the worm support tical (2), - a second radiation sensor (4), which is arranged so that its aiming direction (4a) defines a zenith angle (B) between 15 ° and 45 ° with said longitudinal axis (2 ') of the vertical support (2), in that the viewing directions (3a, 4a) of said first radiation sensor (3) and said second radiation sensor (4) together define a vertical zenith angle (C) of at least 10 °, and in that the solid sensing angle (3b, 4b, 5b) of each radiation sensor (3, 4, 5) covers, on both sides of its associated aiming direction (3a, 4a, 5a), the following angles: - a zenith angle (D) between 5 ° and 15 °, and - an azimuthal angle (E) between 10 ° and 45 °. 2. Measuring device according to claim 1, characterized in that the lateral radiation sensors (3, 4) fitted to the lower stage (8, 9) are arranged so that their sighting directions (3a, 4a) and the longitudinal axis (2 ') of the vertical support (2) together constitute intersecting straight lines at a point (13). 3. Measuring device according to any one of claims 1 or 2, characterized in that the stages (7, 8, 9) each comprise two pairs of lateral radiation sensors (3, 4) which are arranged on either side of the longitudinal axis (2 ') of the support (2), with their viewing directions (3a, 4a) opposite in azimuth and extending in the same vertical plane (11). ). 4. Measuring device according to any one of claims 1 to 3, characterized in that the or the first radiation sensors (3) are oriented so that their respective viewing directions (3a) have a zenith angle (A ) between 57 ° and 58 ° with the longitudinal axis (2 ') of the vertical support (2), and in that the second or the second radiation sensors (4) are oriented so that their viewing directions (4a) respective have a zenith angle (B) between 34 ° and 36 ° with said longitudinal axis (2 ') of the vertical support (2). 5. Measuring device according to any one of claims 1 to 4, characterized in that the upper stage (7) comprises a third radiation sensor (5) whose aiming direction (5a) has a zero zenith angle with the longitudinal axis (2 ') of the vertical support (2). 6. Measuring device according to any one of claims 1 to 5, characterized in that it comprises two lower levels (8, 9) of lateral radiation sensors (3, 4), with (i) a first lower stage (8), underlying, the side radiation sensors (3, 4) are intended to come close to the ground, and (ii) a second lower stage (9), overlying, the side radiation sensors (3, 4) are adjustable in height. 7. Measuring device according to any one of claims 1 to 6, characterized in that the carrier (2) consists of a pole. 8. Measuring device according to claim 7 in combination with claim 6, characterized in that the pole (2) having two ends, one lower (2a2) and the other upper (2b1), the lower floor underlying (8) lateral radiation sensors (3, 4) is provided at said lower end (2a2), and the upper stage (7) of radiation sensors (3, 4, 5) is provided at level of said upper end (2b1). 9. Measuring device according to any one of claims 7 or 8, characterized in that the pole (2) has an adjustable length, advantageously between 2 and 3 m. 10. Measuring device according to any one of claims 1 to 9, characterized in that the radiation sensors (3, 4, 5) are selected from radiation sensors sensitive to visible light. 11. Measuring device according to any one of claims 1 to 10, characterized in that said device (1) is equipped with processing means (15) for acquiring data relating to the amount of incident radiation and to the amount of radiation transmitted, and for determining the value of at least one surface index of said vegetation cover (C) from said acquired data. 12. Measuring device according to any one of claims 1 to 11, characterized in that said device (1) is equipped with means for acquiring its geolocation, its spatial orientation and a time stamp, for collected data. 13. Measuring device according to claim 12, characterized in that the means for acquiring its spatial orientation comprises means for acquiring its orientation with respect to a reference direction, for example magnetic north, and - means for acquiring the inclination of the longitudinal axis (2 ') of the support (2) relative to the vertical. 14.- Method for measuring the directional transmission of light in a plant canopy (C), for determining a surface index of said canopy (C) by means of a measuring device (1) according to one of any of claims 1 to 13, characterized in that it comprises the following steps: (a) a step of positioning said measuring device (1) at a point of origin (T1) of a transect (T ) a plot (P) of vegetation cover (C) to be characterized, and so that the longitudinal axis (2 ') of its vertical support (2) is oriented vertically, and (b) a step of acquisition of data relating to the amount of radiation indicative and the amount of radiation transmitted, during which said measuring device (1) is moved from said point of origin (T1) of the transect (T) to an end point (T2 ) of said transect (T). 15.- Method according to claim 14, characterized in that it further comprises (c) a step of processing the data from the acquisition step (b), for measuring the value of a surface index of said vegetation cover (C). 16.- Method according to any one of claims 14 or 15, by means of a measuring device (1) according to any one of claims 12 or 13, characterized in that the acquisition step (b) , and if necessary the treatment step (c), ensure obtaining a succession of radiation quantity values and surface index (s) which are geolocated and timestamped. 17.- Method according to any one of claims 14 to 16, for the measurement of surface index (s) of a plant cover (C) which consists of a plurality of rows (R) defining aisles ( A) each having a longitudinal axis (A '), characterized in that, during the acquisition step (b), the measuring device (1) is moved so that its vertical plane (11) extends perpendicularly to said longitudinal axis (A ') of the associated aisle (A), the aiming direction (3a, 4a) of the lateral radiation sensors (3, 4) thus being oriented perpendicular to said longitudinal axis (A') of the aisle (A) associated and perpendicular to the direction of travel. 18. A method according to any one of claims 15 to 17, characterized in that the treatment step (c) generates successive groups of values of surface index (s), and preferably of average leaf angle, which groups include: - a value derived from the side radiation sensors (3, 4) of the lower stage (8, 9) or, where appropriate, from each lower stage (8, 9), and situated on the first side of the longitudinal axis (2 ') of the support (2), and - a value originating from the lateral radiation sensors (3, 4) of the lower stage (8, 9), or possibly from each lower stage (8, 9). ), and located on the second side of the longitudinal axis (2 ') of the support (2).