4. Transmission

In contrast to the absorption, transmission is the process by which incident radiations pass through the atmosphere and reach the Earth’s surface. Visible wavelengths are able to penetrate to the Earth's surface with little atmospheric interaction, as shown in Figure 5.7. Microwaves and Radio waves have little interaction with the atmosphere, and easily penetrate to the surface.

Transmission refers to how some types of radiation, such as light, can pass through the atmosphere without significant interference. Unlike absorption, where radiation is taken in and transformed into heat, transmission allows radiation to reach the Earth's surface. For example, when sunlight travels through the air, most of it can reach us because it is not absorbed much by atmospheric particles. This is particularly true for visible light, microwaves, and radio waves, which pass easily through the atmosphere.

Consider a clear glass window. When sunlight hits the glass, most of it passes through into the room without being absorbed by the glass. Similarly, when sunlight enters the Earth's atmosphere, much of it continues to travel unhindered, reaching the surface where we can benefit from its warmth and light.

Atmospheric windows are those regions in the atmosphere where transmission of EMR from source to object and back to the sensor is maximum, and other losses are minimum. In optical remote sensing, many wavelengths within the visible and infrared portions of the electromagnetic spectrum (0.40-2.50 µm) have the ability to penetrate the Earth's atmosphere.

Atmospheric windows are specific wavelengths in the electromagnetic spectrum where radiation can pass through Earth's atmosphere with minimal absorption or scattering. These windows are crucial for remote sensing as they allow satellites to collect clear data. For instance, many wavelengths in the visible range and parts of the infrared spectrum are considered atmospheric windows, as they can effectively reach the Earth's surface and provide valuable information back to sensors.

Think of atmospheric windows like the holes in a sieve. Just as a sieve allows water to pass through while filtering out solids, atmospheric windows let certain wavelengths of light reach Earth while filtering out others. This allows satellites to 'see' through the atmosphere and gather images and data about the Earth’s surface.

Such wavelength regions, as shown in Figure 5.7, white regions in the curve, are considered to be very suitable in remote sensing. In these regions, sensors may be designed to capture the reflected radiations, providing images with good contrast. It is therefore important that a sensor is designed to operate in these regions where the atmospheric losses are minimum.

When designing sensors for remote sensing, it is vital to consider the atmospheric windows because these are the wavelengths that provide the clearest views of the Earth's surface. Sensors operating in these windows can capture better quality images since there are fewer atmospheric losses. This results in higher contrast and clearer data, making it easier to analyze various features on the ground.

Imagine trying to take a photograph through a foggy window. The picture will be blurred due to the fog interfering with the light. However, if you open the window and take the photo outside in clear air, you'll get a far better image. Similarly, sensors effectively capturing data through atmospheric windows can provide clearer images of the features on Earth.

These windows are found in the visible, near-infrared, certain bands in thermal infrared and the microwave regions, as given in Table 5.2.

Atmospheric windows cover specific parts of the electromagnetic spectrum which are optimal for remote sensing. They exist in various ranges, including visible light, near-infrared, some thermal infrared bands, and microwave regions. Understanding these regions helps scientists and researchers select the appropriate wavelengths for monitoring the Earth's surface effectively.

Consider a television remote control: it works best when pointed directly at the television's sensor, as the infrared signals can pass through the air without significant loss. Just like those infrared signals, specific wavelengths in these atmospheric windows can travel through the atmosphere effectively, allowing remote sensors to function similarly for capturing images of the Earth.

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