Optical design requires an accurate knowledge of the dispersion functions for the materials in each lens. For systems to work over a wide range of temperature, knowing the temperature dependence of these functions is as important as knowing the coefficients of thermal expansion. The dispersion curves of several NRL-developed infrared glasses were measured over a temperature range that spanned, at minimum, -20°C to 60°C. Details concerning the data fidelity, data modeling, fit results, and physical implications of the results will be provided in this work.
The market for thermal imaging sensors and cameras has been increasingly focused on higher volumes and lower costs. Precision glass molding (PGM) is a high volume, low cost method which has been utilized for decades to produce lenses from oxide glasses. Due to the recent development of high quality precision-molded chalcogenide glasses, which are transparent at critical thermal imaging wavelengths, PGM has emerged as the enabling technology for low cost infrared optics. Since the price of germanium is high and volatile, it plays a large role in the high price of chalcogenide glasses that contain it. As40Se60 has previously been investigated as a lower-cost alternative to germanium-containing chalcogenide glasses and was found suitable for the PGM process. This paper investigates the composition-dependence of PGM-relevant properties for As38Se62 and standard As40Se60 and presents a comparison of molding behavior and lens performance.
A program has been started at NIST to make high-accuracy measurements of the infrared (IR) index properties of technologically important IR materials, in order to provide the IR optics community with updated values for the highest quality materials now available. For this purpose, we designed and built a minimum-deviation-angle refractometry system enabling diffraction-limited index measurements for wavelengths from 0.12 μm to 14 μm. We discuss the apparatus and procedures that we use for IR measurements. First results are presented for germanium for the wavelength range from 2 μm to 14 μm, with standard uncertainties ranging from 2 × 10-5 near 2 μm to 8 × 10-5 near 14 μm. This is an improvement by about an order of magnitude of the uncertainty level for index data of germanium generally used for optic design. A Sellmeier formula fitting our data for this range is provided. An analysis of the uncertainty is presented in detail. These measurements are compared to previous measurements of Ge.
Precision glass molding has a well-documented decrease in the index of refraction of the glass during the molding process. This index drop must be taken into account in the optical design in order to accurately determine the optical performance of the final lens. Knowing the annealing coefficient of the glass being molded allows the index to be fine-tuned by adjusting the cooling rate during the molding process. While annealing coefficients are available for visible glasses, the validity of using this method for chalcogenide gasses has not yet been investigated. This paper will determine the annealing coefficient for As40Se60 experimentally, and then verify the results by comparing calculated and experimental refractive index values for other cooling rates.
A method for non-contact 3D form testing of aspheric surfaces including determination of decenter and wedge errors and lens thickness is presented. The principle is based on the absolute measurement capability of multi-wavelength interferometry (MWLI). The approach produces high density 3D shape information and geometric parameters at high accuracy in short measurement times. The system allows inspection of aspheres without restrictions in terms of spherical departures, of segmented and discontinuous optics. The optics can be polished or ground and made of opaque or transparent materials.
With the increase in demand for infrared optics for thermal applications and the use of glass molding of chalcogenide materials to support these higher volume optical designs, an investigation of changes to the optical properties of these materials is required. Typical precision glass molding requires specific thermal conditions for proper lens molding of any type of optical glass. With these conditions a change (reduction) of optical index occurs after molding of all oxide glass types and it is presumed that a similar behavior will happen with chalcogenide based materials. We will discuss the effects of a typical molding thermal cycle for use with commercially and newly developed chalcogenide materials and show results of index variation from nominally established material data.
Precision glass molding has a well-documented effect of a decrease in the index of refraction of the glass during the molding process. This index drop has such significant value that optical designs for molded lenses must take into account the index drop to accurately determine the optical performance of the final lens. Widespread adoption of chalcogenide glasses for molded infrared optics has raised a series of questions as to the behavior of these glasses under molding conditions. This paper will investigate the index of refraction changes in two different chalcogenide glasses and determine if these changes are significant enough for optical designers to consider in their designs.