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Knife-edge removal in schlieren imaging

Vimod Kumar, Manish Kumar

Abstract

A knife-edge is recognized as a critical component of the schlieren imaging system. This knife-edge serves as a cutoff element, which is required to achieve high sensitivity in the schlieren imaging setup. This article describes a method for totally removing the knife-edge from a normal schlieren imaging system while maintaining the same sensitivity. In this approach, the camera lens's internal aperture acts like the cutoff element making an external knife-edge redundant. This method simplifies the schlieren setup and also reduces the setup cost due to the reduced number of optical or optomechanical elements. We show flow visualization data that clearly demonstrate that schlieren contrast can be obtained by employing a lens's internal aperture as the cutoff.

Knife-edge removal in schlieren imaging

Abstract

A knife-edge is recognized as a critical component of the schlieren imaging system. This knife-edge serves as a cutoff element, which is required to achieve high sensitivity in the schlieren imaging setup. This article describes a method for totally removing the knife-edge from a normal schlieren imaging system while maintaining the same sensitivity. In this approach, the camera lens's internal aperture acts like the cutoff element making an external knife-edge redundant. This method simplifies the schlieren setup and also reduces the setup cost due to the reduced number of optical or optomechanical elements. We show flow visualization data that clearly demonstrate that schlieren contrast can be obtained by employing a lens's internal aperture as the cutoff.
Paper Structure (13 sections, 8 figures, 1 table)

This paper contains 13 sections, 8 figures, 1 table.

Table of Contents

  1. INTRODUCTION
  2. METHODS
  3. List of components
  4. Conventional knife-edge assisted schlieren alignment
  5. Knife-edge-free schlieren alignment
  6. Centerline height alignment
  7. Imaging the light source in the iris plane
  8. Achieving schlieren contrast using the iris edge cutoff
  9. EXPERIMENTAL DEMONSTRATION
  10. Shadowgraph vs schlieren
  11. Controlling sensitivity and range
  12. A minimalist knife-edge-free schlieren setup using a DSLR camera
  13. DISCUSSION AND CONCLUSION

Figures (8)

  • Figure 1: Schematics for schlieren imaging systems. a) The conventional knife-edge assisted schlieren imaging system, and b) knife-edge free schlieren imaging system.
  • Figure 2: A horizontally offset LED placement on camera lens for knife-edge-free schlieren imaging.
  • Figure 3: Initial assembly of the knife-edge-free schlieren imaging system. All optical elements are aligned at the same centerline height.
  • Figure 4: Steps for aligning the LED image in the iris plane. a) Schematics representing the sequence of steps and adjustments. b) Illustration of the view from the camera's live feed as the LED image aligns with in the iris plane.
  • Figure 5: Realization of the schlieren contrast using a camera lens iris for cutoff. a) Fine control of the LED image position using mirror's yaw and tilt (on left). Shadowgraph vs schlieren positioning of the LED image (on right). b) The LED image cutoff at iris edge (see b1) causes darkening of the mirror view in camera's live feed. A non-uniform darkening indicates non-ideal distance between camera assembly and mirror (see b2 and b3). A uniform darkening indicates schlieren imaging readiness (see b4).
  • ...and 3 more figures