3.2.1 - Visible region

The visible part of the reflectance spectrum (0.40 μm – 0.70 μm) of vegetation is controlled by the pigments in the green leaf chloroplasts that reside in the outer or Palisade leaf, the Chlorophyll pigments – chlorophyll-a and chlorophyll-b. A pigment is any substance that absorbs light. The colour of the pigment is determined by reflected wavelengths. White pigments/light colours reflect all or almost all of the energy striking them. Black pigments absorb all of the wavelengths that strike them. Chlorophyll is the major absorber of radiation in the visible region and its absorption is dominant in the visible red 0.6 – 0.7 μm – red wavelengths; it is called the green pigment and it is common to all photosynthetic cells. Other leaf pigments also have an important impact on the visible part of the spectrum. The carotene (yellow to orange-red pigment responsible for the colour of some flowers, fruits and leaves without chlorophyll) and xantophyll (responsible for the leaf colour in autumn) have strong absorption in the 0.35 – 0.5 μm, blue wavelengths.

Figure 3.4 – Reflectance spectrum for green grass, for VIS wavelengths.

Figure 3.4. shows a typical reflectance spectrum for green vegetation, for VIS wavelengths. Since absorption occurs in red and blue wavelengths, the predominant reflectance of visible wavelengths is concentrated in the green.

Exercise: Why does vegetation appear green?