GEOGRAPHY.

SPECTRAL REFLECTANCE OF VEGETATION

The spectral characteristics of vegetation vary with wavelength. A compound in leaves called chlorophyll strongly absorbs radiation in the red and blue wavelength but reflect green wavelength. The internal structure of health leaves act as diffuse reflector of near infrared wavelengths. Measuring and monitoring the infrared reflectance is one way scientists determine how healthy particular vegetation may be. Leaves appear greenest to us is summer and become red or yellow with decrease in chlorophyll content in autumn. 

SPECTRAL REFLECTANCE OF WATER

Majority of radiation incident upon water is not reflected but either is absorbed or transmitted. Longer wavelength and near-infrared radiations are absorbed more by water than the visible wavelengths. These water looks blue or blue-green due to stronger reflectance at these shorter wavelengths, and darker if viewed at red or near – infrared wavelengths. The factors that affect the variability in reflectance of water body are depth of water, materials within water and surface roughness of water.

SPECTRAL REFLECTANCE OF SOIL

The majority of radiation on a surface is either reflected or absorbed and little is transmitted. The characteristics of soil that determine its reflectance properties are its moisture content, texture, structure iron-oxide content. The presence of moisture in soil decreases its reflectance.

By measuring the energy that is reflected by targets and earth’s surface over a variety of different wavelengths spectral signature for that object can be made. And by comparing the response pattern of different features we may be able to distinguish between them.

SPECTRAL REFLECTANCE CURVES

Is a graph of spectral reflectance of an object is a function of wavelength. Configuration of the spectral reflectance curves gives us or insights into the spectral characteristics of an object. Spectral reflectance curves guides us in selecting wavelengths region(s) in which remote sensing data should be acquired for the given science goal.

Or is a graphical representation of the reflectance of a specified material over several wavelengths of electromagnetic radiation.

It is plotted between reflectance (y-axis) and wavelength (x-axis). It can be obtained from spectral radiometer. Reflectance curve highlights the particular wave length where the absorption is noticed such as where the some part of the radiant energy is absorbed by the object. Reflectance curve for white surface is a straight line as there no absorption.

In reflectance curve there is reflectivity values of elements. Reflectivity means that, the ability of being able to reflect light or the quality of an element to reflect light.

Remote sensing is based on the measurement of reflected or emitted radiation from different bodies. Objects having different surface features reflect or absorb the sun’s radiation in different ways. The reflectance properties of an object depend on the particular material and its physical and chemical state for example moisture. The surface rightness as well as the geometric circumstances example incidence angle of the sunlight. The most important surface features are colour, structure and surface texture.

These differences make it possible to identify different earth surface features or materials by analyzing their spectral reflectance patterns or spectral signatures. There signatures can be visualized in so called spectral reflectance curves as function of wave length.

The spectral reflectance curves contain three basic types of Earth features which are like Green vegetation, dry bare soil and clear water.

The spectral reflectance curve of green vegetation has a significant minimum of reflectance in the visible portion of electromagnetic spectrum resulting from the pigments in plants leaves. Reflectance increases dramatically in the near infrared. Stressed vegetation can also be detected because stressed vegetation has a significantly lower reflectance in the infrared.

The spectral reflectance curve of bare soil is considerably less variable. The reflectance curve is affected by moisture content, soil texture, surface roughness, presence of iron oxide and organic matter. These factors are less dominant than the absorber factors are less dominant than the absorber features observed in vegetation reflectance spectral.

The spectral reflectance curve of water is characterized by a high absorption at near infrared wavelengths range and beyond. Because of this absorption property water bodies as well as features containing water can easily detected, located and delineated with remote sensing data. Turbid water has a higher reflectance in the visible region than clear water. This is also true for water containing high chlorophyll is concentrated. These reflectance patterns are used to detect algae colonies as well as contaminations such as oil spills or industrial waste in water.

                                                       BYEE