Reflectance

Energy Interactions, Spectral Reflectance and Colour Readability in Satellite Imagery

   All matter is composed of atoms and molecules with particular compositions.

   Therefore, matter will emit or absorb electro-magnetic radiation on a particular wavelength with respect to the inner state.

   All matter reflects, absorbs, penetrates and emits Electro-magnetic radiation in a unique way.

   Electro-magnetic radiation through the atmosphere to and from matters on the earth's surface are reflected, scattered, diffracted, refracted, absorbed, transmitted and dispersed.

   For example, the reason why a leaf looks green is that the chlorophyll absorbs blue and red spectra and reflects the green.

   The unique characteristics of matter are called spectral reflectance characteristics.

Refraction, Reflection and Absorption:

When electro-magnetic energy is incident on any given earth surface feature, three fundamental energy interactions with the feature are possible refraction, reflection and absorption.

A full understanding of how this works needs quantum mechanics, but the general idea is as follows:

        There are charges electrons in glass that are able to oscillate in response to an applied external oscillating electric field, but these charges are tightly bound to atoms, and only oscillate at certain frequencies.

        It happens that for ordinary glass none of these frequencies correspond to those of visible light, so there is no resonance with a light wave, and hence little energy absorbed.

        Glass is opaque at some frequencies outside the visible range (in general, both in the infrared and the ultraviolet).

        These are the frequencies at which the electrical charge distribution in the atoms or bonds can naturally oscillate.

        A piece of metal has electrons free to move through the entire solid. This is why metals can conduct electricity. It is also why they are shiny.

        These unattached electrons oscillate together with large amplitude in response to the electrical field of an incoming light wave.

        They themselves then radiate electromagnetically, just like a current in an antenna. This radiation from the oscillating electrons is the reflected light. In this situation, little of the incoming radiant energy is absorbed, it is just reradiated, that is, reflected.

        Soot, like a metal, will conduct an electric current, although not nearly so well.

        There are unattached electrons, which can move through the whole solid, but they keep bumping into things they have a short mean free path.

        When they bump, they cause vibration, like a pinball machine, so they give up energy into heat.

        Although the electrons in soot have a short mean free path compared to that in a good metal, they move very freely compared with electrons in atoms, so they can accelerate and pick up energy from the electric field in the light wave.

        They are therefore effective intermediaries in transferring energy from the light wave into heat.

Spectral Reflectance & Colour Readability

Two points about the above given mechanisms should be noted.

  1. The proportions of energy reflected, absorbed, and transmitted will vary for different earth features, depending upon their material type and conditions. These differences permit us to distinguish different features on an image.
  2. The wavelength dependency means that, even within a given feature type, the proportion of reflected, absorbed, and transmitted energy will vary at different wavelengths. 

    Thus, two features may be distinguishable in one spectral range and be very different on another wavelength brand.

    Within the visible portion of the spectrum, these spectral variations result in the visual effect called COLOUR.

    For example we call blue objects 'blue' when they reflect highly in the 'blue' spectral region, and so on.

    Thus the eye uses spectral variations in the magnitude of reflected energy to discriminate between various objects.

Spectral Reflectance Curves

    A graph of the spectral reflectance of an object as a function of wavelength is called a spectral reflectance curve.

    The configuration of spectral reflectance curves provides insight characteristics of an object and has a strong influence on the choice of wavelength region(s) in which remote sensing data are acquired for a particular application.

     This is illustrated in figure 1, which shows highly generalized spectral reflectance curves of deciduous and coniferous trees. (In the discussion, we use the terms deciduous and coniferous somewhat loosely, referring to broad-leaved trees, such as Oak and Maple, as deciduous and to needle-bearing trees, such as pine and spruce, as coniferous.).

    It should be noted that the curve for each of these object types is plotted as a 'ribbon' (or 'envelope') of values, not as a single line. This is because spectral reflectances vary somewhat within a given material class.

    That is, the spectral reflectance of one deciduous tree species and another will never be identical. Nor will the spectral reflectance of trees of the same species ever be exactly equal.

Figure 2 shows the typical spectral reflectance curves for three basic types of earth feature: 

  • Green vegetation
  • Soil
  • Water.

   The lines in this figure represent average reflectance curves compiled by measuring large sample features.

   It should be noted how distinctive the curves are for each feature. In general, the configuration of these curves is an indicator of the type and condition of the features to which they apply.

   Although the reflectance of individual features will vary considerably above and below the average, these curves demonstrate some fundamental points concerning spectral reflectance.

Colour Discrimination based on Wavelengths of Spectral Reflectances.

 

Landsat 7
Band #

Wavelength Interval

Spectral Response

Applications

1

0.45-0.52 m

(0.45-0.52)*

Blue-Green

Sensitive to sedimentation, coastal water mapping, soil/vegetation discrimination, forest classification,
man-made feature identification

2

0.52-0.60 m

(0.52-0.59)*

Green

Vegetation discrimination and health monitoring, man-made feature identification, rock-soil discrimination, turbidity and bathymetry in shallow waters

3

0.63-0.69 m

(0.62-0.68)*

Red

Sensitive to chlorophyll absorption, plant species identification, man-made feature identification, differentiation of soil and geological boundary

4

0.76-0.90 m

(0.77-0.86)*

Near IR

Soil moisture monitoring, vegetation monitoring, water body discrimination, landform/geomorphic studies.

5

1.55-1.75 m

Mid-IR

vegetation moisture content monitoring

6

10.40-12.50 m

Thermal IR

Surface temperature, vegetation stress monitoring, soil moisture monitoring,
cloud differentiation, volcanic monitoring

7

2.08-2.35 m

Mid-IR

Mineral and rock discrimination, vegetation moisture content

* Wavelength interval of IRS-IA/IB LISS I and LISSII
Spatial Resolution of Linear imaging self scanning (LISS): LISS-I (72.5 m) and LISS-II (36.25m)

Notes & Handouts

The Himalayas

Kumaon Himalayas

Askot Basemetals

University

   


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S. Farooq

Department of Geology

Aligarh Muslim University, Aligarh - 202 002 (India)

Phone: 91-571-2721150

email: farooq.amu@gmail.com