See Air Currents with Shadowgraphs and Schlieren Images

Abstract

Have you ever wondered what the air currents look like around a candle? What about cold air flowing around an ice cube, or warm air rising from your hand? What about when you sneeze? We are surrounded by air currents all the time, but these subtle movements are completely invisible to the human eye. In this science project, you will take advantage of small changes in air density to make air currents visible in regular photographs and videos, using techniques called shadowgraphy and schlieren imaging. 

Summary

Areas of Science

Photography, Digital Photography & Video
Physics

Difficulty

Method

Engineering Design Process

Time Required

Average (6-10 days)

Prerequisites

Experience with adjusting manual camera settings is helpful for this project.

Material Availability

Readily available

Cost

Low ($20 - $50)

Safety

Adult supervision required when using a razor blade or open flame. 

Credits

Ben Finio, PhD, Science Buddies

Science Buddies is committed to creating content authored by scientists and educators. Learn more about our process and how we use AI.

https://www.youtube.com/watch?v=9FZRSmbO_as

Objective

Visualize air currents in various scenarios using shadowgraphy and schlieren imaging. 

Introduction

We know that air moves all the time. We can feel the breeze on our skin and hear leaves rustling in the wind. We cannot always see air moving, but there are some exceptions. On a particularly windy day, sometimes you can see fine particulate matter (like dust, snow, or sand) blowing along with the wind. We can see smoke rising up from a campfire or a candle. Scientists can intentionally release smoke in a wind tunnel to see how air moves over an airplane wing. Techniques that help us see how air moves are called flow visualization.

In this project, you will try two different flow visualization techniques that do not require any smoke. They both take advantage of the fact that light refracts, or changes direction, when it passes through materials with a different refractive index (Figure 1). You may have noticed this if you have ever observed how a straw appears to bend when placed in a glass of water. This happens because air and water have different refractive indices. 

Image Credit: ajizai / Public Domain

Figure 1. A ray of light refracting as it passes through a glass block.

The refractive index of air is not constant. It depends on the density of the air. This means the path of a light ray can change as it passes through air, even if it does not pass through another material like water or glass. For example, hot air above a road can create a mirage like the one in Figure 2 as air refracts through it.

Image Credit: Michael from Calgary, AB, Canada / Creative Commons Attribution 2.0 Generic

Figure 2. Mirage created by hot air above a road.

With a special camera setup, light refracted through air can create brighter and darker areas in an image. The bright areas are spots where more light was refracted to, and the dark areas are spots where more light was refracted away from. The resulting image is called a shadowgraph. Shadowgraphs can be used to view things normally invisible to the human eye, like the shockwaves around a bullet (Figure 3). Schlieren images (Figure 4) look very similar to shadowgraphs at first glance, but they have a slightly different setup and can be more sensitive, revealing more subtle changes in air density. Note that schlieren is German for "streaks." It is not someone's name, so it is not capitalized unless it is at the beginning of a sentence. 

Image Credit: Daniel P.b. Smith / Creative Commons Attribution-Share Alike 3.0 Unported

Figure 3. Shadowgraph of shockwaves and turbulent wake behind a bullet.

Image Credit: Gary Settles / Creative Commons Attribution-Share Alike 3.0 Unported

Figure 4. Schlieren image of hot air rising from a candle. 

So, how do you create these images? Figure 5 shows a basic setup. A point light source is placed at a distance twice the focal length away from a concave spherical mirror. The light source is offset slightly to the side of the optical axis. A camera is positioned on the other side of the optical axis, so that light reflected from the mirror is aimed at the camera. Light passing through pockets of higher or lower density air is refracted on its way from the light source to the camera, resulting in a shadowgraph. Since the light is effectively aimed directly into the camera's lens, the camera must typically be set to a low exposure to avoid overexposing or washing out the image. 

Image Credit: NathanHagen / Creative Commons Attribution-Share Alike 4.0 International

Figure 5. Optical setup for a single-mirror schlieren system. The setup for a shadowgraph is similar, but it omits the knife edge used as a light block.

For a schlieren image, a light block is added to the setup, typically the edge of a razor blade. This edge must be placed exactly where the point light source is focused. This results in some refracted light rays being blocked completely by the light block, while others are refracted past it, increasing the contrast of the resulting image (When set up properly, this does not result in simply cutting off half the image; see the Bibliography for more details about the physics). While this setup is more difficult due to the careful alignment required for the light block, it makes schlieren imagery more sensitive than shadowgraphy to subtle changes in density and air currents. 

