Experimental demonstration of coherence effects and linearity in microdensitometry

Area of Science:

• Optical Physics
• Metrology
• Image Science

Background:

• Microdensitometer measurements of photographic film transmittance at high spatial frequencies reveal inherent nonlinearity.
• Instrument response is influenced by spatial frequency and illumination mode, impacting measurement accuracy.

Purpose of the Study:

• To investigate the nonlinear behavior of conventional microdensitometers.
• To develop and demonstrate a linear microdensitometer for accurate high spatial frequency measurements.
• To characterize spatial coherence in microdensitometry.

Main Methods:

• Utilized a Wollaston prism shearing interferometer to measure spatial coherence in typical microdensitometers.
• Developed a linear microdensitometer that bypasses the imaging step and collects all transmitted light.
• Employed a cw laser source and microscope objectives on an optical bench for instrument assembly and testing.

Main Results:

• Documented the presence of partial coherence in microdensitometers, affecting measurements.
• Demonstrated that the linear microdensitometer's spatial frequency response is limited only by the scanning spot size.
• Verified linearity through edge width measurements and flat response to a phase edge.

Conclusions:

• Conventional microdensitometers exhibit nonlinearities due to partial coherence, limiting their accuracy at high spatial frequencies.
• The developed linear microdensitometer offers improved performance by avoiding imaging and collecting all transmitted light.
• This linear system provides a more accurate method for analyzing photographic film transmittance, especially at high spatial frequencies.