Redefining the Directional-Hemispherical
Reflectance and Transmittance of Needle-
Shaped Leaves to Address Issues in Their Existing
Measurement Methods
Jun Wang, Jing M. Chen, Lian Feng, Jianhui Xu, and Feifei Zhang
Abstract
The directional-hemispherical reflectance and transmit-
tance 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 incom-
ing radiation. Through theoretical analysis, we found that
needle directional-hemispherical reflectance and transmit-
tance measured by two existing techniques, namely Daugh-
try’s method and Harron’s method, could be significantly
biased. This finding was proved by ray-tracing simula-
tions 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 con-
vex needle surfaces should be avoided, and multiple scat-
tering of radiation among needles should be minimized.
Introduction
Needle-leaved plants represent a significant fraction of
natural terrestrial ecosystems. For example, the boreal forest,
the second-largest needle-leaf-dominate
world (Astrup
et al.
2018), covers ~50%
can boreal zone (Brandt
et al.
2013) and
the Earth’s forest area (MacDicken
et al.
ing the temporal and spatial variation of needle-leaved plants
is in the interest of studies of global change. The advent of
spectroscopy and remote sensing has made such monitor-
ing more efficient, convenient, and intuitive. The accuracy
of relevant results, as well as the validity of corresponding
conclusions, heavily depends on the fidelity of collected leaf
spectra, which are the basic data required for retrieving leaf
biochemical and biophysical traits.
An integrating sphere is a device commonly used in the
remote-sensing community for leaf spectral measurements,
due to its solid theoretical basis (Jacquez and Kuppenheim
1955; Miller and Sant 1958). It is a highly reflective cavity in
appearance, with several holes or ports reserved for attaching
samples, holding a light source or a white reference. When
connected with a spectrometer, signals within the sphere can
be captured so as to produce a reflectance, transmittance, or
absorption curve. In most circumstances, the sample port of an
integrating sphere can be completely covered by broad leaves,
whose reflected and transmitted radiation can be clearly sepa-
rated in this way (Figure 1a). However, needle-shaped leaves
have distinct morphological characteristics. They are always
too narrow to completely cover the sample port of an integrat-
ing sphere. Existing techniques, namely Daughtry’s method
and Harron’s method, measure the directional-hemispherical
reflectance (
DHR
) and transmittance (
DHT
) of needle-shaped
leaves by putting an array of needle samples into a sample
holder, which has to be no smaller than the sample port to
be attached (the sample holder is also called a
carrier
in other
studies). As already mentioned, an integrating sphere only
Jun Wang and Lian Feng are with the School of Environmental
Science and Technology, Southern University of Science and
Technology, Shenzhen, China (
).
Jing M. Chen is with the International Institute for Earth
System Science, Nanjing University, Nanjing, China
(
).
Jianhui Xu is with the Key Laboratory of Guangdong for
Utilization of Remote Sensing and Geographical Information
System, Guangdong Open Laboratory of Geospatial
Information Technology and Application, Guangzhou
Institute of Geography, Guangzhou, China; and the Southern
Marine Science and Engineering Guangdong Laboratory,
Guangzhou, China.
Feifei Zhang is with the Department of Computer Science,
Guangdong University of Education, Guangzhou, China.
Photogrammetric Engineering & Remote Sensing
Vol. 86, No. 10, October 2020, pp. 627–641.
0099-1112/20/627–641
© 2020 American Society for Photogrammetry
and Remote Sensing
doi: 10.14358/PERS.86.10.627
Figure 1. Spectral measurements of (a) a broad leaf and (b)
a needle with an integrating sphere. A magnifier is used
to zoom in on the needle in (b). The needle, with convex
surfaces, is narrower than the sample port of the integrating
sphere. Part of the reflected radiation (pointed out by purple
circles) goes in the forward direction away from the light
source and is mixed with transmitted radiation. As a result,
it is misclassified.
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
October 2020
627
