The Fourier Transform Infrared Spectrophotometry laboratory contains several systems that are used to measure infrared reflectance and transmittance for wavelengths between 1 µm and 20 µm. Near normal incidence absolute spectral reflectance and transmittance of both specular and diffuse samples can be measured from near ambient to 200 °C using a custom integrating sphere and a Fourier transform infrared (FTIR) spectrometer. Additional capabilities for specular samples include transmittance down to 10 K using an optical cryostat, as well as variable angle transmittance and reflectance using a custom goniometer and polarizers.
Special test measurements are performed for both NIST and external customers in government and industry on a wide variety of materials including mirrors, filters, windows, coatings and cavity structures through Calibration Service 38075S. Emittance and absorptance are obtained indirectly through combining the reflectance and transmittance under identical conditions. Also, a facility is available for the measurement of directional spectral emittance for sample temperature up to 1000°C, accomplished through comparison of radiance with reference blackbody sources.
Our overall approach can be summarized in the following 5 key elements:
- Development of a comprehensive measurement capability for the infrared spectral region encompassing a wide range of optical properties, for the characterization of a wide range of material types (including specular & diffuse, transmissive & opaque), and covering a wide range of critical parameters (including temperature, beam geometry, polarization state, wavelength range, and property level);
- Evaluation and development of absolute measurement methodologies and instrumentation for high accuracy, as we have done for directional-hemispherical reflectance, by the use of an integrating sphere method for specular samples, and the use of Monte Carlo modeling techniques for analysis and optimization of the instrumentation designs;
- Investigation and understanding of major sources of measurement error, both hardware and software related, including detector spatial non-uniformity and non-linearity, sample-detector-spectrometer inter-reflections, etc. A critical element to this effort is the last key concept in which comparisons using different methods helps to elucidate any overlooked error contributions.
- Development, design and implementation of custom accessory instrumentation for absolute measurements, based on minimization of errors, instrument characterization and modeling analysis. Systems we have developed include: a goniometer for variable angle absolute specular reflectance and transmittance, an integrating sphere for specular and diffuse samples, an Infrared spectral emittance system, a cryostat for variable temperature reflectance and transmittance, a polarimeter for polarization dependent and Mueller matrix measurements, a temperature-dependent emittance measurement system, as well as laser-based systems, for bi-directional reflectance distribution function measurements and black cavity emittance/absorptance;
- Intercomparison with other techniques and other Laboratories, as for example, comparison of transmittance measured with FTIR and laser systems, and emittance with reflectance from separate systems. Comparisons of transmittance, reflectance and emittance have been made with other NIST facilities as well as with other National Metrological Institutes from around the world.