NIST Developments in Support of CLARREO Reflected Solar ...

NIST Developments in Support of CLARREO Reflected Solar ...

NIST Developments in Support of CLARREO Reflected Solar Instruments Joe Rice* Steven Brown, Ping Shaw, John Woodward, Brian Alberding NIST, Gaithersburg, MD *Contact Author Ph: 301.975.2133 Email: [email protected] Focus Areas Exploring new approaches to ACR-based calibrations Automated sources Supercontinuum source-monochromator (a.k.a. laser-line tunable filter: LLTF) kHz OPO-based

Exploring new Working Standards Spectrographs Low noise pyroelectric detectors (if time) Exploring a bandpass correction approach to improve the resolution of spectrographs & scanning monochromators Developing new sources for use with ACRs in scale transfers Motivation: SIRCUS tunable lasers can be finicky, requiring someone to be in attendance at all times Hand tuning required over much of the spectral range Searching for sources that can be tuned automatically

Reduces the labor involved Enables calibrations to be run overnight Two sources being considered Supercontinuum source-Laser Line Tunable Filter kHz OPO system Power Responsivity Scale Realization Using POWR and Super Continuum Source-Laser Line Tunable Filter POWR Main Stage POWR: Primary Optical Watt Radiometer - A cryogenic electrical substation radiometer) DUT & XY Stage

POWR LLTF: Laser Line Tunable Filter - A monochromator Trap Shutter Super Continuum Source LLTF Polarizer

Monitor Polarizer Stabilizer Spectrograph Current wavelength range: 475 nm to 1000 nm (Replace LLTF for 1000 nm to 2500 nm range) Beam size: ~1.5 mm Bandwidth< 1.5 nm (FWHM) Estimated uncertainty: ~0.04% Beam Profile from SC/LLTF at 650 nm

Measured Transmittance of the Brewster Window of the POWR Measured External Quantum Efficiency (EQE) of Trap Detectors kHz OPO: Absolute Measurement of Time-Dependent Light Sources Absolute Cryogenic Radiometer developed to measure stabilized light sources by electrical substitution radiometry EKSPLA Laser tunable from 205 nm to 2300 nm pulsed low-frequency fluctuations, 5 %

rms Low-frequency fluctuations are the difficulty! radiometer electronics must be modified to respond to time dependent sources while maintaining accuracy. New Working Standards Motivation Looking for standards for the 1.7 mm to 2.5 mm region Improve spectral standards for broad-band sources Reduce the uncertainty in disseminated radiance standards Low-noise pyroelectric detectors Spectrographs as Working Standards Stability measurements Working Standard Spectrograph Development

Motivation NIST Irradiance Scale NIST Radiance Scale During NASAs Earth Observing System-era, a series of source radiance validation campaigns were planned and executed by the EOS Project Office with the goal of validating the radiances assigned to laboratory calibration sources, principally lampilluminated integrating spheres, and establishing an uncertainty budget for the disseminated radiance scale. Issued Lamps, k = 2 uncertainty approximately 0.6 % @ 900 nm

0.9 % @ 500 nm 1.25 % @ 350 nm H. Yoon and Charles Gibson, Spectral Irradiance Calibrations, NIST Special Publ. 250-89 (July 2011). Based on an analysis of 7 years worth of data, Butler et al.1 assigned an uncertainty in disseminated radiance scales of 2% to 3% in the Vis/NIR (silicon) region, increasing to 5 % in the short-wave infrared region. Uncertainties too large for CLARREO Butler, J. J., et al., Validation of radiometric standards for the laboratory calibration of reflected-solar Earth observing satellite instruments, Proc. SPIE 6677, 667707 (2007). 1

Motivation The SIRCUS facility has demonstrated that moving from source-based scales traceable to primary standard blackbodies to detector-based scales traceable to low temperature cryogenic radiometers offer opportunities to reduce the uncertainties in disseminated standards. Expanded Uncertainties (k=2) 0.08 % Yoon, H. W., et al., Appl. Opt. 46, 2870 (2007) Absolute Calibration of a Reference Spectrograph FEL-Lamp calibration often the single largest source of uncertainty Solution: Map out the Single Pixel Responsivity of every pixel using SIRCUS Single Pixel Spectral Response for Pixel 240

Single Pixel Responsivities Uncertainty: 0.2 % or less (k=2) Si range 7000 Response (Arbitrary Unit) 6000 5000 4000 3000 2000 1000 0 530 535

