Inferring spectral reflectances of plant elements by simple inversion of bidirectional reflectance measurements

• Published in: Remote sensing of environment 1987-03, Vol.21 (2), p.215-228
• Main Authors: Otterman, J., Strebeli, D.E., Ranson, K.J.
• Format: Article
• Language: English
• Subjects: ABIES BALSAMEA; Agronomy. Soil science and plant productions; Animal, plant and microbial ecology; Biological and medical sciences; BIOMASA; BIOMASS; BIOMASSE; CANOPY; COUVERT; CRECIMIENTO; CROISSANCE; CUBIERTA VEGETAL; Earth Resources And Remote Sensing; Fundamental and applied biological sciences. Psychology; General aspects. Techniques; Generalities. Biometrics, experimentation. Remote sensing; GLYCINE MAX; GROWTH; OPTICAL PROPERTIES; PROPIEDADES OPTICAS; PROPRIETE OPTIQUE; Remote sensing; Teledetection and vegetation maps; ZEA MAYS
• ISSN: 0034-4257; 1879-0704
• DOI: 10.1016/0034-4257(87)90054-X

Inverting previously developed explicit expressions for a vertical architecture, bidirectional reflectances measured over corn viewing from the solar quadrant at azimuths near the principal plane are used to determine the spectral reflectances of plant elements. The leaf reflectance values extracted in three visible bands at viewing zenith angles of 70°, 60°, and 45° agree closely with laboratory-measured reflectances of corn leaves. At viewing zenith angle of 30°, the inversion breaks down, inasmuch as the inferred plant element reflectances are too high. Satisfactory results are also achieved when the same approach is applied to bidirectional reflectances measured over potted balsam firs, but when applied to soybeans reflectances, the procedure yields unreasonably high leaf reflectances. The failure in this case is attributed to the nonvertical architecture of the soybean canopy. However, for this canopy, inversion based on horizontal architecture is possible: The bidirectional reflectances measured from the solar quadrant, at viewing angles appreciably far from “hot spot” viewing, approximately equal in magnitude half of the leaf reflectance. The 0.5 ratio is predicted by a previous analysis of opaque horizontal Lambertian facets, as the asymptotic value for a dense canopy at any viewing angle. For soybeans, this ratio applies very closely at 15° viewing zenith angle. The results suggest that inversion based on simple architecture, applying explicit expressions, might be of value, either in itself or as a preliminary step before inversion applying complex models.