Optical characterization of rigid endoscopes and polarization calibration methods

Polarization imaging can reveal orthogonal information with respect to color about the structural composition of biological tissue, and with the advance of superior polarimeters its use for biomedical applications has proliferated in the last decade. Polarimetry can be used in pre-clinical and clinical settings for the early detection of cancerous tissue. Polarization-based endoscopy with the complementary near-infrared fluorescence imaging modality improves the early diagnosis of flat cancerous lesions in colorectal tumor models. With the development of new polarization sensors the need to use standard laboratory optics to create custom imaging systems increases. These additional optics can behave as polarization filters effectively degrading and modifying the original tissue's polarization signatures leading to erroneous judgments. Here, we present a framework to characterize the spectral and polarization properties of rigid endoscopes for polarization-based endoscopic imaging. We describe and evaluate two calibration schemes based on Mueller calculus to reconstruct the original polarization information. Optical limitations of the endoscopes and minimum polarimeter requirements are discussed that may be of interest to other researchers working with custom polarization-based imaging systems.