Jinan Zeng

A Comparison of Optical Properties between High Density and Low Density Sintered PTFE

Materials with similar chemical compositions often exhibit different optical properties due to their structural composition. PTFE is widely used in many applications for both its mechanical and optical properties. Low density sintered PTFE has optical properties that make it desirable for use as a white diffuser in applications such as remote sensing. The contrast between the commonly available high density material and the low density material may be useful for those interested in optical modeling of scattered light. Additionally, some applications may find high density PTFE suitable for some optical applications. This paper describes measurements of BRDF, 8º/hemispherical reflectance, and directional hemispherical transmittance for both high density (HD) and low density (LD) sintered PTFE.

Extension of the NIST spectral responsivity scale to the infrared using improved-NEP pyroelectric detectors

Routine NIST spectral responsivity calibrations are needed for the infrared range. Low-NEP pyroelectric radiometers have been developed for traditional monochromator applications to extend the responsivity scale to the infrared. After NEP tests, the best pyroelectric detectors were converted to transfer-standard radiometers. The relative spectral responsivities were determined from spectral reflectance measurements of the organic black detector coatings. The absolute tie points were measured against a domed pyroelectric radiometer standard and a sphere-input extended-InGaAs transfer-standard radiometer. The infrared spectral power responsivity scale has been extended for the 2 µm to 14 µm wavelength range. A single-grating monochromator has been adapted to the calibration facility and used to characterize and calibrate infrared detectors.

Low NEP pyroelectric radiometer standards

Verification of the low measurement uncertainty of a group of eight newly developed pyroelectric detectors at the output of a traditional monochromator is described. The frequency compensated hybrid detector-amplifier packages have fixed 1010 Ω feedback resistors. The characterizations verified that the 3 dB upper roll-off frequencies of the signal-gain curves are close to 100 Hz and the temperature coefficient of responsivity is 0.14 %/oC. The hybrid packages were tested for noise performance in the f/8 beam of a grating monochromator between 900 nm and 2.7 μm. The monochromator output beam-power, the output signal, and the output noise of the hybrid packages were measured and compared. The NEPs were between 3.3 nW/Hz1/2 and 10 nW/Hz1/2. The relative standard uncertainty of the noise measurements was 20 % (k=1). The noise tests were utilized to select hybrid packages with NEPs that are one order of magnitude lower than that of traditional pyroelectric detectors and current-amplifiers. The power responsivity of one hybrid was calibrated against an absolute cryogenic radiometer. With this detector, the measured signal-to-noise ratios were higher than 400 between 1.1 μm and 2.1 μm and 250 at 2.5 μm using a lock-in integrating time-constant of 1 s. The noise test results show that using a hybrid detector with an NEP equal to the group average of about 6 nW/Hz1/2, spectral responsivity measurements with a relative standard uncertainty of 0.2 % to 0.4 % (k=1) can be achieved.

An infrared laser-based reflectometer for low reflectance measurements of samples and cavity structures

An instrument, the Complete Hemispherical Infrared Laser-based Reflectometer (CHILR), has been designed and built for the accurate characterization of the total reflectance of highly absorbing samples and cavity structures down to the level of 10-5. The design of CHILR employs a number of the same features of Total Integrated Scatter (TIS) measurement devices, but is used for total reflectance (both specular and diffuse components), rather than only the diffuse component. A number of features of CHILR include spatial uniformity and angular dependence of reflectance measurement capability, multiple wavelength laser sources, and the ability to measure a wide range of sample sizes and cavities with aperture sizes, ranging from 3 mm to 51 mm. We address several basic issues of alignment, background and externally scattered light, reference measurement, and laser drift, for the CHILR. We also present results of several examples, including cavities for blackbody sources, and radiometer cavities.

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.

Calibration of the spectral radiant power responsivity of windowed pyroelectric radiometers from 785 nm to 14 μm

As part of the optical detector characterization program at the Optical Technology Division at NIST, we have established a capability for the calibration of spectral radiant power responsivity in the infrared from 785 nm to 14 μm. We have used our facilities to characterize two commercial pyroelectric radiometers with KRS5 windows. The calibration methodology consists of the determination of the responsivity at single wavelength tie points together with relative spectral responsivity measurements. Responsivity tie points at 785, 1064, 1320, 1600, 2000 and 10600 nm are obtained against the NIST ACR-L1 absolute cryogenic radiometer as well as a domed pyroelectric transfer standard, using an OPO tunable laser and a stabilized CO2 laser as sources. The spatial variation of the responsivity of the test radiometers has also been measured. This enables us to minimize the uncertainties due to interference fringes from the KRS5 window. The relative spectral responsivity curves for the two radiometers are obtained indirectly through measurement of the detector absorptance with an FTIR-based integrating sphere reflectometer. In order to obtain the actual absorptance at the detector from the reflectance measurements, an individual KRS5 window and a bare detector were also measured. The results are compared and the uncertainty budgets will be discussed.

Development and calibration of pyroelectric radiometer standards at NIST

The reference spectral power responsivity scale of NIST is being extended from the silicon range to the infrared (IR) using pyroelectric radiometers. Two transfer standard pyroelectric radiometers have been developed at NIST. The main design consideration was to obtain only a minimal increase in the measurement uncertainty during the responsivity scale extension. Domain engineered LiNbO3 and regular LiTaO3 pyroelectric detectors were used in the two radiometers. Both detectors are gold-black coated and temperature controlled. Reflecting domes are attached to the radiometer inputs to decrease the reflectance loss and to improve the spatial uniformity of responsivity in the infrared. Four commercial pyroelectric detectors have been added to the group and used as working standards. The relative spectral responsivity of all pyroelectric detectors was determined from spectral reflectance measurements. The radiant power responsivity tie points were derived from Si trap and single element detectors traceable to the NIST reference responsivity scale. The pyroelectric radiometers have been characterized for frequency and temperature dependent responsivity, noise, spatial non-uniformity of responsivity, angular responsivity, and linearity. The expanded (relative) uncertainty of the spectral power responsivity calibrations ranged between 0.5 % and 1.2 % (k=2) within the 1 μm to 19 μm range.

Increased responsivity pyroelectric radiometer with dome input and temperature control

Pyroelectric radiometers with noise-equivalent-power (NEP) close to 1 nW/Hz1/2 have been developed to measure less than 1 μW radiant power levels at room temperature to 25 μm. The radiometers will be used as transfer standards for routine spectral responsivity calibrations in the infrared range at the output of a traditional monochromator. Dome inputoptics are used with multiple beam reflections to increase absorptance and to minimize structures in the spectral responsivity function. The temperature of the pyroelectric detector is stabilized with a thermoelectric cooler/heater. Spectral power responsivity calibrations were performed with two different methods. A Fourier Transform Spectrometer using its Infrared Reference Integrating Sphere System based method was validated with a continuously variable filtermonochromator based detector-comparison method using earlier developed pyroelectric radiometer standards. A responsivity uncertainty of 1.4 % (k=2) was obtained between 2 μm and 14 μm.