Multispectral imaging with optical bandpass filters: tilt angle and position estimation

Johannes Brauers; Til Aach

doi:10.1117/12.804985

19 January 2009 Multispectral imaging with optical bandpass filters: tilt angle and position estimation

Johannes Brauers, Til Aach

Proceedings Volume 7241, Color Imaging XIV: Displaying, Processing, Hardcopy, and Applications; 72410Z (2009) https://doi.org/10.1117/12.804985
Event: IS&T/SPIE Electronic Imaging, 2009, San Jose, California, United States

Abstract

Optical bandpass filters play a decisive role in multispectral imaging. Various multispectral cameras use this type of color filter for the sequential acquisition of different spectral bands. Practically unavoidable, small tilt angles of the filters with respect to the optical axis influence the imaging process: First, by tilting the filter, the center wavelength of the filter is shifted, causing color variations. Second, due to refractions of the filter, the image is distorted geometrically depending on the tilt angle. Third, reflections between sensor and filter glass may cause ghosting, i.e., a weak and shifted copy of the image, which also depends on the filter angle. A method to measure the filter position parameters from multispectral color components is thus highly desirable. We propose a method to determine the angle and position of an optical filter brought into the optical path in, e.g., filter-wheel multispectral cameras, with respect to the camera coordinate system. We determine the position and angle of the filter by presenting a calibration chart to the camera, which is always partly reflected by the highly reflective optical bandpass filter. The extrinsic parameters of the original and mirrored chart can then be estimated. We derive the angle and position of the filter from the coordinates of the charts. We compare the results of the angle measurements to a ground truth obtained from the settings of a high-precision rotation table and thus validate our measurement method. Furthermore, we simulate the refraction effect of the optical filter and show that the results match quite well with the reality, thus also confirming our method.

Citation Download Citation

Johannes Brauers and Til Aach "Multispectral imaging with optical bandpass filters: tilt angle and position estimation", Proc. SPIE 7241, Color Imaging XIV: Displaying, Processing, Hardcopy, and Applications, 72410Z (19 January 2009); https://doi.org/10.1117/12.804985

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KEYWORDS

Optical filters

Cameras

Bandpass filters

Calibration

Image filtering

Imaging systems

Mirrors

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