What would happen if you put a mirror in a scanner?

What would happen if you put a mirror in a scanner? by Archie D'Cruz

Answer by Archie D'Cruz:

Let’s find out, shall we?

I took this fairly large round mirror and scanned it using my Canon flatbed. The result was…let’s say, not very mirror-like at all. In fact, the glass area was almost black.

So what happened? Where’s the shiny glass mirror we were hoping to see?

To understand why we got the result we did, we first need to look at how a scanner works.

There are many variations of this setup, but at its most basic, here’s what happens after you place a paper document on the glass bed and press Scan:

  1. A light source illuminates the document through the glass bed. The light source moves along a track as the document is being scanned.
  2. The document is reflected into an angled mirror, which also moves along a track at the same speed.
  3. The first mirror’s reflection is captured by a fixed mirror at the other end of the scanner.
  4. The image from the fixed mirror is captured by a CCD (charge-coupled device) and can then be saved to a computer hard-drive.

This is another way of looking at it, with a three-mirror setup.

Now here’s what happens when you replace the document with a mirror:

Everything works the same way as before, except that what the scanner’s movable mirror sees now is not a document, but a reflection of what is below. And what's below is the inside base of the scanner, which is black.

So what your scan will capture is essentially a dark reflection, and maybe some fingerprints or light scratches…but no shiny objects.

Note: Illustrations updated based on suggestions by Frank Heile. (Thanks, Frank!)

Added March 27: Comment worth noting made by Shriram Krishnamurthi, Professor of Computer Science at Brown University: My colleague John Hughes (of graphics book fame) agreed with this answer and added, "The only missing point is that there may be some optics that want to focus right about the scanning-surface plane, so that the reflected image of the inside of the scanner might well also be rather blurry, because it's way outside the depth-range at which the optics are focused."

What would happen if you put a mirror in a scanner?


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