Birefringence, Photoelasticity, Anisotropic Materials, Iridescence and the Rainbow Effect - Part 3

In the final blog post of this trilogy we will discuss Iridescence and how it can cause a rainbow effect on abrasion resistant coated Polycarbonate sheet. We will also discuss how the rainbow effect can be minimized.

Iridescence is the rainbow or oil slick type pattern that often appears on the surface of a Polycarbonate sheet particularly under artificial lighting conditions.

Coated Polycarbonate sheet has a thin film of coating on the surface of the sheet in order to protect the sheet against abrasion damage. It is this thin film of coating material that causes the problem, in much the same way that a thin film of oil on the surface of a pool of water exhibits the rainbow patterns. The effect is due to the process known as interference.

As discussed in previous blog posts, whenever light travels from a material with one refractive index to another material with a different refractive index, some of the light is reflected. In the case of the coated Polycarbonate sheet, when light moves from the air into the coating some of the light is reflected. Then, when the light moves from the coating into the actual Polycarbonate, some more of the light is reflected. When the light that is reflected from the first surface comes into contact with the light that is reflected from the second surface the light waves recombine.

Depending on how thick the coating layer is, the light waves may be in sync when they recombine or may be out of sync when they recombine. If they are in sync the two waves will added together and will have constructive interference. If they are out of sync the two waves will start to cancel each other out and will have destructive interference.

Since visible light has wavelengths of 380nm(violet) to 750nm (red) and a typical hard coat has a thickness of 4 to 7 microns [4000 to 7000 nm], the coating thickness is an order of magnitude thicker than the wavelengths of visible light. A small percentage variation in the coating thickness can therefore change whether the constructive interference or destructive interference occurs. If there is variation of coating thickness over a small area of sheet, even if the variation is only tens or hundreds of nanometers, then there will be areas of constructive interference and areas of destructive interference. This variation in the interference patterns is part of the cause of the iridescence or rainbow effect.

The question then becomes, how do we eliminate the variation in coating thickness? Abrasion resistant coatings are often added to sheet by a process known as flow coating. The sheet is hung vertically and coating solution is allowed to run down the surface of the sheet from top to bottom under gravity. The solvents are then allowed to evaporate. If the sheet is allowed to move before most of the solvents have evaporated, the coating surface can become uneven. However, we need to remember that the coating surface is not the only surface that we need to be concerned about - there is also the sheet surface that is reflecting light. The sheet is extruded between large chrome rolls which are powered by motors. If there is any variation in the motor speed of these motors or the motors pulling the sheet, there can be variation in the thickness of the sheet. While the variation in the thickness will be small, it only requires very small variation to cause iridescence.

The reality is that neither the sheet producers or the abrasion resistant coaters have the ability to control their processes to the level of 10-100nm thickness. If we look at sheet producers, many of them state that their thickness specification is plus or minus 10%. On a 0.118" thick sheet that corresponds to 300,000 nanometers. While this is an overall thickness tolerance and not a measure of local variation of thickness, it does give some idea of the magnitude of the problem. Using this information we can determine that the sheet producers and coaters cannot prevent the problem. In many cases sheet producers often blame the coaters for the problem and coaters often blame the sheet producers. This then leaves us with the question of how do we solve the problem?

To answer the question we will briefly move to another topic - different types of lights. Traditional incandescent light bulbs have a relatively smooth light spectrum across the visible region and are similar to sunlight in this respect. When sunlight is split into its component wavelengths (such as in a rainbow) there is a smooth transition from violet through the various colors to red. There are no wavelengths missing. An incandescent light bulb behaves in the same way (as do some full spectrum LED bulbs).

Fluorescent bulbs, mercury bulbs, sodium bulbs and non full spectrum LEDs are different. When the light is split into its component parts, there are peaks at some wavelengths and gaps at other wavelengths. For example, a low sodium bulb emits an almost monochromatic light source at 589.3nm and a standard fluorescent bulb has 22 peaks with the main four being Mercury at 437nm, Terbium at 543nm, Mercury at 547nm and Europium at 611nm. These wavelengths of a fluorescent bulb combine to yield a light that looks like natural light but has discrete wavelengths rather than the continual spectrum of natural light.

Having a light source composed of discrete wavelengths rather than a continuous spectrum is a major problem for iridescence; when the light is reflected from the two surfaces the discrete wavelengths make the problem much larger as there are no intermediate colors to cancel out the iridescent effect. In short, the light source can make the problem of iridescence much greater.

The best way to reduce the effect of iridescence is to change the lighting source to a full spectrum light source such as incandescent bulbs or full spectrum LEDs. If the only option is to use fluorescent bulbs, it is better to use a bulb with more emission peaks to more closely resemble full spectrum light.

Another option to completely resolve the problem is to use what is known as an index matched abrasion resistant coating. The Polycarbonate sheet has a refractive index of 1.585 and most coatings have a refractive index of 1.49. If an abrasion resistant coating with a refractive index of 1.585 is used, the light will treat the coated Polycarbonate sheet as a single layer material and the effect of iridescence will be completely eliminated. While this process sounds great (and HighLine Polycarbonate can offer index matched abrasion resistant coated products) there is a significant downside - index matched coatings are very expensive. In most applications it is better to install full spectrum bulbs to reduce the problem.

Finally, to illustrate the effect of lighting on the visual appearance of iridescence we will recount a case study about the problem. A rail car manufacturer was experiencing oil slick like patterns on the Polycarbonate windows of their railcars. The manufacturer of the windows was inspecting the windows prior to sending them to the rail car manufacturer to try and identify the problem. They were unable to detect the issue as their factory was lit with incandescent lights. When the windows were installed in the railcars, the oil slick appearance was easily visible because the internal lights on the rail car were fluorescent bulbs.

The most practical solution would have been to change the bulb type on the railcar, but unfortunately the window manufacturer did not understand the problem. They told the rail car manufacturer that the problem was due to birefringence, which, as anyone who has read these three blog posts knows, was not the cause of the problem. By understanding the cause of the problem it is easier to recommend a solution to the customer.