Omron Rings & Color
The following post contains a hypothesis regarding the design, function and execution of the Omron rings. The author is a professional graphic artist and an amateur banknote collector. The author does not have access to any information that is not publicly available, nor does he work for a security printer.
The Omron rings, in their first public incarnation (on the Euro notes), were printed in bright yellow (with the sole exception of the 10 Euro note).
In their current use on other currencies, they are still very frequently printed in yellow. The rare deviations are almost always in a closely-related color: bright green or bright orange.
Why is this?
I believe that the answer has to do with the differences between machine and human vision.
The human eye perceives all the visible colors at once. We cannot mentally ‘filter out’ certain colors. Machine vision (by which, I mean digital imaging devices like scanners and digital cameras) can very easily filter out certain colors. Software can analyze full-color images or consider each individual color component separately.
To the naked human eye, the following image will be very difficult to read:
By stripping away all other colors and examining the previous image in only the blue channel, we can see a much higher-contrast image:
For the Omron rings, yellow printing on a white background is an excellent choice. It makes them nearly invisible to humans, but readily visible to computer software. Subtle and non-distracting to the naked eye, they can be applied almost anywhere on a banknote – even printed over the watermark, or overprinted with a blue background pattern. To currency-detecting software, they can remain as bold as black on white.