10.3 What Happens to Energy When It Hits a Target on the Ground?

After so much energy has been absorbed and scattered by the atmosphere, many of the wavelengths that correspond to the windows finally make it to Earth’s surface and interact with targets there. One of three things can happen to that energy (on a wavelength-by-wavelength basis)—either it can be transmitted through the target, it can be absorbed by the target, or it can be reflected off the target. Transmission occurs when a wavelength of energy simply passes through a surface to interact with something else later. Think of light passing through a windshield of a car—most of the light will pass through the glass to affect things in the car rather than the windshield itself.

Absorption occurs when the energy is trapped and held by a surface rather than passing through or reflecting off it. Think of walking across a blacktop parking lot during a hot summer day—if you don’t have something on your feet, they’re going to hurt from the heat in the pavement. Since the blacktop has absorbed energy (and converted it into heat), it’s storing that heat during the day. Absorption also explains why we see different colors. For instance, if someone is wearing a bright green shirt, there’s something in the dyes of the shirt’s material that is absorbing all other colors (that is, the portions of the visible light spectrum) aside from that shade of green, which is then being reflected to your eyes.

Different surfaces across Earth have different properties that cause them to absorb energy wavelengths in various ways. For instance, we see a clear lake as a blue color—it could be dark blue, lighter blue, maybe even some greenish blue, but it’s nonetheless a shade of blue. We see it this way because water strongly absorbs all electromagnetic wavelengths except for the range of 0.4 to 0.5 micrometers (and some lesser absorption past the edges of that range), which corresponds to the blue portion of the spectrum. Thus, other wavelengths of energy (such as the red and infrared portions) get absorbed and the blue portion gets reflected.

What remote sensing devices are really measuring is the reflectance of energy from a surface—the energy that rebounds from a target to be collected by a sensor. Just as different objects and surfaces absorb energy in different ways, so different objects and surfaces reflect energy differently. The total amount of energy that strikes a surface (the incident energy) can be calculated by adding up the amounts of energy that were transmitted, absorbed, and reflected per wavelength by a particular surface as follows:

where I is the incident energy (the total amount of energy of a particular wavelength) that strikes a surface and is made up of R (reflection) plus A (absorption) plus T (transmission) of that particular wavelength. Since remote sensing is focused on the reflection of energy per wavelength, let’s change that equation to focus on calculating what fraction of the total incident energy was reflected (and to express this value as a percentage instead of a fraction, we’ll multiply by 100):

where ρ is the portion of the total amount of energy composed by reflection (rather than absorption or transmission) per wavelength. This final value is the spectral reflectance—the percentage of total energy per wavelength that is reflected off a target, makes its way toward the sensor, and is utilized in remote sensing.