The usefulness of a military optical system is defined as that part of time when it is meaningful, with respect to the sight, to use the system. The usefulness is dependent not only on meteorological conditions, but also on sight reductions caused by activities on the battlefield or during march In this report is studied the basic data available in form of meteorological statistics and measurements of sight reducing effects concerning IR sights and MAW systems carried on combat vehicles. Further an investigation of reports on sight (visual) during battles compiled by Brigadeer Hugo Cederschiöld has been studied. The meteorological basic data is good. Discrete sources of sight reduction are also registered. From the battle reports is seen that grenade fire causes fog formation. The fog is coloured brown by particles from the ground. It is therefore desirable to study grenade bursts with respect to generation of heat and fog at damp bed. The throw-ups from moving vehicles should be studied further with respect to the character of the bed. The basic facts for this study contains no data for battles in built up areas. This kind of battle is considered to be the most frequent one in future threatening pictures. Since IR-systems in such environments are considered to be superior to visual systems should be studied thoroughly.