Xm21 & AN/PVS2 starlight (passive) NV
The later NV set is shown here, this is an AN/PVS2 starlight, passive scope, the basic difference between the earlier AN/PAS4 IR (active) scope & the AN/PVS (1-3) (passive) scopes are:
Active IR NV devices produce (emit) their own light source which can be easily detected by electronics countermeasures, the AN/PVS series starlight scopes (these are termed 1st generation scopes & there was the AN/PVS1, 2, 2B &3) do not emit a light source, they use an electronic devise in the scope body to amplify what available light there is, the more light, usually moon or starlight (hence the term starlight scope) the better they work, on a moonless, very dark night they're very ineffective, the good point of course is they're not detectable w/countermeasures (at least at the time of the VN war, I don't know much about the later, 2nd gen & newer sets)
Active IR NV devices produce (emit) their own light source which can be easily detected by electronics countermeasures, the AN/PVS series starlight scopes (these are termed 1st generation scopes & there was the AN/PVS1, 2, 2B &3) do not emit a light source, they use an electronic devise in the scope body to amplify what available light there is, the more light, usually moon or starlight (hence the term starlight scope) the better they work, on a moonless, very dark night they're very ineffective, the good point of course is they're not detectable w/countermeasures (at least at the time of the VN war, I don't know much about the later, 2nd gen & newer sets)
Other Sizes: Original