Method for distinguishing similar subjects using discriminating holograms

• US 3,899,240 A
• Filed: 12/09/1970
• Issued: 08/12/1975
• Est. Priority Date: 08/13/1965
• Status: Expired due to Term

• Current Assignee: Dennis Gabor
• Original Assignee: Dennis Gabor
• Inventors: Gabor, Dennis
• Primary Examiner(s): Stern, Ronald J.
• Application Number:
  US05/096,376
• Time in Patent Office: 1,707 Days
• Field of Search: 350/3.5 340/146.3F,146.3P
• US Class Current: 359/10
• CPC Class Codes: G03H 1/12 Spatial modulation, e.g. ghost imaging
• CPC Class Codes: G03H 1/16 using Fourier transform G03H1/10, G03H1/12, G03H1/14 take precedence; analogue computers G06G, e.g. G06G7/19
• CPC Class Codes: G03H 2001/2675 Phase code multiplexing, wherein the sub-holograms are multiplexed according to spatial modulation of the reference beam
• CPC Class Codes: G06V 10/88 Image or video recognition using optical means, e.g. reference filters, holographic masks, frequency domain filters or spatial domain filters
• CPC Class Codes: G11C 13/042 using information stored in the form of interference pattern
• CPC Class Codes: Y10S 359/90 Methods

Abstract

A method for making a discriminating hologram which can be used to distinguish similar subjects, and a method for distinguising these similar subjects using the discriminating hologram. The discriminating hologram is formed by a process in which emphasis is placed on the distinctive portions of the similar subjects. For example, the letter Q and the ltter O are similar except for the tail of the letter Q. Thus, the tail is emphasized when preparing the discriminating hologram. Time-phase relationships are used to subtract light corresponding to similar portions of the letters Q and O, so that the dissimilar portion is emphasized. As an example, a discriminating hologram is prepared by having the phase between a first light beam incident on the common parts of the letters Q and O with respect to a reference beam be different than the phase between a second light beam incident on the distinguishing parts of the letters Q and O, with respect to the same reference beam.

Claims

1. In a process for forming a reference hologram of a pattern having both an element in common with any other pattern and a distinctive element that comprises the steps of:

interfering on a recording medium two beams of light for a suitable exposure one of which beams is modulated by the pattern to be recorded, thereby forming on the recording medium a set of interference fringes, a first part of which is related to the common element in the pattern to be recorded and a second part of which is related to the distinctiive element;

the fringes having a location on the recording medium that is dependent on the phase relation between the two beams;

the improvement characterized by;

forming the first part of the set of interference fringes by projecting onto the recording medium a first and a reference beam of light, the first beam being modulated by the common element of the pattern to be recorded; and

forming a different second part of the set of interference fringes by projecting onto the medium a third and a reference beam of light, the third beam being modulated by the distinctive element of the pattern to be recorded; and

the phase relation between the third and reference beams being different from the phase relation between the first and reference beams.

2. The process of claim 1 wherein forming a different second part of the set of interference fringes by changing the phase relation between the third and reference beams from that between the first and reference beams comprises the step of:

transmitting one of the beams through a one-half-wave plate.

3. The process of claim 1 wherein forming a different second part of the set of interference fringes by changing the phase relation between the third and reference beams from that between the first and reference beams comprises the steps of:

forming a first transparency of the pattern composed of a second transparency of the common element of the pattern having a first optical path length and a third transparency of the distinctive element of the pattern having a second optical path length;

the difference between the two optical path lengths being other than an integral multiple of the wavelength of the first and third beam of light; and

using the first transparency to modulate the first and third beams of light.

4. A process for recognizing one or more than one pattern where the pattern to be recognized has both an element similar to at least part of another pattern and a distinctive element that comprises the steps of:

forming a reference hologram of the pattern to be recorded comprising the steps of;

interfering on a recording medium two beams of light for a suitable exposure one of which beams is modulated by the pattern to be recorded, thereby forming on the recording medium a set of interference fringes, a first part of which is related to the similar element in the pattern to be recorded and a second part of which is related to the distinctive element;

the location of the fringes on the recording medium being dependent on the phase relation between the two beams;

the contrast between a light and dark fringe being dependent on the exposure of the recording medium to the fringes; and

determining whether an unknown pattern is the same as the pattern recorded comprising the steps of;

forming a cross correlation of a beam of light modulated first by the unknown pattern and then by the reference hologram;

detecting the intensity of the crOss correlation;

comparing the intensity of the cross correlation of the beam with the intensity of the cross correlation of a beam of light that is modulated by the pattern recorded and its reference hologram; and

indicating that the unknown pattern is the same as the pattern recorded when the intensities are substantially the same;

the improvement characterized by;

forming the first part of the set of interference fringes by projecting onto the recording medium a first and a reference beam of light, the first beam being modulated by the similar element of the pattern to be recorded; and

forming a different second part of the set of interference fringes by projecting onto the medium a third and a reference beam of light, the third beam being modulated by the distinctive element of the pattern to be recorded;

the phase relation between the third and reference beams being different from the phase relation between the first and reference beams;

as a result of which, a ratio of the intensity of the cross correlation formed when the unknown pattern is the same as the pattern recorded on the improved reference hologram to the intensity when the unknown pattern is not the same is different from the intensity ratio when the reference hologram of the two parts is formed without varying the phase relation.

