Minimum OD?

Hi, I would like to know what is the minimum necessary Optical Density needed for a red diode laser if it is powered from 250mW and ahead... I don't have a clue about this and I am afraid of becoming blind.

Thank you in advance

The OD value is a log value. If you aren't interested in the math, I'll tell you that 250mW would require at least a level of OD3 in order to ensure safety; that would make the 250mW laser appear as a 250/1000=0.25mW laser, which is safe (so long as you don't point it directly at your eyes of course).