In brief, NV's primary role is to allow you to see in darkness (obv.) in order to do this the unit (in your case Gen1) gathers all the available ambient light through the image intensifier, too much light or over exposure will damage or render the tube unusable (commonly known as 'blowing it) you have a dedicated unit which can only be used at night, there are advanced, military spec units on the civilian market which alow you to switch from night to day but you'll need a fairly decent budget.
Martin