George P. Eppeldauer

Facility for spectral irradiance and radiance responsivity calibrations using uniform sources

Detectors have historically been calibrated for spectral power responsivity at the National Institute of Standards and Technology by using a lamp-monochromator system to tune the wavelength of the excitation source. Silicon detectors can be calibrated in the visible spectral region with combined standard uncertainties at the 0.1% level. However, uncertainties increase dramatically when measuring an instruments spectral irradiance or radiance responsivity. We describe what we believe to be a new laser-based facility for spectral irradiance and radiance responsivity calibrations using uniform sources (SIRCUS) that was developed to calibrate instruments directly in irradiance or radiance mode with uncertainties approaching or exceeding those available for spectral power responsivity calibrations. In SIRCUS, the emission from high-power, tunable lasers is introduced into an integrating sphere using optical fibers, producing uniform, quasi-Lambertian, high-radiant-flux sources. Reference standard irradiance detectors, calibrated directly against national primary standards for spectral power responsivity and aperture area measurement, are used to determine the irradiance at a reference plane. Knowing the measurement geometry, the source radiance can be readily determined as well. The radiometric properties of the SIRCUS source coupled with state-of-the-art transfer standard radiometers whose responses are directly traceable to primary national radiometric scales result in typical combined standard uncertainties in irradiance and radiance responsivity calibrations of less than 0.1%. The details of the facility and its effect on primary national radiometric scales are discussed.

Gold-black coatings for freestanding pyroelectric detectors*

We describe the process of depositing gold-black on thin, freestanding pyroelectric detector substrates and compare this with previous work documented in the literature. We have evaluated gold-black coatings on thin, freestanding pyroelectric detector substrates by means of scanning electron microscope, Fourier transform infrared spectrophotometer reflectance, and spectral responsivity measurements. Spectrophotometric measurements indicate that reflectance at normal incidence varies by less than 1% at wavelengths shorter than 2.5 µm and by less than 10% at 10 µm. These results are correlated with the spectral responsivity of the detector and demonstrate that radiation not reflected by the gold-black is absorbed by the detector element. We have evaluated gold-black coatings as a function of position at two wavelengths and found variations of less than 1% at 1.25 µm and less than 5% at 10.3 µm, which demonstrates that spatial uniformity can be coating dependent. Gold-black coatings exposed to a 193 nm wavelength excimer laser were evaluated by visual inspection for damage and determined to have a damage threshold of approximately 38 mJ cm−2.

NIST facility for Spectral Irradiance and Radiance Responsivity Calibrations with Uniform Sources

A laser-based facility has been developed to provide high-flux, monochromatic, Lambertian radiation over the spectral range 0.2 µm to 18 µm. The facility was designed to reduce the uncertainties in a variety of radiometric applications, including irradiance and radiance responsivity calibrations. The operational characteristics of the facility are discussed and the results of detector responsivity calibrations over the spectral range 0.406 µm to 0.920 µm are presented.

Fourteen-decade photocurrent measurements with large-area silicon photodiodes at room temperature.

Recent improvements in commercial silicon photodiodes and operational amplifiers permit electrical noise to be reduced to an equivalent of 0.1 fA of photocurrent when a measurement time of 400 s is used. This is equivalent to a photocurrent resulting from fewer than 800 photons/s, and it implies a dynamic range of 14 orders of magnitude for a detector circuit. We explain the circuit theory, paying particular attention to the measurement bandwidth, the causes of noise and drift, and the proper selection of circuit components. These optical radiation detectors complement the primary radiometric standards. These detectors may replace photomultiplier tubes that have been used traditionally and or that were too costly to be used.

LED-based Spectrally Tunable Source for Radiometric, Photometric, and Colorimetric Applications

A spectrally tunable light source using a large number of LEDs and an integrating sphere has been designed and is being constructed at the National Institute of Standards and Technology. The source is designed to have a capability of producing any spectral distribution, mimicking various light sources in the visible region by feedback control of individual LEDs. The output spectral irradiance or radiance of the source will be calibrated by a reference instrument, and the source will be used as a spectroradiometric as well as a photometric and colorimetric standard. A series of simulations have been conducted to predict the performance of the designed tunable source when used for calibration of display colorimeters. The results indicate that the errors can be reduced by an order of magnitude when the tunable source is used to calibrate the colorimeters, compared with measurement errors when the colorimeters are calibrated against Illuminant A. The source can also approximate various CIE daylight illuminants and common lamp spectral distributions for other photometric and colorimetric applications.

Linearity of InGaAs photodiodes

In radiometry or pyrometry, radiometers are often used to assign the spectral radiance or radiance temperatures of sources using ratios of signals which can differ by several decades. For performing ratios between such sources with low uncertainties, the linearity of the detectors used in the transfer radiometers needs to be characterized. The linearity of InGaAs photodiodes has been studied using the flux-addition method using a broadband infrared source with a visible-blocking filter. Using this technique, 18 InGaAs photodiodes from four different vendors were studied without spectral filtering in a broad wavelength region from 900 nm to 1700 nm with the diodes underfilled by the incident flux. The linearity of InGaAs photodiodes was determined within the range of photocurrents from 10−8 A to 10−4 A. All the InGaAs photodiodes demonstrated linearity from 10−7 A to 10−4 A within the expanded uncertainties of 0.08% (k = 2). The uncertainty in the linearity measurement below 10−8 A is increased due to the increased noise in the photocurrent arising from the feedback resistance of the transimpedance amplifier being greater than the shunt resistance of the photodiode.

