IT201900001891A1 - Method for monitoring plant surfaces - Google Patents

Description

Description of the invention entitled: Method for monitoring plant surfaces

Background of the invention

The present invention refers to the field of environmental diagnostics since a method for monitoring the physiological state of lawns and other types of natural coverings that can also be used for an accurate characterization and management of plant coverings such as sports fields, areas natural or artificial greens.

State of the art

The study of the spectral behavior of a natural surface consists in the characterization of the so-called spectral signature of the investigated surface, i.e. the quantification of the radiometric behavior of a surface, generally expressed in terms of the percentage of radiant energy (solar or artificial light) that is reflected at the different wavelengths in the domain of the electromagnetic spectrum between the visible (VIS) and short-wave infrared (SWIR) regions. This percentage, called reflectance, can be considered as a unique signature of the surface observed under the same intrinsic conditions such as humidity, organic content, mineralogical composition of soils and water status, chlorophyll content and leaf structure in the case of vegetation, the variations of which are reflected directly in the signature itself.

The spectral signature of vegetation in the visible bands depends on the presence of chlorophyll pigments that absorb light radiation with a wavelength around 450 nm (blue) and 650 nm (red) and partially reflect green radiation (wavelength of 550 nm).

The leaf structure is instead responsible for the spectral behavior in the near infrared (NIR) bands, between 700 and 1350 nm, and causes a very high reflection, in the order of 50-70%, of the incident energy, while the leaf water content is responsible for the absorption in the short wave infrared domain (SWIR), between 1350 and 2200 nm, which is highlighted in the spectrum with the so-called "absorption holes" centered around 1400 and 1900 nm (Gomarasca M., 1997. Introduction to remote sensing and GIS for the management of agricultural and environmental resources, Published by A.I.T., Italian Remote Sensing Association).

The spectral indices of vegetation (VI), using characteristic and specific values of the spectral signature, allow to numerically express certain physiological states of the vegetation.

For example, from the combination of the reflectance values in the red (RRED) and infrared (RNIR) bands, the index called Normalized Difference Vegetation Index (NDVI) is defined, which expresses the vigor of the vegetation cover through the following expression:

The NDVI index assumes a value between 0 and 1 (Rouse, J.W., Jr., R.H. Haas, J.A. Schell, and D.W. Deering. 1973. Monitoring the vernal advancement and retrogradation (green wave effect) of natural vegetation. Prog. Rep. RSC 1978-1, Remote Sensing Center, Texas A&M Univ., College Station, 93p. NTIS No. E73-106393) and is able to indirectly estimate physiological parameters of vegetation such as the Leaf Area Index (LAI) ), the soil cover factor (Fc, Fraction Cover) and the crop coefficients (Kc, Crop Index).

Additional radiometric indices are for example the EVI index (Enhanced Vegetation Index), the SAVI index (Soil Adjusted Vegetation Index), the GI green index (Green Index).

Such further indices, similarly to the NDVI index, are however based on different algebraic combinations of the spectral reflectance in the VIS-NIR domain.

An extensive database of vegetation indices currently proposed in the scientific literature is available, also depending on the type of application and / or monitoring (University of Bonn, Institute of Crop Science and Resource Conservation - INRES, http: // www. indexdatabase.de).

In addition to remote sensing techniques for indirect estimation of vegetation indices, portable and punctual meters for direct measurement of vegetation indices have been developed in the last decade. The systems currently available differ from each other for the type of source used, natural light or artificial light, for the bands of the electromagnetic spectrum explored, for the type of sensor used, point sensor or pixel matrix sensor, for the type of data processed obtainable, as for example single numerical value or image with associated numerical values.

Among the known systems, those based on the measurement of the reflection of solar radiation from the surfaces under observation are classified as passive systems. The measurement obtainable from passive systems must take into account the temporal and meteorological variability of the incident solar radiation and therefore appropriately subjected to correction. Passive systems include point gauges without image acquisition for the estimation of chlorophyll or a single vegetation index (FieldScout CM 1000, Spectrum Technologies, inc.), Applications based on the acquisition of images in the green band only ( FieldScout GreenIndex +, Spectrum Technologies, inc.), Systems based on the use of multispectral digital cameras that acquire images in natural lighting conditions (Tetracam ADC, Tetracam inc.), Generally installed on remote platforms such as drones, balloons and kites.

