A STUDY ON ABSORPTION AND REFLECTION OF INFRARED LIGHT BY THE UNCOATED AND AL COATED SURFACES OF POLYMER FILMS TECHNIQUES - Polymers and Polymeric Composites - page 9

FIGURE 1.7 Specular Reflectance of Spectra of the film 1 (1) polypropylene surface (2)
Al coated surface of the film 1.

1.3.3.2 THICKNESS OF COMMERCIAL FILM 1

From the number of the fringes the thickness of the films can be calculated
using Eq. (1) [19].

1 N

(1)

b = x

2 ( n ) ( n − n )

1 2

where b = film thickness, n = refractive index of sample, N = number of
fringes within a given spectral region, ν 1 , ν 2 = start and end point in the
spectrum in cm -1 .
To count the number of fringes, select starting and ending points both
as minima or maxima of the spectrum and then count the number of op-
posing minima or maxima. In other words if we select minima values for
starting and ending points in the spectrum, then select maxima points to
count the number of fringes.
The refractive index of polypropylene is 1.49. There are 5 refraction
fringes in the region of 1600 cm -1 to 2650 cm -1 in Fig. 1.5. Thus the film
thickness, b , is found as 16 µm from Eq. (1).