Hi,

I have the following setup: A polychromatic light source, a uv-cured thin layer of a transparent uv-varnish on quartz-glass (6mm). The light beam has an angle of 0 degree to the sample. The spectrum is also recorded under 0 degree using a UV-VIS-spectrometer .

If I now create a reference against the base substrate (without the varnish on top) and measure in a second step the applied varnish layer (approximately 20 microns thick), I get a transmission reading with clear interference frings.

Using this formula:

d = m / 2 D square root (n^2 - sin^2 theta)

with

d = thickness

D = Wavenumber-region

n = refraction index

theta = angle of incident

I'm able to calculate the actual layer thickness.

That's working fine.

NOW, I created a new sample but this time I referenced against the UV-cured varnish layer on top to the quartz-glass. An immediate measurement shows (as expected) 100% transmission over the whole visible wavelength range. Anyhow, after a dozen seconds fringes are becoming visible as if measured like before. After a few minutes the amplitude of these fringes is significantly increased. After 10 minutes the fringes are even higher in amplitude but no more change is observed for t = 10+x.

Why do these fringes appear appear at all and why do they increase in amplitude over time. The fringes count is constant and of the same number as counted in the first part of the experiment?

Thanks a lot,

Daniel