Colour, greyscale, or bitonal for receipts

Every scanner offers three ways to record tone: full colour, shades of grey, or pure black and white with nothing in between. The third is called bitonal or line art, it produces by far the smallest files, and it is the default on a lot of document scanners. It is also the setting most likely to quietly delete the part of a receipt you needed.

What bitonal actually does

Bitonal makes one irreversible decision per pixel. The device measures a brightness for each sample, compares it against a threshold, and writes either black or white. Everything above the line becomes paper; everything below becomes ink. There is no third option and no record of how close the call was.

On a laser-printed invoice this is nearly lossless in the practical sense, because the print is dense black on bright white and every pixel is an easy call. The resulting file is tiny and the text is crisp. This is why bitonal is a sensible default for office documents and why it ships as one.

On a receipt it is a gamble. Thermal print that has begun to fade sits close to the threshold, so a character can be resolved into black in one place and white in another, breaking strokes and hollowing out digits. Worse, the failure is not recoverable: the greys that would have told you a faint 8 was not a 3 were discarded at capture time. You cannot post-process them back.

Where bitonal fails on real slips

Four specific situations turn a bitonal scan of a receipt into a worse record than the paper.

• Faded thermal print. The whole image drifts toward the threshold and characters come out broken or absent. This is the main case, and it is covered in more depth in scanning faded thermal paper.
• Coloured or shaded backgrounds. Some tills print onto tinted stock, or the slip carries a shaded promotional block behind text. A single threshold either keeps the background as black and loses the text in it, or drops the background and takes the text with it.
• Creases and shadows. A fold casts a darker line. In greyscale it reads as a shadow you can ignore. In bitonal it becomes a solid black bar across whatever it crossed.
• Handwriting. A pencil annotation, a tip written in on the line, a signature — all lower in contrast than the machine print around them, and all first against the wall when a threshold is applied.

That last one matters more than it sounds. The note somebody wrote on the receipt is often the only record of why the spend happened, and it is the part bitonal is most likely to erase.

Greyscale is the sensible default for receipts

Greyscale keeps the intermediate tones and costs a modest amount of file size relative to bitonal. Everything bitonal decides at capture — where the line between ink and paper falls — remains adjustable afterwards, which means a slip that looked marginal can be brightened, contrast-stretched, or re-thresholded later without going near the paper again. That optionality is the whole argument.

It also behaves better for automated text reading, which generally prefers to make its own decisions about what is ink rather than inherit yours. Handing a recognition step a greyscale image gives it more to work with than handing it the result of a threshold applied by a device that knew nothing about the document.

When colour earns its size

Colour is the largest of the three and most of the time you do not need it, because a receipt’s information is monochrome print. There are exceptions where it is the right call.

Colour preserves the distinction between a printed line and a handwritten one, which greyscale can blur when the pen is similar in darkness to the print. It preserves stamps, highlighter, and any part of the document whose meaning is carried by its colour — a red-ink correction, a coloured “paid” mark, a card scheme logo. And for anything you are capturing as a one-off record of a physical object rather than as a source of text, colour is the honest capture.

A reasonable policy: greyscale for the recurring stream, colour for anything annotated, stamped, unusual, or irreplaceable.

The interaction with resolution and compression

These three settings are not independent and it is easy to fix one problem by creating another. Raising resolution on a faint slip resolves paper noise along with the print, which makes a subsequent threshold worse rather than better — the mechanism is in what dpi does to a receipt scan. Meanwhile the compression applied to a colour or greyscale image can introduce its own artefacts around small text, discussed in compression and scan quality.

Bitonal sidesteps compression artefacts entirely, since there are only two values to store, and this is a genuine point in its favour. It is just not worth the information it destroys on the paper this site is about.

Find the default and change it deliberately

Most devices ship in a mode chosen for office documents, and most people never look. Find out what yours is doing before you build a habit on top of it. Scan one faint slip in each of the three modes, view them at the same size, and see which ones still have a legible total and a legible date. It takes a few minutes once, and it will usually move you off the factory default permanently.

If your device supports saved settings, make the greyscale receipt configuration the one attached to whichever button or profile you actually use, so the right mode is the lazy option rather than the deliberate one. And if the device gives you a brightness or threshold control, note where it sits — that single slider is more consequential for a faded slip than any other setting on the machine.

None of these choices bear on whether a scan is an adequate substitute for the original. Whether a captured image satisfies a record-keeping obligation, in what form, and for how long the paper has to exist alongside it, varies by country and by the kind of entity keeping the records. Ask your own tax authority or an accountant who works where you do.