Spectral satellite sensors perceive the Earth as grey. This is due to the fact that they detect each wavelength range separately via so-called bands. Only the differences in light intensity are stored here. Blue light is stored in the blue band, green light in the green band and red light in the red band. The higher the number of bands of a remote sensing sensor, the higher the spectral resolution of a satellite. Conventional sensors like the sensors of the satellite Landsat have a spectral resolution of 7 bands (fig.). Three are storing the visible light, three infrared light and one thermal radiation of the Earth's surface.
Bands of a characteristic multispectral sensor. Here, the blue, red, and green light as well as the near and normal infrared range is recorded.
Using several bands for earth observation, these techniques are called multispectral remote sensing. But there are sensors with 200 or more bands as well; those systems are not called multispectral but hyperspectral. Two fundamental conditions have to be considered in order to determine which spectral bands are appropriate for a sensor: On the one hand, the atmospheric window and, on the other hand, the spectral signature of the object in question.
Windows and bands
Spectral satellite sensors can only store those parts of the radiation scattered back from the Earth that can pass the atmosphere. The Earth's atmosphere contains gases and small particles like ice crystals, water drops, or dust. These so-called aerosols absorb and scatter some parts of the electromagnetic spectrum. Those wavelength ranges lost to the satellite sensor due to the presence of the atmosphere are called absorption bands. Those getting through to the sensor are called atmospheric window. The figure below shows the percentage of radiation passing the atmosphere. It is depicted how intense the solar radiation is per wavelength range. It can be seen that the intensity, i.e. the radiation energy, decreases the closer we get to the long-wave range of the electromagnetic spectrum.
Absorption bands of the Earth's atmosphere. The wavelength ranges highlighted in red cannot be perceived by a satellite sensor - it seems to be almost blind. To get information on the mid infrared range, the bands are extended.
Apart from atmospheric windows, the selection of spectral bands has to consider the reflection characteristics of objects on the Earth's surface. As you can see in the animation below, different object on the surface of the Earth reflect differently in the various parts of the electromagnetic spectrum.
Clicking on the buttons on the lower edge of the screen you can read the spectral fingerprints of water, soil and different states of vegetation.
Dry soil reflects highly in the infrared range, whereas water reflects in the visible range of light only. Like every human has its own individual fingerprint, the objects on the surface of the Earth have a spectral fingerprint due to absorption and reflection of light.
So we have to consider whether we need a sensor specialized on distinct objects or not and, according to this, what the spectral resolution should be. The visual light as well as the infrared range is important for multispectral remote sensing. The infrared range follows the range of visible light (0.3-0.7 micrometer) in the electromagnetic spectrum and its wavelengths are between 0.7 und 1000 micrometer. Infrared radiation is invisible to the human eye and is divided into near (NIR), short-wave (SIR) mid-wave (MIR), long-wave (LIR) and thermal (TIR) infrared.
The infrared signal of remote controllers or thermal radiation is an example for infrared spectra. Infrared waves have greater wavelengths than the visible light and are invisible to humans.
As can be seen in the animation above, the reflection characteristics of vital vegetation are very interesting. Vital plants reflect green light greatly, red and blue light partially. This is caused by chlorophyll using the blue and red range of light for photosynthesis but rejecting the green light. The reflection curve has a steep slope in the infrared range and is steady on a high level until it drops down in the range of mid-infrared because of absorption due to the high moisture content in vital vegetation. Why do we see such a leap from red to near infrared, the so-called red edge?
A leaf reflects infrared light twice as much as green light - this is caused by the inner structure and chemical makeup of the leaf.
This leap is caused by the cell walls which reflect infrared light within the cells several times. Due to the high reflection values in the infrared range we see that vital plants (much chlorophyll and stable cell walls) are prominent in the infrared band of satellite images. If humans were able to see infrared light, leaves would not be green but infrared to us.
The spectral fingerprint of a leaf in the course of time.
The animation shows the changes the spectral fingerprint of a leaf undergoes in the course of time. It can be seen clearly that the red edge described above is very pronounced in vital plants. The red edge is reduced as soon as the leaf withers.
Heat radiation and remote sensing
Well-known from infrared lamps, heat radiation is infrared radiation as well. Other than near to long-wave infrared, thermal infrared is not a kind of reflected sun radiation. It is the radiation belonging to the objects on the Earth surface itself originating in the process of absorption. Every object, alive or anorganic, emits heat to its surroundings, some more than others. Remote sensing sensors can detect this radiation via thermal bands. The images have a coarser resolution than other bands because the thermal radiation of the Earth is not as intensive as the reflected sun radiation.
Red and thermal band in the swipe tool. Which is which? (Images by courtesy of USGS/NASA Landsat Program)
The swipe above contains a multispectral image of Berlin in summer. We can see the red and thermal band; the red band does not show great differences between city areas and forest, the thermal band shows the city as a great heated area sticking out from the surroundings. This is caused by the materials used to build a city (asphalt, concrete, cement, bricks a.s.o.) as those materials have a lower albedo and store a great amount of heat. Especially at night, cities are warmer than their surroundings. Because of this heat gradient (= heat difference), cities are called "heat islands".
How remote sensing benefits from thermal radiation can be seen here.
Spectral remote sensing systems store every wavelength in separate bands, producing one grey-scale image per band. The more bands a spectral sensor has, the higher its spectral resolution. Assigning a primary colour to three grey-scale images, a true-colour or false-colour image is formed. Objects reflect and absorb wavelengths of the electromagnetic spectrum differently and, thus, have a characteristic spectral fingerprint. Vital vegetation is characterized by the red edge, meaning the steep rise of the reflection curve between red and infrared light. The spectral resolution of many sensors includes the thermal range of the electromagnetic spectrum.