Sergey Mekhontsev

Infrared spectral emissivity characterization facility at NIST

A new facility for the measurement of spectral emittance (emissivity) of materials that employs a set of blackbody sources is being built at NIST. This facility has also been used to investigate the capabilities of Fourier transform (FT) spectrometers to characterize the spectral emissivity of blackbody sources. The facility covers the spectral range of 1 μm to 20 μm and temperatures from 600 K to 1400 K. The principle of operation involves the spectral comparison of an unknown source with a group of variable temperature and fixed point reference sources by means of the FT spectrometer and filter radiometers. Sample surface temperature is measured by non-contact method using a sphere reflectometer. The current reflectometer setup allows measurements of opaque samples, but it is planned to include semitransparent materials at a later stage.

Monte Carlo modeling of an integrating sphere reflectometer

The Monte Carlo method has been applied to numerical modeling of an integrating sphere designed for hemispherical-directional reflectance factor measurements. It is shown that a conventional algorithm of backward ray tracing used for estimation of characteristics of the radiation field at a given point has slow convergence for small source-to-sphere-diameter ratios. A newly developed algorithm that substantially improves the convergence by calculation of direct source-induced irradiation for every point of diffuse reflection of rays traced is described. The method developed is applied to an integrating sphere reflectometer for the visible and infrared spectral ranges. Parametric studies of hemispherical radiance distributions for radiation incident onto the sample center were performed. The deviations of measured sample reflectance from the actual reflectance as a result of various factors were computed. The accuracy of the results, adequacy of the reflectance model, and other important aspects of the algorithm implementation are discussed.

Chapter 5 Calculation of the Radiation Characteristics of Blackbody Radiation Sources

Publisher Summary The chapter presents a discussion on the computational characterization of radiation emitted by blackbodies. The chapter focuses on the work of the last two decades (1990s and 2000s), while still including earlier milestone work. The chapter also describes the terminology used and provides definitions of principal quantities. Conventional, deterministic methods for calculation of the radiation characteristics of isothermal and non-isothermal cavities are discussed. The applications of these methods are primarily determined by the model of radiation characteristics (diffuse and specular) adopted for the cavity walls. The chapter describes the application of the stochastic (Monte Carlo) ray-tracing method to computer modeling of blackbody radiators. This computational method has become prevalent in the last decades of the 20th century because of its great generality, flexibility, and a number of other advantages. A comparison of some results obtained by various methods is described. Calibration of radiation thermometers is performed using blackbody radiation sources. Radiation thermometry metrology is impossible without a reliable determination of the radiation characteristics of such sources.

Water heat pipe blackbody as a reference spectral radiance source between 50°C and 250°C

Realization of a radiometric temperature scale for near ambient temperatures with accuracy at the 20 to 50 mK level is crucial for a number of demanding military and commercial applications. In support of such measurements, radiation sources with high stability and spatial uniformity must be developed as reference and working standards. Traditionally, the temperature scale, maintained at the National Institute of Standards and Technology (NIST), relies on water bath and oil bath blackbodies in this temperature range. Recently, a water heat pipe blackbody was used at NIST as a spectral radiance source in a spectral emissivity measurement facility. Now a new, more versatile high emissivity water heat pipe blackbody was designed and characterized to be used as a reference radiance source for the radiometric temperature scale realization between 50 °C and 250 °C. Furthermore, it will serve as a reference source for the infrared spectral radiance measurements between 2.5 μm and 20 μm. The calculated spectral emissivity of the painted copper alloy cavity was verified by reflectance measurements using a CO2 laser at 10.6 μm wavelength. The spatial thermal uniformity and stability of the blackbody were characterized. Two independent realizations of the radiometric temperature scale were compared in order to verify the accuracy of the scale. Radiance temperature, calculated from the cavity temperature measured with a calibrated PRT contact thermometer and from the emissivity of the cavity, was compared to the radiance temperature, directly measured with a reference pyrometer, which was calibrated with a set of fixed point blackbodies. The difference was found to be within measurement uncertainties.

Low Scatter Optical System for Emittance and Temperature Measurements

The development and evaluation of an optical system for a new spectral directional emittance facility at NIST is reported. The imaging quality and signal contributions due to out‐of‐field‐of‐view scattering, commonly characterized by the “size‐of‐source effect” (SSE) parameter, have been measured across the spectral range of 0.65 μm to 4 μm by three independent methods. The SSE measurement results of scatter levels not exceeding 2 to 3 parts in 104 are consistent and exceed the design targets. The potential application of the optical system to construction of a portable instrument with low scatter/emission and an operating spectral range of 0.65 μm to 20 μm are discussed.

