EMR PRINCIPLES

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ELECTROMAGNETIC RADIATION PRINCIPLES Introduction Energy recorded by a remote sensing system undergoes fundamental interactions that should be understood to properly interpret the remotely sensed data. The process of the energy when its travels to target can be summarizing as:

Is radiated by atomic particles at the sources (the sun)

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Travels through the vacuum of space at the speed of light

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Interacts with the earth’s atmosphere

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Interacts with the earth surface

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Interacts with the earth’s atmosphere once again and

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Finally reaches the remote sensors, where it interacts with various optics, filters, film emulsions, or detectors.

Electromagnetic Radiation EMR was define as an “electromagnetic wave that travels through the space at a speed of light” (James Cleark Maxwell, 1860s). Light, electricity, and magnetism are manifestations of the something called electromagnetic radiation (EMR). Sunlight and energy is a form of EMR. This energy also comes in many forms that are not detectable with our eyes such as infrared(IR), radio, X-rays, ultraviolet(UV), and gamma rays, etc. The form of energy that our eyes can detect is called “visible” or “optical” light.

Electromagnetic radiation is energy that travels in waves. Waves has measureble properties that help us describing the radiation, icluding wavelength, amplitude, frequency and velocity.

The point of maximums of upwards diaplacements of a wave is called crest and the area of maximums downward displacement is called a through. A wave amplitude is defined as the magnitudes (or distance) of the vertical displacements caused by the wave, or as the height of the wave. The wavelength of the wave is defined as a distance between two successive crests or (or between two successive troughs). Frequency is measure of how many wave pass a fixed point at a given unit of time, and is therefore dependent on the speed (velocity) at which the wave is travelling. Since all electromagnetic energy travel at a constant speed, the wavelength and frequency of different energy types are inversely related. In other words, for electromagnetic energy, general rules states that, the longer the wave, the lower the frequencies and the shorter the wave, the higher the frequency. All electromagnetic radiation has fundamental properties and behaves in predictable ways according to the basics of wave theory. Electromagnetic radiation consists of an electrical field (E) which varies in magnitude in a direction perpendicular to the direction in which the radiation is traveling, and Siti Nor Maizah Saad JSUG / FSPU / UiTM Perlis

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a magnetic field (M) oriented at right angles to the electrical field. Both these fields travel at the speed of light (c).

Figure 1: Electromagnetic Radiation Model

â–ş wavelength and frequency are related to the following formula:- c = ĘŽv Electromagnetic Spectrum The

sensors

onboard

remote

sensing

platforms

record

energy

transmitted

as

electromagnetic radiation. The sensors contain detectors to record specific wavelengths within the Electromagnetic Spectrum (EMS). In remote sensing terminology, portions of the EMS are often called bands, or spectral bands (See Figure 2).

Figure 2: Electromagnetic Spectrum Model

The Electromagnetic Spectrum is defined by wavelength. It ranges from very short wavelength cosmic rays to the long wavelength of standard radio waves. Typical aerial photography and infrared aerial photography are taken in the visible and photographic infrared bands, which range from 0.4 to 0.9 micrometers.

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As electromagnetic radiation moves from one medium to another it can be reflected, absorbed, transmitted, or refracted. Different types of land cover absorb and reflect different portions of the electromagnetic spectrum.

Different devices, however, can measure most of the major spectral regions.The division of the spectral wavelength is based on the devices which can be used to observe particular types of energy, such as thermal, shortwave infrared and microwave energy.

Visible/Optical Spectrum - The light which our eyes can detect forms the visible spectrum. It is important to note how small a portion of the electromagnetic spectrum is represented by the visible region.

Figure 3: Visible Portion

The visible wavelengths cover a range from approximately 0.4 µm to 0.7 µm. the longest visible wavelength is red and the shortest is violet. Common wavelength is of what we perceive as particular colours from the visible portion of a spectrum are listed below. It is important to note that this is the only portion of the spectrum that we can associate with the concept of colours.      

Violet (0.4 – 0.446 µm) Blue (0.446 – 0. 500 µm) Green (0.5 – 0.578 µm) Yellow ( 0.578 – 0.592 µm) Orange (0.592 – 0.620 µm) Red (0.592 – 0.7 µm)

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Ultra Violet (UV) UV portion have the shortest wavelength. This radiation is just beyond the violet portion of the visible wavelengths.

Ultra Violet Region

Infra Red The infra red IR region covers the wavelength range at approximately 0.7µm to 100 µmmore than 100 times as wide as visible portion.

The infrared can be divided into two

categories: the reflected IR or emitted or thermal IR. Radiation in the reflected region is use for remote sensing purposes in ways very similar to radiation in visible portion. The reflected IR cover wavelength from approximately 0.7 to 3.0 µm. the thermal region is quite different than visible and reflected IR portions, as this energy is essentially the radiation that emitted from the earth surface in the form of heat. The thermal IR cover from 3.0 to 100 µm.

