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On-line Sensing of Birefringent Webs

IP.com Disclosure Number: IPCOM000177085D
Publication Date: 2008-Dec-04
Document File: 9 page(s) / 137K

Publishing Venue

The IP.com Prior Art Database

Abstract

This article discloses an apparatus and method for on-line, real-time measurements of optical retardance and principle axis orientation in birefringent polymer films. With this apparatus and method, data rates in excess of 10 kHz are readily achievable, fast enough to resolve millimeter scale structures on webs moving up to 5 m/s (~1000 fpm). The present invention is calibrated using samples of known retardance and orientation, as measured with an off-line technique, but with the design described, the calibration remains stable over days or weeks for the production of a given film or set of films whose properties fall with specified ranges. The skilled artisan will recognize that this method can be generalized to measure out-of-plane retardance values as well.

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Page 1 of 9

On-line Sensing of Birefringent Webs

In the manufacture of certain films intended for optical products, e.g. cast webs and oriented films, an assessment of optical retardance and principle axis orientation may be desirable. Retardance and orientation affect how polarized light interacts with birefringent media, and depend upon thickness of the web or film as well as the alignment or crystallinity of polymer chains. Besides describing the optical performance of the film, they can also provide meaningful feedback regarding the process or processes used to make the film.

This article discloses an apparatus and method for on-line, real-time measurements of optical retardance and principle axis orientation in birefringent polymer films. With this apparatus and method, data rates in excess of 10 kHz are readily achievable, fast enough to resolve millimeter scale structures on webs moving up to 5 m/s (~1000 fpm). The present invention is calibrated using samples of known retardance and orientation, as measured with an off-line technique, but with the design described, the calibration remains stable over days or weeks for the production of a given film or set of films whose properties fall with specified ranges. The skilled artisan will recognize that this method can be generalized to measure out-of-plane retardance values as well.

A basic discussion of the measurement of these optical properties can be found in Gerald
G. Fuller, Optical Rheometry of Complex Fluids, Oxford University Press, New York, 1995. Fig. 1 defines the variable symbols used in the discussion presented here, and also depicts a basic layout of a sensing module designed to measure properties of a birefringent film. For simplicity, the coordinate system is chosen such that the x- and y- axes are aligned with principle in-plane axes of the film, and the beam propagates normal to the film. Without loss of generality, assume the x-axis is the slow axis and the y-axis is the fast axis.

β

Linear polarizer with transmission axis at angle
β to x-axis of film coord system

β x

Film, thickness d, refractive indices nx, ny, nz

Analyzer (2nd linear polarizer) transmission axis crossed with polarizer (i.e. at β+90 to x-axis)

y'

y

y

Incident Beam

y

y

z

β

x

x

x

For birefringent film, beam becomes elliptically polarized

Detector

x'

d

Fig. 1: Schematic of polarizer/analyzer setup for measuring birefringent film properties.

Page 2 of 9

Let E0 be the magnitude of the electric field vector that has passed through the first linear polarizer, where the transmission axis of the polarizer makes an angle β with film coordinate system. This derivation takes exp(-iωt) time dependence of fields as implicit.

y

E0

β

x

Fig. 2: Coordinate axis in plane of film.

Assuming perfect linear polarizers (100% transmission and no leakage), then the field components incident on the fil...