A cover image for a piece about Brewster's fringes.

What are Brewster’s Fringes?

Clayton Team | 04 May 2021
minute read

What are Brewster’s Fringes? Ever wondered why your windows sometimes display an oily rainbow effect when viewed in certain lights? Well wonder no more. What you’re seeing is the optical phenomenon known as “Brewster’s Fringe’s”. So what are Brewsters Fringes? In this short piece, we will explain what Brewster’s Fringes are, what causes them and why you need not worry about their effect on the performance of your windows.


Brewster’s Fringes are represented visually by an oily-looking rainbow effect that sometimes occurs in double-glazed glass units under certain lighting conditions.


Brewster’s Fringes occur in a double-glazed glass unit where the thicknesses of the two glass panes are the same. Here’s the complicated bit – if two beams of white light were to cross optical paths in a normal scenario, they would be in phase. This means that they will not produce any colourful phenomenon.

But when white light is shined through glass at a certain angle, it refracts. This causes the direction of the light to change slightly when passing through the glass. When this refracted light meets the incident light entering the glass, these two beams of light can become out of phase.

This causes the white light to separate into its constituent parts which, if you remember from your science classes in school, is the full visible light spectrum, with different colours representing different wavelengths. Because these components of light are of different wavelengths, they will be refracted by different amounts, causing the rainbow effect to be seen.

What Are Brewsters Fringes? Brewsters Fringes form in a similar way to a rainbow, with light refracting, reflecting and dispersing, although, in the instance of a rainbow, this occurs through water droplets in the air rather than glass.

Reflections from two parallel surfaces can also cause differences in the optical path. If the difference in optical path happens to be an exact multiple of a particular wave length (i.e colour) then the intensity of this colour is increased because the superimposed beam coincides with the original.

If, on the other hand, the difference in path is half a wavelength, the intensity of this colour is reduced, and if the two beams are of equal intensity, they will cancel each other out. Obviously, since white light consists of a mixture of the two spectral colours (or wavelengths), it follows that where these differences in optical paths occur, some colours will be intensified and some diminished, and since light is not normally in the form of discrete narrow rays, but rather large ‘bundles’ of rays, the spread of the interference pattern will be such that all the spectral colours become visible in bands.

When Brewster’s Fringes are seen in units in the form of more or less straight bands of colour, this is due to reflection and refraction when two glasses of similar substance having flat and parallel surfaces are placed close together, such in a double-glazed unit. They are not normally visible when looking through the glass, but if they do occur, they are generally seen when looking obliquely at the glass from a bright exterior, with the opposite side comparatively dark.


So we’ve answered the questions: “what are Brewsters Fringes?” and “what causes Brewsters Fringes?”, but what does their presence mean for the integrity of a glass unit? Are Brewsters Fringes an indicator of impaired performance?

No. If anything Brewster’s Fringes are a sign of a unit that is functioning correctly, being both parallel and made of a smooth, high-quality glass. The presence of Brewster’s fringes is therefore not an indicator of poor performance.

When any double-glazing manufacturer makes units from glass that is extremely flat and parallel, there is a risk of the event of Brewster’s Fringes being present. The phenomenon will apply to window makers the world over – being that it is an occurrence related to the physical properties of light.

In short, Brewster’s Fringes are no cause for concern.