PN 350-010000-05
P a g e
| 6
10,000 (m)
Δ ·
2
·
(n
2
-sin
2
θ
)
1/2
Reflectance Spectra and Applications
Reflectance spectroscopy is useful for the measurement of films on metallic substrates, the thickness of
free standing transmitting films and for the characterization of samples which do not transmit IR
radiation and so cannot be measured by other techniques.
Free Standing Films
Free standing films exhibit a fringe pattern in the collected spectrum due to constructive and destructive
interference caused by reflection from both sides of the thin film. Measurement of the period of the
fringes will give the thickness of the film as long as the refractive index is known. For an angle of
incidence and a refractive index n, the equation governing this measurement is:
where m is the number of fringes counted,
Δ
is the wavelength difference between the first and last
fringe (cm
-1
), and θ is
the angle of incidence. This interference fringe effect can create problems when
measuring thin film samples. The interference fringes can hide spectral features and cause quantitation
problems since the fringe position and spacing is sensitive to the thickness of the sample. Note the low
level of the spectrum, since only a small percentage of the infrared energy is reflected from the sample.
Figure 5: Reflectance fringe pattern of polyethylene film
Thickness (µ) =
