speed measurements

I don't remeber when I found a simple program which might be helpful to assess darts and other ammos speed.
below I attached a telling picture illustrating how itworks.
Its name is dartspeed (1.37 MB), freeware, found on some site about blowguns (I'm sure I'm not telling anything new).
Personally I've never tried it, but watching Joerg's videos over and again I decided to dive in the world of testing.
Any operational comments higly welcomed.
slingor