While they require patience and careful setup, shadowgraph and schlieren techniques can result in some truly amazing images. While many images you can find online result from setups with professional laboratory-grade equipment, you can quickly get a basic setup working at home just using a cell phone and a makeup mirror. Depending on your budget, time available, and what materials you have access to, you may be able to devise a higher-quality setup to produce better images. Once you have a setup working, you can open the door to visualizing many of the normally invisible phenomena that surround us every day!

Terms and Concepts

• Flow visualization
• Refract
• Refractive index
• Density
• Shadowgraph
• Schlieren image
• Point light source
• Focal length
• Concave spherical mirror
• Optical axis
• Exposure
• Light block

Questions

• What causes light to refract?
• How are shadowgraphs and schlieren images created?
• What is the difference between a shadowgraph and a schlieren image?
• What normally invisible things can you see with a shadowgraph or schlieren image?

Materials and Equipment

There are different ways you can set up this experiment depending on your budget and what you already have available. You may wish to get a basic setup working before you invest money in more expensive equipment.

• To produce basic shadowgraphs, you will need:
  • A phone with a built-in camera and LED flashlight
  • A magnifying makeup mirror. If you have a two-sided mirror and one side is flat, make sure you use the magnifying side. 
• To produce higher-quality schlieren images, you will need:
  • A camera
  • A separate light source, like a flashlight
  • A concave spherical mirror. In general, larger-diameter mirrors with longer focal lengths will produce better images, but these mirrors can get expensive. Search online for concave spherical telescope mirrors or schlieren mirrors to see your options.
  • A light block with a sharp, defined edge. You can use a piece of cardstock or an index card, but you may get higher-quality images with a razor blade.
• For either of the above options, you will also need:
  • Materials to modify your light source to create a point light source:
    • A flat, opaque material, like aluminum foil or black cardstock
    • Scissors
    • A pin to poke a small hole
    • Tape
  • Supplies that allow you to carefully position and adjust your camera, light source, mirror, and light block: 
    • For a basic setup, you can just prop things up on books, pieces of cardboard, etc., and use tape to hold them in place.
    • For a more advanced setup, you will need to carefully align all the parts and make fine adjustments of just a few millimeters at a time: 
      • A tripod may be helpful for holding your phone or camera.
      • You can build a custom wooden mount to hold your mirror (see some of the YouTube videos in the bibliography for ideas).
      • Laboratory stands with clamps can be useful for carefully positioning things. 
  • Objects to test:
    • For a basic shadowgraph setup with low-cost equipment, try a lighter or candle, which will create flames with large temperature and density changes
    • For a more advanced schlieren setup, you can look at passive air flow around objects like your hand or an ice cube

This project follows the Engineering Design Process. Confirm with your teacher if this is acceptable for your project, and review the steps before you begin.

1. Create a point light source.
   1. If you are using a phone (Figure 6):
      1. Cut a very small piece of cardstock or aluminum foil, big enough to completely cover the phone's LED, but small enough not to block any of the nearby camera lenses.
      2. Use a pin to poke a small hole in the center of the aluminum foil or cardstock.
      3. Tape the material onto the phone such that the hole is centered on the LED. Make sure the tape does not cover any of the camera lenses. You may need to remove the phone's case to do this. 
   2. If you are using a separate flashlight (Figure 7):
      1. Cut a piece of cardstock or aluminum foil large enough to cover the entire front of the flashlight.
      2. Use a pin to poke a small hole in the center of the aluminum foil or cardstock.
      3. Tape the material onto the front of the flashlight. Make sure light does not come out around the edges. Light should only come out of the pinhole in the middle.

Image Credit: Ben Finio / Science Buddies

Figure 6. A piece of black cardstock with a pinhole poked in it taped over a phone's LED.

Image Credit: Ben Finio / Science Buddies

Figure 7. A piece of black cardstock with a pinhole poked in it taped over a flashlight.

