Abstract

The character recognition method described here is based on the principle of incoherent spatial matched filtering. The input to this matched filter is not the unknown character itself, but its Fraunhofer diffraction pattern. The intensity distribution in this diffraction pattern is insensitive against shifting of the unknown character, avoiding the need for character registration. The incoherent matched filter is easier to implement than the coherent matched filter, since only binary rather than continuous-tone masks are required. The theory and some experiments will be discussed and compared with other optical character recognition methods.

© 1965 Optical Society of America

Full Article  |  PDF Article

References

  • View by:
  • |
  • |
  • |

  1. L. P. Horwitz, G. L. Shelton, Proc. Inst. Radio Engrs. 49, 175 (1961).
  2. E. Baumann, J. Soc. Motion Picture Television Engrs. 60, 344 (1953).
  3. A. Maréchal, in Communication and Information Theory in Optics (G. E. Co., Syracuse, N. Y., 1960); E. O’Neill, IRE Trans. IT-2, 56 (1956); J. Tsujiuchi, Progress in Optics (Wiley, New York, 1963), Vol. II, L. Cutrona et al., Inst. Radio Engrs. IT-6, 391 (1960).
  4. A. Lohmann, Optica Acta 5, 293 (1958); Optica Acta 6, 319 (1959); in Communication and Information Theory in Optics (G. E. Co., Syracuse, N. Y., 1960).
  5. E. Leith, J. Upatnieks, J. Opt. Soc. Am. 53, 1377 (1963).
    [CrossRef]
  6. A. Vander Lugt, IEEE Trans. IT-10, 140 (1964).
  7. D. H. Kelly, J. Opt. Soc. Am. 51, 1095 (1961).
    [CrossRef]
  8. E. A. Trabka, P. G. Roetling, J. Opt. Soc. Am. 54, 1242 (1964).
    [CrossRef]
  9. A. Lohmann, H. Wegener, Z. Physik 143, 431 (1955). IBM Tech. Rept., available from the authors on request.
    [CrossRef]
  10. G. Toraldo di Francia, Electromagnetic Waves (Interscience, New York, 1953).
  11. P. M. Duffieux, L’Intégral de Fourier et ses Applications à l’Optique (Chez l’ Auteur, Université de Besançon, Besançon, 1946); H. H. Hopkins, Proc. Roy. Soc. (London) A217, 408 (1953).
  12. J. Armitage, A. Lohmann, Appl. Opt. 4, 399 (1965).
    [CrossRef]
  13. E. Leith, Phot. Sci. Eng. 6, 75 (1962).
  14. D. Delwiche, J. Clifford, W. Weller, J. Soc. Motion Picture Television Engrs., 67, 678 (1958).
  15. D. Paris, IBM Tech. Rept., available from the author on request.

1965

1964

1963

1962

E. Leith, Phot. Sci. Eng. 6, 75 (1962).

1961

L. P. Horwitz, G. L. Shelton, Proc. Inst. Radio Engrs. 49, 175 (1961).

D. H. Kelly, J. Opt. Soc. Am. 51, 1095 (1961).
[CrossRef]

1958

A. Lohmann, Optica Acta 5, 293 (1958); Optica Acta 6, 319 (1959); in Communication and Information Theory in Optics (G. E. Co., Syracuse, N. Y., 1960).

D. Delwiche, J. Clifford, W. Weller, J. Soc. Motion Picture Television Engrs., 67, 678 (1958).

1955

A. Lohmann, H. Wegener, Z. Physik 143, 431 (1955). IBM Tech. Rept., available from the authors on request.
[CrossRef]

1953

E. Baumann, J. Soc. Motion Picture Television Engrs. 60, 344 (1953).

Armitage, J.

Baumann, E.

E. Baumann, J. Soc. Motion Picture Television Engrs. 60, 344 (1953).

Clifford, J.

D. Delwiche, J. Clifford, W. Weller, J. Soc. Motion Picture Television Engrs., 67, 678 (1958).

Delwiche, D.

D. Delwiche, J. Clifford, W. Weller, J. Soc. Motion Picture Television Engrs., 67, 678 (1958).

Duffieux, P. M.

