Down to Earth: optical measurement technique

How has the space race has brought us better contact lenses and made cataract surgery safer? Here's Stuart Higgins to tell us...

Stuart - Welcome to Down to Earth from the Naked Scientists. The mini series that explores the spinoffs from space technology that are being used on Earth. I’m Dr Stuart Higgins.

This episode: how a method used to measure how liquids and gases behave in zero G is being used to check the safety of contact lenses back on Earth.

The flow of a liquid or gas through a pipe might seem simple but, in reality, it’s extremely complicated, in particular when there’s a mixture of both liquid and gas at the same time. Such mixtures are often found in the cooling system used on board spacecraft and, as spacecraft get bigger, scientists and engineers want to understand better how these mixtures behave when they’re not experiencing gravity.

One company working with the European Space Agency (ESA) helped build the tools to measure the behaviour of liquids and gases in an experiment on board the International Space Station. Their system can measure the complex flow of liquids simply by looking at how a light passes through them. It’s based around Schlieren imaging, a technique dating back to the 1800s.

Light passing through a fluid at an angle will change direction, and the amount it shifts is determined by the material’s refractive index. Water has a higher refractive index than air, which is why things look distorted when you look through a glass of it. However, the refractive index can change with the density of fluid too.

Schlieren imaging passes light through a fluid and then focuses it onto a knife edge or a grid, blocking out about half the light. Behind this block sits a camera. Changes in the refractive index of a fluid, say from a shockwave, will cause some of the light to bend more strongly hitting the block and showing up as a dark shadow in the camera. The camera sees a shimmering pattern of moving fluid which represents areas of different density allowing its behaviour to be studied.

The company working with ESA invented a new variation of the Schlieren technique called Phase Shifting Schlieren, which also looks at how the phase of light changes as it passes through the fluid allowing them to measure with even greater precision. And this seemingly niche piece of technology found an unexpected application in helping to create better and safer contact lenses.

It began one day when an engineer tried placing their glasses inside one of the measuring systems to see what would happen. They found that their Phase Shifting Schlieren technique allowed them to map both the surface and inside of lenses with much greater precision than existing methods. The company built a new tool around the technique that can be used to inspect contact lenses, and also the artificial lenses used by surgeons in cataract patients helping to ensure there are no defects before the lense is surgically implanted.

So that’s how measuring complex fluid flows in space can be used to improve the quality of contact lenses and cataract implants back on Earth.