With the randomness and immeasurability properties of zero point, the conventional
self-supporting film thickness measurement model must calibrate the
distance between two chromatic confocal sensors using a standard part
whose thickness needs to be measured by other methods in advance. The
measurement performance is easily disturbed by the calibration
process, and by the accuracy of sample thickness or its uniformity. In
order to overcome these limitations, a new thickness measurement model
was developed by adding an auxiliary transparent film in the initial
position of the dispersion field. The lower plane of the reference
film is not only applied as the zero point of the first sensor but
also can be measured by another sensor, whose value is equal to the
sensor distance. Theoretical analysis and simulation showed that the
proposed method does not change the linear relationship of the
displacement coefficient. In order to verify the proposed measurement
model, a laboratory thickness measurement system was developed based
on two commercial chromatic confocal sensors with a displacement
accuracy less than 0.2 µm. A set of self-supporting film was measured
using the proposed system, the traditional method, and the reference
system. These experiments indicated that the standard deviation of the
calibration results of the sensor distance based on the proposed
method was reduced to 0.1 µm, which can be concluded that its
stability was improved significantly compared to the conventional
model. In addition, the proposed method was able to achieve a
measurement accuracy of 0.4 µm, which can demonstrate its efficiency
and practicability.