Bright versus dark schlieren imaging: quantitative analysis of quasi-sinusoidal phase objects

Area of Science:

• Optical imaging techniques
• Phase contrast microscopy
• Wave optics

Background:

• Schlieren imaging is a widely used optical technique for visualizing phase objects.
• Understanding the relationship between object phase and image intensity is crucial for accurate phase retrieval.
• Traditional Schlieren methods can introduce nonlinearities that complicate quantitative analysis.

Purpose of the Study:

• To investigate the effect of the zero-order spatial component on Schlieren imaging of quasi-sinusoidal phase objects.
• To determine the conditions under which direct phase retrieval is possible in Schlieren processing.
• To explore methods for contrast enhancement in Schlieren imaging.

Main Methods:

• Analysis of the Fourier spectrum of phase objects.
• Simulation and experimental investigation of Schlieren imaging with and without blocking the zero-order component.
• Mathematical derivation of intensity-phase relationships.

Main Results:

• When the zero-order component is not blocked, image intensity is a linear function of phase amplitude.
• When the zero-order component is completely blocked (dark Schlieren), image intensity becomes a quadratic function of phase.
• Direct phase retrieval is not feasible with dark Schlieren processing due to quadratic dependence.

Conclusions:

• The presence or absence of the zero-order component significantly impacts the linearity of the phase-to-intensity mapping in Schlieren imaging.
• Optimizing Schlieren processing by controlling the zero-order component is essential for quantitative phase retrieval.
• Further research into contrast enhancement techniques for Schlieren imaging is warranted.