|Remote Sensing technology is changing rapidly as the profession moves away from analog (film) cameras to much more sophisticated digital aerial camera systems. It is important for everyone impacted the by this profession to understand how these changes affect geospatial data and services available to them. As the profession moves away from film toward digital camera “systems” they bring with them much more complexity and capabilities. This short series of articles will discuss some of these more important ideas.|
Aerial remote sensing, for many years, has primarily focused on black & white, and now more common, three color (RGB) images of the earth. In recent years, infrared imagery has grown in prominence and importance. But what exactly is infrared imagery and how is it different than the imagery you have used before, and why should I care?
Remote– (away from or at a distance) sensing (detecting a property or characteristic) require an energy source to illuminate the target. This energy is in the form of electromagnetic radiation and is derived primarily from the sun. The energy consists of high energy like cosmic and gamma rays at one end to ubiquitous energy like visible light and microwaves at the other end of the electromagnetic spectrum.
Figure 1. The electromagnetic spectrum. Not visible here are the very short and high energy wavelengths of gamma rays and x-rays.
As depicted in Figure 1, any point along the spectrum is described by its “wavelength”. Very small wavelengths of energy are dangerous and can penetrate solid material. Very long wavelengths like radio waves are everywhere and seem to cause no problems to life. As we all learned in physics courses, these wave/particles travel at about 670 million miles per hour or 186,234 miles per second.
Human eyes can perceive only a small part of the electromagnetic spectrum called “visible light”. Energy in this very narrow band has wavelengths of 04. – 0.7 microns. Energy with much shorter wavelengths like ultraviolet, x-rays, and gamma rays as well, as energy with longer wavelengths like infrared, microwaves, and radiowaves, are invisible to the human eye. But many of the newer digital aerial cameras can now “sense” and accurately record not only the visible light but also portions of the near infrared spectrum. This major new development is important to consumers of geospatial information.
The “typcial” RGB aerial orthoimage is pictured on the left, with an infrared aerial orthoimage on the right.
Aerial cameras capable of acquiring color infrared (CIR), (also commonly called “false-color”) have been used for many decades with special film, lenses, and filters to capture reflected near infrared energy. Objects that are normally red appear green, green objects (except vegetation) appear blue, and “infrared” objects, which normally are not seen at all, appear red. This shifting of color bands results in a photograph which emphasizes vegetation (healthy green vegetation appears bright red) and is therefore much more useful than black & white or color imagery for a wide variety of applications. Historically, applications for CIR was far less common than regular aerial film because it was expensive and generally required two flights to capture “normal” black & white or color imagery and a second for the CIR imagery.
New digital aerial cameras, on the other hand, use filters and CCDs to capture and record near infrared energy simultaneously with visible light in the same flight using the same equipment. The costs of CIR imagery are now dramatically lower than they have been historically. It is important to note that these camera systems do not “see” thermal (far) infrared (the energy that leaks from warm houses or you feel sitting around a fire), because it has longer wavelengths and are not detected by the equipment (although there are cameras that are used to detect this energy).
Figure 2. Reflectance of visible and infrared electromagnetic energy.
In Part 2 of this series, we will discuss why this new technology that enables beautiful, high resolution, and inexpensive imaging in the near infrared is important to the geospatial profession. It is becoming incumbent on the users of geospatial information to exploit this technology and from it.