# How were these pictures of spacecraft wind tunnel testing taken?

The picture below from this answer shows four spacecraft wind tunnel tests from the 1950s. In addition to the test object, the bow shock and turbulence are clearly visible.

How is this visualization accomplished?

• Is the medium air, or a denser fluid such as water?
• Is there a pigment added to the fluid to help visualization?
• Is there special lighting, such as polarized light?
• Any notable camera tricks?
• Is the 3-D nature of the test object more significant than being a thick 2-D object?

• One of my former college instructor's PhD thesis was manned vehicle re-entry in the 1950s or 1960s after Sputnik. He said he simulated re-entry by firing a small projectile and piercing a membrane. He told me the US Air Force tried to get Florida State to cancel his funding because his experiments costs about \$30,000 or$50,000 at the time; while the US Air Force budget for the same was multi-million dollars. After his PhD and space program experience, he went on the the NSA. Operation Ivy Bells was one of his programs.
– jww
May 28 '19 at 10:09

These appear to be Schlieren photography. Schlieren photography uses a collimated light source to highlight the refractive index changes brought on by density gradients in a fluid - in other words, it can visualize the shockwaves in air that are produced by different shapes.

The simplest technique shines collimated light past the object of interest, and places a knife edge at the focal point to block a portion of the light. Light beams that pass through density gradients are bent away from the path, and get blocked by the knife edge. The higher density zones are left as dark regions on the image, as their light was intercepted by the knife edge.

(image NASA)

• Schlieren photography is a fairly old method developed in 1864 by the german chemist and physicist August Toepler. This is why the german word 'Schlieren' is used. It may be used in transparent liquids too.
– Uwe
May 26 '19 at 14:25
• Both Veritasium and Smarter Every Day Youtube channels have done some great videos on (using) the Schlieren effect: 1, 2, 3, 4 and 5
– user10509
May 27 '19 at 20:12

@Saiboogu's answer is of course correct, this looks to be classic Schlieren photography.

It is notable that NASA has also developed two methods to generate faux-Schlieren images or "synthetic Schlieren" images without the need of special optical systems and collimated beams of light inside wind tunnels.

These techniques can be applied to real high speed aircraft or spacecraft moving in the Earth's atmosphere. Instead of generating light/dark bands from refraction of the collimated, uniform-intensity beam directly, they instead image a finely textured background as the craft passes in front of it. By using "optical flow" or other techniques they map small displacements of the texture due to the refraction, and plot the deflection map as intensity. The result can in some cases be better than traditional Schlieren imaging because they can reconstruct the vector direction of the displacement; the air density gradient.

NASA pioneered the use of the Sun, filtered through a narrow line filter (hydrogen-alpha or calcium-K) to produce the textured background. This could then be imaged from the ground by the craft passing between the ground cameras and the Sun.

Recently however they have also published results using a textured ground pattern in the desert, with a down-looking camera viewing from an aircraft above the craft to be measured.