The ghost on the page
You lay a thin sheet on the table — an old airmail letter, a page from a pocket Bible, a carbon-copy receipt, a leaf of onion-skin typing paper — and the scan comes back haunted. Behind the words you meant to capture, a second set of words floats faintly, reversed and grey, like a photograph of the back of the page bleeding forward. The text is readable, more or less, but it looks dirty. On a double-sided document it can be worse: run it through OCR and the software starts mistaking the ghost letters for real ones.
The instinct is to blame the scanner, or the camera, or the app. But this isn't a software failure. It's physics, and once you understand the single mechanism behind it, the fix takes about one second and costs nothing.
Show-through is a lighting problem, not an ink problem
Start with the paper itself. What makes a sheet feel "thin" isn't only thickness — it's opacity, the measure of how much light a sheet blocks versus lets pass. Paper is made of felted plant fibers with air gaps between them, and light scatters at every fiber-to-air boundary. A thick, heavily loaded sheet scatters light so many times that almost none makes it through; it reads as solid white. A thin sheet lets a fraction of the light travel clean through to the other side.
Here is the part that matters. When you photograph or scan a translucent page, light doesn't only bounce off the front. Some light comes from behind the page — reflecting off the table, the platen, or whatever surface the sheet is resting on — passes through the paper, and on its way it illuminates the ink printed on the back side. That back-side ink, now lit from behind, is what your camera sees glowing faintly through the fibers. Papermakers call this show-through, and it is purely optical: the ink hasn't moved anywhere.
It's worth separating show-through from its uglier cousin, strike-through, where wet ink physically soaks into and through the sheet, leaving a real stain on the far side. Strike-through is a printing defect baked into the paper — nothing you do at scan time will remove it. But show-through, the far more common problem, is just a trick of the light. And light is something you control.
Why a black sheet behind the page fixes it
Think about where the ghost's light is coming from. It enters from behind the page, passes through, and lifts the back-side ink into visibility. So the fix is to kill the light behind the page before it can make the round trip.
Put something dark and opaque directly behind the sheet you're scanning — a piece of black construction paper, a black folder, a matte black card. A black surface absorbs light instead of reflecting it. With the black backing in place, very little light is bouncing up through the paper from behind, so the ink on the reverse side is no longer illuminated. The ghost simply goes dark. Meanwhile the front of the page is still lit by your room light or your phone's environment, so the text you actually want stays bright. You've widened the gap between the two: front stays high-contrast, back falls into shadow.
This is the same reason a document on a glossy white desk shows more bleed-through than the same document on a dark desk. Most people have witnessed the fix without noticing it. Slip a dark sheet under the page and watch the reverse text visibly fade before you even take the shot.
A few practical notes on the backing. Matte beats glossy — a shiny black surface can throw a specular reflection back up into the page and undo the effect. Genuinely opaque beats merely dark; a single sheet of grey paper isn't enough. And keep the page flat against the backing, because an air gap lets stray light sneak in from the sides.
When you can't change the backing
Sometimes the page can't be moved — it's bound into a book, or it's a fragile letter you don't want to press. You still have two levers.
The first is your light. Show-through gets worse when the only strong light is coming from behind or from a low angle that rakes across the sheet. Light the front of the page generously and evenly, from roughly the same side as the camera, and the front-side ink wins the contrast battle. A page lit brightly from the front and sitting in dim surroundings will show far less of its reverse than a page held up toward a window.
The second lever is the scan settings, used carefully. Because show-through is faint and grey while the real text is dark, a black-and-white or high-contrast mode with the threshold raised will often clip the ghost to pure white while keeping the true text black. This works precisely because there's a brightness gap between the two, and thresholding draws a line through that gap. The caution: push the threshold too far and you'll start thinning out light pencil, faint stamps, or the delicate strokes of a signature along with the ghost. Nudge it, check a full-resolution crop, and back off if real detail starts disappearing. Contrast is a scalpel, not a hammer.
A note on the paper that got it right
If you've ever handled a good India-paper Bible — pages so thin they're almost translucent, yet you can barely see the next page through them — you've held the counterexample to everything above. That paper is deliberately loaded with high-opacity mineral fillers, often titanium dioxide, which scatter light so aggressively that a very thin sheet still blocks most of it. It's proof that thinness and show-through aren't the same thing; opacity is engineered, and cheap thin paper simply skips the expensive filler. When you're fighting a ghost, you're fighting the absence of that filler, and the black backing is how you compensate for what the papermaker left out.
The scan you keep is the one you don't have to redo
Spend thirty seconds getting the backing and the light right and you get a clean sheet the first time — no grey ghost, no OCR reading phantom letters, no returning to the shoebox next month to shoot the same fragile letter again because the first pass was unusable. The cheapest scan is the one you only do once.
This is the part of scanning that no amount of processing power fixes after the fact, which is why LumenScan puts real controls where you can reach them: a true black-and-white mode with an adjustable threshold, per-page contrast, and a clear high-resolution preview so you can see the ghost fade — or catch a signature you're about to lose — before you commit the page. And because its OCR runs entirely on-device, that thin, personal letter you're finally digitizing never leaves your phone to get read.
If you've got a drawer of onion-skin letters or carbon copies waiting, a dark sheet of paper and the right mode are most of the battle. LumenScan handles the rest — quietly, and without uploading a thing: lumenscan.lumenlabs.works.