Grey background on a scanned PDF
The grey is the paper, photographed. To remove it without losing faint letters, the background has to be evened out across each page first, then the print turned black, rather than turning up the contrast.
Key takeaways
- A scanner records the paper as well as the ink. Aged, thin or tinted paper comes out grey or yellow, and the spine adds a darker band.
- E-ink shows a limited range of greys, so grey text on grey paper looks faint on an e-reader even when it looked fine on a monitor.
- Brightness and contrast act on the whole page at once. The background isn't even, so one setting leaves the gutter dark and burns away thin strokes.
- The fix is to measure the background across each page, level it to white, then decide what is ink.
- Pages with photos or engravings should stay in greyscale, or the pictures turn into black blobs.
Where the grey comes from
Nothing went wrong with the file. A scanner photographs everything on the glass: the ink, but also the paper’s tint, the stains, the print showing through from the other side, and the shadow where the page bends into the binding. On an old book all of that adds up to a page that is grey, cream or brown, with text that is dark grey rather than black.
Typical sources, from lightest to worst:
- Modern paper, scanned in greyscale. Light grey wash, mostly even.
- Older book paper, yellowed with age. A cream or yellow cast, darker towards the edges.
- Library and archive downloads. Often scanned in colour for preservation, with every tint kept.
- Phone photos of pages. Uneven light: one corner bright, the opposite one dim, a shadow from your hand or phone.
- Photocopies of photocopies. Grey speckle across the page and thickened or broken letters.
Why it looks worse on an e-reader
A monitor is lit from behind and shows a huge range of tones, so dark grey print on pale grey paper still has plenty of separation. An e-ink screen reflects room light and has far fewer shades to work with. The gap between the grey of the paper and the grey of the print narrows, and the page looks washed out. Add a dim front light and it gets harder still.
That’s why people who read scans on Kindle, Kobo or reMarkable ask for black text on a white page, even when the book looked fine on their laptop.
Why the contrast slider doesn’t fix it
The obvious move is to open the PDF in an editor and push contrast up and brightness up. It fails in a predictable way, because the background isn’t one shade:
- You raise brightness until the middle of the page is white.
- The gutter shadow is darker than the middle, so it stays grey or turns black.
- To clear the gutter you raise it further, and now the thin strokes of the letters, the serifs and the dots on i’s, fade to nothing.
- Any photograph or engraving on the page turns into a solid dark shape.
One setting for the whole page can’t be right in the bright corner and the shadowed corner at once, let alone across a 400-page book where every page lies a little differently on the glass.
What works instead
The approach that does work treats each part of the page against its own background:
- Measure the background. For each area of the page, estimate the shade of the bare paper, ignoring the letters.
- Level it. Divide the page by that background so bright corners and the shadowed gutter all come out the same white.
- Decide what is ink. Now that the paper is even, a local threshold turns the print solid black and leaves the rest white.
- Remove specks. Small isolated dots that aren’t near letters are taken out.
- Protect pictures. A page with real areas of shading or colour is kept in greyscale, levelled, instead of being forced to black and white.
FixMyScan runs these steps on every page of the book as part of its whole-book clean-up, after one margin crop and with pages straightened, and then reads the cleaned page to add searchable text.
What other tools offer
Some general PDF tools include scan clean-up options. PDF24’s OCR tool offers optional deskew while it adds text. Adobe Acrobat has an Enhance Scanned Document feature that community posts discuss alongside its deskew. We haven’t tested how either handles grey or yellowed paper on a whole book, so we can’t compare the results here.
Before you start
Best suited for: grey or yellowed book scans you mean to read, search or print, rather than archive.
If you are keeping a scan as an archival record, keep the original colour file too. A black-and-white reading copy is made for reading. It deliberately throws away the paper’s tone, which an archivist may want.
Steps
- Upload the scan. PDF or DjVu, up to 1,000 pages.
- Draw one crop past the spine shadow and the dark scanner edge.
- Let the pages clean. Paper goes white, print goes black, pictures stay in greyscale.
- Check flagged pages. Pages marked for pictures, faint text or odd crops are easy to compare with the original, and any can be swapped back.
- Download the PDF and send it to your e-reader.
The upload is deleted 24 hours after the book is ready.
Questions
How do I remove the grey background from a scanned PDF?
Use a tool that evens out the background before it turns the page black and white. FixMyScan does this on every page of the book: paper to white, print to black, pictures kept in greyscale.
Can you make PDF text darker?
On a scan, yes, by deciding which pixels are ink and making them solid black. Done without levelling the background first, the dark parts of the paper turn black too.
Why does my scan look fine on my computer but grey on my Kindle?
A monitor shows thousands of tones and is lit from behind. An e-ink screen shows far fewer greys, so the small gap between grey print and grey paper shrinks further.
Why is there a dark stripe down one side of every page?
That is the gutter, where the page curves into the spine and falls into shadow on the scanner glass. Levelling the background removes most of it, and a crop past the spine removes the rest.
Will whitening the page remove pencil notes?
Light pencil may fade or go, because it is faint grey like the paper. If the notes matter, use the original page for those pages after processing, or choose Clean greyscale.
My book is full of photos. Can it stay grey?
Yes. Choose Clean greyscale in your settings. The paper is still evened out and the pages straightened, but nothing is turned to pure black and white.
Is it the scanner's fault?
Partly. Scanning old books in colour or greyscale is normal practice for archives, because it keeps the most information. It just isn't the best form for reading on e-ink.