Introduction


Human beings have been interested in light since the beginning of their existence. As scientific knowledge and technology progressed, the many possible applications of the elusive electromagnetic wave have spurred inventions ranging from Polaroid cameras to laser eye surgery. But humans are actually at a disadvantage to lower creatures like spiders and squids when dealing with light. Unlike humans, these creatures and several others are aware of the polarization of light. One species of spider can actually navigate through its detection of polarization orientation [2]. As easily as we locate the origin of a sound using two ears, these simple bugs can determine the difference in polarization of scattered sunlight between their eyes, using this information to locate the position of the sun overhead. This one characteristic of light escapes our eyes' detection but can still reveal so much information about the world around us. For example, polarized light has long been used to characterize thin films in a process known as ellipsometry. More recently, the biomedical world has started a large campaign to use light in lieu of X-rays, scalpels or needles, seeing the non-invasive process light allows as safer, faster and more accurate. Exciting prospects include a non-invasive means of counting a person's blood-sugar, helping the millions of people worldwide who have diabetes. Because the human eye is unable to distinguish polarized light, all results must be processed in some way to form an image, so that our minds can interpret data that our retinas cannot see. The last decade has seen a movement to unite optics, computer technology, biology, and medical techniques to fully understand the possibilities polarized light offers.

The concept of combining polarized light images in matrix form was introduced in 1997 by Andreas Hielscher and collaborators at Los Alamos National Laboratory [1]. (Dr. Hielscher is now at Columbia University, New York, New York [3] ) Because Mueller matrices give the most complete description of the effect objects have on polarized light, they were the best choice for representing complex media such as human tissue or other biological matter. Mueller calculus is also preferable to other options such as Jones calculus because it can show circular polarization without involving imaginary numbers. There are numerous instances where biological matter is termed ``chiral'' and can cause elliptically or circularly polarized light to be created. For example, D-glucose and L-glucose are named for the terms dextro, meaning right, and levo, meaning left, respectively. While very similar in structure, these sugars are called ``optical isomers'' and interact with right and left-handed circularly polarized light in different ways [4].

The technology needed to create these Mueller matrix images has steadily improved over the last few years. Modern CCD elements allow even invisible frequencies of light (near infrared) to be measured with high spatial resolution and an accurate response to light level. The camera circuits have been improved upon so greatly that kits are even available that allow amateur astronomers to easily create their own camera with a purchased CCD element [5]. Finally, computer technology is now so advanced that even a modest personal computer is more than adequate for manipulating images with millions of pixels.

The use of modern CCD cameras and imaging software has allowed Mueller matrices of highly scattering media to be created quickly and efficiently. These matrices have allowed cancerous and non-cancerous call suspensions to be differentiated by changes in color patterns [1]. The technique of imaging scattered light is currently being used in all areas of scientific study and will likely one day lead to many advances in our knowledge of ourselves and the world around us.






[Title Page] [Abstract] [Polarized Light]