Rainbow effect on scans

David Hicks

Hi all
On some of my scans I am getting a rainbow effect ( see below)



The stamp looks fine - At first I thought it was a reflection from the hawid type strip or even something on the glass of my scanner - but having cleaned both of these there is no difference

Anyone any ideas or experienced this please ?

It is not on every stamp , nor is it in any particular part of the scanner glass

David


Many thanks

David
https://www.hipstamp.com/store/the-stamp-cave

Harry Patsalos

David,
It might be space between the stamp and the scanner top. Try to add minimal pressure on the top while scanning BUT NOT TOO MUCH, you don't want to break the glass.
Harry

David Hicks

Thanks Harry

Have tried placing the stamp directly on the glass, then a book on top of that - alas no change

Have tried scanning in an area of glass that I do not usually use - same effect

Have scanned another stamp in same part of glass - perfect




Have tried scanning upside down - no difference, but when I scan sideways - it appears OK ????




Weird or what ?

Many thanks
David
https://www.hipstamp.com/store/the-stamp-cave

Harry Patsalos

David,
Must be something about the design of the stamp or maybe the color that causes this. Interesting. when I have time I will experiment with a similar item.
Harry

Harry Patsalos

David,
Just had an idea, what resolution are you using to scan?, if you are using the standard 100dpi try 200.
Harry

David Hicks

Good thought - have been using 150

Have just tried 300 - perfect

Well done kind sir - Still do not comprehend why sideways should work at 150 though - might have to postpone my funeral till I sort this one - I may never get to rest otherwise LOL

David
https://www.hipstamp.com/store/the-stamp-cave

Harry Patsalos

Chalk it up to 20 years of hit or miss scanning.
Glad I could help.

Michael Generali

It is a refraction of the scanner light against the horizontal engraving lines and the color of the ink based on your lower scan quality. When you turned the stamp 90 degrees, the horizontal scanning scanned the engraving lines at a different angle to where the refraction did not occur. That's how I understand what happened, but please don't ask me to give a precise scientific reason!

Ted Tyszka

Michael's explanation is pretty much correct, from what I've read. You may also see a similar result scanning photogravure (or rotogravure) stamps, where the image is produced from a half-tone screen. At scanning resolutions of 300 dpi or less, the dots comprising the stamp image are "out of sync" (so to speak) with the dots per inch of the scanner, which results in a moire pattern on the scanned image. At higher scanning resolutions, 600-1200 dpi, the scanner dots are finer than the screen dots, and thus moire patterns are avoided.

With stamps produced by lithography, or other continuous-tone method, you can scan at any resolution, as there are no ridges to cause diffraction, or screen dots to cause moire.