, a light meter
that can measure light from a flash
. This can not be done with a normal continuous light meter, because the light from an electronic flash
system is a very short blast.
A flashmeter, knowing the ISO/ASA rating of the film you are using, will tell you what f-stop is appropriate (in the light metering sense of appropriate). Of course, it will not tell you the exposure time: since, compared to the shutter, the flash is almost instantaneous, the time will not influence flash exposure.
The exposure time, though, will influence ambient light exposure, though: and this is the whole trick behind fill flash.