Active systems, on the other hand, are based on the use of their own source whose radiant flux is defined and constant. The measurement obtainable from active systems does not require corrections due to the variability of the radiant source and allows to obtain calibrations in reflectance independent from the conditions and from the moment of measurement and therefore univocal.

The active systems already developed are mainly point systems, which use specific light sources capable of measuring unique and pre-set spectral indices but without image acquisition support (FieldScout TCM 500, Spectrum Technologies, inc .; GreenSeeker Crop Sensing System, Trimble, inc.).

For both passive and active systems, the accuracy of the measurements that can be obtained also depends on whether or not specific internal calibrations of the sensors are used to transform the measured signal into a spectral reflectance and / or vegetation index value. For example, both point and image-supported passive sensors are generally calibrated to directly obtain measurements of the NDVI index, without providing the reflectance value in the observed bands. On the other hand, active systems, available on the market, are generally equipped with an internal calibration capable of providing an average value of NDVI, but do not provide images capable of highlighting the variations of NDVI within the observed surface.

Currently, among the active systems there are no tools capable of deriving spectral information on multiple wavelengths in the form of an image and equipped with internal calibrations capable of estimating both the spectral reflectance and one or more vegetation indices derived from it. In fact, the only active system equipped with image support found in literature is the so-called "Light box" which uses only white light sources obtained with fluorescent lamps, which operates exclusively in the visible range and does not have NIR information, as it uses a standard digital camera (Ghali, I. E., G. L. Miller, G. L. Grabow, and R. L. Huffman. 2012. Using Variability within Digital Images to Improve Tall Fescue Color Characterization. Crop Sci. 52: 2365-2374. doi : 10.2135 / cropsci2011.10.0553; Richardson, M.D., D.E. Karcher, and L.C. Purcell. 2001. Quantifying turfgrass cover using digital image analysis. Crop Sci. 41: 1884–1888. Doi: 10.2135 / cropsci2001.1884).

The system described above does not provide information in the infrared domain, and only allows the discrimination, within the images, of the share of soil covered by vegetation (fraction cover).

Technical problem

The devices known in the art have numerous drawbacks. For example, passive systems, both punctual and diagnostic imaging (imaging), depend on the temporal and meteorological variability of the incident solar radiation and consequently require calibration for each measurement. Instead, the active systems, currently available, allow to obtain spectral information limited to the visible range.

Furthermore, there are no systems equipped with absolute radiometric calibration capable of directly allowing the estimate of the reflectance values from which to accurately calculate the vegetation indices.

The system proposed by the present invention allows to overcome the drawbacks and limitations of the devices known in the art.

The method proposed by the present invention allows the calculation of the most suitable vegetation indices for monitoring plant surfaces, and allows to obtain radiometrically correct output data, allows to monitor the physiological state of natural coverings and turf in order to identify any conditions anomalous or plant diseases in any environmental and lighting conditions, therefore both in the laboratory and in the open field and without the need for complex calibration operations, thus overcoming the limits of the systems known in the art.

Object of the invention

With reference to the attached claims, the technical problem is therefore solved by providing a method for the calculation of spectral indices of plant coverings to be monitored, obtained from the measurements of the spectral reflectance obtained with the device object of the present invention comprising the following stages:

a) positioning of a device for measuring the spectral reflectance of plant surfaces consisting of a multispectral chamber operating in the region of the visible and infrared domains comprising a chamber, externally covered with shielding and optically reflective material, and inside white, having an exit wall open to the outside and an entrance wall, opposite it, which houses a night vision camera and sets of white and monoband LEDs in the spectral domains of red, green, blue and near infrared, arranged on inclined planes. placing it on the surface of a plant cover to be monitored;

b) activation of white LEDs and subsequent acquisition of real color images of the vegetation cover to be monitored;

c) activation of red, green, blue and near infrared monoband LEDs and subsequent acquisition of images of the vegetation cover to be monitored;

d) devignetting and cropping of the images obtained in b) and c);

e) radiometric calibration of images obtained in d) and calculation of reflectance values;

f) calculation of vegetation indices.

Further characteristics of the present invention will become clear from the detailed description which follows with reference to the attached figures and the experimental tests provided.

Brief description of the figures

Figure 1 shows a longitudinal section of the device used in the method of the present invention.

Figure 2 shows the detail of the internal surface of the inlet wall of the device shown.

