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Resolution and Illuminations

Resolution

The basic task of a travel document reader system is the reading of MRZ data described in the ICAO 9303 document. About 150 dpi resolution is enough to recognize the text in this area. Greater resolution is recommended for reading the data of the VIZ area and OCR reading from other document types.

The reading ability is affected by the character size, the reduced character distance, the printing quality, the background pattern, the hologram pattern and the damages of the covering foil.

The required character size for a lower case is 16-32 pixels vertically; the required barcode resolution is 1.5-2 pixels per module. For PDF417 type barcodes the vertical module size must be 4 pixels or more.

Resolution property

The default resolution of the captured images can be changed using the ctrl/resolution property (as a pixel/meter value). Setting the resolution to lower values results in smaller image size, which e.g. eases the insertion into a database.

White light

The image captured in white light is a simple photocopy of the document. It is usable for human inspection, examination of background pattern or face photo, or as secondary source for the conventional recognizing tasks. The primary source of the conventional recognising processes (OCR, barcode reading, signature checking) is the images captured in infrared light, however in some cases - when the printing quality is low - better results can be achieved using images captured in white light.

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Sample image: White

Infra light

The ICAO 9303 document states that for reading text and barcodes, images captured in infrared light (wavelength: 900 nm) shall be used. In this illumination the background patterns are hidden, so the recognizing process gives better results.

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Sample image: IR

UV light

Images captured with Ultra Violet illumination can be used to check some authenticity features. In UV light the graphics and text printed with special fluorescent ink are visible.

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Sample image: UV

Coaxial light

This is a direct light and it is used to examine a special foil. In this light the damages and defections of this foil can be seen easily. Only a few document types have such foil.

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Sample image: Difference between white and coaxial light

Kinegram visualization

Kinegrams are simple holograms that illustrate some kind of motion on the documents. The presence of a hologram can be tested by using an alternative optical system to capture another white image. Holograms can be observed by comparing the two white images, and can be visualized by alternating the images or by generating a differential image.

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Sample image: Alternating images and differential image

Working with lights

In the Passport Reader system the user can set one or more pages (of a document) to be captured in one capture process. Different light settings can be configured for each page. By default one page is set and all the available lights are enabled. The settings can be changed with the SetPageLight and LightMask functions:

  pages[0] := LightMask(PR_LIGHT_WHITE) + LightMask(PR_LIGHT_INFRA)
  pages[1] := LightMask(PR_LIGHT_WHITE) + LightMask(PR_LIGHT_UV)
  SetPageLight(pages)

Recognizing and reading a registered type of document is accomplished with the Recognize function. Based on the database of the system this function uses all the images that were captured in order to recognize the type of the document and to read all its registered text fields - MRZ and VIZ fields - and barcodes. Data from a document of an unregistered type can be read with the GetMrz and GetBarcode functions that process only one captured image specified by the user.


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