540 545 Laser Wavelength (nm) 550 Spectrograph Characterizations to evaluate its potential for use as a Working standard Absolute spectral responsivity (SIRCUS) Wavelength scale (SIRCUS) Stray light correction (SIRCUS) Stability (FEL lamps)

Bandpass correction (SIRCUS) Linearity Temperature dependence Wavelength scale Comparison between SIRCUS and the Instrument Vendor Vendor Wavelength uncertainty: 0.5 nm Bandwidth of a Spectrograph Vendor spec: 3 nm Pixel to pixel spacing ~ 0.6 nm 5.6 5.4 5.2 5.0

6000 Response (Arbitrary Unit) FWHM Bandpass [nm] 7000 4.8 5000 4000 3000 2000 1000 0 530

535 540 545 550 Laser Wavelength (nm) 4.6 4.4 4.2 300 400

500 600 700 Wavelength [nm] 800 900 1000 Stray Light

Spectral Response of Pixel 240 Smeas= [I+D]SIB OOB OOB Stray light correction algorithm Smeas= [I+D]SIB Stray Light Distribution Function, D Describes the scattering properties of the spectrograph Y. Zong, et al., Simple spectral stray light correction method for array spectroradiometers, Appl. Opt. 45(6), 1111 1119 (2006). Stability is the Key for use of Spectrographs as

Candidate Working Standards Came out of work at the Whipple Observatory, Mt. Hopkins, Amado AZ* Spectrograph Characteristics Radiometric Stability v an FEL-lamp - CCD-based fiber-fed slit spectrograph - 380 nm to 1040 nm, 4 nm resolution - Temperature-stabilized CCD Calibration setup not maintained; reproduced for each measurement. 1.006

from 11/2012 6/2014 Event where water spilled onto the instrument and it was left outside for a while to dry Ratio to Mean Deployed to Mt. Hopkins and returned to NIST several times 1.004 1.002 1 0.998 0.996 0.994

400 500 600 700 800 900 1000 Wavelength [nm] Repeatability of Fiber insertion into spectrograph Fiber plug-in repeatability Mt. Hopkins Stability Data Most of the observed variability from fiber insertion into the spectrograph Stability measurements: Weekly measurements with FEL lamp over 2 mos.

Stability measurements: E head Water Measurements continue on a monthly basis for 6 mos. to a year. Developing Protocols to characterize and calibrate Spectrographs Validate Instrument Responsivity in the field based on working standard detectors Monochromatic Light from Supercontinuum Source-pumped Laser Line Tunable Filter Vis-NIR Detector-based Scale held on Si photodiodes Scale held on Si Working Standard Detector(s)

Si MonPD Spectrograph Range (nm) FWHM Vis-NIR 400 - 1000 2.5 nm SWIR 1000 - 2300 4 nm UV: replace SC source with

Laser-driven Xe arc source DataLogger Wavelength scale verified by high resolution spectrograph Validation Source SC source-pumped LLTF LLTF output fibercoupled to a 2 integrating sphere equipped with a monitor photodiode Test spectrographs about 30 cm away Spectrograph integrates for 10 s

Measurements from 450 nm to 1000 nm every 10 nm for 5 consecutive days. Output of SC-LLTF source ~1.1 nm FWHM LLTF-Based Stability Measurements made on 5 consecutive days Uncertainty Estimate Component Absolute responsivity Wavelength Stability

Stray light Field Validation Other Total Uncertainty (k=1) % 0.1 <0.1 0.02 <0.01 0.05 0.1 0.15 Uncertainty

(k=1) % 0.2 0.02 0.1 0.1 0.245 May be possible to achieve 0.5 % (k=2) uncertainties for A Working Standard Spectrograph Implications: Irradiance scale Uncertainty in NIST Irradiance Scale Disseminated Standards (FEL lamps) Wavelength [nm] 250

350 450 555 654.6 900 Unc (k=2) [%] 1.74 1.27 0.91 0.77 0.69 0.57 Spectrograph uncertainty target: 0.5 % k=2 or less over full spectral region H. Yoon and Charles Gibson, Spectral Irradiance Calibrations, NIST Special Publ. 250-89

(July 2011). Implications: Radiance Scale Potential impact on lamp-Illuminated Integrating Sphere uncertainties From Butler et al.1 the uncertainty in disseminated radiance scales are 2% to 3% in the Vis/NIR (silicon) region. Includes uncertainties in the reference radiance meters (not negligible) Uncertainties in a Working Standard Spectrograph on the order of 0.2 % to 0.3 % (k=1) or less

