Optical system for producing an image for a set of characters

• US 4,731,854 A
• Filed: 07/17/1986
• Issued: 03/15/1988
• Est. Priority Date: 07/17/1986
• Status: Expired due to Term

• Current Assignee: Perceptics LLC
• Original Assignee: Perceptics LLC
• Inventors: Gonzalez, Rafael C.
• Primary Examiner(s): Boudreau, Leo H.
• Application Number:
  US06/887,545
• Time in Patent Office: 607 Days
• Field of Search: 350/394, 350/403, 350/407, 358/101, 358/108, 358/244, 358/225, 358/93, 354/129, 354/130, 354/135, 354/150, 382/1, 382/65
• US Class Current: 382/105
• CPC Class Codes: G06K 2207/1018 Source control
• CPC Class Codes: G06K 7/10851 Circuits for pulse shaping, amplifying, eliminating noise signals, checking the function of the sensing device basic electronic circuitry H03
• CPC Class Codes: G06K 7/10861 sensing of data fields affixed to objects or articles, e.g. coded labels postal sorting B07C3/14, conveying articles B65G47/48
• CPC Class Codes: G06V 10/145 Illumination specially adapted for pattern recognition, e.g. using gratings
• CPC Class Codes: G06V 10/147 Details of sensors, e.g. sensor lenses fingerprint or palmprint sensors G06V40/13; vascular sensors G06V40/145; eye sensors G06V40/19

Abstract

An optical system produces an image of a set of characters on a background. A flash light source directs uniform illumination toward the set of characters for a predetermined flash period having an intensity chosen to exceed the intensity of corrupting light from the environment, such as sun light, street lights or car lights. A first polarizer is positioned to polarize the illumination from the flash light source prior to the light impinging on the set of characters, and a second polarizer is positioned to receive light reflected by the set of characters and the background. The second polarizer is cross-polarized with respect to the first polarizer so that it filters out polarized light from the flash light source that is reflected by the set of characters and background. An optical receiver is positioned to receive light from the second polarizer and produces an image of the set of characters and the background. A shutter is used to control the light received by the optical receiver. The shutter is synchronized with the flash light source and the optical receiver to open and close during the flash so that the image produced by the optical receiver is dominantly produced by the de-polarized flash light reflected from the set of characters and background.

Claims

1. An optical system for producing an image of a set of characters on a background in the presence of corrupting light, comprising:

a flash light source for directing light toward the set of characters for a predetermined flash time period;

a first polarizer for polarizing the illumination of said flash light source prior to the light impinging upon the set of characters, the illumination from said first polarizer impinging upon the set of characters having an intensity exceeding the intensity of the corrupting ambient light;

a second polarizer positioned to receive light reflected from said set of characters and background, said second polarizer being cross-polarized with respect to said first polarizer whereby said second polarizer is operable to filter out polarized light from said first polarizer that is specularly reflected by said set of characters and background;

an optical receiver for receiving light from said second polarizer and producing an image of said set of characters and said background; and

a shutter synchronized with said flash light source and optical receiver to open and close during said flash time period to admit light into said optical receiver, whereby the image produced by said optical receiver is dominantly produced by de-polarized light reflected by said set of characters and background from said flash light source.

2. An optical license plate imaging system for producing an image of a license plate on a vehicle in the presence of corrupting light in the environment, said license plate bearing a set of characters comprising:

a flash light source for directing light toward the license plate for a predetermined flash time period;

a first polarizer positioned between said flash light source and the license plate for polarizing the light from said flash light source the light from said first polarizer incident upon the license plate having an intensity exceeding the intensity of the corrupting ambient light;

a second polarizer positioned to receive light reflected from said license plate, said second polarizer being cross-polarized with respect to said first polarizer whereby said second polarizer is operable to filter out polarized light from said first polarizer that is specularly reflected in a polarized state by said license plate;

an optical receiver for receiving light from said second polarizer and producing an image of said license plate; and

a shutter synchronized with said optical receiver and flash light source to open and close at a selected shutter speed during said predetermined flash time period to admit light into said optical receiver, whereby the image produced by said optical receiver is dominated by light from said flash light source that is de-polarized when reflected by said license plate.

3. The system of claim 2 wherein said flash light source comprises a plurality of flash light units that simultaneously flash to produce a uniform flash.

4. The optical system of claim 2 wherein said shutter speed is within the range of 10^-6 to 4×

10^-3 seconds.

5. The optical system of claim 2 wherein said shutter speed is about one ten-thousandth of a second.

6. The optical system of claim 2 wherein said shutter comprises an electronic liquid crystal shutter.

7. The optical system of claim 2 further comprising a continuous light source for masking the flash illumination from said flash light source.

8. The optical system of claim 2 wherein said optical receiver comprises a television camera that is synchronized with said shutter and said flash unit to begin integrating light to form an image immediately prior to the opening of the shutter.