Infra red Region

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Microwave Microwave radiation covers the wavelength portion from about 1mm to 1m. This covers the longest wavelength used for remote sensing. The shorter microwave wavelength have properties similar to thermal infra red region while the longer used for radio broadcast. This region has special interest which is important in remote sensing.

Microwave Region

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We feel infrared light as heat and our radios pick up the messages encoded in radio waves emitted by radio stations.

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Ultraviolet light has high enough energy to damage our skin cells, so our bodies will produce a darker pigment in our skin to prevent exposure of the deeper skin cells to the UV. –The special bulbs called “black lights”' produce a lot of UV and were used by hospitals to kill bacteria, amoebas, and other micro-organisms.

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X-rays are produced by very hot things in space. X-rays have more energy than UV, so they can pass through skin, muscles, and organs. –They are blocked by bones, so when the doctor takes your X-ray, the picture that results is the shadow image of the X-rays that passed through your body. –Because X-rays have such high energy, they can damage or kill cells. A few brief exposures to low-intensity X-rays is okay. –The X-ray technician would be exposed to thousands of X-ray exposures if s/he did not use some sort of shielding.

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Gamma rays are the most energetic form of electromagnetic radiation and are produced in nuclear reactions.

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Spectral Signatures/ Spectral Reflectance Features on the surface of the Earth have reflectance characteristics which can help identify a feature. One important characteristic is the object's albedo, which is the percentage of the energy that is reflected from an object. Objects with high albedo are brighter; those with low albedo are darker. The reflectance characteristics of earth surface features may be quantified by measuring the portion of incident energy that is reflected. By measuring the energy that is reflected by targets on earth’s surface over a variety of different wavelengths, a spectral signature for that object can be made. And by comparing the response pattern of different features we may be able to distinguish between them.

Factors Affecting Spectral Reflectance Pattern of the Object They are three major factors that can affect the spectral reflectance pattern when the energy hit the object. These factors is relates the contents of the object.

1) Surface Roughness / texture of the features When a surface is smooth, we will get specular or mirror-like reflection, where all (or almost all) of the energy was directed away from the surface in a single direction.

Diffuse reflection occurs when the surface is rough and energy is directed almost uniformly in all directions. Most earth surface features lie somewhere between perfectly specular or perfectly diffuse reflectors. Whether a particular target reflects specularly or diffusely depends on the surface roughness of the feature in comparison to wavelength of the incoming radiation. If the wavelength is much smaller than the surface variations, or the particles size that make up the surface, diffuse reflection will dominate. For example, fine-grained sand would appear fairly smooth to long wavelength microwave but will appear quite rough to the visible wavelength.

The internal structure of healthy leaves act as excellent diffuse reflectors of near infrared wavelength. If our eyes were sensitive to near-IR, trees would appear extremely bright to us at this wavelength. In fact, measuring and monitoring the near-IR reflectance is one way to determine how healthy or unhealthy the vegetation might be.

The topography of water surface (rough, smooth, floating materials) also led to complications for water related interpretation due to potential problems of specular reflection and other influence on colour and brightness.

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2) Organic or Inorganic content in features A chemical compound in leave called chlorophyll strongly absorbed radiation in the red and blue wavelength and reflect energy the green wavelength. Leave appear greenest to us in summer, when chlorophyll content is maximum. In autumn, the time when chlorophyll content in a leave is at its minimum, less energy absorption occur in red (more reflection in red wavelength), so the leaves turn to red or yellow colour (combination of red and green).

Water typically looks blue or blue green due to stronger reflectance at these shorter wavelength and darker if viewed at red or near IR wavelength. If there is suspended sediment present in the upper layer of water body, this will allow better reflection and brighter appearance of the water. The appearance colour of the water will show a slight shift to longer wavelengths. Suspended sediment might be looked same as shallow clear water since these phenomena appear very similar. Chlorophyll in algae absorbs more the blue an red energy and reflect green energy at green wavelength cause the water appear greener in colour if the algae present in that water.

3) Moisture content in the features High moisture content in vegetation will make it appear darker in near infra red region.

Moisture content also affects the reflectance properties of soil. Coarse soil will have low water content will reflect highly the energy and well drained soil will have high water moisture reflect less energy because water condition in the soil will absorbs more energy. So, coarse texture or dry soil will appear brighter and moisture soil will appear darker.

Spatial and Temporal Effects to Spectral Response Pattern Temporal effect is any factor that changes spectral characteristics of a feature over time. For example the spectral characteristics of many species of vegetation are in a nearly continual state of change throughout a growing season. These changes often influence when we might collect a sensor data for a particular application.

Spatial effect refer to factors that cause the same types of features (e.g. corn plants) at any given point in time to have different characteristics at a different geographic locations. In small areas analysis, the geographic locations may be meters apart and spatial effects may be negligible. When analyzing satellite data, the locations may be hundreds of kilometers apart where entirely different soil conditions, climates and cultivation practices might exist.

