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Martin High School CSI:DI Gadget

This device was constructed to use the properties of polarized light. Basically, light passed through a polarized filter, then through clear tape and then through another polarized filter that was rotating. The clear tape is a "birefringent" material. The molecules in the tape are stretched parallel to the length of the tape. Light polarized parallel to the stretch of the molecules travel through the tape more slowly than light polarized perpendicular to the stretch.

The different rates of transmission through the materials result in different colors showing up. When a second piece of polarized material is placed over the tape and rotated, it transmits different colors at different angles. This results in the different color combinations that you see at a given angle, and for the changes in color as the polarizer is rotated.

The actual device was constructed by making a wooden light box with a clear plastic (non "birefringent") lid. We used a compact fluorescent light bulb because they do not get as hot as a regular incandescent bulb. Our lamp socket was from a flood light with the shield removed. We wired it to a 120v electrical switch on the side of the box. We placed a piece of polarizing film over the clear plastic cover.

The rotating polarizing filter was constructed using a small toy motor. We made a wooden circular frame to hold the filter material. We then built a square box that the circular frame would fit inside. We installed a small pulley on the toy motor and wrapped a rubber band inside the pulley grooves to have something that would have some gripping power. We covered the outside of the circular wooden frame with masking tape to give the rubber band on the pulley something to grip to. The circular frame rested on top of the pulley. The motor was controlled with a simple electrical switch on top of the square frame.

The lifted "fingerprint" was then placed on top of the filter on the box and the rotating filter was placed in front of it to reveal a colorful parrot feather.