HP 5501A Operating And Service Manual page 33

-- -
Figure 2-7. Basic Measurement System
One of the most important concepts which must be understood for successful application of
the Laser Transducer System i s that it measures only relative position change between the
interferometer and the retroreflector. I t does not measure absolute position. In addition, the
only components sensitive to motion are the interferometer and the retroreflector (Figure 2-2).
If the interferometer and retroreflector are fixed in position with respect to each other, there
is no knowledge of the absolute distance between the components.
Figure
2-2.
SIC
~nterrerometr~c Measurements
I n the measurement of relative position, it does not matter which component moves a s long
a s one is fixed with respect to the other. If the interferometer is fixed and the retroreflector
is
the moving component (toward or away from the interferometer) only motion with respect
to its original position is detected. Conversely, if the retroreflector is fixed the interferometer
can be the moving component.