Background
Automatic interferometers offer high precision length measurement for the
calibration of gauge blocks. Displacements are measured by quantifying the
interference fringes from frequency stabilised laser light. The changes in
refractive index of the air within the optical path with variation in ambient humidity
need to be carefully monitored.
Car manufacturers or motor research associations need to carry out stringent
tests of car engines to reduce emissions into the air. As part of these tests for
engine combustion efficiency, the ambient air entering the inlet manifold is
monitored for compensation of NOX analysis to dry gas basis.
In the USA the Environmental Protection Agency advise the use of cooled mirror
instrumentation to make this measurement.
National laboratories providing high accuracy mass standards and mass transfer
standards need to compensate their balance systems for air buoyancy, which is
affected by the humidity of the air.