Jussi Hagman from Finland writes:

I was just left wondnering whether the 18-bit test image should have
been dithered, the display manufacturers could perhaps use some kind of HW based dithering to give an illusion of a better color depth.

Good question, and if I get the time I’ll do a follow-up on exactly that issue.

I started by looking at my brother’s PowerBook, where he did a quick test gradient in Illustrator. We saw banding on the machine, so it looked 18-bit, not dithered.

I’ve since started to question this initial test, since everyone seems to report that their computer looks fine. Adobe has a long history of doing things their way, and it’s possible that Illustrator is 18-bit on an allegedly 24-bit laptop.

At some point, time permitting, I hope to post an 18-bit dithered test image. I’ve done a quick test on my desktop (Ubuntu Linux with a generic desktop LCD) which makes me suspect that hardware manufacturers are doing built-in dithering. But I’ll need a better test image to be sure.