Figure 3 shows a block diagram schematizing the method of the present invention.

Figure 4 graphically shows the frequency histograms and statistical values relating to maps of NDVI values obtained for three different plots P1, P2 and P3.

Figure 5, in panel A shows in graph the reflectance values measured with a spectroradiometer on the plots P1, P2 and P3 in the electromagnetic spectrum region of VIS-NIR (between 300 and 900nm), in panel B shows the comparison between the values of NDVI obtained with the device and those obtained from the spectral signatures measured with the spectroradiometer in the same plots.

Figure 6 graphically shows the trend of NDVI values with respect to the sowing density on plots of L. perennial var. subjected and not subjected to fertilization.

Detailed description of the invention

Definitions

In the context of the present invention, by spectral reflectance we mean, at different wavelengths, the percentage of radiant energy reflected with respect to the incident radiation (solar or artificial light.

In the context of the present invention, green cover means the vegetation covering a surface such as, for example, sports fields, golf courses, recreational areas and green spaces, natural and artificial pastures.

In the context of the present invention, by shielding and optically reflective material is meant a material of limited thickness but sufficient to ensure the opacity of the walls of the chamber to external solar radiation, for example a metal sheet, preferably of aluminum.

In the context of the present invention, Normalized Difference Vegetation Index (NDVI) means the index that expresses the physiological state of the vegetation cover,

which takes a value between 0 and 1.

In the context of the present invention, EVI (Enhanced Vegetation Index) means the index that expresses the physiological state of the vegetation cover,

which takes on a value between -∞ and ∞

In the context of this invention, the SAVI index (Soil Adjusted Vegetation Index) means the index that expresses the rate of vegetation cover on the ground,

which takes on a value between -∞ and ∞

In the context of the present invention, the green index GI (Green Index) means the index that expresses the rate of chlorophyll pigments on the plant surface,

which takes on a value between -∞ and ∞

In the context of the present invention, the green index GLI (Green Leaf Index) means the index that expresses the rate of chlorophyll pigments on the plant surface,

which takes on a value between -∞ and ∞

In the context of the present invention, the VARI index (Visible Atmosperically Resistant Index) means the index expresses that the physiological state of the vetation,

which takes on a value between -∞ and ∞

In the context of the present invention, by pixel we mean the elementary unit that constitutes a digital image (pixel matrix).

In the context of the present invention, digital number values (digital number, DN) mean the numerical value corresponding to each pixel.

In the context of the present invention, geometric resolution means the number of pixels that make up a digital image, expressed as the number of rows per number of columns.

In the context of the present invention, system calibration means the execution of one or more imageprocessing procedures to be conducted univocally in the laboratory during the preparation of the instrument in order to define the numerical parameters used in the normal acquisition phase.

In the context of the present invention, a multispectral camera means a device capable of providing digital images consisting of pixels to which the spectral reflectance values of the surface imaged at different wavelengths are associated.

In the context of the present invention, a mask means a digital image processed during the calibration phase, with a resolution equal to that of the images acquired by the system, in which the numerical values associated with the individual pixels have been suitably determined to correct any geometric aberrations. and radiometric operations by means of mathematical operations between corresponding pixels of the mask and of the acquired images to be corrected.

In the context of the present invention, the vignetting effect means the reduction of the brightness of the image that is recorded gradually and radially in the peripheral pixels with respect to those of the central area.

In the context of the present invention, devignetting means a procedure capable of correcting the DN values of the acquired image to cancel the vignetting effect by applying a predetermined corrective mask during the calibration phase of the system.

In the context of the present invention, cropping means a procedure that transforms the acquired image by eliminating the peripheral bands consisting of a certain number of pixels obtained during the calibration phase, generally affected by excessive optical aberrations.

In the context of the present invention, by radiometric correction, or spectral calibration in reflectance, is meant a procedure which, for each spectral band, R red, G green, B blue, IR infrared, transforms the Digital Number values of the image pixels acquired by the system, in spectral reflectance values obtained by applying a specific corrective mask predetermined during the system calibration phase.

