CORRESPONDENCE

It is appropriate to point out that our coherent optical systememployed standard components, and we did not use eitherprecision grade lenses or specially designed, so-called Fourieroptics. No careful precautions were taken with respect topositioning the film samples in the optical system. Once thefeature-selection transformation matrix is determined, the onlysignificant computation in processing a CWP film is in applyingthe k-nearest-neighbor algorithm. A special purpose dedicatedcomputer could be used for this task.

In the ODC technique, as described here, the most time-con-suming job and the major source of errors was the photographicreduction of the original film onto 35 millimeter film. Photo-graphic film is a "wet" medium that is difficult to control. Anyphotographic transfer process is likely to introduce artifactand grain noise and reduce the details in the original. Ouroptical system could not handily accommodate the originalfilm, and this necessitated the transfer to 35 millimeter format.There are, fortunately, several solutions to this problem. Themost direct would be to design an optical system that wouldaccommodate the original film directly. This would essentiallyrequire larger (and therefore better) lenses. The design of an

optical system to handle, for example, 14 x 17 in. radiographsis entirely feasible, since the whole image need not be viewed or

Fourier transformed simultaneously; separate zones could behandled sequentially. Although a phase-matching oil bathapparatus to handle 14 x 17 in. film may not be readily available,there is no reason why such a device could not be designed forthis purpose; its design would certainly not require a state ofthe art breakthrough. Another solution would be to scan theoriginal film with a TV vidicon camera and reconstruct a trans-parent image in a coherent optical system that is capable ofmodulating a coherent wavefront. Casasent's electronicallyaddressed light modulator does precisely this [16] and may

offer an attractive solution to the problem.

ACKNOWLEDGMENT

The authors are grateful to Dr. William Crawford for furnish-ing the films of CWP and giving helpful advice and to Dr. E. L.Hall for making important contributions in the early phases ofthis research. The helpful suggestions furnished by the reviewers,which included identifying the correct author of the trace

criterion, is appreciated.

REFERENCES[1] IEEE Trans. Computers (Special Issue on Optical Computing), vol.

C-24, pp. 337-456, April 1975.[2] R. P. Kruger, W. B. Thompson, and A. F. Turner, "Computer diag-

nosis of pneumoconiosis," IEEE Trans. Systems, Man, and Cybernetics,vol. SMC-4, pp. 40-49, Jan. 1974.

[3] E. L. Hall, R. P. Kruger, and A. F. Turner, "An optical-digital systemfor automatic processing of chest X-rays," Optical Engineering,June 1974.

[4] E. L. Hall, W. 0. Crawford, and F. E. Roberts, "Computer classifica-tion of pneumoconiosis," IEEE Trans. Biomed. Eng., vol. BME-22,pp. 518-527, Nov. 1975.

[51 J. R. Jagoe and K. A. Paton, "Measurement of pneumoconiosis bycomputer," IEEE Trans. Comput., vol. C-25, pp. 95-97, Jan. 1976.

[6] J. W. Goodman, Introduction to Foureir Optics. New York: McGraw-Hill, 1968, pp. 154-155.

[7] M. Born and E. Wolf, Principles of Optics. New York: PergamonPress, 1965, p. 188.

[8] H. Stark, "An optical digital computer for parallel processing ofimages," IEEE Trans. Compuit.. vol. C-24, pp. 340-347, April 1975.

[9] R. N. Sutton and E. L. Hall, "Texture measures for automatic clas-sification of pulmonary disease," IEEE Trans. Comput., vol. C-21,pp. 667-676, July 1972.

[10] W. B. Meisel, Computer-Oriented Approaches to Pattern Recognition.New York: Academic Press, 1972, pp. 12-15.

[11] Y. T. Chien and K. S. Fu, "On the generalized Karhunen-Loeveexpansion," IEEE Trans. Inform. Theory, vol. IT-13, pp. 518-520,1967.

[12] K. Fukunaga and W. L. G. Koontz, "An application of the Karhunen-Loeve expansion to feature selection and ordering," IEEE Trans.Comput., vol. C-19, pp. 311-318, April 1970.

[13] D. H. Foley and J. W. Sammon, Jr., "An optimal set of discriminantvectors," IEEE Trans. Comput., vol. C-24, pp. 281-289, March 1975.

[14] K. Fukunaga, Introduction to Statistical Pattern Recognition. NewYork: Academic Press, 1972, pp. 260-264.

[15] K. S. Fu, "Pattern recognition in remote sensing of the earth'sresources," IEEE Trans. Geosci. Electron., vol. GE-14, pp. 10-18,Jan. 1976.

[16] D. Casasent and W. M. Sterling, "An optical/digital processor:Hardware and applications," IEEE Trans. Comput., vol. C-24, pp.348-358, April 1975.

Correction to "A Simulation Approach to the Evaluation ofTwo Telephone Switchboard Systems"

RICKIE H. LUK AND GERALD F. RABIDEAU

In the above correspondence,' the sentence beginning at thebottom of the right column on page 312 should read: A furthercriterion assumed was that, in order to provide good services,99 percent of the customers should not be kept waiting for morethan 15 s.

Manuscript received June 28, 1976.R. H. Luk was with the Department of Systems Design, Faculty of En-

gineering, University of Waterloo, Waterloo, Ont., Canada, on study leavefrom the Department of Design Interpretives, Bell-Northern ResearchLaboratories, Ottawa, Ont., Canada. He is now with the Department ofSystems Engineering, Northern Telecom Limited, Ottawa, Ont., Canada.G. F. Rabideau is with the Department of Systems Design, Faculty of

Engineering, University of Waterloo, Waterloo, Ont., Can-ada.1 R. H. Luk and G. F. Rabideau, IEEE Trans. Syst., Man, Cybern., vol.

SMC-6, pp. 310-315, Apr. 1976.

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