Supposing to multiplex a LED matrix, after a certain number of columns the LEDs power-on time becomes too low in order to ensure a good LEDs brightness so you need to increase the current in order to compensate the decrease of the power-on time. Until now, in this situations, I always did empirical tries in order to understand what's the correct ratio between power-on time and current. Is there any general rule that allows you to calculate the right current starting from the power-on time and vice versa in order to obtain a good brightness? Or is the empirical method the only way?
On the Internet I see that, in multiplexing, some people don't use resistors for the LEDs saying that if you turn on the LED without any current limitation for a very low time it will not damage. Obviously in this case the LED is very bright but is this technique completly safe for the LED or affects its lifetime? In the case where there is a safe time for which the LED can be turned on without current limitation, which is it?