Optical remote sensing involves detecting objects based on their characteristic electromagnetic radiation emissions and/or reflections. Visible light, with its familiar rainbow of colors, makes up the portion of the electromagnetic spectrum to which we are most commonly accustomed. However, there is an incredibly broad range of other “colors” that make up the electromagnetic spectrum. While the technologies for detecting or generating electromagnetic energy vary significantly across the spectrum, the underlying physical laws describing the radiant energy are the same.
Objects that emit light, heat, or other electromagnetic energy are actually producing photons, and therefore can potentially be measured using an optical receiver. By definition, any object with a temperature greater that absolute zero emits photons. This constant emission of photons by material objects is, for example, why thermal imaging cameras are effective at night -- they "see" the photons emitted by the objects themselves.
Photon physics is a fascinating subject, and the details are well beyond the scope of this discussion. However, in a nutshell, photons are massless particles with the intriguing characteristic that they also exhibit wavelike properties. The wavelength of a photon is inversely proportional to the energy it contains. In addition, specifying the wavelength of a photon is the same as specifying the color of light that is associated with it -- for instance, a wavelength of 632 nanometers corresponds to red light.
We are accustomed to thinking of light in terms of what we can see with our eyes. However, we also regularly encounter invisible light. More energetic photons with correspondingly shorter wavelengths produce ultraviolet light, which, among other things, causes sunburn. Less energetic photons with longer wavelengths produce infrared light. In our everyday world, infrared light most often manifests itself as heat -- for example, you cannot see that an electric skillet is turned on, but you can feel the heat from it.
An optical detector is a device that produces a electrical signal when photons are incident on its active area. In general, the electrical signal is proportional to the intensity of the incident light. Since the electrical signals produced by detectors are typically very small, they are amplified electronically to bring them into a measurable range.
Detectors come in many shapes and sizes, and the material used in producing a detector determines the wavelengths of light over which it will operate. For example, to measure visible light, silicon detectors are usually used. However, in the infrared region, other types of detector materials are required depending on the wavelengths of interest.
Detectors are also packaged in numerous formats, including "discrete" detectors that have a single active area, linear arrays that consist of a few to several thousand detectors lined up in a row, and two-dimensional area arrays which are the fundamental components of video cameras. Each of these package types can be obtained with the detector material needed to measure light of the desired wavelength.
An optical receiver typically consists of a detector (in any of the formats described above) along with some type of optical assembly located in front, although in some cases the detector and its associated electronics alone constitutes the entire receiver. The optical assembly is designed to achieve two objectives -- it enhances the optical signal of interest, and it focuses the light onto the detector(s).
The optical assembly can be as simple as a single lens element that focuses light to the detector, or as complex as a spectrometer that separates light into its constituent spectral regions so that each detector measures a different color. In fact, some unique applications require even more exotic optical front ends, such as in "active" system designs where the optical assembly both projects a light source such as a laser to illuminate the object and also receives the resultant photons that are reflected from the object.
In some applications the overall receiver package must be small and lightweight, such as for night-vision equipment that might be carried by a soldier in the field. This puts an additional constraint on the optical design. Designs that are amenable to prototyping on a laboratory benchtop, where size and weight are typically not limiting factors, may not scale down for more realistic measurement environments. Thus, the required form factor is as significant as any other constraint in the design process.
By its very nature, remote sensing implies that the source being measured is some distance away from the optical receiver. The atmospheric path between the source and receiver will attenuate the source’s signature and is likely to change its spectral shape. These changes have important implications for developing remote sensing systems and interpreting their data. For example, deriving the required sensitivity for an optical receiver will depend on the magnitude of atmospheric losses encountered in the expected measurement conditions and distances. Fortunately, atmospheric transmission is well understood and can be modeled reasonably well with computer models.
There are two physical mechanisms by which the atmosphere causes these transmission losses. Photons can be absorbed and converted to heat by atmospheric gas and aerosols, or they can be redirected (scattered) into another direction away from the receiver. These transmission losses are affected by the composition of particles and gases in the atmosphere. In the visible wavelength region, light of all colors is transmitted with relatively high efficiency. However, in other regions, the transmission is strongly dependent on the particular wavelength (as in the above plot of transmission versus wavelength). The transmission is also dependent on the propagation path. A horizontal path through the dense, lower atmosphere will cause more attenuation than a vertical path of equivalent length that passes through higher altitude, thinner atmosphere.
|More on Scattering
As described above, light scattering by atmospheric gases and molecules will attenuate optical signals between the source and receiver by redirecting photons from their propagation paths. These same phenomena can also increase the background signals observed by a remote sensing system by redirecting solar radiation along the propagation path towards the receiver. On hazy days, there are many large particles (usually from water absorbed on the airborne particulates) that scatter the visible light very effectively. By producing background signal along the observation path, the haze reduces the contrast of the objects under observation. This is why visibility is poor under hazy conditions. The signals from the objects being observed are attenuated and their natural contrast is “washed out” by scattered signals along the observation path.
The scattering from a single particle is a function of the particle size, the wavelength of light, and the scattering angle (the picture at right shows the scattered-light profile resulting when a laser is incident on a single particle). The scattered signals are stronger in the forward scatter angles, which is why the apparent glare is higher when one looks towards the sun versus away from it on a hazy day. The scattering is also higher for shorter wavelengths, which accounts for why the sky is blue. In fact, if our eyes were sensitive to UV light, the world would always appear hazy since the scattering phenomena is much stronger for the shorter wavelength UV photons.
Sometimes in remote sensing, optical scattering is the source of the signature being measured. For example, aerosol scatter from a laser beam can be detected readily and used to characterize the atmospheric conditions and wind velocity. From an entertainment perspective, consider how uninspiring laser light shows would be without aerosol and molecular scatter! A light detection and ranging (LIDAR) system transmits a laser beam and measures the signal that gets scattered back to the receiver. LIDARS are typically used to characterize vertical atmospheric profiles in temperature, gas species concentration and pressure.
|Wine Tasting or Atmospheric Physics?
A precise description of optical scattering theory is very involved and requires sophisticated mathematics. However, several important scattering concepts can be captured in a simple demonstration, as illustrated in the picture at right. For the demonstration, a few drops of milk were added to a wine glass filled with water. A laser was positioned so that its red beam entered the glass from the left at an incidence angle selected to give total internal reflection of the beam from the top surface.
The drops of milk added to the water introduced enough scatterers to the liquid that the beam became visible from the side. In other words, the scatterers allow the optical receiver (in this case, the digital camera used to take the photo) to record the laser signal. The fact that light is scattered out of the main beam means that the transmitted light that leaves the glass on the right side has been attenuated by the same amount.