When some optical principles such as the interference

diffraction and polarization of light are used to measure physical quantities such as displacement rotational angle velocity and angular velocity

the errors of measurement are increased due to the undulation of the output intensity of light from the light source and the irregular change of the energy dissipa- tion during the transmission. If the change of the intensity of the incident light entering the measuring and testing system is monitored by a beam of reference light which is separated from the incident light by utilizing a spectroscope

the message and the reference light are transformed into voltage signals V

1

and V

2

by a photodetector

the quotient of two signals V

0

=V

1

/V

2

can be got by a divider. Then

the interference mentioned above is being eliminated in the process of divid- ing. So the signal is resumed to its true colours.