Below is a short list of reasons why OCR on historic documents is a real challenge.

Image Quality

• Old documents are hard to scan, but good scan quality is important for good OCR results. Problems that you may encounter:
  • Curled paper
  • Pages are stuck together
  • Wired layouts
  • Curved lines of text when the book has to be treated carefully

Layout detection

• Historic books/documents often have a different layout structure.
  Accordingly algorithms that were designed for “modern” layouts might not be able to deliver proper results on these layouts
• Old newspapers can also be very tricky
  • Small Fonts
  • Complex Layouts
  • Reading order

Texttypes Used

• Old font types are used - standard character recognisers cannot read gothic/fraktur fonts
• Quality of the characters that should be OCRed is often very bad
  • Broken characters
  • Mixed with noise and dirt or writing
• There are characters in old documents that are not available in modern computer fonts

Language-Issues

• Historically spelling was not unified and consequently there are many different writing variants
• There are no historic dictionaries available

Further Information:

• The challenges of historical materials and an overview on the technical solutions in IMPACT
  Sven Schlarb of the Österreichische Nationalbibliothek (Austrian National Library)
• OCR in libraries – some practical remarks
  Günter Mühlberger of Universitäts- und Landesbibliothek Tirol (University and Regional Library Tyrol)

Sources: Images were taken from the linked presentations