IR spectral characterization of customer blackbody sources: first calibration results

We summarize recent progress in our infrared (IR) spectral radiance metrology effort. In support of customer blackbody characterization, a realization of the spectral radiance scale has been undertaken in the temperature range of 232 °C to 962 °C and spectral range of 2.5 μm to 20 μm. We discuss the scale realization process that includes the use of Sn, Zn, Al and Ag fixed-point blackbodies (BB), as well as the transfer of the spectral radiance scale to transfer standard BBs based on water, Cs and Na heat pipes. Further we discuss the procedures for customer source calibration with several examples of the spectral radiance and emissivity measurements of secondary standard BB sources. For one of the BBs, a substantial deviation of emissivity values from the manufacturer specifications was found. Further plans include expansion of the adopted methodology for temperatures down to 15 °C and building a dedicated facility for spectral characterization of IR radiation sources.

Reciprocity Principle and Choice of the Reflectance Model for Physically Correct Modeling of Effective Emissivity

In the last two decades considerable progress has been made in numerical modeling of isothermal and non‐isothermal cavities by the Monte Carlo method, in particular, by the use of the uniform specular‐diffuse reflection model and backward ray tracing techniques. However, this technique has no essential theoretical foundation. In the present paper a comparative numerical analysis of forward and backward ray tracing algorithms shows agreement with earlier analytical results. Numerical experiments show that the use of uniform specular‐diffuse reflection model, which obeys the reciprocity principle, leads to excellent agreement between effective emissivities, computed by forward and backward ray tracing. At the same time, results obtained for the non‐reciprocal Phong’s model demonstrate substantial discrepancies.

Optical design and initial results from NIST's AMMT/TEMPS facility

The National Institute of Standards and Technologys (NIST) Physical Measurement and Engineering Laboratories are jointly developing the Additive Manufacturing Measurement Testbed (AMMT)/ Temperature and Emittance of Melts, Powders and Solids (TEMPS) facilities. These facilities will be co-located on an open architecture laser-based powder bed fusion system allowing users full access to the systems operation parameters. This will provide users with access to machine-independent monitoring and control of the powder bed fusion process. In this paper there will be emphasis on the AMMT, which incorporates in-line visible light collection optics for monitoring and feedback control of the powder bed fusion process. We shall present an overview of the AMMT/TEMPS program and its goals. The optical and mechanical design of the open architecture powder-bed fusion system and the AMMT will also be described. In addition, preliminary measurement results from the system along with the current status of the system will be described.NIST’s Physical Measurement and Engineering Laboratories are jointly developing the Additive Manufacturing Measurement Test bed (AMMT)/ Temperature and Emittance of Melts, Powders and Solids (TEMPS) facilities. These facilities will be co-located on an open architecture laser-based powder bed fusion system allowing users full access to the system’s operation parameters. This will provide users with access to machine-independent monitoring and control of the powder bed fusion process. In this paper there will be emphasis on the AMMT, which incorporates in-line visible light collection optics for monitoring and feedback control of the powder bed fusion process. We shall present an overview of the AMMT/TEMPs program and it goals. The optical and mechanical design of the open architecture powder-bed fusion system and the AMMT will be also be described. In addition, preliminary measurement results from the system along with the current system status of the system the will be described.

Temperature dependent emissivity metrology development at NIST in support of RTP needs

Measurement instrumentation and methodology for temperature dependent emissivity of solid materials are under development in NISTs Fourier Transform Spectrophotometry Laboratory. The effort is directed to support US industrial needs for emissivity data and standards for a broad range of applications including rapid thermal processing (RTP). The measurement approach and instrumentation design for several systems under construction are described. In particular, a vacuum goniometer for reflectance and transmittance measurement has been designed for the characterization of the emissivity of RTP samples.

Establishing traceable radiation thermometry with in-line imaging system at the NIST AMMT Facility (Conference Presentation)

The paper describes efforts to establish traceable measurements of radiance temperature on laser-induced heated metal surfaces on the NIST Additive Manufacturing Metrology Testbed (AMMT). Knowledge of radiance temperature with a well understood uncertainty budget is a necessary initial step towards an ultimate project goal of traceable emittance and true surface temperature across the heat affected zone, which is a key objective in additive manufacturing research, and the subject of another paper at this conference. Reliable measurements of radiance temperature with an imaging system require (1) calibration of its responsivity at select radiance levels, (2) establishing a calibration equation that interpolates between these levels, (3) dealing with finite spectral bandpass and spatial non-uniformity of the sensor responsivity, and (4) ability for compensate effects of imperfect optical imaging and readout electronics on spatial distribution of the target. The developed system includes an integrating sphere-based calibration source, a pyrometer for its calibration against external blackbody, and an imaging system co-axially aligned with the heating laser, each of which using identical narrow band filters. This paper describes the evaluation of an 850 nm band, with additional wavebands planned for the future. This paper presents experimental results, description of measurement equation and processing algorithm, as well as a framework for establishing an uncertainty budget, including current estimates and future performance goals.