Measured comparison of the crossover periods for mid- and long-wave IR (MWIR and LWIR) polarimetric and conventional thermal imagery.

We report the results of a multi-day diurnal study in which polarimetric and conventional thermal imagery is recorded in the mid- and long-wave IR to identify and compare the respective time periods in which minimum target contrast is achieved. The data shows that the chief factors affecting polarimetric contrast in both wavebands are the amount of thermal emission from the objects in the scene and the abundance of MWIR and LWIR sources in the optical background. In particular, it has been observed that the MWIR polarimetric contrast was positively correlated to the presence of MWIR sources in the optical background, while the LWIR polarimetric contrast was negatively correlated to the presence of LWIR sources in the optical background.

Improved integrating-sphere throughput with a lens and nonimaging concentrator.

A reflectometer design utilizing an integrating sphere with a lens and nonimaging concentrator is described. Compared with previous designs where a collimator was used to restrict the detector field of view, the concentrator-lens combination significantly increases the throughput of the reflectometer. A procedure for designing lens-concentrators is given along with the results of parametric studies. The measured angular response of a lens-concentrator system is compared with ray-trace predictions and with the response of an ideal 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.

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.

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.

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