Schlieren Imaging

Schlieren imaging is designed to detect changes in the refraction index of objects. Index of refraction is a property of all substance that describes the phase shift experienced by a beam of light that passes through the boundaries of the substance. Our eyes and ordinary cameras cannot detect phase differences in a beam of light, but only the amplitude and the color contrast. Schlieren methods enable us to see the phase differences by converting them to amplitude and sometimes color differences. When a beam of light passes through a refractive index gradient, it bends or refracts. By using suitable methods, these deflected rays can be visualized by a normal camera. This technique enables for example visualization of:

  • Temperature gradients (heating processes)
  • Phase differences (mixing of liquids, gas flows, evaporation)
  • Small particles (efficient light scattering)
  • Pressure gradients (shock waves in air or liquid)

There are various ways of performing Schlieren imaging. We introduce three options that are well compatible with CAVILUX laser illumination. Combination of Schlieren imaging and laser illumination opens some additional possibilities that are described in the end of this document. This combination enables for example:

  • Improved accuracy (monochromatic light, high intensity)
  • Freezing of any terrestrial motion (ballistics, combustion engines)
  • Benefits of polarized light

Option 1: One Mirror Schlieren Imaging

Pro:

  • Very high accuracy possible
  • Easy to setup

Contra:

  • Requires a lot of space (setup is long in one direction)
  • Sensitive to vibrations
  • Ghost images (seeing object double depending on angle of mirror)
schlieren_one_mirror_curved_v2

Schlieren technique has been utilized e.g. in the monitoring of protection gas in laser welding process. The measurement setup is shown in the figure above. Bright laser light source, CAVILUX Smart, was connected to a light guide and around 1 mm pinhole was placed in front of the light guide in order to form a point source. A concave spherical mirror was used to focus the light back to a small spot in size of the light source. An opaque mask, also in the size of the light source, was placed in front of the camera to block all the light rays which do not experience refraction by the object (i.e. and ).

The pinhole and the mask were set in a distance of two focal lengths from the concave mirror. The angle between the pinhole and the mask was relatively small, around one degree.

Welding process causes changes in the refraction index and as mentioned above this will cause deflection to the light rays. In this case the light rays which experience refraction caused by the object ( and ) will pass the mask and they are observed by the camera. The laser band pass filter has to be used to suppress disturbing process light.

Hints for adjustment:

  • Place the light source to the distance of 2F (parameter of the mirror). Turn the laser on and find the reflected image of the laser spot. Adjust the laser so that the image of laser spot is by the side of laser and there is enough room for the camera.
  • Fine-adjust the camera mask according to the visible laser spot (use IR-card when using IR laser) instead of fine-adjusting the mirror or the laser.
  • Camera mask can be as simple as blue-stack attached to the camera lens (or protective glass of it)
  • You can minimize ghost image by placing the light source as close to camera mask as possible. If you place the camera to e.g. 2F+3 cm then the right distance for the light source is 2F-3 cm and you can place the light source in front of the camera lens (distance between light source and camera can be 1 cm or even less).
  • Rotate the laser and you’ll see the effect of polarization direction on your Schlieren image!

Option 2: Two Mirror Schlieren Imaging

Same principles as the one mirror Schlieren only that the light path is guided via two mirrors.

Pro:

  • No ghost images
  • Accurate

Contra:

  • Sensitive to vibrations
schlieren_two_mirror_curved_v2

Option 3: Double Lens System

In this option the laser is focused directly to the camera mask.

Pro:

  • No ghost images
  • Low space requirements
  • Easier set up than mirror Schlieren

Contra:

  • Sensitive to vibrations
  • Not that accurate as mirror Schlieren
schlieren_two_lens_curved_v2

The focal length of the second lens can vary. A shorter focal length provides a smaller spot, therefore a smaller mask can be used and the set up is more sensitive. The spot size of a larger focal length is bigger and easier to align.

Combination of CAVILUX Illumination and Schlieren Imaging

Schlieren with flame and bullet

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