We are all familiar with colour and brightness, but there is a third property of light – the “polarisation” that tells us the orientation in which light waves are oscillating. Animals, like bees and ants, use the polarisation patterns in the sky as a navigation aid. But few people, even in the scientific community, are aware that humans can perceive the polarisation of light with the naked eye.

In previous studies LCD screens have been used to test polarisation sensitivity in aquatic organisms. Our study tested the limits of human polarisation sensitivity, developing special filters to vary the percentage of polarised light reaching the eye from 0% to 100%.

Polarization oflight

Light is the single most crucial element of photography. Knowing the two types of reflected light and how to use them is vital when honing your skill. Here’s why.

When light strikes a medium and bounces back, that’s reflection. Any object you can see is visible because of reflected light, even if you don’t think of the object as “reflective” like a mirror. We’ll get to the specific types of reflected light in a moment. But first, reflection is commonly confused with refraction, which is when light passes through a medium and changes direction and speed.

polarization中文

Humans perceive polarised light using “Haidinger’s brushes”, a subtle visual effect which appears like a yellow bow-tie at right angles to the polarisation angle. You may also see a bluish bow-tie at right angles to the yellow one. The effect originates within the eye itself and is not an image of a real external object, so Haidinger’s brushes usually fade in a couple of seconds as your brain processes them out. This is one of the reasons that few people notice them day-to-day, and why they have previously been fairly difficult to study.

The ability to see Haidinger’s brushes is caused by circularly symmetric organisation of carotenoid pigments in the macula (an area that covers and protects the central part of the retina). Blue light which is oscillating parallel to these pigment molecules is strongly absorbed. White light, which is depleted in blue, appears yellow, which explains the yellow bow-tie effect. The blue parts of the brushes are thought to be generated by the brain in response to the unexpected presence of yellow.

Polarization

This was to establish the minimum percentage polarisation at which Haidinger’s brushes could be detected. Among 24 people, the average polarisation sensitivity threshold was 56%. Some people could still see Haidinger’s brushes when the light was less than 25% polarised – not quite as good as cuttlefish but still better than any other vertebrates tested to date.

Reflected light is light that’s reflected off a subject. No matter what direction the light is coming from, it will be affected by what it’s bouncing off. The subject will determine which type of light will be reflected. Keep in mind that reflection of light isn’t the same as refraction.

Haidinger’s brushes also provide a demonstration of the physics of light and the anatomy of the human eye. By taking the polaroid layer off an old LCD screen you can make your own simplified version of our test; black and white letters turn into contrasting polarisation angles once the polarising film is removed.

Circular polarization

So if polarised light is actually pretty common outdoors, in your home and in your office – how come you didn’t notice anything special before now?

Skylight polarisation patterns, caused by light scattering in the atmosphere, are such that the long axis of the yellow bow-tie will point approximately towards the sun.

Without light, photography doesn’t exist. And knowing how light works and interacts with your subject — and with your camera — will make you a better photographer.

When it comes to light, there are two types you should be familiar with: incident light and reflected light. In this post, we’ll take a look at reflected light.

Might it be possible to use our polarisation powers for good? The risk of acquiring age-related macular degeneration has previously been associated with low carotenoid pigment density in the macula.

As AMD is currently the leading cause of blindness in the developed world and finding an early stage diagnostic indicator of this before any actual sight loss is incurred is a research priority. It is our hope that polarisation sensitivity could be used to investigate and ultimately monitor any changes in the organisation of the pigments happening in the early stages of this degenerative eye condition. More work is needed to assess the medical potential of these kinds of tests.

In research we’ve just published in Proceedings of the Royal Society B, we used an experiment that was originally designed to test the visual abilities of octopuses and cuttlefish to investigate our human ability to perceive this polarised light.

Light is the most important element in photography, and the better you understand how it works, the better your photos will be. By mastering the reflection of light, you’ll be able to take better shots and avoid common issues like red eyes, hot spots, and blown-out exposures.

Circularlypolarized light

Ever fancied having a superpower? Something you can call upon when you need it, to hand you extra information about the world? OK, it’s not X-ray vision, but your eyes do have abilities that you might not be aware of.

polarization极化

Imagine a skipping rope is a light wave travelling through space. If you move the rope from side to side, the wave you make is horizontally polarised. But if you shake it up and down you create a vertically polarised wave. Generally, light is a mixture of polarisations, but sometimes – for example in parts of the sky, on your computer screen and in reflections from water or glass – a large percentage of the waves are oscillating in the same orientation. This light is described as being strongly polarised.

You will probably have come across technology that is built around polarised light before. For example, “Polaroid” sunglasses work by blocking out polarised light which is reflected from shiny surfaces such as car bonnets or the surface of water. This is possible because light reflected into our eyes from horizontal surfaces is horizontally polarised and the sunglasses have a structure like a picket fence, so they only let in vertically polarised oscillations, blocking out the horizontally polarised bright reflections. Polarised light is at the heart of modern 3D cinema and LCD computer screens, smart phones and tablets.

At a recent science festival I tried to get people to take an “octopus eye test” by reading the hidden letters using their polarisation sensitivity alone. It went down a storm, except with one little boy, who was terrified of the accompanying octopus headdress. Time to work on a less intimidating super-costume.

To see Haidinger’s brushes for yourself, look at a blank white portion of an LCD screen on a computer, tablet or phone. Tilt your head from side to side and faint yellow and blue bow-ties, slightly larger than your thumb, should become visible. With practice, you can then see them in the blue parts of the sky at 90 degrees from the sun, particularly at sunrise and sunset.

Electric polarization

The reflection of light can be broken into two categories: diffuse reflection and specular reflection. Diffuse reflection is light that hits a surface and is reflected in many different directions. Almost all light you see on a daily basis is diffusely reflected.

A shiny nose or forehead, glasses, windows, or other highly reflective surfaces can cause these spots. Using the law of reflection, we know that moving the light source or camera to a different angle will help eliminate these spots. The same goes for red eyes.

Linear polarization

Our results shows that your cornea can dramatically affect how you perceive polarised light. As the optical properties of the cornea vary between individuals, this may also explain why people often report their experience of seeing Haidinger’s brushes differently.

Knowing how light reflects off different subjects will allow you to overcome potential problems like hot spots or red eyes. Hot spots are areas in an image that are much lighter and brighter, and they’re caused by specular reflection.

Light is so important to photography. Tell us what you think! Leave a comment below or start a conversation on Twitter, Instagram, and Facebook.

By using LCD screens capable of constantly refreshing the effect, we were able to make the first measurements of the dynamics of Haidinger’s brushes, confirming the prediction that some individuals would perceive the orientation of the bow-tie to “flip-flop” as the polarisation angle is rotated.

Now that you know the difference between the two, let’s jump into the reflection of light and how it applies to photography.

A good example is when you use a refraction ball or shoot through a water droplet. The subject or landscape on the other side is inverted and skewed by the shape of the refracting object. Light passing through a prism or a camera lens is another example of refraction.

Specular reflection is light that hits a smooth surface and is reflected at the same angle. An example of specular reflection in photography is taking a picture of a reflection in a still body of water. In the photo above, the water is creating a specular reflection of light, and the forests and mountains are creating diffuse reflections of light.

Juliette McGregor does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.