Sergey Mekhontsev

National Institute of Standards and Technology

Optical Technology Division, Gaithersburg, MD 20899

Infrared Spectral Radiance Scale Realization and Transfer

“NIST IR Radiometry for Climate Benchmark Traceability” MeetingGaithersburg, MD, June 12, 2008

Outline

• Spectral Radiance: Definition and Significance

• Spectral Radiance Scale Realization Principles

• IR Spectral Radiance Scale at the AIRI Facility

• CBS3 Facility and CLARREO mission support

Definition: Radiant flux [] per unit projected area [A], per unit spectral interval, and per unit solid angle [] incident on, passing through, or emerging in a specified direction from a point in a specified surface

Symbol: L(

Unit: W/(cm3·sr)

L

Spectral Radiance - Definition

A

cos

)()(

2

dAd

dL

Radiance is an invariant property of any lossless optical system.

Typically, if radiometric device contains a pair of apertures separated

by some distance, and both apertures are filled by the source, it measures radiance.

Detector Based IR Radiance Scale Realization

42 Tn

LBB

1. Can be used to measure BB temperature assuming it has unity emissivity, using Stefan-Boltzmann law

(2) or (3) can be used with element-level calibration or together with (1) and tunable monochromatic sources

2. Can be used to measure radiance of a monochromatic source (could be explored)

Source Based IR Radiance Scale Realization

AIRI - National Primary Standard of IR Spectral Radiance

Recently established Advanced Infrared Radiometry and Imaging (AIRI) Laboratory, among other

functions, is enabling a national level traceability for measurements of absolute spectral radiance and

spectral emissivity of BB sources and targets at near ambient

temperatures at ambient environment.Fixed Point BB Bench

Variable Temperature/Spectral Bench Scene plate/Scatter Tool

Tunable Filter Comparator TFC

In te rna lB B

C ircu la r Variab le

F ilte r

P o in tin gL ase r

Tem p era tu re -C o n tro lled F ro n t P la te

R eflec tiv eF ie ld S top

E llip ticP rim aryM irro r

L ase rF o ld in g M irro r

A p ertu reS top

R eflec tiv eC h op per

In S b , M C T o r sa n d w ich

d e tec to r(sh o w n

ro ta ted a t 9 0fo r c la rity )

°

E llip ticR ela y M irro r

Lyo tS top

TFC has capacity to consistently measure temperature with standard deviation from 5 to 25 mK in the range -50 °C to 250 °C across spectral regions to 3-5 and 8-12.5 micron

0

0.05

0.1

0.15

0.2

0.25

3.4 3.6 3.8 4 4.2 4.4

Example of CVF Filter Transmittance for several angles.

Measured at NIST using FTIR Spectrometer

244254264274

Ab

so

lute

Tra

nsm

itta

nce

Wavelength, Microns

Spectral Resolution

Spatial Scatter

0

0.0001

0.0002

0.0003

0.0004

0.0005

0.0006

0.0007

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0 50 100 150 200 250 300 350

3.7 microns, InSb, relative 12 mm diameter source,source 50 C, background 23 C

SSE, signal

SSE correction, degrees

SS

E in

radi

ance

, abs

olut

e

SS

E correction, degrees

Source Diameter, mm

Measured spectra

Known Spectral Radiances of Ambient and Variable Temperature Reference BB’s

AAC

AC

Auutuut LLL

VV

VVL

FTIR-Based IR Spectral Radiance ComparatorUncertainty Evaluation at 10 °C using FT Spectrometer

InSb (80 cycles ABABA) at 10 C + NPSL MCT (40 cycles ABABA)

-0.5%

-0.4%

-0.3%

-0.2%

-0.1%

0.0%

0.1%

0.2%

0.3%

0.4%

0.5%

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Wavelength, microns

Dev

iatio

n fr

om 1

InSb (80 cycles ABABA) + NPSL MCT (20 cycles ABCBA)

9.950

9.975

10.000

10.025

10.050

3 5 7 9 11 13 15 17

Wavelength, microns

Tem

pera

ture

, C

Random Uncertainty of Comparison, Radiance

Random Uncertainty of Comparison, Temperature

-FT capability realized both at NIST and NPSL (Navy Primary Standards Lab) -Shown results are obtained at NPSL setup with SR-80 blackbodies at 10 °C

Water Bath BB Characterization

WBBB#1: Stability of Radiance TemperatureMeasured at 4.7 and 10 microns vs. WBBB#2 and Flat Plate BB.

