1. UoL MSc Remote Sensing Dr Lewis plewis@geog.ucl.ac.uk
Radiative Transfer Theory at Optical wavelengths applied to vegetation canopies: part 1
UoL MSc Remote Sensing
Dr Lewis

2. Aim of this section
Introduce RT approach as basis to understanding optical and microwave vegetation response
enable use of models
enable access to literature

3. Scope of this section Introduction to background theory
RT theory
Wave propagation and polarisation
Useful tools for developing RT
Building blocks of a canopy scattering model
canopy architecture
scattering properties of leaves
soil properties

4. Associated practical and reading
Course notes for this lecture
Reading list

5. Why build models? Assist data interpretation Study sensitivity
calculate RS signal as fn. of biophysical variables
Study sensitivity
to biophysical variables or system parameters
Interpolation or Extrapolation
fill the gaps / extend observations
Inversion
estimate biophysical parameters from RS
aid experimental design
plan experiments

7. Radiative Transfer Theory
Applicability
heuristic treatment
consider energy balance across elemental volume
assume:
no correlation between fields
addition of power not fields
no diffraction/interference in RT
can be in scattering
develop common (simple) case here

8. Radiative Transfer Theory
Case considered:
horizontally infinite but vertically finite plane parallel medium (air) embedded with infinitessimal oriented scattering objects at low density
canopy lies over soil surface (lower boundary)
assume horizontal homogeneity
applicable to many cases of vegetation

9. Building blocks for a canopy model
Require descriptions of:
canopy architecture
leaf scattering
soil scattering

10. Canopy Architecture
1-D: Functions of depth from the top of the canopy (z).

11. Canopy Architecture
1-D: Functions of depth from the top of the canopy (z).
1. Vertical leaf area density (m2/m3)
the leaf normal orientation distribution function
(dimensionless).
3. leaf size distribution (m)

12. Canopy Architecture Leaf area / number density LAI
(one-sided) m2 leaf per m3
LAI

13. Canopy Architecture
Leaf Angle Distribution

14. Leaf Angle Distribution
Archetype Distributions:
· planophile 
· erectophile 
· spherical 
· plagiophile 
· extremophile 

15. Leaf Angle Distribution
Archetype Distributions:

16. Leaf Dimension RT theory: infinitessimal scatterers
without modifications (dealt with later)
In optical, leaf size affects canopy scattering in retroreflection direction
‘roughness’ term: ratio of leaf linear dimension to canopy height
also, leaf thickness effects on reflectance /transmittance

17. Canopy element and soil spectral properties
Scattering properties of leaves
scattering affected by:
Leaf surface properties and internal structure;
leaf biochemistry;
leaf size (essentially thickness, for a given LAI).

18. Scattering properties of leaves
Leaf surface properties and internal structure
optical
Specular
from surface
Smooth (waxy) surface
- strong peak
hairs, spines
- more diffused

19. Scattering properties of leaves
Leaf surface properties and internal structure
optical
Diffused
from scattering at
internal air-cell
wall interfaces
Depends on refractive index:
varies: nm
Depends on total area
of cell wall interfaces

20. Scattering properties of leaves
Leaf surface properties and internal structure
optical
More complex structure (or thickness):
- more scattering
- lower transmittance
- more diffuse

21. Scattering properties of leaves
Leaf biochemstry

22. Scattering properties of leaves
Leaf biochemstry

23. Scattering properties of leaves
Leaf biochemstry

24. Scattering properties of leaves
Leaf biochemstry

25. Scattering properties of leaves
Leaf water

26. Scattering properties of leaves
Leaf biochemstry
pigments: chlorophyll a and b, a-carotene, and xanthophyll
absorb in blue (& red for chlorophyll)
absorbed radiation converted into:
heat energy, flourescence or carbohydrates through photosynthesis

27. Scattering properties of leaves
Leaf biochemstry
Leaf water is major consituent of leaf fresh weight,
around 66% averaged over a large number of leaf types
other constituents ‘dry matter’
cellulose, lignin, protein, starch and minerals
Absorptance constituents increases with concentration
reducing leaf reflectance and transmittance at these wavelengths.

28. Scattering properties of leaves
Optical Models
flowering plants: PROSPECT

29. Scattering properties of leaves
Optical Models
flowering plants: PROSPECT

30. Scattering properties of leaves
leaf dimensions
optical
increase leaf area for constant number of leaves increase LAI
increase leaf thickness - decrease transmittance (increase reflectance)

31. Scattering properties of soils
Optical and microwave affected by:
soil moisture content
soil type/texture
soil surface roughness.

32. soil moisture content Optical
effect essentially proportional across all wavelengths
enhanced in water absorption bands

33. soil texture/type Optical
relatively little variation in spectral properties
Price (1985):
PCA on large soil database
99.6% of variation in 4 PCs
Stoner & Baumgardner (1982) defined 5 main soil types:
organic dominated
minimally altered
iron affected
iron dominated

34. Soil roughness effects
Simple models:
as only a boundary condition, can sometimes use simple models
e.g. Lambertian
e.g. trigonometric (Walthall et al., 1985)

35. Soil roughness effects
Rough roughness:
optical surface scattering
clods, rough ploughing
use Geometric Optics model (Cierniewski)
projections/shadowing from protrusions

36. Soil roughness effects
Rough roughness:
optical surface scattering
Note backscatter reflectance peak (‘hotspot’)
minimal shadowing
backscatter peak width increases with increasing roughness

37. Soil roughness effects
Rough roughness:
volumetric scattering
consider scattering from ‘body’ of soil
particulate medium
use RT theory (Hapke - optical)
modified for surface effects (at different scales of roughness)

38. Summary Introduction Canopy model building blocks
Examined rationale for modelling
discussion of RT theory
Scattering from leaves
Canopy model building blocks
canopy architecture: area/number, angle, size
leaf scattering: spectral & structural
soil scattering: roughness, type, water