Better PDF OCR: ClearScan is smaller, looks better

By Rick Borstein – January 26, 2010

 

Join Rick Borstein as he presents the advantages and disadvantages of ClearScan over Searchable Image OCR in Acrobat 9. Optical Character Recognition (OCR) converts scanned paper documents into searchable PDF documents. This technology has been available in Acrobat for about ten years. While OCR accuracy and language support have improved over the years, the default OCR "flavor"— Searchable Image— was the only useful choice.

Searchable Image retains the underlying scanned image and adds an invisible layer of text on top, which may be selected:

00_selected_text_on_image_001

Searchable Image OCR has some shortcomings:

  1. File Size
    For 300 dpi black and white scans, a typical file size is 15-40K per page. Scanning at higher resolutions (600 dpi Vs. 300 dpi) increases file size about three to four times.
  2. Print Speed
    Because of the image-heavy content, searchable image PDFs can take a long time to print.
  3. Visual Quality
    At 300 dpi, scanned documents are easily distinguishable in quality from computer-generated files.

In Acrobat 9, Adobe engineers added a new flavor of OCR called ClearScan. ClearScan offers improved text quality with a decrease in file size:

001a_clearscan_search_000

I've recently completed some benchmarking which shows dramatic file size decreases and quality gains. Read on to learn about size comparisons, how to use ClearScan OCR and a bit more about how it all works.

Testing Methodology

I created two test documents:

  • 78-page image-only PDF document scanned at 300 dpi
  • 78-page image-only PDF document scanned at 600 dpi

I ran OCR and compared file sizes on my ThinkPad W500. The test machine ran Vista Enterprise in 32-bit mode and has 4GB of RAM. In addition to Acrobat, I also had Excel running. The W500 is a current model laptop which runs an Intel Core 2 Duo CPU at 2.8 GHz. The test machine has an IBM standard 320GB laptop hard drive running at 7200 rpm.

Visual Results and Total File Size

02_400-percent

Timing and File Sizes

Note: Numbers rounded.

At 300 dpi, ClearScan offered improved visual quality at about one-third the total file size. At 600 dpi, the ClearScan file was seven times smaller and looked better.

How does ClearScan work?

ClearScan works by turning the images which represent text characters on the page into smoothed vector outlines. Each character on the page is compared and all matching characters are replaced with a an outline character:

Original
ClearScan
003b_G 003a_G
800% View in Acrobat
300 dpi scan

ClearScan does not replace the font with your system fonts. Rather, a custom font it is created to match the visual appearance of the pixels. In fact, if you run ClearScan OCR and choose File—> Document Properties and click on the Fonts tab, you'll see that custom fonts are created:

004_fonts_tab

Besides better visual appearance, print time is reduced. Instead of sending large images to the printer, Acrobat can send the compact font information instead.

How can I try ClearScan OCR?

ClearScan OCR is not the default in Acrobat 9, so you'll need to change a setting to use it. Here's how.

  1. Choose: Document—> OCR Text Recognition—> Recognize Text using OCR . . .
  2. Click the Edit . . . button in the OCR window:

    005_ocr_window_000

  3. Change the PDF Output Style to ClearScan.

    006_clearscan_choice_000

  4. Click OK twice to OCR the document.

ClearScan Q&A

Here are a few answers to the most common questions about ClearScan OCR.

  1. Is OCR accuracy any different between ClearScan and Searchable Image styles?
    No. The accuracy will be identical for input files of the same dpi. However, since a ClearScan files are so much smaller, you might consider using a 600 dpi input file as a starting point since there is little downside other than processing time.
  2. Can I make changes to the text in a ClearScan file?
    No. The Touchup Text Tool does not currently work on ClearScan files.
  3. Are ClearScan files admissible in court?
    To our knowledge, there has never been a challenge. ClearScan files are generally an accurate representation of the original document.
  4. What fonts does ClearScan use?
    ClearScan creates a custom font to match the character shape. It does not rely on system fonts or any other font that may be installed on your system.
  5. Are ClearScan files always smaller?
    For single page typical legal documents, you may not see much difference in file size. Multiple page documents will show the greatest file size reduction. Documents which vary greatly in the type and number of fonts will show less return on file size.
  6. Is ClearScan slower than Searchable Image OCR?
    No. The speed is virtually identical.
    vWhat is the benefit of scanning at 600 dpi then using ClearScan?
    Since ClearScan bases its shapes on the original image, a 600 dpi input file yields better looking text than one scanned at 300 dpi:

