OCR Accuracy: The Seven Things That Actually Affect It

People tend to think of OCR quality as a property of the software. It is much more a property of the input. The same engine, on the same page, can go from near-perfect to unusable based on decisions made before the document ever reached it.

1. Resolution

300 DPI is the sweet spot for standard body text. Below 200 DPI, character strokes merge and the classic confusions appear: rn reads as m, cl reads as d, 0 reads as O. Above 400 DPI you gain almost nothing and pay in processing time. If you control the scanner, set it to 300 and stop thinking about it.

2. Contrast and lighting

Phone photos of documents fail more often than scans, and it is almost always lighting. A shadow across the page from your own hand shifts the local threshold and swallows a whole column. Shoot in even, indirect light, or use a scanning mode that flattens and thresholds per-region.

3. Skew

Even two or three degrees of rotation hurts, because line-detection assumes horizontal baselines. Most engines deskew automatically, but they do it better when the page edges are visible — so do not crop tightly before OCR.

4. The font itself

Standard serif and sans-serif body faces are what engines are trained on. Condensed fonts, decorative headers, and anything below 8pt are materially worse. Handwriting is a different problem entirely and needs a handwriting model, not a print OCR model.

5. Language selection

OCR uses a language model to resolve ambiguous glyphs. Running a French document through an English model produces confident, plausible, wrong words — accented characters get silently normalised away. Always set the language, and for genuinely bilingual documents, run each language over the relevant pages separately.

6. Layout complexity

Multi-column layouts, sidebars, footnotes and tables are where OCR output becomes technically accurate but practically useless: every character is right, but the reading order interleaves two columns into nonsense. If you care about downstream text, check the reading order before you check the spelling.

7. Prior compression

A scan that has already been aggressively JPEG-compressed carries ringing artefacts around every character edge. The engine sees those artefacts as ink. This is the single most common cause of 'the scan looks fine to me but OCR is terrible' — your eye corrects for the artefacts and the algorithm does not.

A realistic quality checklist

• Scan or photograph at 300 DPI equivalent, in even light.
• Keep the page flat and roughly square to the sensor.
• Do not compress before OCR. Compress after.
• Set the correct language.
• Spot-check numbers first — digits have no dictionary to fall back on and fail silently.

How to verify without reading everything

Search the OCR'd document for three words you know appear in it, including one that is unusual. Then search for a number you can verify. If all four hit, the page is broadly sound. If the unusual word misses, the engine is guessing from its dictionary, and you should treat every proper noun in the document as suspect.