2. Position your mirror so the optical axis is horizontal, parallel to the ground. Clamp your mirror in place, or mark its position with tape, in case you bump it and need to reposition it later. 
3. Find your mirror's focal length. Remember that you need to set up your point light source at a distance twice the focal length from the mirror (Figure 5). If you purchased a laboratory-grade mirror, this information should come with the mirror. If you purchased a makeup mirror, you will need to find it experimentally:
   1. Starting far away from the mirror, aim your point light source at the center of your mirror, holding it slightly to one side of the optical axis.
   2. Hold a piece of black paper next to the light source. 
   3. Look for a spot of light on the black paper. 
   4. Keeping them next to each other, slowly move both the light source and the black paper closer to the mirror. 
   5. Watch for the spot of light to get smaller and more in focus. 
   6. If the spot of light starts to expand again or get blurrier, you have gone too far. Go back again (increase the distance) until the spot of light is as small and focused as possible.
   7. Mark this position (on the floor or table) with tape. This distance is twice your mirror's focal length. 
4. Set up your light source and camera at a distance that is twice the focal length away from the mirror. If you looked up your mirror's focal length, remember to double that distance. If you determined it experimentally, then use the distance you measured in step 3 (which is already twice the focal length). 
   1. If you are using a phone, the camera and light source are attached to each other, so you only need to position your phone and the mirror (Figure 8).
      1. Use something (a tripod, a phone stand, a custom rig made from cardboard, etc.) to hold your phone.
      2. Aim your phone at the mirror.
      3. Zoom in on the mirror as much as possible so it fills the image frame. 
      4. Make sure the plane of the mirror is in focus, not the reflected image of the phone in the mirror. You may need to set your camera to manual focus, since autofocus can have difficulty focusing on a mirror.
      5. Make sure your phone's flashlight is on.
      6. Watch for the reflected spot of light on the surface of your phone.
      7. Carefully adjust your phone and/or the mirror until the reflected light goes directly into the camera lens. Note: Most modern smartphones have more than one camera lens, so you will have to determine which one is being used for your current zoom level.
   2. If you are using a separate camera and light source, you will need to position them separately (Figure 9). Follow the same procedure as step 4.a, but place the camera and light source as close together as possible. 
   3. If you do not have a tripod, clamps, or stands, you can use cardboard or paper shims to help aim your camera, flashlight, and mirror. 
   4. Be careful not to bump anything once you have your setup aligned. It may help to tape things in place.

Image Credit: Ben Finio / Science Buddies

Figure 8. Basic shadowgraph setup with a cell phone and a makeup mirror. The phone is held in a clip stand. The phone's LED is aimed at the mirror such that the reflected light goes directly into the camera lens. The mirror has a built-in tilt stand so it can be aimed. 

Image Credit: Ben Finio / Science Buddies

Figure 9. Shadowgraph setup with a separate flashlight and camera. The mirror is aimed left to right by rotating the cardboard box and aimed up and down using a paper shim under the box. The flashlight and camera are propped up on pieces of cardboard.

5. To make it easier to align the light, mirror, and camera:
   1. Set your camera to manual or pro mode.
   2. Set your camera to a low exposure setting. The image preview should be very dark.
   3. When everything is perfectly aligned and the reflected light is going directly into the center of the camera, the mirror will look like a bright circle with dark surroundings (Figure 10).

Image Credit: Ben Finio / Science Buddies

Figure 10. A makeup mirror that is aligned with a camera and light source for a shadowgraph setup. A window with full daylight outside is just barely visible in the background and appears much darker than the mirror. The dark spots in the circle result from imperfections in the mirror.

6. Try producing your first shadowgraphs. Figure 11 shows a shadowgraph of a grill lighter made with the setup in Figure 8. 
   1. Carefully position a flame from a lighter or candle in front of the mirror.
   2. Take a picture or a video. Be careful not to bump your camera and move it out of alignment. 
   3. Try adjusting your camera settings and/or improving the alignment of your system for better lighting. If the image is washed out, reduce your camera's exposure. Can you improve your image quality?
   4. What types of air flow or heat can you visualize with your system?

Image Credit: Ben Finio / Science Buddies

Figure 11. Shadowgraph of a grill lighter as it ignites. The shadowgraph reveals fluid flow that is normally invisible to the naked eye.

7. Try adding a light block to your setup to produce schlieren images (Figures 12 and 13). 
   1. Since this process is more sensitive in general, it may also be more sensitive to imperfections in your mirror. You may need a higher-quality mirror to get good results. 
   2. This will require a separate camera and light source so the camera can be positioned slightly behind where the light is focused. This will not work with a phone (to our knowledge) since the camera and LED are right next to each other.
   3. Position the light block edge opposite the point light source (on the other side of the optical axis), where the light is focused, so it cuts off half the light. When properly aligned, this should result in a higher-contrast image, but should not simply cut off half the image. This alignment can be difficult. Take your time and be careful!
   4. Try testing the same object(s) you did for your shadowgraphs. How do the resulting images or videos differ?
   5. Try holding your hand or an ice cube in front of the mirror. Can you see the air flowing around it? What else can you visualize with the schlieren setup that you could not see with the shadowgraph setup?

Image Credit: Ben Finio / Science Buddies

Figure 12. Schlieren setup with light block (a piece of black cardstock held in place by a clip) added just in front of the camera lens. 

Image Credit: Ben Finio / Science Buddies

Figure 13. Close-up view of the light block, which is aligned to cut off roughly half of the point of focused light just in front of the camera lens.

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