Why Don't Yellow Highlights Show up in My Photocopies?
Who doesn’t enjoy highlighting things? But when you use a photocopier to duplicate a document marked with yellow or other brightly coloured, fluorescent highlighter, the highlights don’t show up!
Why does this happen?
Typical black & white photocopiers use only black toner, and can therefore only print black. Even grey can only be produced with the help of dithering, which creates the illusion of greater colour depth.
When copying colours with a black and white photocopier, the machine must convert every colour into either black or white. Yellow highlighter is a very pale and light shade. The copy machine sees it as being very close to white. It is smoothed out, and therefore does not show up in copies.
“It’s a feature, not a bug”
Ever wonder why typical office highlighters are yellow or green? Perhaps not, but here’s the answer anyway: it’s precisely because it will not show up when photocopying. This allows you to mark sections of a document without the highlights blacking out or obscuring text in copies.
Just for fun, try highlighting some text with a pink Sharpie and then copying it. Depending on your photocopy machine’s resolution and settings, it may come out entirely black.
Read more about colours.
What if you do want highlights to show up?
If you’d like your highlighted text to reproduce in grey when copied, choose darker colours or select a pigment-based (rather than dye-based) highlighter.
As mentioned above, resolution also matters. If your copier's resolution under 300 dpi (dots per inch), it might have trouble correctly reproducing highlighted sections.
With modern colour photocopiers, this is not usually an issue. The colours of the original, including yellow, will be exactly reproduced in copies.