A continuation of the previous replay.
The theory of reflection of a transparent film on a bulk substate should be available from a physics or optics textbook. I will approach this from a practical point of view. What we need to apply here is that at different wavelengths, there is a series of constructive and destructive interference effects within the film. Constructive interference effects give wavelenghts of maximum reflection and destructuve effects give reflections of minimum reflection.
Let us take a TiO2 film on glass as an example. A film has a physical thickness (d), quarterwave optical thickness (4nd), and refractive index . Refractive index is made up of a real (n) and complex component (k). For a thin, non-absorbing film, we will let the complex component (k) equal zero. For TiO2 this is about true and let us say the real component (n) equals 2.0 (a dimensionless number).
If the TiO2 film has a physical thickness of 100nm (1000Angstrom) we can calculate that the quaterwave optical thickness (QWOT) is 4nd or (4)(2.0)(100nm) which equals 800nm. This is just in the near IR.
As you made a wavelength scan from say 1000nm to 200nm on a spectrophotometer, you would see a maximum in reflection at 800nm. This is the QWOT where constructive interference takes place. As you follow the curve down from 800nm, you would see a minimum in reflection at 400nm (800/2). This is destructive interference. As you continue down, you will see another maximum reflection point at 267nm (800/3). Basically all the odd quarter waves (i.e. QWOT, 3QWOT, 5QWOT) are maximum reflection points and all the even quarterwaves (2QWOT, 4QWOT) are minimum reflections.
If the layer is very thin, no maximum in reflection is seen. Beyond the first QWOT, the film reflection slowly decreases to zero as the wavelength increases. If the layer is very thick, the first QWOT may be in the infared (IR) and not easily measureable due to lack of instrument range or absorption in the coating.
You can calculate the film thickness in this case by determining the ratio between the maximums and minimums and determining which QWOT's you are seeing.
For example, say the TiO2 film is 400nm thick. The first QWOt will be at 4nd= (4)(2.0)(400nm) = 3200nm. You can calculate that the 2QWOT is at 1600nm, the 3QWOT is at 1067nm, the 4QWOT is at 800nm, the 5QWOT is at 640nm, the 6QWOT is at 533nm, etc. Other effects will prevent this from being as simple as I show here. The largest effect is from dispersion. This is the property where a material's refractive index changes with wavelength. TiO2 has a high dispersion and SiO2 has almost none. As the refractive index changes, you can see that it changes the result of the calculation. however, the calculation works well for most materials in the visible and near IR range. Many materials have very high dispersion in the UV. They also often become very absorbing in the UV. This is where the complex refractive index (k) increases rapidly from being close to zero in the visible or IR.
Most chemistry labs or water analysis labs have a spectrophotometer that can be adapted to measuring reflectance. A beam bending mirror attachment may need to be made. For non-absorbing films, it is often easier to put them on a transparent substrate and then directly measure transmission. i.e. Glass for visible, silicon for IR.
hope this helps a little - have fun - bob