Image Processing Reference
In-Depth Information
far into either the ultraviolet or infrared bands. 2 Thermal infrared light strongly
interacts with the silicon dioxide molecules in window glass, and thus we cannot
see through glass windows with a MWIR or LWIR thermal imaging system. 3
Figure 2.10 shows two views of a man sitting in a car, one in visible light and
the other in MWIR light. These images illustrate a limitation of thermal imaging:
the optical properties of glass place limits on thermal imaging for surveillance,
which is a boon to privacy advocates everywhere!
Reflectivity of Materials
Some surfaces are good mirrors in the thermal IR band, but are not good mirrors
in the visible band. For a metal surface, the surface texture determines the degree
to which it will act as either a diffuse reflector or a specular reflector in different
wavebands. A nice example of this is a stainless steel refrigerator door shown
in Fig. 2.11. The surface finish is what machinists call “grained.” The grooves
and texture have length scales on the order of 0.5 micrometers. Thus, visible
light is highly scattered by the surface—it acts like a diffuse reflector. In the 3-5
micrometer band, the door becomes an excellent reflector, showing the person
reflected in it. Many metal surfaces are excellent thermal IR mirrors.
Midwave and Longwave IR Imaging Systems
As we attempt to image the world around us with electromagnetic energy that
departs significantly from the wavelength range of visible light, we can no longer
use ordinary cameras with special films and filters to make images as we could
in the near-infrared and near-ultraviolet bands. The imaging devices we need
to make thermal images still use lenses, but they are made of special materials
(such as silicon and germanium) that have very different optical properties from
those of glass. Ordinary camera lenses, like the car windshield in Fig. 2.10 are
opaque to MWIR and LWIR light. In addition, ordinary photographic film is
not sensitive to these long wavelengths of light, so we need another means of
detecting the focused image. There are different ways to accomplish this but all
use very specialized detectors. Some of the original thermal imaging devices were
developed for military use in the 1950s. They used single electronic detectors or
linear arrays of detectors that were scanned (typically by a rapidly moving mirror)
in such a way as to build up a two-dimensional image of a scene. Today, almost all
thermal imaging systems are designed around focal plane arrays (FPAs), which are
two-dimensional “mosaics” of electronic detectors. Each detector corresponds to a
pixel, or picture element, in the final image. A lens focuses infrared energy onto the
FPA, which then converts the energy into electronic signals that can be transmitted
2 The American physicist Richard Feynman watched the first atomic bomb test through a car
windshield, knowing that the glass would not transmit harmful UV light to his eyes.
3 There are glass-like materials that are transparent to both visible light and infrared light, but they are
expensive and hard to fabricate into large sheets.
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