ABSTRACT for the 2009 Celebration of Research and Creative Activity

An Investigation of `Spectrum' Polarizing Plates

Himadri (Soni) Kakran, Laser Teaching Center, Department of Physics and Astronomy, Stony Brook University

This project was motivated by a fascination with Liquid Crystal Display (LCD) screens. LCD screens consist of many pixels whose brightness can be varied electronically. Less well known is that this variability depends on the properties of polarized light. Thus our overall goal is to understand the fundamental concepts of polarization and polarized light in detail in order to better understand LCD displays.

Polarization of light refers to the orientation of the electric field vector in a light wave. In the most general case the electric field moves in an ellipse with time, creating elliptical polarization. The extremes of elliptical polarization are linear polarization and circular polarization, where the electric field vector moves in a straight line or a circle respectively. Elliptically polarized light is obtained by passing linearly polarized light through a retarder (waveplate). The retarder creates a phase shift between orthogonal components of the electric field vector as it has different indices of refraction for these two components. The retardance of a waveplate varies in proportion to its thickness and inversely with the wavelength of the incident light. The different states of polarized light and the action of polarizers and retarders can be described and predicted using 2x1 vectors and 2x2 matrices with complex elements (the "Jones calculus").

A few weeks into the spring semester, by chance we noticed that a new item was available from Surplus Shed [1]: `Spectrum Plates' (item number L3692) that had been removed from some obsolete display device. They were said to "work exactly like polarizing sheets but instead of just getting darker the plates change to different colors." We decided to purchase these unknown plates and investigate their properties.

We started out by making some general observations. We soon deduced that the plates consist of a linear polarizer and a retarder sandwiched together. Also, the two plates seem to be identical except for the orientation of the optical elements with respect to the edge of the plate. The beautiful saturated colors are observed when white light is passed first through the linear polarizer side of either plate and then a linear polarizer (analyzer). As the analyzer is rotated through 90 degrees the color changes to a complementary one - for example, deep blue changes to yellow, or green changes to red. This happens because the plates create different types of elliptically polarized light for different colors. Currently we are making measurements to determine the retardance as a function of wavelength. The quantity measured is the transmission of several types of laser light through first the spectrum plate and then an analyzer, as a function of the analyzer angle.

[1] Surplus Shed, Fleetwood, PA. Surplus Shed

Himadri Kakran
February 2009
Laser Teaching Center