Reference PRT Reading 56.206 C

56.194

56.198

56.202

56.206

56.21

56.214

12:00:00 12:30:00 13:00:00 13:30:00

Time, hh:mm:ss

Rad

ianc

e Te

mpe

ratu

re,

C

10 Microns

4.7 Microns

Radiance Temperature Comparison of Water Baths BB #1 and #2 at 50 C

-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015

3 5 7 9 11 13

Wavelength, microns

(Me

as

ure

d -

Se

t P

oin

t), B

ath

#

2,

de

gre

es

AM series

PM series

WBBB#1: Spatial Uniformity of Radiation TReference PRT reads 56.206. Measured at 10 microns

56.19

56.195

56.2

56.205

56.21

56.215

56.22

132 142 152 162 172 182

Relative Distance. mm

Rad

ianc

e T,

deg

. C

Vertical Scan

Horizontal Scan

- Current serve as Primary Standard at the Temperature Range 10 °°C to 75 °°C

- Availability of two WBBB is instrumental for system uncertainty evaluation

- So far performance is still comparator-limited

Ammonia Heat Pipe BB Characterization

Refrigerated Bath,Refrigerated Bath, -65 -65 °°C to +55 to +55 °° CC

CavityCavity geometrygeometry

Blackbody DesignBlackbody Design

Gas heat Gas heat ExchangerExchanger

-45.250

-45.225

-45.200

-45.175

-45.150

4 6 8 10 12

TFC Random Uncertainty EvaluationUsing Auto-referencing of Ammonia BB @-45.18 C

and water bath @ 11 C

Ap

pare

nt T

emp

era

ture

, de

gre

es

C

Wavelength, microns

30.04

30.05

30.07

30.08

4 6 8 10 12

Measured Radiance Temperature of the Ammonia BBReference PRT Reading 30.06

App

are

nt

Te

mp

era

ture

, d

egre

es

CWavelength, microns

10.22

10.24

10.26

4 6 8 10 12

Measured Radiance Temperature of the Ammonia BBReference PRT Reading 10.24 C

S1

S2

S3

S4

L1

App

are

nt

Te

mp

era

ture

, d

egre

es

C

Wavelength, microns

BB Ammonia Stability, PRT temperature

-45.192

-45.188

-45.184

-45.18

-45.176

0 500 1000 1500 2000 2500 3000 3500 4000

Time, seconds

Tem

per

atu

re, C

Gold coated Al reflector

Polished CopperCavity, coated with Z302 paint

Gallium in a Teflon Enclosureor Methanol Heat Pipe in a Copper Enclosure

Multilayer Foil Insulation

Foil heater for use in cryogenic chamber

Heat exchanger for use in laboratory conditions

with refrigerated bath

Large Aperture Ga Fixed Point BB

Ga FP BB (29.765 °C) is an important tool for further reduction of the uncertainty of near-ambient BB measurements. It is also expected to become a prototype for large

aperture vacuum-compatible water (0.01 °C) and Mercury (-38.834 ° C) FP blackbodies

Gallium Melting Plateau measured with TFC at 10 micron relative water bath BB, ITS-90 value 29.765 C

29.745

29.75

29.755

29.76

29.765

29.77

29.775

14:00:00 18:00:00 22:00:00 26:00:00

Time, hh:mm:ss

Ra

dia

nc

e T

em

pe

ratu

re,C

Ga BB: Spectral Radiance Measured with TFC

29.75

29.755

29.76

29.765

29.77

29.775

3 5 7 9 11 13

Wavelength, microns

Ra

dia

nc

e t

em

pe

ratu

re,

C

Internal (FTIS / AIRI) Comparisons

0.99

0.991

0.992

0.993

0.994

0.995

0.996

2 4 6 8 10 12 14

Reflectometry, DIGILABARM

Em

issi

vity

Wavelength (um)

Pyramid (Structured) Target Emittance

Diffuse Black Target Emittance

Radiometric Characterization of Target Emittance: Experimental Implementation

Target Side View Radiometer Side View

Target Plate Emissivity Measurements

Scene Plate with Alternating Temperature(e.g. 20 C and 75 C)

Through Hole Larger Than Radiometer Nominal Spot Size

20 C Emitting Plate

Layer of Insulating Foam

IR Radiometer (spectrometer)

Target PlateUnder Test

Calibration Artifacts

Pyramid Target and Coupon(Diffuse Black Paint)

Flat Target and Coupons (Diffuse Black and Diffuse Silver Paints)

Spectrophotometric Charactrization of Target Emittance: Integrating Sphere Reflectometer

0.96

0.965

0.97

0.975

4 6 8 10 12

ARM, 13 C background2TM, 13C / 20 C backgrounds2TM, 20 C and 27 C backgroundsARM, 27 C BackgroundReflectometry, Digilab FTIR

Em

issi

vity

Wavelength (um)

Radiance Temperature Measurement Validation

Observed Radiance Temperatureof the diffuse black target measured by the Absolute Radiance Method (ARM) at background 13 C well agrees with predictions based on the data from ARM at BG 27 C and DIGILAB FTIR-based Reference Reflectometer

This indicates a good agreement between thermistor-based and actual surface temperature values

Specifications• range: 1.0 - 18 µm• 6 inch diameter

• gold-electroplated plasma -sprayed metal

coating

• MCT detector w/ concentrator optics

• baffling in sphere

• 8° incidence angle

Capabilities• Reflectance, Transmittance & Emittance

• Temperatures 15 - 200 °C

• absolute & relative, specular & diffuse

• uncertainties (2):

Ø specular: = 0.3%Ø diffuse: 1.5 - 3.5%Ø larger for angle dependent structure

19.8

19.82

19.84

19.86

19.88

19.9

3 5 7 9 11 13

Wavelength (um)