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Temporal and spatial effects influence virtually all remote sensing operations. These effects normally complicate the issues of analyzing spectral reflectance properties of earth resources. Again, however, temporal and spatial effects might be the keys to reveal the information sought in an analysis. For examples, the process of change detection is premised on the ability to measure temporal effects. An example of this process is to detecting the change in suburban development near a metropolitan area by using data obtained by a different dates.

An example of a useful spatial effect is the change in the leaf morphology of trees when they are subjected to some form of stress. For examples, when the tree becomes infected with Dutch elm disease, its leaves might begin to cup curl, changing the reflectance of tree relative to healthy tress that surround it. So even though a spatial effect might cause differences in the spectral reflectance of the same type of feature, this effect may be just what is important in a particular application.

Spectral Reflectance Curves of Common Land Cover Figure 5 below shows typical spectral reflectance curves of three basic types of earth features, healthy green vegetation, dry bare soil and clear water. In general, the configuration of these curves is an indicator of the type and condition of the features to which they apply.

Although the reflectance of individual features will vary considerably above and below the average, these curves demonstrate some fundamental point concerning spectral reflectance.

Figure 5: Spectral Reflectance Curves of Vegetation, Water and Soil

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Green Vegetation Spectral reflectance curve for healthy green vegetation almost always manifest the “peak and valley” configuration. The valleys in visible portion of the spectrum are dictated by the pigments in plant leaves. Leaves contain chlorophyll, which strongly absorb energy in wavelength band range at 0.45 µm and 0.67µm (visible region).our eyes detect healthy vegetation as a green in colour because of the very high absorption of blue and red energy by plant leaves and very high reflection of green energy. If plant to subject to some form of stress, which interrupt the normal growth and productivity, it may decrease the amount of chlorophyll. So we will see the plant or leaves in yellow color.

At near infrared portion (0.7µm), the reflectance of healthy vegetation increases dramatically.

From range of 0.7 µm to 1.3 µm, plant leaf typically reflects 40 to 50 percent of the energy incident upon it. Plant reflectance at this ranges results from the structure of plant leaves.

Because this structure is highly variable between plant species, reflectance measurements in these ranges allow us to discriminate between species, even if they look same in visible wavelength. Beyond 1.3 µm, energy is essentially absorbed or reflected. The decreasing of reflectance occurs at 1.4, 1.9 and 2.7 µm because water in the leaves absorbs strongly at these wavelength. If less water contents in the leaves, it will reflect more, so the reflectance peaks occur at 1.6 and 2.2 µm. At this range (1.3 to 2.7 µm), the reflectance curve depends on water present in a leaf.

Dry Bare Soil The soil in the curve shows less “peak and valleys” variation in reflectance. The characteristics of soil that determine its reflectance properties are its moisture content, texture or surface roughness, structure iron-oxide content. For examples, the presence of moisture in soil will decrease its reflectance. As with vegetation, this effect is great at water absorption bands (1.4, 1.9 and 2.7 µm). So we will see the curve occur as a valley at that ranges. Soil moisture content is strongly related to the soil texture. Coarse, sandy soils are usually well drained, resulting in low moisture content and relatively high reflectance. Poorly drained fine textured soils will generally have lower reflectance. We can see the soil curve decrease uniformly from visible (0.4 – 0.7 µm) to infrared (0.7 – 1.3 µm) wavelength and the best range to discriminates the content of the soils is at water absorption bands.

Clear Water Considering the spectral reflectance of water, probably the most distinctive characteristics is energy absorption at near IR wavelength and beyond. In short, water absorbs energy on these wavelengths, whether that is a water features such as a lake water or water content in Siti Nor Maizah Saad JSUG / FSPU / UiTM Perlis

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vegetation and soil. Locating and delineating water bodies with remote sensing data are done most easily in near IR wavelength because of this absorption property. Clear water absorbs relatively little energy having wavelength less than about 0.6 Âľm. However, because of the turbidity of water changes (organic or in organic materials), the reflectance curve changes dramatically. For examples, water containing large quantities of suspended sediments resulting from soil erosion normally has much higher visible reflectance than other clear water at the same area. The reflectance of water also changes if the chlorophyll concentrated involved. Increasing in chlorophyll concentration, tend to decrease water reflectance in blue wavelength and increase in green wavelength. Generally, majority of the radiation incident upon water is not reflected but either is absorbed or transmitted. Longer visible wavelengths and near-infrared radiations are absorbed more by water than the visible wavelengths. Thus water looks blue or blue-green due to stronger reflectance at these shorter wavelengths and darker if viewed at red or near-infrared wavelengths. The factors that affect the variability in reflectance of a water body are depth of water, materials within water and surface roughness of water.

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