公开(公告)号：US3623817A

公开(公告)日：1971-11-30

申请号：US3623817D

申请日：1970-02-24

Applicant: CIBA LTD

Inventor： CELIO TINO

IPC: G01J1/16 ,  G01J3/48 ,  G01N21/22

CPC classification number: G01J1/16

Abstract: This invention relates to a densitometer for measuring small differences in density. The densitometer comprises a light source providing a reference light beam for imaging on a reference object and a measuring light beam for imaging on an object whose density relative to that of the reference object is to be measured. Light reflected from the two objects is transmitted to a photodetector via an optical chopper that sequentially interrupts the transmitted beams. The photodetector produces sequential pairs of voltages which are applied to a logarithmic element in the form of a diode which produces pairs of logarithmic voltages for application to a polarity reverser which changes the polarity of one logarithmic voltage of a pair relative to the other voltage of the same pair. The voltages of each pair are then applied to an indicating instrument where they are utilized to give an indication of the difference in density between the two objects. The pairs of logarithmic voltages produced by the logarithmic element are also applied to a control circuit which controls either the intensity of the source or the amplification of the photodetector so that the voltages produced by the photodetector and applied to the logarithmic element are always within a predetermined range of amplitudes.