    007a_300_dpi_m
    007a_600_dpi_m
    300 dpi input file
    ClearScan 800% view
    600 dpi input file
    ClearScan 800% view

  7. Can I re-OCR a Searchable Image file to turn it into a ClearScan file?
    Yes.
  8. Can I turn a ClearScan file into a Searchable Image file?
    No. This will trigger a "renderable text" error. You could export it to TIFF, reassemble and then OCR or print it to an image file.


Was this tutorial helpful?

Please Log in to provide feedback on this tutorial.

Rate this tutorial

Please Log in to rate this tutorial.

Rating:

Did you know?

  • You can ask a question and get an answer from one of our experts.
  • You can search our database of over 800 tutorials by product and/or topic.
  • You can leave a comment below for the author of this tutorial.

Products covered:

Acrobat 9

Related topics:

Scanning & OCR

Top Searches:

Create PDF, Scan to PDF, Electronic signatures, OCR PDF

7 comments

Lori Kassuba

10, 2013-03-11 11, 2013

Hi Heather,

You’ll need to correct the OCR suspects to correct the misidentified characters. Here is a tutorial that explains how to do this:
http://acrobatusers.com/tutorials/how-to-find-and-correct-ocr-errors

Thanks,
Lori

Heather

12, 2013-03-05 05, 2013

Interesting. I should have known this before! It still works with Acrobat XI. My question is what happens with misidentified characters? The resulting text is sometimes different. Will this wrong result be displayed instead? Thanks.

Lori Kassuba

3, 2013-02-19 19, 2013

Hi Steven,

I would recommend using the Optimize Scanned PDF command modify your scanned settings. Here is a tutorial on the subject:
http://acrobatusers.com/tutorials/how-do-i-optimize-a-scanned-pdf-document

Thanks,
Lori

Steven Hubert

2, 2013-02-15 15, 2013

It seems that the reason it could take more than an hour to print an OCR’d document is the initial scan setting. The way I understood this, if the scan quality is not too good, OCRing it will produce a lont-time-to-print PDF. To avoid the issue in the future I thought to change my scan settings (on a Fujitsu ScanSnap fi-5110EOXM) to “Excellent” which means slower, like to 10 pages per minute. But what about converting all of those slow PDF’s into a faster-printing ClearScan document? Can I re-OCR a batch of Searchable Image files to turn them into ClearScan files?

Patty Friesen

4, 2012-11-13 13, 2012

Hi Jim,

If you’re interested in how to edit text in a scanned PDF, check out:
http://acrobatusers.com/tutorials/how-do-i-edit-text-in-a-scanned-pdf

Hope this helps,

Patty

Jim Dunlap

11, 2012-11-13 13, 2012

The link I clicked was how to Edit text in a scanned PDF… this tutorial is great for Recognizing text… not editing it…

Michael Brennan

5, 2012-06-26 26, 2012

I was successfully redacting a number of scanned PDF files, but there were a couple pop up boxes.  I checked the boxes so they would not be shown again.  Now when I go to search the scanned docs for SSN patterns, Adobe says it is not finding any documents to scan.

I replied to this feed because the last thing I saw before successfully executing my redaction was the same “Recognize text” pop-up you show above.

How do I “uncheck” the boxes I asked to never see again and/or redact these documents now that I’ve checked the boxes?


Can you post your question on our User-to-User security forum so that we can help you interactively?
http://forums.adobe.com/community/acrobat/security_&_digital_signatures

Thanks,
Lori

Leave a reply:

Have an urgent question? Post your question to our Ask an Expert forum for a faster response.

Fields marked with * are required.

Learn how to
edit PDF.

Get started