Security printing

Hi everyone - here's a challenge for the experts out there…

I'm trying to find a way of producing an 'anti-photocopy' pattern in our prints. We use Xerox digital presses and would like to stop people photocopying work we have produced.

We currently have some stock paper that is litho printed and looks like a grey paper. Our client would like something that is less grey and ideally is hidden in the text or image.

Does anyone know of any system or technique that we could use?

Thanks