P. M. Duffieux, L’Intégral de Fourier et ses Applications à l’Optique (Chez l’ Auteur, Université de Besançon, Besançon, 1946); H. H. Hopkins, Proc. Roy. Soc. (London) A217, 408 (1953).

Horwitz, L. P.

L. P. Horwitz, G. L. Shelton, Proc. Inst. Radio Engrs. 49, 175 (1961).

Kelly, D. H.

Leith, E.

Lohmann, A.

J. Armitage, A. Lohmann, Appl. Opt. 4, 399 (1965).
[CrossRef]

A. Lohmann, Optica Acta 5, 293 (1958); Optica Acta 6, 319 (1959); in Communication and Information Theory in Optics (G. E. Co., Syracuse, N. Y., 1960).

A. Lohmann, H. Wegener, Z. Physik 143, 431 (1955). IBM Tech. Rept., available from the authors on request.
[CrossRef]

Maréchal, A.

A. Maréchal, in Communication and Information Theory in Optics (G. E. Co., Syracuse, N. Y., 1960); E. O’Neill, IRE Trans. IT-2, 56 (1956); J. Tsujiuchi, Progress in Optics (Wiley, New York, 1963), Vol. II, L. Cutrona et al., Inst. Radio Engrs. IT-6, 391 (1960).

Paris, D.

D. Paris, IBM Tech. Rept., available from the author on request.

Roetling, P. G.

Shelton, G. L.

L. P. Horwitz, G. L. Shelton, Proc. Inst. Radio Engrs. 49, 175 (1961).

Toraldo di Francia, G.

G. Toraldo di Francia, Electromagnetic Waves (Interscience, New York, 1953).

Trabka, E. A.

Upatnieks, J.

Vander Lugt, A.

A. Vander Lugt, IEEE Trans. IT-10, 140 (1964).

Wegener, H.

A. Lohmann, H. Wegener, Z. Physik 143, 431 (1955). IBM Tech. Rept., available from the authors on request.
[CrossRef]

Weller, W.

D. Delwiche, J. Clifford, W. Weller, J. Soc. Motion Picture Television Engrs., 67, 678 (1958).

Appl. Opt.

IEEE Trans.

A. Vander Lugt, IEEE Trans. IT-10, 140 (1964).

J. Opt. Soc. Am.

J. Soc. Motion Picture Television Engrs.

D. Delwiche, J. Clifford, W. Weller, J. Soc. Motion Picture Television Engrs., 67, 678 (1958).

E. Baumann, J. Soc. Motion Picture Television Engrs. 60, 344 (1953).

Optica Acta

A. Lohmann, Optica Acta 5, 293 (1958); Optica Acta 6, 319 (1959); in Communication and Information Theory in Optics (G. E. Co., Syracuse, N. Y., 1960).

Phot. Sci. Eng.

E. Leith, Phot. Sci. Eng. 6, 75 (1962).

Proc. Inst. Radio Engrs.

L. P. Horwitz, G. L. Shelton, Proc. Inst. Radio Engrs. 49, 175 (1961).

Z. Physik

A. Lohmann, H. Wegener, Z. Physik 143, 431 (1955). IBM Tech. Rept., available from the authors on request.
[CrossRef]

Other

G. Toraldo di Francia, Electromagnetic Waves (Interscience, New York, 1953).

P. M. Duffieux, L’Intégral de Fourier et ses Applications à l’Optique (Chez l’ Auteur, Université de Besançon, Besançon, 1946); H. H. Hopkins, Proc. Roy. Soc. (London) A217, 408 (1953).

A. Maréchal, in Communication and Information Theory in Optics (G. E. Co., Syracuse, N. Y., 1960); E. O’Neill, IRE Trans. IT-2, 56 (1956); J. Tsujiuchi, Progress in Optics (Wiley, New York, 1963), Vol. II, L. Cutrona et al., Inst. Radio Engrs. IT-6, 391 (1960).

