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Sensors 
When a 'normal camera' takes a picture, its film is exposed to visible radiation (light); it is then developed to generate a
picture, in colour or black and white. So when you take a photograph of a house, what you see on the picture is what you see in reality. The electromagnetic spectrum In remote sensing, sensors are also able to acquire information that the human eye cannot normally see using radiation in other parts of the electromagnetic spectrum than the visible. An example that you might know about are the special viewers that enable users to see in the dark just like in spy and action movies. These viewers enhance what is seen using infrared illumination.
Do you know why these glasses are called infrared viewers?
If you do not know the answer, read the following paragraphs, then try again. * See Answer 1 at the bottom of the page Objects normally reflect some of the light that reaches them. This part of light generally 'gives' the objects their colour. For example, if you see that a plant is green, it is because the plant reflects part of the light that the eye sees as green and an apple is red because it reflects part of the light that corresponds to red. Objects not only reflect light that reaches them, but also emit 'radiation', mainly because of their temperature. For example, a fire emits warmth and light, so even when it is dark you can see the fire. By comparison, when it is dark, you cannot see an apple. This is because an apple only reflects light, whereas fire emits light as well.
Can you give three other examples of objects that emit radiation and three examples of objects that reflect radiation?
* See Answer 2 at the bottom of the page More information on the radiation of bodies can be found on the 'Radiation of bodies' viewgraph. Light is emitted and reflected by objects in the form of 'radiation'. Radiation is a propagating wave in space with magnetic and electric components. It is also called electromagnetic radiation. Electromagnetic radiation carries energy, which may be imparted when it interacts with matter. In fact radiation is a way to transfer energy from one point to another. Electromagnetic waves are characterised by different wavelengths, like waves in the sea. For instance, the different colours of light have a different wavelength each. Therefore, in order to measure radiation emitted or reflected by objects, it is necessary to measure their energy at different wavelengths. Objects reflect or emit wavelengths that cannot be seen by the human eye, like, for example, infrared.
The collection of all possible wavelengths is called the 'electromagnetic spectrum'. In order to exemplify the spectrum's contents, scientists have summarised the electromagnetic spectrum as shown in the graphic below.
Can you give three more examples of objects from which the radiation emitted cannot be seen by the naked eye?
* See Answer 3 at the bottom of the page.
In remote sensing, many different sensors are used that have varying sensitivities to radiations at different wavelengths in the electromagnetic spectrum. For example, some sensors are designed to receive all 'green' wavelengths, whilst others are more targeted towards infrared wavelengths. The infrared viewer, for example, is specially made to 'see' objects emitting infrared radiation, even in the dark.
In general terms, sensors that use external energy sources to 'observe' an object (for example the sunlight to observe the Earth) are called 'passive sensors'.
Passive sensors' main disadvantage is that if the sky is covered with clouds, they cannot be used to observe the Earth's surface or the oceans. Therefore, another type of sensor must be used. This is called an 'active sensor' because it relies on its own sources of radiation to 'illuminate' objects so that the energy reflected and returned to the sensor may be measured.
The most common active sensor used in remote sensing is the radar.
* Answer 1: Infrared viewers are devices made to 'see' objects in the dark by emitting infrared radiation.
* Answer 2: Emission examples: light bulb, microwave oven, the Sun .
* Answer 3: Ultra-violet radiation of the Sun, microwaves from a mobile phone network, gamma rays emitted by radioactive rock, X-rays used in
hospitals, etc. Last update: 7 December 2009
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