1st Workshop BOOK - project RHEA

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Hyperspectral imagery to discriminate weeds in wheat the relative response matrix for each detector element (or pixel) and the camera spectral sensitivity. The resulting float type value CN[i,j] for each line pixel is then a luminance signal. It will be noted L(x,y,�) in the following, where x is the spatial coordinate i in the line, y is the spatial coordinate of the line along track, and � is a given wavelength. This luminance correction can be made by post-processing, or in real-time during image acquisition. An example of corrected spectrum is given in Fig. 2.b. Reflectance correction In order to be able to compare spectral data collected in different outdoor conditions, we need to have hyperspectral images independent of illumination, i.e. reflectance images. The reflectance of a given material is the ratio of reflected light to incident light. So, we need to know solar lighting at each acquisition time. To this end, Spectralon� (Labsphere, Inc., New Hampshire, USA.) is generally used because it is a lambertian surface and it reflects 99 % of received signal whatever the wavelength. Therefore, it provides a good approximation of solar incident light in outdoor conditions. However, in our case, we have chosen to use a commercial ceramic plate, which is more robust to damage or dirt due to field experiment conditions. Also, since integration time is optimised for vegetation, a grey ceramic plate was required, in order to avoid saturated ceramic pixels. This plate has been set in the field of view for every image acquisition (see Figs 5 and 7). The bidirectional reflectance distribution function (BRDF) of the ceramic has been measured in laboratory. As for many ordinary materials, it is the summation of a lambertian term and a specular term (shininess), this last one depending on the incident and viewing angles. However, because the specular term is directive enough, it can be totally neglected in our field operating conditions (horizontal plate observed with a zenithal view under non zenithal solar lighting incidence, according to the latitude and season). We can thus consider the ceramic plate as a lambertian material with a known hemispheric reflectance Rc(�). Finally, for a given luminance image, the average luminance Lc(�) measured on the ceramic plate can be used to compute the horizontal irradiance E(�) on the scene: 38 E( �) � Lc( �) / Rc( �) (2) This allows to apply the following reflectance correction to every pixel in the image: L( x, y, �). Rc( �) R( x, y, �) � L( x, y, �) / E( �) � (3) Lc( �) where (x,y) are the pixel coordinates.
Robotics and associated High technologies and Equipment for Agriculture Fig. 2. Example of 12 bits raw spectrum (a), luminance spectrum (b) and reflectance spectrum (c) for a vegetation pixel The effect of successive luminance and reflectance corrections on a vegetation pixel are given in Fig. 2. The reflectance spectrum clearly shows how lighting a) b) c) 39
Page 1: RHEA-2011 ROBOTIICS AND ASSOCIIATED
Page 5: The research leading to these resul
Page 9 and 10: FOREWORD These proceedings are the
Page 11 and 12: CONTENTS Weed management A Five-Ste
Page 13: A Five-Step Approach for Planning a
Page 16 and 17: A Five-Step Approach for Planning a
Page 26 and 27: Effect of flaming with different LP
Page 37 and 38: Robotics and associated High techno
Page 45: A Five-Step Approach for Planning a
Page 48 and 49: Hyperspectral imagery to discrimina
Page 60 and 61: Strategies for video sequence stabi
Page 73 and 74: Camera System geometry for site spe
Page 79 and 80: 4. Methods Robotics and associated
Page 85 and 86: Camera System geometry for site spe
Page 91 and 92: 3. Experiments Robotics and associa
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Robotics and associated High techno
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Techniques for Area Discretization
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2.3 Temperature sensor cards Roboti
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2.5 Principal points of interest Ro
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Temperature (ºC) Temperature (ºC)
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4. Conclusions Robotics and associa
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3. Results and Discussion Robotics
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1.3. Time management in Webots Robo
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3.2 Risk assessment Robotics and as
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References Robotics and associated
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1st Workshop BOOK - project RHEA

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