5.13.3. Based on the wavelength

Optical Sensors operate in the visible, near infrared, middle infrared and short wave infrared portions of the EMS within a range 0.30 µm to 3.0 µm, e.g., bands of IRS P6 LISS IV sensor work in optical range of EMS.

Optical sensors are devices that capture light energy in various wavelengths, specifically from 0.30 µm (ultraviolet range) to 3.0 µm (infrared range). They are sensitive to visible light and other forms of electromagnetic radiation, allowing them to detect and quantify how much light is reflected by different surface materials. For instance, when sunlight hits a surface, part of that light is absorbed, and part is reflected. The optical sensors measure the reflected light to gather information about the object below. IRS P6 LISS IV sensor is one example that utilizes these wavelengths.

Imagine using a pair of sunglasses with special filters that only let certain colors through. When you look at different objects, the colors appear differently depending on how much of each color is reflected back. Similarly, optical sensors ‘see’ the earth in various light wavelengths, allowing scientists to interpret the conditions or materials present on the surface, like vegetation health, soil type, or water quality.

Thermal infrared energy is emitted as heat by all the objects, both natural and manmade, that have a temperature greater than absolute zero. Even in complete darkness and poor weather conditions, thermal imaging is able to detect small temperature differences.

Thermal sensors detect the infrared radiation emitted by all objects based on their temperature. Every object that has a temperature above absolute zero emits some thermal energy. This type of energy is not visible to the human eye. Thermal sensors can pick up this energy and measure the temperature differences between objects, even when it is dark outside or when other optical signals are blocked by clouds and weather. For example, these sensors can be used to monitor the heat emitted by urban areas to study urban heat islands or to detect wildfires.

Think of how you can feel the heat from a warm cup of coffee even when it's dark. In the same way, thermal sensors pick up on this ‘heat’ energy given off by objects. Just as you can tell which objects are warmer or cooler by feeling their temperature or looking for glowing lights in darkness, thermal sensors offer insights into temperature variations of large areas through satellite or aerial imaging.

The microwave region falls between the IR and radio wavelengths, and has a long range extending from approximately 0.1 cm to 1 m. The microwave sensors, e.g., RADARSAT, having their own sources of energy, record the backscattered microwaves and operate independent of weather and solar energy.

Microwave sensors operate in the microwave region, which lies between infrared radiation and radio waves. These sensors emit microwave energy to illuminate objects and then measure the energy that is bounced back, or backscattered, to them. Their ability to penetrate clouds, fog, and other atmospheric conditions means they can operate day or night, regardless of weather. For instance, RADARSAT is an example of a satellite equipped with microwave sensors that can provide images and data about Earth's surface features despite challenging observation conditions.

Consider how you can use a flashlight to see in a dark, foggy room. The flashlight allows you to light up the space and observe what's around. Microwave sensors work similarly; they send out microwave pulses to illuminate objects and then listen for the echoes that bounce back, creating an image of what is there, even if traditional light is not useful.

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