The present invention relates, with reference to the claims and the attached figures, to a method for monitoring and characterizing vegetation cover which provides for the following stages:

a) positioning of a device 1 for measuring the spectral reflectance of plant surfaces comprising a chamber 2 equipped with an entrance wall 3 an open exit wall 4 and side walls having the external surfaces 6 coated with a shielding material and optically and the internal surfaces 7 of white color in which in the central part of the entrance wall is fixed a night vision video camera 8 operating in the spectral range between 400 and 1100 nm and with a geometric resolution of not less than 1296x972 pixels on the sides of which two planes 10,11 are positioned which are symmetrical to each other and inclined by an angle ɑ between 25 and 35 degrees with respect to the perpendicular of the entrance direction which have a LED lighting system on the surfaces 12,13 facing the entrance wall consisting of: at least one LED or a series of white light LEDs, arranged in series or parallel, which emits in the interval of wavelength between 400 and 800 nm; at least one LED or a series of LEDs, arranged in series or parallel, with emission peak at 460 nm and FWHM width at half height (Full Width at Half Maximum) of 20 nm; at least one LED or a series of LEDs, arranged in series or parallel, with emission peak at 520 nm and FWHM width at half height (Full Width at Half Maximum) of 20 nm; at least one LED or a series of LEDs, arranged in series or parallel, with an emission peak at 630 nm and a Full Width at Half Maximum of 20 nm; at least one LED or series of LEDs, arranged in series or parallel, with emission peak at 830 nm and FWHM width at half height (Full Width at Half Maximum) of 30 nm

placing it on the surface of a plant cover to be monitored;

b) Activation of white monochromatic LEDs, which emit in the wavelength range between 400 and 800 nm, to acquire real color images of the vegetation cover to be monitored;

c) Activation of non-band LEDs, blue with emission peak at 460 nm and FWHM width at half height (Full Width at Half Maximum) of 20 nm, green with emission peak at 520 nm and FWHM width at half height (Full Width at Half Maximum) of 20 nm, red with peak emission at 630 nm and FWHM width at half height (Full Width at Half Maximum) of 20 nm, and infrared with emission peak at 830 nm and FWHM width at half height (Full Width at Half Maximum) of 30 nm, to obtain monoband images for each type of LED of the plant cover to be monitored;

d) devignetting and cropping of the images obtained in b) and c)

e) Radiometric calibration of the images obtained in d) by applying radiometric calibration masks, to transform the digital number values of the individual pixels of each image obtained in stage d) into reflectance values (R).

f) calculation of vegetation indices:

in which RNIR is the image containing for each pixel the reflectance values in the infrared band obtained in point e).

RRED is the image containing for each pixel the reflectance values in the red band obtained in point e).

RGREEN is the image containing for each pixel the reflectance values in the green band obtained in point e).

RBLUE is the image containing for each pixel the reflectance values in the blue band obtained in point e).

In stage a) the at least one LED or a series of white light LEDs, arranged in series or parallel, which emits in the wavelength range between 400 and 800 nm, are white light;

In stage a) the at least one LED or series of LEDs, arranged in series or parallel, with emission peak at 460 nm and FWHM width at half height (Full Width at Half Maximum) of 20 are blue monochromatic;

In stage a) the at least one LED or a series of LEDs, arranged in series or parallel, with an emission peak at 520 nm and a Full Width at Half Maximum of 20 nm; they are monochromatic green;

In stage a) the at least one LED or a series of LEDs, arranged in series or parallel, with emission peak at 630 nm and FWHM width at half height (Full Width at Half Maximum) of 20 nm; they are monochromatic red;

In stage a) the at least one LED or a series of LEDs, arranged in series or parallel, with an emission peak at 830 nm and FWHM width at half height (Full Width at Half Maximum) of 30 nm, emit in the infrared;

In stage a) laterally to the night vision video camera 8 there is also a through hole 9 and said through hole allows to optionally house an optical fiber of a spectroradiometer. Said optical fiber is used for any periodic check operations on the radiative flux emitted by the LED system.

In stage a) the device can further comprise suitable power supply means for the individual LEDs i) to v) and for the video camera.

In stage a) the device can further comprise suitable means which connect it to a data acquisition and storage system.

In step a) preferably the angle ɑ is equal to 30 °.