Using a Working Standard Spectrograph in situ (at the time of measurement) may reduce the uncertainties in the disseminated Radiance Scale an order of magnitude, to a level that meets or exceeds most satellite sensor laboratory calibration uncertainty requirements. Approach to improve the resolution of spectrographs or scanning spectrometers (Bandpass correction algorithm) Measurement Equation Ignoring details about the In-Band Responsivity s1 r11 0 s 0 r 22 2 s3 0

0 0 sn 0 0 0 r33 0 0 e1 0 e2 0 e3 rnn enn

7000 We have thrown away information about the in-band responsivity. What if we put that information back into the Measurement Equation? Response (Arbitrary Unit) 6000 5000 4000 3000 2000 1000 0 530

535 540 545 Laser Wavelength (nm) 550 Measurement Equation including IB Responsivity after Stray Light Correction S = R. e 0

0 Knowing S and R, can we solve the system of linear equations for e? What does e look like? Proof-of-Principle Simulations Source Distribution Assume a Gaussian source distribution s=1 FWHM = 2.35*s Single Pixel Responsivity Assume a Gaussian source distribution s=3 FWHM = 7.05*s Simulation:

Look at the achievable resolution Resolution achievable may be ~equal to the pixel-to-pixel spacing, not the instrument single pixel bandpass (our instrument, ~0.6 nm pixel to-pixel spacing, ~4 nm bandpass, so a factor of ~7 increase in resolution with no loss in throughput) *In a scanning system, pixelto-pixel spacing corresponds to the step size. Implication is that you can improve the resolution in a scanning instrument by reducing the step size with no loss of throughput.

Problem: Noise amplification Some type of regularization necessary to reduce sensitivity of the solution to noise? We are currently working on this. See Eichstaedt, et al., in references on the next slide, for example. Bandpass Correction References of Note: Nevas, S., et al ., Simultaneous correction of bandpass and stray-light effects in array spectroradiometer data, Metrologia 49, S43-S47 (2012). Eichstaedt, S., et al., Comparison of the Richardson-Lucy method and a classical approach for spectrometer bandpass correction, Metrologia 50, 107-118 (2013). CIE Technical Report, Effect of instrumental bandpass function and measurement interval on spectral quantities, CIE 214:2014. Stearns, E. I. and Stearns, R. E., An example of a Method for Correcting

Radiance Data for Bandpass Error, Color Res. & Appl. 13, 257-259 (1988). Wooliams, E. R., et al., Spectrometer bandwidth correction for generalized bandass functions, Metrologia 48, 164-172 (2011). Ohno, Y., A flexible bandpass correction method for spectrometers, AIC05, Granada, SP (2005). Gardner, J. L., Spectral deconvolution applications for colorimetry, Color Res. & Appl. 39(5), 430-435 (2006) Low-noise pyroelectric detector The responsivity of a pyroelectric detector is proportional to (1-R), where R is the total hemispherical reflectance. Gentec pyroelectric detector Gentec responsivity Tradition pyroelectric detectors have Noise Equivalent Powers (NEPs) on the

order of 60 nW/Hz^1/2 to 80 nW/Hz^1/2, too large for uses as Transfer Standard radiometers for GLAMR. Recently, pyroelectric detectors have been developed with Noise Equivalent Powers (NEPs) on the order of 1 nW/Hz^1/2, opening up the possibility of developing pyroelectric detector-based NIR and SWIR transfer standards for GLAMR. Low noise pyroelectric detectors A spectral responsivity reference scale has already been realized from 600 nm to 19 mm. The reference-scale uncertainty is 1 % (k=2). NEP ~ 6 nW/Hz^1/2 Calibrate on SIRCUS in Ti:S range ~ 800 nm and use in the SWIR from 1.6 mm to 2.5 mm. Low noise pyroelectric detectors Gentec pyroelectric detectors have NEPs 1 nW/Hz1/2 Infratec pyroelectric detectors have NEPs 0.3 nW/Hz1/2

Incident flux > 1 mW to keep noise contribution to the uncertainty budget < 0.1 %. Summary LLTF leading to full spectral, fast automation of dissemination of primary scale Vis-NIR demonstrated Moving to SWIR and near-UV LLTF combined with stable spectrographs being developed to improved transferred irradiance uncertainty by factor of 10 Stray light correction algorithms being extended to enhance spectral resolution of spectrographs and spectrometers

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