Rad

ianc

eT

empe

ratu

re,C

AR M , BG 13 C (observed)

AR M , BG 27 C (converted)

FT IR D ata (pred icted)

Thermal IR Scales – Internal Validation

International Comparisons of IR Spectral Radiance and Radiance Temperature

Transfer Standard Pyrometer TSP and Source TSS2

Schematic and General View of the Transfer Standard Source TSS1

Participants:

Artifacts:

Range:

Status:

Two Transfer Standard Blackbodies, one Pyrometer

-40 to 300°C °C, 3 m to 13.5 mμ μ

In progress (expected completion May 2008)

NIST (pilot),NPL (UK), PTB (Germany), NRC (Canada)

Many laboratories operate near-ambient tempertaure BB sources with potential or claimed total uncertainty at the level of 15-25 mK (k=2)

.

Certain applications, such as climate monitoring, have generated additional interest in establishing and confirming such levels of uncertainties at the NMI level. We are not aware of any inter-laboratory comparison which was able to demonstrate uniformity and equivalence (or lack thereof) of local radiance temperature scales at the required level of 0.1 ̊ C or better.

The main objective of this comparison is to establish uniformity and equivalence of local scales of radiance temperature at temperatures from -40 ˚C to 300 ˚C with a goal of achieving uncertainties appropriate to such applications as climate monitoring, demanding uncertainties at the level of 0.1 ̊ C (2 sigma) for the near-ambient range.

Successful implementation of the current comparison may help us to proceed with: - a full-fledged comparison of spectral radiance in the thermal IR; - a radiance temperature comparison under thermovacuum

and controlled background conditions such as used to calibrate spaceborne instrumentation

in the atmospheric transmission bands from 3 to 5 and 8 to 14 μm μm

Goals and objectives

COMPARISONS NIST / NRC / NPL / PTB

Transfer Standard -40 -20 0 15 35 50 100 150 200 250 300

Raytek TRT LT X X X X X X X X X X X

SR-800 BB X X X X X

HART-9132 BB X X X X X

Thermal IR Scales – International Comparisons

AIRI Near Ambient Scale Realization - Summary

• A systematic realization of IR spectral radiance and radiance temperature scales has been performed.

• Implemented AIRI capabilities include: – characterization in absolute spectral radiance and

radiance temperature across the spectral range 2.5 µm to 13.5 µm and temperature range - 50 °C to 150 °C with typical comparison uncertainty 25 to 50 mK (k=2);

– characterization of spatial uniformity and stability;– background radiation correction and emissivity

(reflectance) evaluation for flat plate calibrators.

• Internal and international comparisons are being conducted to validate the uncertainties

What AIRI Has to, but Cannot Do?

- Measure outside atmospheric transmission windows

- Calibrate targets at set points and radiation backgrounds below 18 °C

- Calibrate thermal IR targets which are designed to operate in vacuum

- Expand to far-IR spectral range

Simplified Conceptual Design

Configuration 3

Spectral Directional Emissivity-55 °C to 250 °C

Controlled Background Center-mount Sphere

Reflectometer

Sample

Reference BB 2

Reference BB 1

Reference BB 2

Reference BB 1 Optical

Interface Unit

FTIRSpectrometer on a Rotating

Platform

INFRARED MATERIALS CHARACTERIZATION

Configuration 2

Radiance Temperature -55 °C to 250 °C

CustomerPyrometer/Radiometer

RADIOMETER CALIBRATION

Configuration 1

Spectral Radiance / Emissivity -55 C to 250 °C°

Optical Interface

Unit

BLACKBODYCALIBRATION

Customer BB

FTIRSpectrometer on a Rotating

Platform

Temperature Controlled Shroud (-70 to +50 )° °C C

Temperature Controlled Shroud (-70 to +50 )° °C C

ReferenceBB

Proposed Solution

Top View

Some details of design of the proposed multipurpose controlled background vacuum emissometer

CBS3 Concept

Chamber Hardware

Chamber Specifications:

- 4’ x 4’ x 8’ box with two 4’ x 4’ doors

- Modular temperature controlled shroud, 200 K to 350 K

Anticipated CBS-3 Role in CLARREO Traceability

Summary• Proven technical solutions and experience with IR spectral radiance

realization at the AIRI facility demonstrated feasibility of building an environment–controlled extension – CBS3 facility.

• Along with continued support to NIST TXR and existing NASA customers,

CBS-3 will support spectrally resolved radiance temperature measurements for targets at 190 K to 520 K at backgrounds from 190 K to 320 K across spectral band from 2.5 µm to 100 µm.

• It is anticipated that after moderate modifications, existing primary BB sources will be shared between the AIRI and a new CBS3 facility.

• CBS-3 will also accommodate a novel variable angle emissometer / reflectometer for characterization of materials and coatings over the aforementioned spectral and temperature ranges.

• The intended range and uncertainties of the spectral radiance / radiance temperature and material characterization capabilities make CBS-3 facility a cornerstone of the NIST proposed strategy for establishing nationwide traceability of IR remote sensing measurements, including climate change studies such as proposed CLARREO mission.