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SPIE Professional July 2017

Picture this: Optics in the Air

Rainbow, corona, glory, sundog, or halo? Not all scattering from water droplets or ice crystals are rainbows, but SPIE Fellow Joseph Shaw finds all types of sunlight transformed into previously hidden colors “visually irresistible.”

Observing and photographing optical phenomena in the atmosphere has been Shaw’s lifelong passion. Now, 30 years after receiving his BS in electrical engineering under the auroras of Alaska and 21 years after earning his PhD in optical sciences in Arizona, where he vowed to learn everything possible about natural optical phenomena, Shaw’s photographic collection and explanation of “optical beauty” has been published in a new book.

Optics in the Air: Observing Optical Phenomena through Airplane Windows is Shaw’s illustrated introduction to how to see the rich array of colors and optical effects that occur in nature. Although mainly about observation and photography from aloft, readers who have their “heads in the clouds” and their feet firmly planted on the ground will find many of the photography tips and descriptions of optical phenomena useful and accessible.

Simple line diagrams and a glossary of optical terms supplement more than 200 photographs depicting everyday sky and sunset colors, auroras, noctilucent clouds, and shadows. In addition, Shaw says he hopes to inspire others with “a whole world of too-often-ignored occurrences such as sun glitter patterns on bodies of water, colorful ringed glories and coronas, rainbow-like patterns that cling to the clouds below a high-flying airplane, and ice halos that spring up as an airplane passes through high-altitude ice clouds.”

WINDOWS ON A PLANE

The book is divided into chapters on different types of optical phenomena and begins with an introduction to photography and observation techniques aboard an airplane. Shaw explains how to create – or avoid — unusual effects through airplane windows, which are often scratched and do not have high-quality optical surfaces.

“Most airplane windows have two thick panes of stretched acrylic, separated by an air gap and mounted in a pressure seal,” he writes. “The thickest outer pane is designed to withstand the large stress of cabin pressurization and is sometimes curved to match the external aircraft body. The middle pane has one or more breather holes to allow cabin air to reach the inner surface of the outer pane.

“Without this hole, moist air reaching the inner surface of the outer pane would condense on the window because the outside air at flight altitude is extremely cold (typically –40 to –60°C).” Nevertheless, Shaw says passengers can sometimes see window condensation or ice during a descent into more humid air.

Since the stretched acrylic material of the airplane window is birefringent, it bends and transmits light differently for light waves oscillating in different directions, i.e., light of different polarization states, Shaw explains. And it does this differently for different colors.

“In fact, a well-known optical method of testing for material stress involves placing the material between two crossed polarizers, i.e., polarization filters with orthogonal transmission axes, and observing the resulting pattern of pastel colors,” Shaw writes.


Choosing a window seat is important but not always necessary to frame your photo.

Shaw advises photographers to remove their polarizing filter on the camera lens when taking a picture through the airplane window unless your intention is to produce dramatic colored patterns with the “stress birefringence” in the window.

The book contains several examples of skylight polarization, including a surreal image of the Eiffel Tower taken with a polarizing lens filter through a birefringent transit-car window in Paris.


Photographing the famed Eiffel Tower through a birefringent transit-car window with a polarizing filter over the camera lens creates a surreal effect.

A chapter on shadows is illustrated with photographs of eclipses as well as shadows from clouds, mountains, and airplane contrails, some with a bonus glory.

Shadows might seem like trivial optical phenomena, he writes in chapter 6, “but they actually contain a variety of brightness and color if you observe carefully.”

PHOTO CONTEST AT SPIE OPTICS + PHOTONICS

SPIE Press author Joseph Shaw is a cochair of the Light in Nature and Polarization Science and Remote Sensing conferences at SPIE Optics + Photonics in August. A polarization photo contest is being held in conjunction with the latter conference, and prizes will be awarded at a 3:10 pm ceremony 8 August.

Deadline for submissions is 4 August.

Read more about the Polarization Photo Contest.

SPIE Fellow Joseph Shaw is the director of the Optical Technology Center at Montana State University (USA) and a professor in the university’s College of Engineering. He develops optical remote-sensing instruments for applications ranging from airborne laser mapping of fish to polarimetric imaging of the atmosphere. He is the recipient of a US Presidential Early Career Award for Scientists and Engineers, the Vaisala Award from the World Meteorological Organization, and an Award for Excellence from the University Economic Development Association.

His book is available through SPIE Press.


DOI: 10.1117/2.4201707.11

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