PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
December 2014
1101
What ’s Hot about Thermal Imaging?
If you Google the phrase “thermal imaging” in the fall of 2014,
you will find links to the release of cell phone compatible thermal
cameras to assist with ground based energy audits of your home
or office. For most of us, we are more familiar with viewing color
imagery in the visible spectrum between 0.4 µm and 0.7 µm (to
the naked eye that would be the range of colors from violet to
red). When you think of the term thermal, thermal can cover
the electromagnetic spectrum from near infrared to long wave
infrared. Near infrared is collected in the [ 0.7 µm to 2.5 µm].
One application for using the NIR range is to detect the amount
of chlorophyll in vegetation for reliable plant condition analy-
sis. The long-wave infrared (LWIR) range of the electromagnetic
spectrum is detectable within a wavelength of 8 µm to 15 µm,
which we feel as warmth, but don’t see. To put this into per-
spective, humans at normal body temperatures radiate near 10
µm. This is why thermal imaging is a critical tool in search and
rescue missions both in use by the military and civilian entities.
For an aerial thermal camera to capture a digital LWIR
image, the camera will be detecting energy intensity that is
radiated from objects visible to the sensor in the 8 µm to 15 µm
range. The amount of thermal radiation emitted depends on
the emissivity values of the structure’s surface (i.e., a painted
metal roof versus a nearby water pond). Emissivity ranges in
values of 0 to 1 as it represents the ratio of existence from a
real object to the existence of a perfect black-body at the same
temperature. A black-body is an ideal material that allows all
incident radiation to pass into it with no energy reflected from
it. In nature there are no materials with an emissivity of 1 but
within the geographic environment there are a few features
that are close to 1 and water is one of them.
Water features like lakes and ponds can be used as relative
black-body features for control objects in a thermal imagery
project. Thermal IR imagery is typically flown at night when
solar loading is at a minimum. Water with an emissivity of
0.97 will be absorbing thermal energy during the day. Water
also has a small diurnal temperature variance as water warms
slowly after sunrise and cools slowly after sunset reaching a
minimum near dawn. Due to the nature of natural features
near the water (soil and vegetation as shown in Figure 1a),
the water will appear warmer than the surrounding land (see
Figure 1b). The varying shades of orange (in Figure 1b) are an
indicator of differing amounts of sediment in the water body.
Thermal imagery proves to be a great method for stream
location when vegetation covers areas of hydrography features.
Figure 2a shows tree canopies over parts of the stream channel.
Thermal imagery in figure 2b can be used to aid in the location
of the stream since water will be a warmer feature in the
imagery as compared to the surrounding geographic features.
Figure 2a. Several locations of the linear hydrography
feature are hidden by tree canopies. **
Figure 2b. Delineation of streams can be derived from
thermal imagery. **
Figure 1a. A natural color image showing water/
vegetation area at NASA Langley Research Center. **
Figure 1b. During a night-time thermal imagery
capture, water can be used as a black-body source to
validate temperatures of features in the imagery. **
