Redefining the Directional-Hemispherical Reflectance and Transmittance of Needle-Shaped Leaves to Address Issues in Their Existing Measurement Methods

The directional-hemispherical reflectance and transmittance of needle-shaped leaves are redefined in this study. We suggest that the reflected and transmitted radiation of a leaf should be distinguished by the illuminated and shaded leaf surfaces rather than the usual separation of the two hemispheres by a plane perpendicular to the incoming radiation. Through theoretical analysis, we found that needle directional-hemispherical reflectance and transmittance measured by two existing techniques, namely Daughtry's method and Harron's method, could be significantly biased. This finding was proved by ray-tracing simulations intuitively as well as by inversions of the PROSPECT model indirectly. We propose the following requirements for needle spectral measurement in an integrating sphere: needles should be fully exposed to the light source, the interfusion of reflected and transmitted radiation on convex needle surfaces should be avoided, and multiple scattering of radiation among needles should be minimized.