Depolarizing metrics for plant samples imaging

Abstract

Optical methods, as fluorescence microscopy or hyperspectral imaging, are commonly used for plants visualization and characterization. Another powerful collection of optical techniques is the so-called polarimetry, widely used to enhance image contrast in multiple applications. In the botanical applications framework, in spite of some works have already highlighted the depolarizing print that plant structures left on input polarized beams, the potential of polarimetric methods has not been properly exploited. In fact, among the few works dealing with polarization and plants, most of them study light scattered by plants using the Degree of Polarization (DoP) indicator. Other more powerful depolarization metrics are nowadays neglected. In this context, we highlight the potential of different depolarization metrics obtained using the Mueller matrix (MM) measurement: the Depolarization Index and the Indices of Polarimetric Purity. We perform a qualitative and quantitative comparison between DoP- and MM-based images by studying a particular plant, the Hedera maroccana. We show how Mueller-based metrics are generally more suitable in terms of contrast than DoP-based measurements. The potential of polarimetric measurements in the study of plants is highlighted in this work, suggesting they can be applied to the characterization of plants, plant taxonomy, water stress in plants, and other botanical studies.

Fig 1

Plant sample used for the polarimetric analysis: (a) Hedera maroccana plant; (b) Hedera maroccana measured leaf (Sample A).

Fig 2. Scheme of the imaging polarimeter used to measure the Mueller matrices of plant leaves.

Fig 3

(a) Nonpolarized light intensity image of the Hedera maroccana (Sample A) obtained on transmission. (b)-(d) DoP image for an input polarization: (b) linear polarization at 0°; (c) linear polarization at 45°; (d) left-handed circular polarization. Input polarization is marked in red at the top-right corner of the images.

Fig 4. Poincaré sphere showing the location of the input polarizations (N = 200) used to study the dependence of the input polarization on the DoP-based images contrast.

Fig 5. DoP values as a function of the input linear polarization orientations.

Orange (squared), red (circle), blue (triangle) and purple (inverted triangle) curves correspond to the segments of the same color in Fig 3A.

Fig 6. Visibility values calculated from DoP images corresponding to 200 different input polarizations (k parameter).

(a) Orange curve and blue curve provide the visibility related to the orange and blue segments in Fig 3A, respectively; and (b) Mean visibility (black curve) as a function of the input polarization, calculated from 10 different segments (orange, blue and 8 green lines in Fig 3A) arbitrarily selected along the leaf. The corresponding standard deviations values are given by the upper and lower dashed black lines.

Fig 7

Hedera maroccana (Sample A) images obtained by using different depolarizing based indicators: (a) Depolarization Index P_Δ; (b)-(d) Indices of Polarimetric Purity (IPPs), P₁, P₂ and P₃. Sample A images obtained by combining different IPP channels: (e) P₂-P₁; and (f) P₃-P₂.