Well, crazy or not Hippynerd, I did the test, ( as you suggested in the second post of this thread ? ) and the difference is like chalk and cheese.
As I said, there are so many contradictory theories out there when I Googled about multiplex / brightness /pwm, that I had to check for myself.
Have a look at the photos, they are as they appear to the eye, the second 12% multiplexed row of LEDs are supposed to be only half the perceived brightness of the middle 100% row.
I do not have the time to measure the results unfortunately, but while I might get away with the 100% row at 50 meters into the sun, with a bit of filtering , there is no way the 12% row will work. ( I don't have the budget to use the power LEDs that the big video screens use )
I have no idea how much extra current at 12% would be needed to bring the " brightness " up to the level of the 100% LEDs, but the manufacturer of my LEDs does not recommend pulsing them at more than 25mA anyway !
Sorry to say, multiplexing will not work for my project, it would have saved me 80% of the cost of driver chips , ( which I had budgeted for anyway ) but I would have had to go for a double sided pcb and some Mosfets.
And then there might have been problems running the high speed SPI data to each of the 24 displays over 2 meters.
With Shiftout the fastest response I need is to change the timer display every second , and I dont have to worry about the receivers interrupt.
I see a lot of videos people put on you-tube of their LED multiplex projects, but many record them with the room lights off.
Take the board outdoors on a clear day, with the LEDs facing into the sun, and let me see the results, this is the worst case scenario I have to cater for with my outdoor projects.
Thanks everyone for your inputs, it was worth a try, and very interesting.