10.0 MEASURING PRINCIPLE
Reflection ratio of gas mixture is determined by the kinds of the composing gases and the mixture ratio. As long as the
kinds of composing gases are known, the mixture ratio (concentration) can be determined by measuring the reflection
ration.
The optic interferometer applied in the FI-21 displays “Interference Stripes” on the CCD. The Interference stripes move
proportional to reflection ratio. The amount of the movement is measured by solution of the interference stripes on
CCD with Fourier analysis. And the result is converted to the reflection ratio.
Concentration can be displayed by adding some data like “measuring gas” and “base gas” to the reflection ration
determined by high accuracy. Sensitivity of the optic interferometer depends on the length of the chamber. Since the
length of the chamber is physically unchanged by elapsing time, the high accuracy is maintained for a long time.
