Chapter 6:
Sweeping through the Spectrum: Comparative Imagery

Having toured the known spectrum of light, from radio waves to gamma rays, one can appreciate the tiny slice of the spectrum that our unaided eyes can see is a fairly specialized band that permits vision in our water-drenched world. Yet the visible waveband provides an incomplete picture of the physical universe. Imaging technology permits us to transcend the limitations of our anatomy and see the universe with "alien vision."

What would it be like if human eyes had an adjustable color response? Our eyes automatically adjust their focus and their sensitivity in response to varying object distance and light levels, yet their color response is fixed by their chemical structure. We can overcome this limitation with artificial eyes, each designed for a particular waveband of the electromagnetic spectrum. Consider a familiar object, a human face. The appearance of the face is highly dependent on wavelength - there are many "faces" to a face. I have been imaged in many wavelengths of light, from the millimeter-wave to the x-ray band. My familiar face, the one I see in the mirror, is only one of a larger set.

The millimeter-wave image seen in Fig. 6.1 was made with an imaging system that operates at a wavelength of 3.3 mm (3300 μm), corresponding to a transmission window in moist air. The image was taken outdoors under a clear sky. The cold sky reflects off my cheekbones, nose, ears, and forehead, giving the image a high degree of thermal contrast. This grayscale image is inverted so that black indicates hot and white indicates cold - it is easier to see the facial features this way. The image has a low resolution compared with some other imaging technologies, since the high-speed detector array has many fewer detector elements than an infrared or visible sensor, for example.

The infrared waveband contains a multitude of sub-bands, each with its own distinctive look. We see a different aspect of a human face in each sub-band - the LWIR and MWIR images convey the thermal emission of the skin with no external illumination required, while the near-IR band gives us insight into the reflectivity of the skin and eyes at wavelengths that are several times longer than visible light.

The LWIR face in Fig. 6.2 glows with its own light in the 8-9 micrometer waveband. A cooled infrared camera with a QWIP FPA captured this image, which is rendered in grayscale with white indicating hot and black indicating cold.

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