Thin Film Interference Effect

You see it on a soap bubble, on oil spills, sometimes it is seen on metallic surfaces. Also it is used for coating glasses and much more. The idea is that there is a really thin layer on top of material that interfers the light and make colored reflection, as its thickness is comparable to the wavelength of light.

Let’s start with a simple metallic ball. Make a new scene, add a sphere and assign silver material to it:

Silver ball in Owlet

Then open “Silver” material in the material editor, open its layer and locate the “Specular” option there. Check Thin-Film interference box to activate the effect:

Thin film interference activated in Owlet

Here’s what you see below:

  • Thickness - the thickness of the film in nanometers. This must be around 150-500 for better effect;
  • Min Thickness - this is activated when you load a texture to Thickness slot, then end thickness is calculated by adding thickness from the texture to this minimal thickness;
  • IOR - the drop-down lets you choose how the film’s index of refraction is defined: by just providing N, both N and K, or by loading an IOR-file;
  • N, K, IOR-File - these fields are shown depending on IOR field and help controlling the index of refraction of the film.

So basically you need to configure the index of refraction of the film and then adjust its thickness to get the apperance you want. Let’s change our material and set N to 1.5:

Setting index of refraction of the thin film to 1.5

That looks better, right? Now let’s change the thickness of the film to 250:

Setting the thickness of the thin film to 250 nm

Then change it to 300:

Setting the thickness of the thin film to 300 nm

Here’s how to make it green:

Making thin film green

As you see, you can make absolutely any color or gradient by playing with Thickness and N parameters. The next time you render colored sunglasses - just use this effect.

Variable Thickness

Now let’s make it even more interesting. We’ll need a texture like this:

Cloud-noise texture for thin-film thickness modulation

Load it to Thickness slot and see what happens to the ball:

Modulated thin-film thickness in Owlet

Owlet uses minimal thickness (100) for black parts of the texture and maximal thickness (100 + 400 = 500) for its white parts. You can provide noise, gradient or ornament maps to get the coating you need.