David B. Chenault

Measurements of linear diattenuation and linear retardance spectra with a rotating sample spectropolarimeter

A technique to measure linear diattenuation and retardance spectra of infrared materials in transmission is described. A sample is rotated between two stationary linear polarizers in the sample compartment of a Fourier transform infrared spectrometer. The intensity modulation that results from the rotation of the sample is Fourier analyzed, and the linear diattenuation and linear retardance of the sample are calculated from the Fourier series coefficients for each wavelength. The advantages of this technique include immunity of the measurement to instrumental polarization, to circular diattenuation, and to circular retardance. The rotating sample polarimeter does not require retarders. Compensation for systematic errors from polarizers with diattenuation less than one is included in the data reduction. This technique is useful for the calibration of retarders and polarizers and hence for the bootstrap calibration of more elaborate polarimeters such as Mueller matrix polarimeters. We present as an example of t technique the caliration spectra 3-14 µm of an infrared quasi-achromatic retarder whose fast axis orientation oscillates with wavelength.

Mueller matrix algorithms

A method for the correction of systematic errors generated by large orientational and retardance errors in the polarization optics in the dual rotating retarder polarimeter is presented. Small orientational and retardance errors (<1 degree(s)) can lead to large errors in the measured Mueller matrix (> 10% in some matrix elements). We incorporate correction terms for large orientation and retardance errors into the dual rotating retarder data reduction algorithm. Using these data reduction algorithms and a calibration step, the associated systematic errors are calculated and removed from the measured Mueller matrix. This procedure is especially useful for spectral and multi-wavelength systems in which the retardance and often the orientation of the retarders are wavelength dependent. The equations, the procedure to calculate the orientations of the polarization elements and the retardances of the retardation elements, and the method to correct for any errors are presented here. The effect of these errors on the calculated Mueller matrix elements and their correction is shown analytically and through experimental data taken on an infrared spectropolarimeter.

Near-infrared imaging polarimetry

A near IR Stokes imaging polarimeter is described. Basic principles of operation are presented, system specifications are given, and polarization elements are characterized. System control software and data reduction techniques are briefly explained. Examples of scenes collected as visible photos, visible and near IR intensity images, and visible and near IR polarization images are presented.

Infrared birefringence spectra for cadmium sulfide and cadmium selenide

Measurements of the birefringence spectra for cadmium sulfide and cadmium selenide from 2.5 to 16.5µm obtained with a rotating sample spectropolarimeter are presented. Because of the similarity in the birefringence spectra for cadmium sulfide and cadmium selenide, a highly achromatic IR retarder can be constructed from a combination of these materials. The ordinary and extraordinary refractive indices for cadmium sulfide are estimated in the region from 10.6 to 15 µm and for cadmium selenide from 10.6 to 16.5 µm by combining these birefringence data with an extrapolation of previous dispersion relations.

Imaging polarimeter development and applications

Recent interest in the utility of polarimetry for military and commercial applications has led to the development of many different imaging polarimetric systems. Much of the attention has focused on time-sequential acquisition systems. These systems use a rotating retarder or polarizer with data acquisition times that range anywhere from 10 to 100 seconds. This length of time is needed to capture the required frames of data used to construct a polarimetric image. Consequently, scene changes during data collection will either induce false polarization effects or induce mis- registration effects. These can occur in scenes where the target, background or sky is changing intensity rapidly or in cases where the target or sensor is in motion. To overcome these limitations SY Technology Inc. is developing and testing a 4 camera polarimetric imaging system capable of simultaneously capturing the needed frames to produce a full-stokes polarimetric image. The system operates in the visible to near IR spectrum and is capable of a 25Hz Stokes image frame rate. This paper will summarize the design, components and calibration of a 4 camera polarimetric system.

Electro-optic coefficient spectrum of cadmium telluride

A spectrum for the electro-optic coefficient of cadmium telluride measured from 3 to 14 µm is reported. The spectrum shows that the quantity n(3)r(41) has a nearly constant value of 1.09 × 10(-10) m/V over this spectral band, with a slight (5%) dip at the weak absorption band centered at 6 µm. Measurements were performed with an infrared Mueller matrix spectropolarimeter. Transmission spectra of the Mueller matrix were acquired at a set of applied voltages. Retardance spectra were calculated from Mueller matrix spectra, and then the electro-optic coefficient was calculated at each wavelength by a least-squares fit to the resulting retardance as a function of voltage.

Infrared linear diattenuation and birefringence spectra of ferroelectric liquid crystals

Linear diattenuation and linear birefringence spectra of three ferroelectric liquid-crystalline materials, 764E, SCE4, and SCE9, are presented for the wavelength region from 2.5 to 12 microm. Relatively high birefringence (Deltan >/= 0.1) was found in the transmission bands from 2.5 to 3.2 microm and 3.6 to 5.7 microm, with only a small amount of interference from linear diattenuation (D </= 2%). The potential for these materials as broad-spectral-band infrared modulators in these regions is suggested by the relatively constant retardance and small linear di-attenuation. Substantially larger values of linear diattenuation and retardance are found in the narrower transmission bands beyond 5.8 microm. The linear diattenuation and linear birefringence spectra display the expected anomalous dispersion near the absorption bands.

Automated registration of polarimetric imagery using Fourier transform techniques

Polarimetric imagery that is collected from time-sequential and multiple image format sensors all have potential for image misregistration. Since polarization is usually measured as small differences between radiometric measurements, it is highly sensitive to misregistration, especially at regions of high contrast. The general consensus in the polarization community is that image misregistration on the order of 1/10th of a pixel can introduce artifacts in polarization images. If the registration is not achieved and maintained to this resolution, the data must be registered in software. Typically, rotation and translation (horizontal and vertical) are the main transformations that need to be corrected. It is desirable to have a registration algorithm that determines rotations and translations to 1/10th of a pixel, does not require user intervention, takes minimal computation time, and is based on analytical (non-iterative), automated calculations. This paper details an analytical, automated registration algorithm that corrects for rotation and translations by using a Fourier transform technique. Examples of images registered with this algorithm, and estimates of residual misregistrations are presented. Typical processing times are also given.

Polarimetric characterization of Spectralon

A polarimetric characterization of the reflective standard material Spectralon is presented. Samples of Spectralon with reflectances of 2 percent, 50 percent, 75 percent and 99 percent were examined. The characterization was accomplished using the Air Force Research Laboratorys spectropolarimeter in reflection mode. Data are presented for the spectral region .65 to 1.0 micrometers. Polarizance was measured for the four Spectralon samples at eight input beam incidence angles. All observations were made from normal to the Spectralon. It was found that as the incidence beam angle increases, the polarizance increases; and as the reflectance of the samples decreases, the polarizance increases.

Evaluation of a selection of commercial polarizers and retarders at visible and near-infrared wavelengths

Complete specification s for commercially available polarizers and retarders are often not available, incomplete, or inaccurate. We analyze several commercial polarization elements using Mueller matrix polarimetry. Elements are characterized in terms of their diattenuation, retardance, and depolarization. Measurements were made with laser polarimeters and a spectropolarimeter.