Magic or Science - Just how do Colour Shifts work?

Whenever I mix up a fresh batch of one of our Colour Shifts for a production run, I am always mesmerised by the colours that appear in the pigment and how they move and change.

I was mixing up some Colour Shift Extreme Abyss the other day, and it got me thinking – I know how these work, but does everyone else understand what is happening when they look at their models painted in them? Let’s take a closer look at how these amazing pigments work and breakdown of how they trick your eyes!

Colour Shifts are a triumph of physics rather than chemistry. Unlike traditional pigments that absorb certain wavelengths of light and reflect others based on their chemical composition, Colour Shifts rely on a product that is known as thin-film interference.

The secret of how Colour Shifts work lies in the microscopic structure of each individual pigment flake. Instead of a solid chunk of colour, a Colour Shifting flake is a multi-layered "sandwich" usually only a few micrometres wide.

The Layers:

- The Core: Usually, a reflective opaque material like Aluminium or a transparent substrate like Synthetic Mica/Silica. SMS Colour Shifts utilise Synthetic Mica cores.|
- The Dielectric Layer: A transparent, "glass-like" layer (often Magnesium Fluoride or Silica) with a specific thickness.
- The Semi-Transparent Layer: A very thin metallic layer that allows some light to pass through while reflecting the rest.

When light hits the surface of these flakes, it doesn't just bounce off once, it undergoes a process called interference.

- Splitting: A light wave hits the top semi-transparent layer. Some of it reflects immediately, the remainder keeps going.

- Travelling: The rest of the light passes through the transparent dielectric layer, hits the core, and reflects back out.

- The Re-entry: A second wave of light exits the flake and meets the first wave.

Because the second wave has to travel "extra distance" through the dielectric layer, it is now slightly out of phase with the first wave.

If the light waves line up in the same pattern (peak-to-peak), that colour is amplified (Constructive Interference). If the light waves don’t line up and are opposite (peak-to-trough), that colour is cancelled out (Destructive Interference).

By why does the colour "Shift"?

The shift happens because of geometry. When you change your viewing angle, you change the distance the light has to travel through the dielectric layer.

At a 90° angle (straight on), the light travels the shortest path through the layer "sandwich," resulting in one specific colour (Green for example).

At a shallow angle (from the side), the light travels a longer, diagonal path through the dielectric layer. This change in distance shifts the phase of the light waves, stretching them out, causing a different colour to be amplified (Purple for example).

So, in basic terms, light waves enter the pigment and then bounce back at different wave lengths. Then as you view from different angles, the wave lengths change as they are travelling to your eyes at a shorter or longer distance than looking directly from above. This also explains why it is difficult to take a photo of a Colour Shift showing its true look as the camera sensors don’t collect light in the same way our eyes do.

Additionally, by changing the base colour under the Colour Shift, the light that travels through the transparent pigment will also change, reflecting back light at a different wavelength. Sometimes this will work in good effect and other times, not so much. It all depends on the actual Colour Shift pigment and what colours it is designed to show.

There is a lot more to learn about how Colour Shifts work, so if you are curious on learning more, check out the Patent document via this link to read up on all the technical wizardry of how they work :

https://patents.google.com/patent/US6157489A/en

So next time you are airbrushing on some SMS Colour Shifts, take note of the colours and better understand how light plays a vital role in creating some of the most amazing effects you will see on your models.

Keep on modelling!

- Scott