DOCSUPRA › Scan to black and white
Scanning a document to black and white
The short answer. Use bitonal for typed text on plain paper: it typically cuts the file 90% or more and makes the letters sharper. Never use it for anything with a photograph, a coloured seal, a signature that has to be compared, or security printing.
Bitonal — one bit per pixel, pure black or pure white — is the single biggest size reduction available for a text document, and the fastest way to ruin an ID. The difference is entirely about what is on the page.
Why it shrinks so much
A colour pixel takes 24 bits. A bitonal pixel takes 1. Beyond that, the compression used for bitonal images is built for long runs of identical pixels, and a page of text is mostly white space. A 6 MB colour photograph of a typed page often lands between 40 and 90 KB in bitonal, and the text edges come out cleaner, because there is no grey fringe left around the letters.
Where it destroys the document
| On the page | Bitonal? | Why |
|---|---|---|
| Typed text, plain paper | Yes | Sharper and far smaller |
| Photograph or ID portrait | No | A face has no pure black or white — it becomes a blot |
| Coloured seal or stamp | No | A red seal drops to solid black or vanishes entirely |
| Signature to be compared | Careful | Pressure variation is what makes it comparable |
| Highlighted or annotated page | No | Yellow highlighter turns into a black bar over the text |
| Watermark or security print | No | Faint marks are exactly what the threshold removes |
The threshold is where it goes wrong
Bitonal has to decide, for each pixel, black or white. Pick the cut-off too high and the paper texture turns into speckles; too low and thin strokes disappear. A period, a comma, an accent and a decimal point are all small and grey at their edges, which is why a badly thresholded page loses punctuation while looking, at a glance, immaculate.
That is also why a shadowed page must be evened out before bitonal, not after: a single threshold across a page that is bright on one side and dark on the other will white out one half and black out the other.
If the page has both, keep colour. An ID with a portrait and a form field, or a certificate with a seal and typed text, has to be handled as a colour document. Reducing the resolution is the safe lever there — never the bit depth.
Use bitonal where it helps, colour where it matters
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