What Is Duplex Scanning? Both Sides of a Page in One Pass

Duplex scanning records the front and the back of each sheet, so ten sheets printed on both sides become a twenty-page file. Simplex scanning records one side only. To know both sides were captured, compare the page count with the number of sheets and follow the printed page numbers through the file.

What is duplex scanning?

Duplex scanning is a scanner setting that captures both faces of every sheet and saves them as consecutive pages. Simplex is the opposite setting, one face per sheet. The word describes how the scan was made, not how the paper was printed, so a duplex scan of one-sided originals has an empty page after every real one.

Two words do the work in this subject, and the counts only make sense once you keep them apart. A sheet is a piece of paper. A page is one face of it, or one image in the file. The arithmetic follows from that:

Control panels do not all use the word. Look for "2-sided", "both sides" or "double-sided" under the settings for the original. On an office copier that setting is separate from the one for two-sided output, which controls how copies are printed and has no effect on what a scan records.

A second setting travels with duplex: the binding edge. A sheet that turns like a book page has its back the same way up as its front. A sheet that flips over the top, like a legal pad, has its back upside down relative to the front. If the scanner is told the wrong one, every second page in the file is inverted.

Single-pass duplex versus flipping the stack by hand

Single-pass duplex means the feeder images both faces during one trip through the machine. Manual duplex means you scan all the fronts, turn the stack over, scan all the backs, and the software or a person weaves the two runs together. The first is one step. The second is three, and each can go wrong.

A single-pass feeder has a sensor on each side of the paper path. A reversing feeder has one sensor, and after the front is read it draws the sheet back in and turns it over, so the machine makes the second pass itself. Either way the front and back of one sheet are captured together, so they cannot drift apart. Your scanner's manual says which kind you have. The post comparing feeder and glass scanners covers the hardware itself.

Manual duplex is what you do with a simplex feeder or a flatbed. Its weak point is the order of the second run. When you turn a stack over, the last sheet's back is on top, so the backs come out in reverse.

Worked example: five sheets flipped by hand

Dana Whitlock scans a ten-page affidavit printed on five sheets using a simplex feeder.

1. The first run captures the fronts: printed pages 1, 3, 5, 7, 9.
2. She turns the stack over and feeds it again. The second run captures the backs in this order: 10, 8, 6, 4, 2.
3. The scanning software's manual duplex option reverses the second run and interleaves it: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10.

If step 3 is skipped, the file reads 1, 3, 5, 7, 9, 10, 8, 6, 4, 2. If two sheets stick together during the second run and the back numbered 4 is never seen, every back after it is paired with the wrong front. The file then has nine pages, and the interleaving puts page 6 after page 3.

How do the backs of pages go missing from a scanned file?

Backs go missing in three ways. The scan was simplex, so they were never captured. They were captured and then removed, by the scanner's blank-page setting or by a person tidying the file. Or the paper you were given was already a one-sided photocopy of a two-sided original.

The causes, with the question that exposes each:

• Simplex left on. The previous job was one-sided and nobody changed the setting. Ask what the setting was, or count pages against sheets.
• A mixed stack treated as one-sided. A bundle of one-sided letters hides a few two-sided items. Fan the stack and look at the backs before scanning.
• A second run that failed. In manual duplex, the backs were never fed, were fed in the wrong orientation, or lost a sheet to a misfeed.
• Removal after capture. Software or a person deleted pages that looked empty.
• An earlier copy. The sender photocopied one side. No setting on your scanner can recover that, so the question goes back to whoever holds the original.

The backs that matter in a legal file are the ones nobody thinks of as pages. Check the reverse of each of these in your own file before you call the scan complete: a cheque, where an endorsement would be; a recorded instrument, where a recording stamp or label may sit; a pre-printed order form or invoice, where terms and conditions may be printed in small type; a postal receipt card; and any sheet with a handwritten note that says "see over".

Blank-page removal and the back side that holds only a stamp

Blank-page removal is a scanner setting that deletes any image with very little ink on it. It cannot tell an empty back from a back that carries one stamp, one endorsement or one pencilled line, because it measures how dark the image is, not what the marks mean.

The setting works on a threshold. The software counts the dark pixels in each image and discards the image when the count falls under a limit. A date stamp in pale ink, a signature in the corner or a bank's processing marks may sit under that limit on one scanner and over it on another.

Three ways to keep the setting from costing you a page:

1. Turn it off for originals you will not keep. Empty pages are a nuisance. A deleted endorsement is a gap nobody can see.
2. Test it on your lightest real page. Put the faintest stamped or pencilled back from the file in a short stack, scan with removal on, and see whether it survives. Lower the sensitivity until it does.
3. Record what was done. A note such as "scanned duplex, blank removal on, 14 sheets, 19 pages" tells the next reader that nine faces were discarded by the machine and not by a person.

Removal also breaks the sheet arithmetic. Once some backs are gone, page 7 of the file is no longer the front of sheet 4, and the count no longer proves anything. For telling an empty back from a failed capture inside an agreement, see the section on blank, duplicated and out-of-order pages in a contract scan. For where duplex sits among resolution, color and format, see the scanner settings for a digitizing project.

Watch the reverse of a transcript sheet. It may carry a stamp, a party name, or a short notice. Scan one side only and a search for that notice comes up empty. Afterward, hunt for a citation you remember from a back. Silence there means the back never joined the file.

Was a file scanned duplex or simplex? Signs in the page order

You can tell how a file was scanned from the sequence of its pages. Printed numbers that skip by two point to a simplex scan of two-sided paper. A real page followed by an empty one, over and over, points to a duplex scan of one-sided paper.

Printed page numbers are the strongest evidence, but not every document has them. Where they are absent, read across each page break: a sentence that stops at the foot of one page and does not resume at the top of the next marks a gap. Show-through is the other clue. Thin paper lets the back's print appear faintly, reversed, behind the front's text. If you can see it and the next page in the file is not that text, the back was not captured.

A short check for any scan someone else made:

1. Ask how many sheets were in the original, and whether any were two-sided.
2. Compare that with the page count of the file.
3. Follow the printed numbers, or the sentences across page breaks, from start to finish.
4. Open every empty page at full zoom before treating it as empty.
5. Write down what you found beside the file.

Converting the file to text gives you another way to do step 3. When you upload a scan to Schema Law it reads the pages into text, and you can view the parsed pages in order and read the foot of each one against the top of the next. The how it works section shows the steps. Nothing in recognized text marks a page as absent, so the comparison with the paper, or with the person who holds it, is still yours to make.