A deep dive into polarisation

The electric field oscillates due to its interaction with the magnetic field and vice versa. The two waves have the same wavelength and are typically in phase with each other. It’s the combination of these two disturbances that forms the electromagnetic wave.

As you already know, waves are made up of troughs, crests and amplitudes. An electromagnetic wave’s amplitude denotes its intensity (or brightness).

The light spectrum sits within a larger electromagnetic spectrum which ranges from gamma rays through to radio waves.

(The UV, visible and IR spectrums make up a small fraction of the various wavelengths that exist)

So how does polarisation work with these?

The polarisation of a wave indicates the direction of the electric field’s amplitude. Polarised light means that all waves would have the same polarisation.

Unpolarised light would have lots of waves oscillating in different directions. This is illustrated in the cross-sectional plane to the right - where the light would be travelling towards you, out of the screen. Examples of this would be incandescent bulbs or the sun.

Edwin Land, born in 1909 in Connecticut, invented Polaroid, the world's first polarising material for commercial use, in 1929. He founded the Polaroid Corporation in 1937 in Cambridge, Massachusetts. The company initially produced Polaroid Day Glasses, the first sunglasses with a polarising filter.

Source: https://en.wikipedia.org/wiki/Polaroid_Eyewear

Where would this be useful?

The ideal illumination solution for objects that are shiny and curved would involve high end illumination sources including ...

...dome lights with half silvered mirrors or “flat domes” that create diffused reflections from all directions.

These are good solutions, but they can be expensive - and sometimes they don’t work. This is where the most common use of polarising filters is introduced - to reduce direct reflections off an object.

Circular and elliptical polarisation

These are a little bit more complicated. If the phase of the electric and magnetic fields is not the same, then the wave is said to be elliptically polarised. When the two are added, it traces an ellipse around the axis.

Is there quantitative a way to specify the polarisation of light?

Yes, by using the Stokes vector, four numbers that define the direction and magnitude of an electromagnetic waves polarisation - whether its unpolarised, partially polarised, or fully polarised.

They were defined by George Gabriel Stokes in 1852, as a mathematically convenient alternative to the more common description of incoherent or partially polarised radiation in terms of its total intensity (I), (fractional) degree of polarisation (p), and the shape parameters of the polarisation ellipse.

Source: https://en.wikipedia.org/wiki/Stokes_parameters

Using Stokes’ images to visualise stresses in a plastic ruler

The images above respectively represent the first three Stokes’ parameters S0 to S2. The Stokes’ parameter images can be combined into single images, in which the pixel’s intensity represents ...

... the Degree of Linear Polarisation (DoLP) and the Angle of Mean Polarisation (AoMP)

S0, DoLP, AoMP can be mapped to the HSV colour space ... to better visualise the stresses in the structure of the ruler.

Polarisation by reflection

The angle of incidence at which polarised light is perfectly transmitted through a transparent surface, is known as the Brewster angle θB.

Sir David Brewster, a British scientist, is principally remembered for his experimental work in physical optics, mostly concerned with the study of the polarisation of light and including the discovery of Brewster's angle.

Source: https://en.wikipedia.org/wiki/David_Brewster

When unpolarised light strikes the surface at the Brewster angle, it produces a 90 angle between the polarised reflected and the refracted ray.

Heres an example using images of a mobile phone. The polarising filter is used with the camera (that’s taking a picture of the phone) at two different angles.

Area scan sensors with integrated polarisation filters

Area Scan Sensor: SONY IMX250MZR POL

Each calculation unit (4 pixels) has 4 nanowire grids oriented at 90°, 45°, 135°, and 0°.

Cameras that use the Sony Polarisation Sensor

By positioning the polariser underneath the on-chip lens, Sony Semiconductor Solutions reduces the possibility of crosstalk. This occurs when light at a polarised angle is misdirected into an adjacent pixel.

Application examples

Detection of mechanical stress in filter can lids

Film inspection of card decks

Film inspection of card deck tear tabs

Inspection of shiny and reflecting surfaces

Advantages of Polarisation

Polarisation imaging is a powerful technique that offers numerous benefits. It is not just surface roughness, scratches, dents and surface coatings that can change the polarisation state of the light, but also other physical properties such as stress or birefringence. This means that the technique can reveal information and de that simply could not be seen using any other imaging technique.

Please talk to us if this technology may be of interest to you.

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Know-how: Polarisation

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