What is more efficiently: switching the light ON and OFF or leaving it ON?

Q. What is more efficiently: switching the light ON and OFF or leaving it ON?
A. If your lamp is 100W and voltage is 120V the lamp impedance will be 144 ohm.
First 0.1 sec (start up) after turning the light ON the lamp impedance is about 10 times less (~15 ohm) and power will be about 1000W (0.1sec) or 100W (1sec).
It means turning the light ON equal to leaving it ON for 1 sec.
This is a minute amount and no efficiency at all.
So it makes sense to always turn the lamp (an incandescent bulb) off when you leave a room, even if it's only for a few minutes.
But frequently switching ON and OFF will make shorter the bulb life.
Energy Star recommends installing florescent bulbs (CFLs) in fixtures that are used for at least 15 minutes at time. Read more...
So it doesn't make sense to use them in places where you'd need to switch them on and off constantly.