It is an important task to measure optical transmission of the atmosphere when testing the performance of electro-optical
systems such as thermal imagers. Only by knowing atmospheric transmission precisely enough, we will be able to
eliminate effects of the atmosphere on test results. For this reason a new instrument that measures optical transmission in
the atmosphere has been constructed. The transmissometer consists of a transmitter/receiver unit, a reflector and control
software. The instrument measures atmospheric transmission at wavelength of 1 &mgr;m and 8-12 &mgr;m by comparing the
intensity of the beam propagating through the atmosphere and the reference beam inside the transmitter/receiver unit.
Calibration is carried out by the aid of a visibility meter and a special calibration algorithm. An important criterion for
the design was to create an instrument which could be used flexibly in field measurements. The transmissometer was
tested comprehensively in the field in March and June 2006. It can measure extinction coefficients up to 3 - 12 km-1
depending on the span between the transmitter/receiver unit and reflector with accuracy of 10 - 20 %. According to the
test measurements the transmissometer also fulfills the other requirement specifications.