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Automatic exposure when scanning vignetted certificates against colored backgrounds
Depending on exposure, writing, printing and stamps on the backs of certificates often show through to the front. Auto-exposure sometimes hides show-through and sometimes makes it worse.
If you refuse to turn off your scanner's auto-exposure, then at least scan certificates against your scanner's white lid. Many people inadvertently worsen their images by scanning against colored or black backgrounds.
If you are going to scan with auto-exposure, then test its limitations on your scanner. Remember that certificates are not normal documents. They neither have the typical white space of typewritten pages, nor the normal color density of photographs. Vignettes may appear like grayscale images to your eyes, but not to your scanner.
Here are four examples to consider. I scanned this certificate against four different backgrounds using auto-exposure on a high-quality scanner. Except for re-sizing to fit this page, images are unaltered.
Like most certificates, the paper used for this certificate is slightly cream colored, and therefore yellower than ordinary copy paper. The paper is normal thickness, so there is some show-through. I showed part of the colored backgrounds so you can recognize the differences easier.
Certificate scanned with auto exposure against a white background.
This scan fairly accurately reproduces the appearance of the real certificate.
Certificate scanned with auto exposure against an orange background.
In this example, the scanner picked up some orange background, but not a lot. The exposure is a little darker than the previous scan, but is still very acceptable. A slight orange cast is visible, and is most noticeable in the area above the vignette and along the paper margins. The reason the background does not affect overall coloration is because the orange is in the same general color range of the paper.
Certificate scanned with auto exposure against a dark blue background.
This scan clearly shows the effect of show-through from the dark background. This scan appears lighter than the previous scans because the certificate appears darker to the scanner. Since the certificate appears darker, auto-exposure compensates by lightening the whole scan. Consequently, the resulting scan is over-exposed.
Certificate scanned with auto exposure against a black background.
Like the dark blue background example above, the scanner compensated for the black border and the dark show-through, and thereby created an over-exposed image.
I grouped all the images together here to make it easier to compare scan results side-by-side.
In general, I recommend not scanning certificates against dark or colored backgrounds when using auto exposure. If you have a compelling reason for doing so - such as wanting to show edge problems - then be careful NOT to show too much of the background. Better still, turn auto-exposure off and manually adjust for your best result.
Some certificates, particularly generic certificates, were printed on very thin paper. Scanning thin-paper certificates is often challenging. For examples, see Automatic exposure when scanning thin certificates against colored backgrounds.
See also Auto-exposure vs manual exposure for examples of what you can accomplish when you turn auto-exposure off.
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