Atmospheric Window and Reflectance Curve

what is atmospheric window in remote sensing?

The atmospheric Window is very important in remote sensing. we know that not all wavelengths of electromagnetic radiation from the sun reach the earth and not all wavelengths emitted by the earth reach into space. The atmosphere absorbs some of this energy while allowing other wavelengths to pass through. so basically Atmospheric windows are the regions in the atmosphere where the EMR can pass through without intervention.

We can identify the wavelengths that we can use most efficiently for remote sensing by comparing their properties with those of the two most frequent energy/radiation sources—the sun and the earth. Our eyes are most sensitive to the visible spectrum, which corresponds to the sun’s peak energy level and an atmospheric window.

At the gaps in the incoming and outgoing energy, the atmosphere absorbs energy. Even though the atmosphere absorbs the majority of the incoming infrared energy, the earth still emits some of it.
We can view the atmosphere through these “windows” at different wavelengths. All of these channels were picked to present various perspectives of the earth. Based on each satellite’s unique orbit and the technology available at the time each was built, we employ various channels to look into the atmosphere.

Atmospheric Window Definition in Remote Sensing

In remote sensing, the term “atmospheric window” refers to certain regions of the electromagnetic spectrum where the atmosphere is relatively transparent, allowing for the transmission of energy from the observed surface or target. These “windows” allow remote sensing instruments to effectively collect data on the surface without interference from the atmosphere. There are several different atmospheric windows, including the visible and near-infrared regions, the shortwave infrared region, and the thermal infrared region. Each window corresponds to a specific range of wavelengths, and different types of remote sensing instruments are designed to operate within specific windows in order to maximize the quality and accuracy of the data collected.

Atmospheric Window Range

There are several different atmospheric windows in the electromagnetic spectrum that are used in remote sensing, each corresponding to a specific range of wavelengths. The range of wavelengths for each window can vary slightly depending on the specific conditions of the atmosphere and the location of the observation.

• The visible and near-infrared region (sometimes referred to as the “visible window”) is approximately between 400 and 800 nanometers (nm).
• The shortwave infrared region (sometimes referred to as the “shortwave window”) is approximately between 1.2 and 3 micrometers (μm).
• The thermal infrared region (sometimes referred to as the “thermal window”) is approximately between 8 and 14 micrometers (μm)

It is important to note that the specific range for each window is not fixed, it can be affected by the atmospheric conditions, altitude, and topography of the observation location.

Reflectance Curve

Now we got the EMR thrugh the Atmospheric Window, let’s see how its refelect from various objects.

Earth is not a flat place. it has rugged topography, Vegetation, Water, Settlements, Desserts, and lots of things that have different characteristics. So every object reacts differently at a different wavelength, and that’s why we get specific Reflectance from an object. which we call a Reflectac Signature.

If we plot a graph of wavelength and spectral reflectance then we will get a Spectral Reflectance Curve.

The choice of wavelength region in which remote sensing data are acquired for a particular application is strongly influenced by an object’s spectral characteristics.

In order to keep collections of typical curves in “spectral libraries,” reflection curves are often gathered for the optical region of the electromagnetic spectrum. The following examples illustrate some common land cover types’ reflectance characteristics.

Vegetation

The orientation and structure of the leaf canopy, as well as other leaf-related factors, affect how vegetation reflects light. A leaf’s colour, thickness, composition (cell structure), and water content all affect how much radiation is reflected at a given wavelength.

Table illustrates a healthy plant’s optimal reflectance curve. Because the blue and red portions of the incident light are absorbed by the plant (mostly by chlorophyll for photosynthesis), the vegetation reflects proportionally more green light in the visible section of the spectrum. The NIR region exhibits the maximum reflectance, however, the degree varies on leaf growth and cell structure.

Bare soil

It is challenging to provide a single average soil reflectance curve since the reflectance from bare soil depends on so many different variables. The soil’s colour, moisture level, carbonate concentration, and iron oxide content are the key determinants of reflectance.

Water

Water has a lower reflectance than vegetation and soils. Vegetation may reflect up to 50% and soils up to 30–40%, however, water reflects at most 10% of the incident radiation. In the visible and a little amount in the NIR ranges, water reflects electromagnetic radiation. All radiation is absorbed over 1.2 m.

Figure displays spectral reflection curves for water with various compositions.

Water that is turbid or silt-loaded reflects light the most. Due to the presence of chlorophyll, water-containing plants or algae exhibit a noticeable reflectance peak for the green light.

as this way diffident objects refelects diffrent with various bands. Every object have its diffrent Refelectance Signature and it makes diffrent Refelectance Curve.

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References:

• Spectral reflectance curves, Living Textbook, ltb.itc.utwente.nl.
• The Atmospheric Window, National Weather Service, www.weather.gov.
• Fundamentals of Remote Sensing, Canada Centre for Remote Sensing.