We describe a novel method of noncontact mode area measurement at long distance of 11.25 m by borrowing the concept of a circular Dammann grating (CDG). The area of objects can be determined accurately by measuring the circular spectrum diameter of the CDG. This noncontact mode measurement requires neither a large amount of image data nor any pattern recognition approach. The spectrum diameter is derived from simple lens formulas. From the fractional Fourier transform, we find that there exists a linear relationship between the spectrum diameter and the distance traveled by the CDG. Compared with the conventional methods, this technique has the advantages of a simple design with good accuracy of better than 3percent, low cost, noncontact mode, and a more compact design. Finally, we present several experimental results demonstrating the effectiveness of this system.
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