Aerosol transmissometer with an in-process transfer standard

A transmissometer for determining a transmissivity of an atmosphere, comprising:

• a chamber for containing the atmosphere;
• a light source for generating a test beam;
• a light detector for detecting the test beam;
• a periscope movable between a first position which allows the test beam to pass through the atmosphere in the chamber and into the light detector and a second position in which the test beam is deflected to pass into the light detector without passing through the atmosphere in the chamber;
• and
• a processor configured to determine the transmissivity of the atmosphere in the chamber during a fogged condition during a test, the transmissivity being determined from a transmissivity measurement for the test beam obtained by the light detector during a clear air condition in the chamber when the periscope is in the first position, a clear air transfer standard obtained at the light detector when the periscope is in the second position during the clear air condition and an in-test transfer standard obtained at the light detector by deflecting the test beam to be received at the light detector without passing through the atmosphere in the chamber during the fogged condition.