The arrange has an infrared beam source for a measurement beam and a second infrared beam source for a comparison beam process. The beam sources (201,202) are connected with a common power source (203). Circuits (204,205) are connected with a common clock (207,208) which alternately turns on the circuits. The clock is set to a time using the equation: time in divided by time out equals one. It is connected to a microprocessor control unit (210) which controls a change in the time relationship time in 2/time out 2 for one beam source (202) so that the relationship time in1/time out1 for the other beam source is changed by the same amount but with different sign.
