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The inspiration for Alien Vision came from two other illustrated science books that I have long admired. Both are visual explorations of nature that use imaging technology to transcend the limitations of human visual perception. The first is The Powers of Ten, by Phillip and Phylis Morrison, which takes the reader on a pictorial journey through 40 powers of ten in size scale, starting with a one-meter square image of a couple sleeping on a park lawn. Each successive section of the book changes the scale of the image by a factor of ten, zooming out to view the park, then Chicago, then Lake Michigan, then North America, then Earth, and so on, until finally the square image is so large that it encompasses a multitude of galaxies. Then the "camera" zooms in on the man's hand, on a mosquito feeding there, then on bacteria on the mosquito, and so on, stopping at the subatomic particles whirling around in the nucleus of a single atom. There is also a movie version of this book available that includes a sequence where the observer rushes in from viewing distant clusters of galaxies to the hand of the sleeping man! The Powers of Ten explores nature in the scale domain, exploring size scales that are much larger and much smaller than the size scale of human visual perception.

The second book is Stopping Time - The Photographs of Harold Edgerton. The photographs in this book show commonplace events captured with highspeed cameras using electronic flash units and special shutters invented by Prof. Edgerton and his colleagues at the Massachussetts Institute of Technology. Events that happen in thousandths or millionths of a second are captured on film: a bullet passing through an apple, the first atomic bomb test an instant after detonation, a football player kicking a football. Edgerton's work is an exploration of images of the world in the time domain. The Eames Office, makers of the film version of The Powers of Ten, have also produced a film called The Powers of Time which explores the universe in 37 orders of magnitude of time, from the tiny attosecond to 31 billion years. These time scales are much shorter and much longer than the time scale of human visual perception.

My idea was to apply this same idea of a visual exploration of the universe to the electromagnetic spectrum itself, which could be considered the domain of wavelength. Instead of exploring the universe in many size or time scales, my book would take the reader on a tour of all the possible "colors" of light, from long-wavelength radio waves to extremely short gamma rays. These are wavelengths of light that are much longer and much shorter than the narrow wavelength range of human visual perception. It would be as though the readers each had a knob on their heads that they could tune like a radio dial and change the "color response" of their eyes out of the visible spectrum and into the infrared, ultraviolet, and beyond. I worked for a time as an astrophysicist and was always fascinated by the way we observe the sky with instrumentation that extends the human visual sense into new realms of the electromagnetic spectrum beyond the visible portion. An astronomy teacher of mine once remarked that if our eyes could only detect radio waves instead of what we call visible light, then we would not see the stars at all. Instead, we would see a sky full of big clouds and swirls of cold interstellar gas, with compact radio sources sprinkled throughout. The familiar night sky we see is only one of many possible skies overlaid on top of one another. I wondered if hypothetical aliens on some distant planet might see a radio sky with their peculiar visual apparatus. Extraterrestrial aliens aside, with imaging technology we have the ability to synthesize our own version of "alien vision."

This book is not intended as a comprehensive survey of imaging technology. Rather, it is a compilation of images and descriptions of imaging technology that convey a sense of what nature looks like when imaged with "invisible light." Descriptions of the imaging technology (electronic sensors and photographic film) are non-technical in nature, and I include pictures of actual imaging devices only in cases where the layperson can appreciate the design of the device. I have attempted to include a visible-light picture of the same scene or object next to every "invisible light" image, but these visible-light counterparts were not always available.

The majority of the research for this book was carried out through Internet searches, which uncovered many images and articles on imaging. I located the authors of this web content, and these helpful colleagues transmitted additional articles and digital images to me via e-mail from all over the world. I obtained many of the infrared images in Chapters 1 and 2 using cameras provided by my present employer, FLIR in Santa Barbara, California. I have attempted to obtain the highest-resolution images available, but many of the electronic sensors used to image in invisible wavebands of light have limited resolution, and the resulting images can sometimes appear quite grainy in comparison with their visible-light counterparts. Since the first edition of the book, the number of pixels of many thermal IR sensors has increased from 80,000 to over 1 million. I attempted to "upgrade" IR images whenever possible, though in some cases it was not practical or possible to do so. In some cases, the ultimate resolution of an image is limited by the wavelike properties of light, properties that make it impossible to resolve features that are smaller than a wavelength. I have used metric units throughout, and terms or jargon in boldface are included in a glossary at the end of the book. Metric units are very appropriate in a book like this, because I don't think I have ever heard of anyone using Imperial units as a measure of wavelength!

I refer to electromagnetic waves as light throughout this book. Some would question that usage, preferring to reserve the term "light" for visible light only. Everything else would then be called electromagnetic radiation. I dislike this usage of the word radiation to describe lightwaves in a book like Alien Vision. To the layperson, the word radiation calls to mind ionizing particles or gamma rays. Some workers in the infrared industry use the term energy rather than light. They will say "shortwave infrared energy" rather than "shortwave infrared light." I don't like this term much either - it is too vague. Energy has many different forms, but light is a very specific manifestation of energy.

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