Diffractive Optic Monitors Float Glass Quality

Richard Gaughan

Laser measurement techniques are commonly used to monitor the thickness of float glass, but these approaches are subject to error because they use only one pair of reflected spots. A new laser technique using a diffractive optical element promises to improve the accuracy of thickness monitoring.

To produce float glass, silica sand is mixed with other minerals, melted in a furnace, drawn onto a bath of molten tin, annealed, cooled and cut. The final product's thickness may be as small as half a millimeter for the electronics industry and as large as a few centimeters for specialized applications. Variations in the desired thickness produce optical distortion and can make the end product unusable, so accurate measurement is critical.

Researchers Jari Räsänen and Kai-Erik Peiponen of the University of Joensuu have developed an approach that displays an accuracy of ±10 µm. They produced a computer-generated hologram by patterning a 120-nm-thick layer of chrome on a fused silica substrate with an electron-beam writer. They reproduced the resultant master diffractive optical element by contact-copying optical lithography.

An expanded beam from a laser diode passes through the 8 x 8-mm diffractive optical element, which produces a 4 x 4 array of 30-µm spots that are directed to the float glass under test at an angle of incidence of 49°. The pattern is reflected from the front and back surfaces of the sheet of glass and detected with a CCD camera.

Image analysis software identifies the center of each reflected spot and calculates the difference in position between the spots from the two surfaces. The position difference is a function of the angle of incidence, index of refraction and glass thickness. Because the angle of incidence is fixed and the index is unvarying, the difference in spot position indicates the thickness of the glass.

An advantage of the technique, which the researchers reported in the Oct. 1 issue of Applied Optics, is that it not only can verify the quality of finished glass, but also is suitable for in-line measurements, enabling rapid process monitoring and the fine-tuning of production.