Schlieren Photography: How to Photograph the Invisible

feature-schlieren

Ever taken a picture of a cough?

Not just somebody coughing. No, we mean the actual air currents as they’re being expelled.

Well, they just did it at Penn State, thanks to the magic of schlieren photography.

Schlieren” are density variations in a gas or liquid that you can’t see with the naked eye, like air currents caused by heat, movement, or explosions.

The super-crazy, holey-moley, you’re-not-gonna-believe-this part is that you can take these kinds of pictures at home, without a ton of special science-y stuff or fancy equipment.

Click below to learn more about schlieren photography, and learn how you can try it at your very own abode.

Photojojo’s Guide to the Mysterious World of Schlieren Photography

Photo credits: Gary Settles, Science Photo Library

A Little History

cough-smThe phenomenon of visible air currents was first observed in 1665 by Robert Hooke.

(Hooke also happens to be the first guy to see cells under a microscope, plus he invented your car’s universal joint and the iris aperture in your camera. Kind of a smart guy.)

Gary Settles, head honcho of Penn State’s Gas Dynamics Laboratory, currently reigns as the King of Schlieren Photography.

He’s the guy that shot that photo of a cough, as part of a study on how SARS and other airborne diseases spread from person to person.

Wanna See Some Photos?

hairdryer-smYeah yeah, we all know you’re here for the pictures. Here’s where to find more schlieren photos:

How Do They Do That?

diagram-smHere’s how schlieren photography works:

A pinpoint of light is bounced off a concave mirror back toward a camera that sits right next to the light source.

On its way into the camera, the reflected beam of light is interrupted by a colored filter.

And then, um, some magic elves get together with some wizards, and then you can magically see air currents.

OK, so maybe we’re a little hazy on the physics, but that’s basically the setup.

Try This at Home, Kids!

setup-smThe amazing thing is that you can do this ultra-scientific gas dynamics imaging stuff at home. Crazy-pants!

We wouldn’t have believed it if we hadn’t stumbled upon this tutorial by Ian Smith, who cooked up a schlieren photography rig in his basement.

We’re paraphrasing his instructions here, but be sure to check out the original for the fine details.

Even though this is DIY, it’s still science, so there’s a certain amount of precision involved. Be prepared.

Special Gear

light-smApart from a camera with a good zoom lens, you’ll need a couple of pieces of special equipment: a telescope mirror, a pinpoint light source, and a color filter.

Telescope mirrors are fairly expensive when bought new, but can be found on eBay for about $50+. For this setup, a 6″ to 8″ mirror will be fine, but you can go larger if you want to.

A point light source is something you can make yourself by shining a taillight bulb or LED light through a pinhole in a piece of tinfoil. The indomitable Ian Smith provides instructions for making a pinhole light source.

A colored filter is something you can print on transparency at home, or have a slide made from a digital file. Either way, here’s the pattern to print out.

1. Aim the Light at the Mirror

mirror-smSet up your rig in a place with hard floors- carpet can settle and throw off precise alignments. You can set up right on the floor or use a tripod — be prepared to make fine adjustments throughout this setup.

Every telescope mirror has a specific focal length. Set up your light source and camera twice that length away from the mirror. For example, a mirror with a 30″ focal length should be 60″ away from the light source.

Shine the light at the center of the mirror and adjust the placement until the beam of light reflects back almost directly at the light source.

You may need to work in near-darkness since the pinpoint of light is so small.

2. Focus the Beam of Light

focus-smSet the camera up right next to the light source- taping the light source to the lens hood is a good option.

Use a piece of paper to reflect the beam of light, and move it forward and backward until the point of light is in focus.

Try focusing with a flashlight first, since it casts a larger reflection and is easier to see than a pinpoint.

Place the filter at that point of focus and set the camera up right behind it. In fact, you can tape the filter right onto the camera and move the camera and filter at the same time.

3. Zoom In With the Camera

candles-smZoom in with the camera until the circle of light fills the viewfinder.

This may require a hefty telephoto lens, depending on how far away the mirror is. A teleconverter can help you get closer without needing a longer lens.

Now you’re all set and can start taking photos!

4. Take Some Pictures!

bluecandle-smWhat can you take pictures of to see the schlieren effect? How about…

  • the heat waves coming off your hand
  • a candle flame
  • a mug of hot coffee
  • a glass of ice water
  • the airstream from a hairdryer
  • soap bubbles (watch ‘em pop!)

More Resources

Now that we’ve whetted your scientific appetite (ah, we love a good whetting) here are some places to find out more about schlieren photography:

Photo credits: schlieren photos by Gary Settles, except the one of four candles by Andrew Davidhazy. Tutorial photos by Ian Smith and Harvard.

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