Development of a Tunable LED-Based Colorimetric Source.

A novel, spectrally tunable light-source utilizing light emitting diodes (LEDs) for radiometric, photometric, and colorimetric applications is described. The tunable source can simulate standard sources and can be used as a transfer source to propagate photometric and colorimetric scales from calibrated reference instruments to test artifacts with minimal increase in uncertainty. In this prototype source, 40 LEDs with 10 different spectral distributions were mounted onto an integrating sphere. A voltage-to-current control circuit was designed and implemented, enabling independent control of the current sent to each set of four LEDs. The LEDs have been characterized for stability and dependence on drive current. The prototype source demonstrates the feasibility of development of a spectrally tunable LED source using LEDs with up to 40 different spectral distributions. Simulations demonstrate that such a source would be able to approximate standard light-source distributions over the visible spectral range—from 380 nm to 780 nm—with deviations on the order of 2 %. The tunable LED source can also simulate spectral distributions of special sources such as discharge lamps and display monitors. With this tunable source, a test instrument can be rapidly calibrated against a variety of different source distributions tailored to the anticipated uses of the artifact. Target uncertainties for the calibration of test artifacts are less than 2 % in luminance and 0.002 in chromaticity for any source distribution.

Opto-Mechanical and Electronic Design of a Tunnel-Trap Si Radiometer

A transmission-type light-trap silicon radiometer has been developed to hold the NIST spectral power and irradiance responsivity scales between 406 nm and 920 nm. The device is built from replaceable input apertures and tightly packed different-size silicon photodiodes. The photodiodes are positioned in a triangular shape tunnel such that beam clipping is entirely eliminated within an 8 field-of-view (FOV). A light trap is attached to the output of the radiometer to collect the transmitted radiation and to minimize the effect of ambient light. The photodiodes, selected for equal shunt resistance, are connected in parallel. The capacitance and the resultant shunt resistance of the device were measured and frequency compensations were applied in the feedback network of the photocurrent-to-voltage converter to optimize signal-, voltage-, and loop-gain characteristics. The trap radiometer can measure either dc or ac optical radiation with high sensitivity. The noise-equivalent-power of the optimized device is 47 fW in dc mode and 5.2 fW at 10 Hz chopping. The relative deviation from the cosine responsivity in irradiance mode was measured to be equal to or less than 0.02 % within 5° FOV and 0.05 % at 8° FOV. The trap-radiometer can transfer irradiance responsivities with uncertainties comparable to those of primary standard radiometers. Illuminance and irradiance meters, holding the SI units (candela, color- and radiance-temperature), will be calibrated directly against the transfer standard trap-radiometer to obtain improved accuracy in the base-units.

Domain-engineered pyroelectric radiometer

We built a large-area domain-engineered pyroelectric radiometer with high spatial and spectral response uniformity that is an excellent primary transfer standard for measurements in the near- and the mid-infrared wavelength regions. The domain engineering consisted of inverting the spontaneous polarization over a 10-mm-diameter area in the center of a uniformly poled, 15.5 mm x 15.5 mm square, 0.25-mm-thick LiNbO(3) plate. Gold black was used as the optical absorber on the detector surface, and an aperture was added to define the optically sensitive detector area. Our results indicate that we significantly reduced the acoustic sensitivity without loss of optical sensitivity. The detector noise equivalent power was not exceptionally low but was nearly constant for different acoustic backgrounds. In addition, the detectors spatial-response uniformity variation was less than 0.1% across the 7.5-mm-diameter aperture, and reflectance measurements indicated that the gold-black coating was spectrally uniform within 2%, from 800 to 1800 nm. Other detailed evaluations of the detector include detector responsivity as a function of temperature, electrical frequency response, angular response, and field of view.

THE NIST DETECTOR-BASED LUMINOUS INTENSITY SCALE

The Système International des Unités (SI) base unit for photometry, the candela, has been realized by using absolute detectors rather than absolute sources. This change in method permits luminous intensity calibrations of standard lamps to be carried out with a relative expanded uncertainty (coverage factor k = 2, and thus a 2 standard deviation estimate) of 0.46 %, almost a factor-of-two improvement. A group of eight reference photometers has been constructed with silicon photodiodes, matched with filters to mimic the spectral luminous efficiency function for photopic vision. The wide dynamic range of the photometers aid in their calibration. The components of the photometers were carefully measured and selected to reduce the sources of error and to provide baseline data for aging studies. Periodic remeasurement of the photometers indicate that a yearly recalibration is required. The design, characterization, calibration, evaluation, and application of the photometers are discussed.