Stefan K. Andersson

Frequency and temperature dependence of the refractive index of sapphire

Because the index of refraction is temperature dependent, a temperature gradient across a window causes image blur and bore sight error. Prior to this paper, there have been no direct temperature-dependent measurements reported on the mid-infrared refractive index for sapphire, a popular infrared window material of high durability. Measurements of dn/dT are reported on the ordinary ray of sapphire in the 4 μm region for the first time. Accurate temperature and frequency dependent refractive index models can now be constructed from visible measurements of the refractive index, far-infrared reflectance measurements, thermo-optic coefficient measurements, and infrared measurements of the absorption coefficient. Visible measurements determine the contribution to the refractive index from electronic transitions. Far-infrared measurements determine the contributions from fundamental lattice vibrations (phonons). Infrared absorption data are used to determine parameters in a multiphonon sum band model. By applying the Hilbert transform to this multiphonon absorption model, a model for the multiphonon refractivity is obtained. Two- and three-phonon contributions to the refractive index are important for an accurate model that includes temperature dependence. Results for the ordinary- and extraordinary-rays are obtained.

Infrared properties of β-sialon as a function of composition

The infrared properties of β-sialons, Si6−zAlzOzN8−z, prepared by hot, isostatic pressing for nine z-values, as well as a corresponding set of samples containing 1 wt% of yttria have been studied. The samples all have densities over 99% of their theoretical value and were found to be single-phased for z<3.8. Polished and cleaned sialon samples and one fully dense β-Si3N4 sample were used for infrared reflectance measurements. The lattice bands in the infrared reflectance spectra were analyzed for the different compositions. Numerical fits, using the parameters for a maximum of five classical Lorentz oscillators as free variables, were carried out. A strong Si–N resonance at 900 cm−1 was identified, the oscillator strength of which scaled linearly with z. The IR reflectance bands represent a potential for selectively low emittance applications on radomes in the 8–12 μm atmospheric window. The evaluation of the appropriate average window emittance showed a close to linear increase from about 0.5 to 0.9 for 0.35

Infrared properties of CVD β-SiC

Abstract The temperature and frequency dependent infrared properties of polycrystalline CVD β-SiC have been measured. This was accomplished using both broadband and narrowband (laser) measurements as a function of temperature from room temperature up to 900 K. Calculated multiphonon absorption shows good agreement with experiments. Furthermore, the thermo-optic coefficient was measured in the 2.5–5 μm region and the BSDF for CVD β-SiC was measured at 0.6328 μm for the first time.

Temperature dependence of the band emittance for nongray bodies

The general problem of obtaining correct emittance values from broadband IR radiometric measurements on nongray samples is discussed. If the spectral emittance has structure in a band, the emittance, averaged over that band, will be temperature dependent, even if the spectral emittance is insensitive to the temperature change. We point out that a widely used expression, with correction for radiance from the surroundings reflected by the sample, is valid only if the spectral emittance is temperature and wavelength independent, i.e., gray. If the spectral emittance is nongray, the conventional emission factor, as determined by a broadband radiometer, is temperature dependent and the numerical value is significantly different from the averaged band emittance sought. Two algorithms are suggested to extract the correct band-averaged emittance from the temperature-dependent radiometric emission factor obtained with the conventional expression. The algorithms are demonstrated with a step model for the spectral emittance, and it is shown that the agreement with the correct average band emittance is significantly improved.

