Specular Surface Shape Measurement with Orthogonal Dual-Frequency Fourier Transform Deflectometry

Area of Science:

• Optical Metrology
• Surface Metrology
• 3D Shape Measurement

Background:

• Specular surface 3D shape measurement is critical for various applications.
• Existing methods struggle with phase jumping and discontinuities during spatial phase unwrapping.
• Accurate measurement of reflective surfaces remains a significant challenge in optical engineering.

Purpose of the Study:

• To introduce a novel orthogonal dual-frequency fringe for enhanced specular surface shape measurement.
• To develop an improved Fourier transform deflectometry method utilizing this new fringe.
• To address and overcome phase jumping and spatial phase unwrapping ambiguities.

Main Methods:

• Proposed a novel orthogonal dual-frequency fringe pattern for structured light projection.
• Developed an improved Fourier transform deflectometry (FTD) technique incorporating the dual-frequency fringe.
• Implemented spatial phase unwrapping by combining high-frequency and low-frequency fringe components.

Main Results:

• The orthogonal dual-frequency Fourier transform deflectometry (ODD) method achieved high-accuracy reconstruction of flat surfaces (2.16 nm rms error).
• Concave surfaces were reconstructed with high precision (1.86 μm rms error).
• Experimental results showed ODD measurements closely matched those from phase-measuring deflectometry (PMD).

Conclusions:

• The ODD method effectively reduces phase unwrapping ambiguities in specular surface measurement.
• Accurate 3D shape measurement of specular surfaces is achievable with a single fringe sample.
• This technique provides a foundation for future real-time specular surface metrology.