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One of the relatively new advances in commercial LIDAR technology is multibeam systems with different wavelengths for each beam (unlike multibeam with the same wavelength used simply to increase effective pulse rate). These so-called "multispectral" LIDARs are becoming quite common in the bathymetric arena where infrared is needed to detect the water surface and green is needed for water column penetration.
In 2014, Optech introduced a new system ("Titan") specifically aimed at extending the multispectral notion beyond simple infrared and green. Are we at the onset of multispectral active sensors? Let’s casually explore what is meant by multispectral when it comes to LIDARs.
Light is part of what is called the electromagnetic energy (EM) spectrum or electromagnetic radiation (EMR) spectrum. You can think of this energy as having some properties similar to an ocean wave. The distance between the crests of the wave is called the "wavelength." Interestingly, the energy of EMR is related to the wavelength in an inverse way. The shorter the wavelength, the higher the energy. This sort of makes sense because more cycles of waves can be packed into the same space if the distance between the crests is shorter.
Electromagnetic energy can be of any wavelength ranging from virtually infinity (very long wavelength) to very short wavelengths. The light that we see is a small range of wavelengths that extend from the longest we see (deep red) to the shortest we can discern (violet). Above what we can see are ultraviolet light, x-rays and so forth. Just below what we can see is infrared. Radio, gamma, microwave and other wavelengths are in the EM spectrum but far outside of the ranges we perceive with our eyes. The electromagnetic spectrum is depicted in Figure 1.
"White" (or "visible") light contains a mixture of a wide range of wavelengths in the visible region. You may recall experiments with a prism, separating out the color components from white light. To me it is rather amazing that our eyes and visual cortex can distinguish these individual colors. We actually have a very sensitive "spectrum analyzer" built in to our eyes and brains! Creatures that cannot see color (rare) do not have this ability to separate out wavelengths. They just perceive them as levels of brightness; e.g. a grey tone image.
Charged Coupled Devices (CCDs) and Complementary Metal Oxide Semiconductor (CMOS) image collectors used in digital cameras have a wide range of spectral response (they respond to light from the near infrared to the violet). For color cameras, a color lens is placed over the CCD elements, allowing only a narrow range of wavelength to impinge upon the sensor. In standard cameras, these are red, green and blue filters.
Most cameras use what are called "passive" sensors. They rely on the scene being illuminated by some external source of radiation. For example, an aerial camera relies on the sun illuminating the scene. You can think of broad spectrum (e.g. white) light being emitted by the sun, illuminating the surface of the earth, reflecting to an airborne camera lens and then being filtered down to red, green, blue (and sometimes infrared) by the color filters on the CCD. This filtering of multiple "spectral" bands of light is what we call "multispectral" imaging ("more than one color"). Note that if the sun (or some other source of illumination) is not present, we cannot image with the sensor.
A laser scanner, on the other hand, is an active sensor. It illuminates the scene with laser light and then detects the reflected light. Thus it can work night or day since it is providing its own light source. For a variety of reasons, we use a laser as the source of illumination. One of the significant properties of a laser is that the emitted light is of a single, very narrow wavelength. Since a specific color implies a specific wavelength, this means a laser is one pure color. Most topographic lasers use a wavelength of 1,054 nanometers which is in the infrared region. This particular wavelength is used because it is "eye safe."
We can visualize the intensity return from a laser scanner. This visualization looks like a black and white image (well, really a grey scale image). It is actually infrared but we usually simply view it as a grey scale.
Now imagine that you could add a green laser and a blue laser to the mix. Then the return would be somewhat similar to that imaged by a Red-GreenBlue camera. However, the laser is an active sensor, illuminating the scene. This would mean that you could do this RGB illumination at night and effectively have a night time camera. We could think of this as active, 3D color imaging.
The wavelengths of the Optech Titan are depicted in Figure 2. As previously noted, it uses three lasers, each having a different wavelength. Their new system can be called "multispectral" because it is illuminating and detecting with more than one wavelength but it does not necessarily mean that what you would image would resemble a `normal’ RGB scene. For example, even though depicted in yellow, the intermediate wavelength beam at 1,064 nanometers is actually in the infrared band.
Judson Thomas performed extensive experimentation with classifying Titan data using conventional tools aimed at image classification ("Terrain Classification using Multi-Wavelength LIDAR Data", September 2015). While his results look promising, it is obvious that new software algorithms will need to be devised for dealing with these very narrow and widely spaced spectral channels.
These are fascinating developments that will hopefully lead to an entirely new way to do spectral modeling. Perhaps a future development will be a tunable laser that can be set to mission specific bands. It will be an interesting progression to follow.
Lewis Graham is the President and CTO of GeoCue Corporation. GeoCue is North America’s largest supplier of LIDAR production and workflow tools and consulting services for airborne and mobile laser scanning.
A 741Kb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE