comparison of the total solar irradiance radiometer facility cryogenic radiometer against the

Optical Technology Division CalCon TSI TRF Rad at POWR 26Aug2009

Comparison of theTotal Solar Irradiance Radiometer Facility Cryogenic Radiometer

against theNIST Primary Optical Watt Radiometer

Joseph P. Rice and Allan W. SmithOptical Technology Division

National Institute of Standards and Technology (NIST)Gaithersburg, Maryland 20899

Greg A. Kopp, David M. Harber, and Karl F. HeuermanLaboratory for Atmospheric and Space Physics (LASP)

University of ColoradoBoulder, Colorado 80303

Steven R. LorentzL-1 Standards and TechnologyNew Windsor, Maryland 21776

Contact: [email protected]


Motivation for this Talk and for the Next Talk

•There is a well-publicized calibration issue with absolute Total Solar Irradiance (TSI) measurements: an unexplained 0.37% difference

•Aperture area (as discussed on Monday by Jim Butler) not able to explain it, so it appears to be something about the way the power measurements are being done

•Such a problem does not occur with related cryogenic radiometer measurements

•Why not apply cryogenic radiometry to solve the problem?


There is an unexplained 0.37% difference between TIM and VIRGO or ACRIM

Exo-atmospheric Total Solar Irradiance (TSI) Measurements


International Intercomparison of Cryogenic Radiometers•Standards labs can measure responsivity of traps to <1 mW laser power to about 0.02%•This was in the late 1990’s, and NIST numbers are from HACR (predecessor to POWR).

BIPM report:


Cryogenic Electrical Substitution Radiometry

LiquidHe at 2K

LiquidNitrogen

•Thermalized optical laser power is compared to thermalized electrical power in a black cavity•Generally, active cavity radiometers in vacuum at 2 K to 5 K•Primary standard at NIST and in most other industrialized nations for optical power responsivity of transfer detectors such as Si-diode trap detectors•Intercompared internationally via portable transfer detectors at 0.02% (k=2) uncertainty

Primary Optical Watt Radiometer (POWR)


Intercomparison of Present-Day Standard NIST Cryogenic Radiometers

-4

-3

-2

-1

0

1

2

3

4

L-1 POWR LOCR

POWR 27Apr05 Intercomparis

488 nm514 nm633 nm

104 x

Diff

eren

ce fr

om th

e M

ean


Introduction to the Intercomparison Reported in This Talk• LASP has now developed a facility for pre-flight calibration of TSI Instruments

– Total Solar Irradiance (TSI) Radiometer Facility (TRF)– System-level calibration in irradiance mode at TSI irradiance level (68 mW for TIM)– This is the first ever facility capable of this feat at less than 0.1% uncertainty level– Motivated by the need for improved TSI measurement accuracy– Supported by the NASA Glory Project – Used for Glory Total Irradiance Monitor (TIM) (David Harber’s Talk, next)

• The irradiance scale is based upon a new cryogenic radiometer: TRF Radiometer– Cryogenic radiometers are in use worldwide and yield the lowest uncertainty– Typical uncertainty of order 0.01% (k=1) (=100 ppm), but only at 2 mW power level– The TRF Radiometer is optimized for 68 mW power level: first of its kind

• What is the radiant power scale uncertainty of the TRF Radiometer?– 1. Can be determined from the components, as for any active cavity radiometer

AND/OR– 2. Can be assigned based in large part upon transfer from a NIST cryogenic radiometer,

such as the NIST Primary Optical Watt Radiometer (POWR)– This talk describes a scale comparison of the TRF Radiometer with the NIST POWR– Result: NIST Correction of TRF native scale by +306 ppm with an uncertainty of

98 ppm (k=1) is required to calibrate it on the NIST POWR scale


Experiment Description Part 1

BeamFrom

532 nmLaser

TranslationStage

TRF RadiometerBrewster Window

POWRBrewster Window

Beamsplitter 1

Beamsplitter 2

Polarizer

Shutters½ Wave Plate

TrapPhotodiode 2

TrapPhotodiode 1

Intensity Stabilizer

SpatialFilter

2 mW

TRF Radiometer

POWR

•Align translation stage so that laser beam enters POWR.•Adjust ½ wave plate to turn power to 2 mW.•Record POWR shuttered power measurements and both Si trap photodiode signals.


Experiment Description Part 2

BeamFrom

532 nmLaser

TranslationStage

TRF RadiometerBrewster Window

POWRBrewster Window

Beamsplitter 1

Beamsplitter 2

Polarizer

Shutters½ Wave Plate

TrapPhotodiode 2

TrapPhotodiode 1

Intensity Stabilizer

SpatialFilter

68 mW

TRF Radiometer

POWR

•Move translation stage so that laser beam enters TRF Radiometer.•Adjust ½ wave plate to turn power up to 68 mW.•Record TRF Radiometer shuttered power measurements and both Si trap photodiode signals.


Typical Raw Data

0

20

40

60

80

100

120

-40000

0

40000

0 500 1000 1500 2000 2500

Pow

er (m

W)

Serv

o Er

ror (

coun

ts)

Time (s)

Servo ErrorHeater Power

SciData_REC 0806131630 TRF Shutter Cycles

-8

-6

-4

-2

0

0 500 1000 1500 2000 2500

TS0806131630 TRF Shutter Cycle Trap Response

Trap

Res

pons

e (V

)Time (s)

Trap 1

Trap 2

TRF Radiometer Trap Photodiode Signals


Results

'ttPrr

PL VRNP

Shuttered Laser PowerEntering TRF ApertureBased only on POWR

(i.e. what TRF should measure)Trap Photodiode Response (2)

Trap Photodiode Responsivity (1)Corrections

CorrectionsR t V t ' 68.318730 mW 43 ppm 68.320214 mW 58 ppm

Relative WindowTransmittance r 0.999953 - 70 ppm 0.999953 - 70 ppmRelative Scatter r 1.0000025 - 17 ppm 1.0000025 - 17 ppmPOWR Nonequivalence N 1.000000 - 28 ppm 1.000000 - 28 ppmPOWR Absorptance 0.999995 - 5 ppm 0.999995 - 5 ppmPOWR Electrical Scale 1.000000 - 13 ppm 1.000000 - 13 ppmPOWR Corrected Value P L 68.322135 mW 90 ppm 68.323620 mW 97 ppmTRF Radiometer Value P TDVM 68.300210 mW 34 ppm 68.303703 mW 23 ppm

P L / P TDVM 1.000321 - 96 ppm 1.000292 - 100 ppm

P L / P TDVM

Value(June 13, 2008)

Uncertainty Component

Value(June 12, 2008)

UncertaintyComponent

98 ppm1.000306Recommended Value Combined Uncertainty (k=1)


Window Transmittance Scans in Air

0.99935

0.99940

0.99945

0.99950

0.99955

0.99960

0.99965

0.99970

-4 -3 -2 -1 0 1 2 3 4

POWR Window Transmittance Scans

Scan 1Scan 2Scan 3

Tran

smitt

ance

Position (mm)

0.99955

0.99960

0.99965

0.99970

0.99975

0.99980

0.99985

0.99990

-4 -3 -2 -1 0 1 2 3 4

TRF Window Transmittance Scans

Scan 1Scan 2Scan 3

Tran

smitt

ance

Position (mm)

POWR Window TRF Radiometer Window

Relative window transmittance at 0 mm position was corrected.


Stress-Induced Birefringence Changes Window Reflectance This common effect, though small with the 6 mm thick POWR window, was significant with the 3 mm thick TRF Radiometer window, and was corrected for both.

2.5

3.0

3.5

4.0

4.5

5.0

5.5

0

200

400

600

800

1000

0 500 1000 1500 2000

POWR Window Reflectance vs Time

Ref

lect

ance

(ppm

)

Pres

sure

(Tor

r)

Time (s)

Reflectance

ClosedGateValve

Pressure

150

200

250

300

350

0 5 10 15 20

TRF Reflectance Vaccum-Air Cycles

Ref

lect

ance

(ppm

)

Set Number

Vacuum

Atmosphere

TweakedWindow

AlignmentHere

Cycle 1 Cycle 2 Cycle 3

POWR Window Reflectance:Venting from vacuum to atmosphere

TRF Radiometer Window Reflectance:Alternating between vacuum and atmosphere


Summary

• A scale comparison of the NIST POWR and the TRF Radiometer was performed– 532 nm– Radiant power (underfilled apertures), as opposed to irradiance mode (overfilled

apertures)– POWR at 2 mW, TRF Radiometer at 68 mW, two trap photodiodes used as transfer

• The TRF Radiometer shuttered power measurement reads low by the following amount:

306 ppm +/- 98 ppm (k=1)• The TRF Radiometer native scale used here had not been explicitly corrected for its

nonequivalence, cavity reflectance (about 38 ppm), or electrical power scale calibration– Applying the recommended correction above intrinsically corrects for these effects

• A detailed report on this comparison is being written for a published journal article

We thank the NASA Glory Project for supporting this work.

comparison of the total solar irradiance radiometer facility cryogenic radiometer against the

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