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An optical open area ratiometer for measuring the percent open area of perforated sheet materials has been developed. The device uses a technique based on Fourier optics in which a detector is employed to measure a small central portion of the light in the Fraunhofer diffraction pattern of the perforated sheet. This paper discusses the associated problems and results of testing the accuracy of the device. The accuracy is shown to be on the order of 2% of the open area ratio.

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