5. The process of claim 4 wherein the cross correlation of the beam of light is formed in an output plane by inserting between the reference hologram and the output plane a converging lens to image the beam of light onto the output plane situated a focal length away from the lens;

the intensity is indicated by an array of photodetectors;

the intensity of the cross correlation is compared with the intensity of known cross correlations by biasing the photodetectors at predetermined theshold levels; and

recognition is indicated by optoelectronic means.

6. The process of claim 4 wherein more than one pattern is stored in a single reference hologram comprising the steps of:

repeating the step of interfering two beams of light on the recording medium as many times as is necessary to record each pattern to be recorded;

a different angle being used between the two beams for each pattern that is recorded.

7. A process for forming a hologram of a pattern for use in pattern discrimination apparatus that comprises the steps of:

dividing the pattern into a first element comprised of at least those parts that are common to the pattern and to any pattern to be discriminated and into a second element comprised of those parts that are distinctive;

forming a set of interference fringes on a recording medium by projecting onto the medium two beams of light of predetermined characteristics for a suitable exposure one of which beams is modified by the first element of the pattern;

forming a second set of interference fringes on the recording medium by projecting onto the medium the two beams of light for a second exposure one of which beams is modified by the second element of the pattern;

each pair of beams of light being derived from the same light source;

the phase relation of the two beams being different during the second exposure from what it was during the first exposure; and

processing the medium to make at least a temporary record of the interference fringes.

8. A process for forming a hologram of a pattern for use in pattern discrimination apparatus that comprises the steps of:

dividing the pattern into a first element comprised of those parts that are common to the pattern and to any pattern to be discriminated and into a second element comprised of those parts that are distinctive;

forming a set of interference fringes on a recording medium by projecting onto the medium two beams of light predetermined characteristics for a suitable exposure one of which beams is modulated partly by the first element of the pattern and partly by the second element of the pattern;

altering that part of the beam modulated by the second element of the pattern by changing its optical path from the corresponding characteristics of the other part of the beam; and

processing the medium to make at least a temporary record of the interference fringes.

9. An optical method of recognition of images or characters that constitute a set of known different subjects, comprising preparing by holographic technique a different matched filter for each of said known subjects, projecting on to each of said matched filters the diffraction pattern of each subject to be recognized to obtain a set of light-intensity distributions corresponding one to each filter, and determining which of the light-intensity distributions so obtained is the auto-correlation function of the subject with itself, and wherein the preparation of at least one of the matched filters includes the steps of holographically recording in a filter body, by means of an illuminating beam carrying the subject information and a reference beam, a representation of the subject in respect of which the filter is matched, and similarly recording in the same filter body but with a different time-phase relationship between the reference beam and the illuminating beam at least one further representation of a different competing one of the subjects in the set, whereby the completed filter displays a degree of rejection in respect of said competing subject.

10. A method according to claim 9, wherein the filter body employed is a photographic plate which is subjected to plural exposures in a hologram-making apparatus supplied both with the subject to which the filter is to be matched and with competing subjects which are to be rejected, the time-phase relationship between the reference beam and the illuminating beam being different for each exposure.

11. A method according to claim 10, wherein the plural exposures are made simultaneously.

12. A method according to claim 10, wherein the plural exposures are made in succession.

13. A method for distinguishing a first subject from a second subject, comprising the steps of:

making a reference hologram of said first subject by interfering light which is modulated by said first subject with a reference light beam on a recording medium where said light strikes portions of said first subject which are similar to portions of said second subject and strikes other portions of said first subject which are distinct from portions of said second subject, and providing a first phase relationship between light modulated by said similar portions and said reference light beam and a second phase relationship between light modulated by said distinct portions and said reference light beam, sending a light beam modulated by an unknown subject to said reference hologram of said first subject for modulation thereby the light from said unknown subject being in registration with the interference patterns formed on said recording medium when said reference hologram was made, and detecting the intensity of said light after modulation by said reference hologram.

14. A method for distinguishing a first subject from a second subject, comprising the steps of:

forming a reference hologram of the first subject which is to be distinguished from said second subject by interfering on a recording medium for a first exposure two beams of light, one of which is modulated by the parts of said first subject which are similar to the parts of said second subject, interfering on said recording medium for a second exposure two beams of light, one of which is modulated by parts of said first subject which are distinctive from parts of said second subject, at least one of the exposure time, optical path length, and phase relation between said two beams being different during said first exposure from what it is during said second exposure, sending a beam of light modulated by an unknown subject to said reference hologram of said first subjEct for modulation thereby the light from said unknown subject being in registration with the interference patterns formed in said recording medium during said first and second exposures, and detecting the intensity of said light after modulation by said reference hologram.