In step d) corrective masks are used, predetermined during the setup and calibration of the device carried out using conventional imageprocessing techniques as described in Kelcey J. and Lucieer A. (2012). Sensor Correction of a 6-Band Multispectral Imaging Sensor for UAV Remote Sensing, Remote Sens. 2012, 4 (5), 1462-1493. Images are acquired for each spectral band, R red, G green, B blue, infrared IR on a flat and homogeneous reference surface; the analysis of the variability of the digital number values for each of the reference images acquired along the main and diagonal axes of the same is carried out on an analytic-mathematical basis and the verification of the radial symmetry of the vignetting effect is carried out. Then the calculation is performed, for each wavelength, of the devignetting mask obtained by calculating, for each pixel, the dimensionless ratio (∆DN) between the digital number value in the central area of the image, not affected by aberration, and the DN value of the pixel itself. A further corrective cropping mask is determined which for each wavelength eliminates the peripheral bands consisting of a certain number of pixels previously obtained, generally affected by excessive optical aberrations that cannot be corrected during the devignetting phase.

The flat, homogeneous reference surface may be commercially available (Reflectance Reference Targets, Labsphere, US).

In step e) the radiometric calibration masks are predetermined in a device calibration step using conventional techniques as described in Kelcey J. and Lucieer A. (2012). Sensor Correction of a 6-Band Multispectral Imaging Sensor for UAV Remote Sensing, Remote Sens. 2012, 4 (5), 1462-1493.). R red, G green, B blue, infrared IR images of a flat reference surface are acquired by juxtaposing 10 reference panels with known and homogeneous reflectance varying from 2% to 99%. For each reference panel and for each spectral band, the average value of the digital numbers of the relative pixels is extracted and the linear transformation equation between digital number and spectral reflectance is calculated for each spectral band by means of a linear regression technique with the method of least squares as described in Chatterjee, S., Hadi, A. S. and Price, B. (2000). Regression Analysis by Example. 3rd ed., John Wiley & Sons, New York, having the following equation:

where ai and bi are the values of the “intercept” and “slope” parameters obtained with the least squares method for each LEDi source. Finally, for each spectral band, radiometric calibration masks are calculated and stored containing the values of the parameters ai and bi as previously obtained.

Reference panels with known and homogeneous reflectance ranging from 2% to 99%. commercially available SRS-02, SRS-05, SRS-10, SRS-20, SRS-40, SRS-50, SRS-60, SRS-75, SRS-80, SRS-99 (Reflectance Reference Targets, Labsphere, USA).

The method of the present invention can be used for monitoring the effectiveness of crop interventions operated in the management of turf of sports fields, golf courses, recreational areas and green spaces, natural and artificial pastures.

Examples

In order to verify the functioning of the device, an experimental validation campaign was carried out at the Castelnuovo Didactic-Experimental Company of the University of Palermo, where a series of experimental plots measuring 1.1 m x 0.8 were created. m with a substrate consisting of 75% sand, 15% silt and 10% clay, on which a grassy carpet Poacea species, Lolium perenne var. ready. The surveys were carried out on plots with different degrees of ground cover. In addition to the images acquired with the device, spectral measurements were carried out using an optical spectroradiometer (ASD) in order to determine the vegetation indices to be used as a control. Measurements with the device were obtained for three different experimental plots (P1, P2, P3) characterized by different degrees of ground cover and level of turf development.

The comparison between the images (not shown) obtained with the device in a mode similar to the "light-box" system (white LEDs) and those of the respective vegetation indices (NDVI) processed by the device in multispectral mode showed that the images obtained with the device are able to emphasize the areas with total vegetation cover compared to those with low coverage and / or with no coverage. From the images obtained, the NDVI index and its spatial distribution at pixel scale (NDVI map) was quantified.

Figure 3 shows the histograms of frequency, F and statistical values related to the NDVI maps for each parcel P1, P2 and P3.

In order to verify the reliability of the NDVI values calculated with the method of the present invention, a comparison was made between the average values determined by the system on each plot and the corresponding values obtained from the spectral signatures measured at the same time during the experimentation by means of the radiometer spectrum. ASD operating in conditions of natural lighting, sunlight; the results are shown in figure 4. The comparison analysis demonstrates agreement between the values calculated with the method and those of the spectroradiometer, considered as a reference system for the spectral characterization of homogeneous natural surfaces in natural lighting conditions.

Finally, the agronomic value was assessed by comparing the NDVI measured on plots subjected to fertilization with complex fertilizer compared to non-fertilized plots. The results shown in figure 5 show that for all the sowing densities tested, in the fertilized plots the NDVI was always higher than in those not subjected to fertilization, highlighting how this tool can provide valid support in evaluating the effectiveness of specific agronomic / cultural interventions.