D. Paris, IBM Tech. Rept., available from the author on request.

Cited By

OSA participates in CrossRef's Cited-By Linking service. Citing articles from OSA journals and other participating publishers are listed here.

Alert me when this article is cited.


Figures (9)

Fig. 1
Fig. 1

Optical systems for (a) matched filter and (b) correlation operations.

Fig. 2
Fig. 2

Optical system for character recognition, including preprocessing for shift invariance and incoherent matched filtering.

Fig. 3
Fig. 3

Only light which has been twice diffracted at the same angle, by the unknown character and by the filter, will pass through the hole to the photodetector.

Fig. 4
Fig. 4

Conventional system extended to parallel processing.

Fig. 5
Fig. 5

Parallel processing (four patterns) using theta-modulated filters. At the left are the four filters theta-modulated with four different values of theta; at the right are (schematically) the output spectra. The unknown character is an F.

Fig. 6
Fig. 6

The stencil-like photoetched characters used in the experiments. Each character is 1 mm high.

Fig. 7
Fig. 7

Spectra recorded at the output plane, without and with the spot; no filter. Unknown character: (a,b) 1-mm circular aperture; (c,d) letter A; (e,f) letter B.

Fig. 8
Fig. 8

Fraunhofer patterns of entire alphanumeric character font, recorded at two different exposures to show both the low and high spatial frequency regions.

Fig. 9
Fig. 9

Numeric values for correlation coefficients: (a) typical combinations; (b) difficult combinations.

Equations (19)

Equations on this page are rendered with MathJax. Learn more.

I n ( x ) = I ˜ n ( ν ) exp ( 2 π i ν x ) d ν .
I n m ( x ) = I ˜ n ( ν ) T m ( ν ) exp ( 2 π i ν x ) d ν .
I n m ( 0 ) = S n m = I ˜ n ( ν ) T m ( ν ) d ν ,
( S n m S n n ) 2 = { I ˜ n I m * } 2 { I ˜ n I ˜ n * } { I ˜ m I ˜ m * } < 1.
{ I ˜ n I ˜ m * } = { I n I m * } , or I ˜ n ( ν ) I ˜ m * ( ν ) d ν = I n ( x ) I m * ( x ) d x .
a n ( x ) = u ˜ n ( x / λ f ) ; u ˜ n ( ν ) = u n ( x 0 ) exp ( - 2 π i x 0 ν ) d x o .
u ( x , z ) = u ˜ 0 ( ν ) exp [ 2 π i ( ν x + 1 - λ 2 ν 2 z / λ ) ] d ν ,
u ˜ 0 ( ν ) = u ( x , 0 ) exp ( - 2 π i ν x ) d x ;
T m ( ν ) = p m ( x + λ f ν 2 ) p m * ( x - λ f ν 2 ) d x .
I ˜ m * ( ν ) = I m ( x ) exp ( 2 π i ν x ) d x ,
I m ( x ) = a m ( x ) 2 = u ˜ m ( x / λ f ) 2 , u ˜ m ( ν ) = u m ( x 0 ) exp ( - 2 π i ν x 0 ) d x 0 .
I ˜ m * ( ν ) = u m ( x 0 + Δ x + λ f ν 2 ) u m * ( x 0 + Δ x - λ f ν 2 ) d x 0 ,
T m ( ν ) = p m ( x + λ f ν 2 ) p m * ( x - λ f ν 2 ) d x = [ u m ( x 0 + Δ x + λ f ν 2 ) u m * ( x 0 + Δ x - λ f ν 2 ) d x 0 ] 1 S m m .
p m ( x ) = u m ( x + Δ x ) / S m m .
I n ( x ) T s ( x ) exp ( - 2 π i ν x ) d x ,
T m ( ν ) = I ˜ m * S m m = 1 S m m I m ( x ) exp ( 2 π i ν x ) d x
S n m = I n m ( 0 ) = 1 S m m I n ( x ) T s ( x ) I m ( x ) d x .
S m m 2 = I m 2 ( x ) T s ( x ) d x
S n m S n n = I n ( x ) T s ( x ) I m ( x ) d x I m 2 T s d x             I n 2 T s d x .

Metrics