Selective suppression of thermal emission from radomes and materials selection therefor

The possibility of using selectively low emission to reduce the thermal signature of radomes is identified. A material can simultaneously have low radiance in the working range of a detector and cool radiatively to the surrounding atmosphere. If the band of low emission is based on lattice excitation, the signature reduction is compatible with electrically insulating properties and radar transmittance. High-density, polycrystalline beryllium oxide is identified as a material with low emittance in the upper atmospheric window at 8 to 13 μm, owing to a strong reststrahlen band. Radiometer measurements and bulk reflectance spectra are reported for ceramic BeO and are used to calculate the thermal infrared emission factors as well as the radiance for a 50°C BeO surface. The results depend heavily upon the short wavelength threshold of the detector. The calculated values have been compared with radiometer measurements in the 3- to 5-μm and 8- to 13-μm ranges. The possibility of reducing the emittance even further with a second material has been investigated with Fresnel calculations. Very favorable calculated and measured results for a 0.8-μm silicon overlayer on BeO are reported.

Temperature correction of radiometric emission factors for nongray objects

The standard analysis of heat-camera measurements in- cludes emitted as well as reflected radiation, but assumes the same emittance for these two contributions. This is not valid if the sample emittance has spectral structure, i.e., is nongray, in the working range of the detector and near the maximum of the Planck function. The tempera- ture dependence of the averaged emittance is shown to cause a signifi- cant error in the emittance value. Two algorithms are derived, based on linearising the heat-camera emittance values as a function of sample temperature for correction of this error. They are demonstrated to be effective on a spectral step model as well as for experimental data. BeO has a steep emittance edge in the long wave atmospheric window, simi- lar to the step model. As for the model calculations, the conventional heat-camera emittance was significantly different from the corresponding integrated average based on spectral, reflectance data. Heat-camera measurements are compared with integrated spectral reflectance of ce- ramic beryllium oxide to illustrate the corrections. The two correction algorithms improve the agreement significantly, and it is proposed that they should be used when the heat-camera emittance exhibits system- atic temperature variation.

Infrared properties of polycrystalline magnesium fluoride

Abstract Kodak performed an optical characterization of polycrystalline magnesium fluoride (when it was called Irtran 1) in the early 1960s to early 1970s. It has since been improved and is now made by a different manufacturer. The imaginary part of the complex index of refraction in the 3- to 5- μ m region is extrinsic for this material. Therefore, changes in the manufacturing process can significantly alter its optical properties as it is currently produced relative to Irtran 1. Because of these manufacturing changes, as well as the availability of improved experimental techniques, it is important to reexamine the optical properties of polycrystalline MgF 2 .

Infrared properties of CVD B-SiC

The temperature and frequency dependent infrared properties of polycrystalline CVD (beta) -SiC have been measured. This was accomplished using both broadband and narrowband (laser) measurements as a function of temperature from room temperature up to 900 degrees Kelvin. Calculated multiphonon absorption shows good agreement with experiments. Furthermore the thermo-optic coefficient was measured in the 2.5 - 5 micrometer region and the BSDF for CVD (beta) -SiC was measured at 0.6328 micrometer for the first time.

Multiphonon contribution to the reststrahlen band of BaF2

Abstract It is well known that multiphonon processes give an important contribution to absorption at higher frequencies than the fundamental lattice vibration of a material. However, multiphonon processes can also be important within the reststrahlen band. BaF 2 is a typical material where the two-phonon absorption band has its maximum close to the high frequency edge of the reststrahlen band. We have demonstrated, theoretically and experimentally, that higher order harmonics of the fundamental lattice vibrations strongly affect the reststrahlen band of BaF 2 . The reststrahlen band of BaF 2 has been measured from room temperature up to 500°C and we have also used a theoretical model to describe the multiphonon contributions to the complex index of refraction.

Oxidation kinetics for metal-dielectric films using IR optical measurements

The thermal oxidation of small metallic particles have been studied using infrared spectroscopy. The oxidation has been quantified by measuring the absorption of p-polarized light at 60 degree angle of incidence at the wavelengths around the longitudinal optical (LO) phonon mode of the created oxide. The case presented in this report is nickel rods embedded in alumina (Ni-Al2O3) exposed to temperatures in the range between 300 and 500 degrees Celsius from 1 to 500 hours. The rate of oxidation was found to be somewhat lower than previously reported for large particles and bulk nickel.