Claims (3)

CLAIMS 1) Method for the monitoring and characterization of plant coverings which includes the following stages: a) positioning of a device for measuring the spectral reflectance of plant surfaces (1) comprising a chamber (2) equipped with an inlet wall (3) an open outlet wall (4) and side walls (5) having the external surfaces (6) coated with a shielding and optically reflective material and the white internal surfaces (7) in which a night vision camera (8) operating in the spectral range between 400 and 1100 nm and with a geometric resolution of not less than 1296x972 pixels on the sides of which are positioned two planes (10), (11) symmetrical to each other and inclined by an angle ɑ between 25 and 35 degrees with respect to the perpendicular of the direction of which have on the surfaces (12), (13) facing the entrance wall a LED lighting system consisting of: at least one LED or a series of white light LEDs, arranged in series or parallel, which emit and in the wavelength range between 400 and 800 nm; at least one LED or a series of LEDs, arranged in series or parallel, with emission peak at 460 nm and FWHM width at half height (Full Width at Half Maximum) of 20 nm; at least one LED or a series of LEDs, arranged in series or parallel, with an emission peak at 520 nm and a Full Width at Half Maximum of 20 nm; at least one LED or a series of LEDs, arranged in series or parallel, with emission peak at 630 nm and FWHM width at half height (Full Width at Half Maximum) of 20 nm; at least one LED or a series of LEDs, arranged in series or parallel, with an emission peak at 830 nm and FWHM width at half height (Full Width at Half Maximum) of 30 nm and laterally to the night vision camera (8) is arranged a through hole (9) placing it on the surface of a plant cover to be monitored; b) Activation of white monochromatic LEDs, which emit in the wavelength range between 400 and 800 nm, to acquire real color images of the vegetation cover to be monitored; c) Activation of non-band LEDs, blue with emission peak at 460 nm and FWHM width at half height (Full Width at Half Maximum) of 20 nm, green with emission peak at 520 nm and FWHM width at half height (Full Width at Half Maximum) of 20 nm, red with peak emission at 630 nm and FWHM width at half height (Full Width at Half Maximum) of 20 nm, and infrared with emission peak at 830 nm and FWHM width at half height (Full Width at Half Maximum) of 30 nm, to obtain monoband images for each type of LED of the plant cover to be monitored; d) devignetting and cropping of the images obtained in stages b) and c); e) Radiometric calibration of the images obtained in stage d) by applying radiometric calibration masks, to transform the digital number values of the individual pixels of each image obtained in stage d) into reflectance values (R). f) calculation of vegetation indices: in which RNIR is the image containing for each pixel the reflectance values in the infrared band obtained in step e), RRED is the image containing for each pixel the reflectance values in the red band obtained in step e), RGREEN is the image containing for each pixel the reflectance values in the green band obtained in step e), RBLUE is the image containing for each pixel the reflectance values in the blue band obtained in step e). 2) Method for monitoring and characterizing vegetation cover according to claim 1) in which in stage d) corrective masks are used, predetermined during the preparation and calibration of the device, by means of image acquisition for each spectral band, R red, G green, B blue, infrared IR on a flat and homogeneous reference surface, analysis of the variability of the digital number values for each of the reference images acquired along the main and diagonal axes of the same and verification of the radial symmetry of the vignetting effect , calculation for each wavelength of the devignetting mask obtained by calculating for each pixel the ratio ∆DN between the digital number value in the central area of the image and the DN value of the pixel itself, and further corrective masks of cropping which for each wavelength eliminates the peripheral bands made up of pixels affected by aber optical rations that cannot be corrected in the devignetting phase. 3) Method for monitoring and characterizing vegetation cover according to claim 1) in which in step e) the radiometric calibration masks are predetermined in a device calibration phase in which R red, G green, B blue images are acquired , Infrared IR of a flat reference surface obtained by juxtaposing reference panels with known and homogeneous reflectance ranging from 2% to 99%, the average value of the digital numbers of the relative pixels is extracted for each reference panel and for each spectral band and is calculated for each spectral band, of the linear transformation equation between digital number and spectral reflectance by means of a linear regression technique with the least squares method, according to the following equation: in which ai and bi are the values of the intercept and slope parameters obtained with the least squares method for each LEDi source, radiometric calibration masks containing the values of the parameters ai and bi are calculated and stored for each spectral band.