Paul Frost wrote:But as the above image shows, you can still see the flash, so there isn't a problem for those with limited hearing, and those with poor eyesight can hear the beep, so everybody is happy, problem solved.
Paul, please see babs f comment, top of page 2, where a problem was still apparent, and Scotts comment below. I thought the same, your photo is taken indoors and the contrast will be enhanced compared to reality.
My favorite solution here is the one shown by Roger, not perfect because if you approach it from behind you need to be on/past it to see the number.