Measuring double refraction to measure foil thickness - by applying phase plate to sample, measuring intensity of light momentarily passing through, etc.

摘要

Measuring double refraction is as follows: (i) a phase plate (36) is applied to the sample (22) such that the phase difference of the overall sample and phase plate delay is a whole multiple of 2 pi using a measuring beam of a first wavelength; (ii) using a second wavelength measuring beam, which is close to the wavelength of the first, in this conditiion; (iii) measuring the intensity of the light momentarily passing through with the passage of the relatively common rotation of the two phase plates (14, 18) in relation to the sample (22) arranged between the two plates, such that the plates retain their direction of polarity in a constant relationship; (iv) application of a ratio between a max. value (Io) and min. value (Im) of the intensity of the momentarily passing light, to a previously determined ratio between the order (n) of the delay and the ratio, in order to derive the order (n) of sample (22) delay; and (v) calculation of the delay and thus the double refraction using the order (n) derived in this manner. USE/ADVANTAGE - Correct measurement of the delay is also possible at a higher order. Further that measuring a delay at high resolution is also possible if the phase difference is approximately pi. The process facilitates the measurement of the thickness of a plastic foil which has been drawn out.