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Radiometry, apparent optical properties,

measurements & uncertainties

Lecture 1:

Introduction to traceable radiometric measurements

Agnieszka Bialek

IOCCG 2016 SLS, 26th July, Villefranche-sur-Mer

Who am I?

Physicist

MSc and Engineer Degree in Technical Physics (2003, Wroclaw

University of Technology, Poland)

Mertologist

10 years experience at NPL

Focus on Earth Observation – Quality Assurance –

Vicarious Calibration test sites in particular

Part time PhD student University of Surrey, UK

Visiting researcher at LOV

Where does Ocean Colour measurement

start?

Picture courtesy of ESA

Picture courtesy of NASA

Picture courtesy of NIST

Context

Satellite products quality assurance

post launch Vicarious calibration

System vicarious calibration

Cal/Val activities

BOUSSOLE

AERONET-OC

AAOT

RadCalNet – Railroad Valley

Outline

SI System of Units and NMIs role

Principle of radiometric measurement

Radiometers and calibration

Famous quotes…

William Thomson,

Lord Kelvin of Largs (1824 - 1907)

When you can measure what you are

speaking about, and express it in numbers,

you know something about it; but when

you cannot measure it, when you cannot

express it in numbers, your knowledge is

of a meagre and unsatisfactory kind: it may

be the beginning of knowledge, but you

have scarcely, in your thoughts, advanced

to the stage of science.

Higgs boson – 1960 idea

CERN 2013 – experiment (tentatively confirmed)

Metrology

http://jcgm.bipm.org/vim/en/index.html

International System of Units

Pictures in courtesy of BIPM

The Convention of the

Metre:

Created BIPM the intergovernmental organization through which

Member States act together

on matters related to measurement science and

measurement standards.

First signed in 1837 in

Paris by 17 nations

Now 58 countries

members states and 40

associate

http://www.bipm.org/en/about-us/

International System of units

Units definitions

http://www.bipm.org/en/measurement-units/

Traceability

Calibration must be linked to accepted national standards, via an

unbroken chain of calibrations, preferably carried out by an approved

calibration laboratory

Accredited Calibration

Laboratories

auditingprocedures

Transfer

standards

calibration

INDUSTRY

Accredited Calibration

Laboratories

auditingprocedures

Transfer

standards

calibration

INDUSTRY

EUROMET

Regional

comparisons

CONVENTION OF THE METRE

Key comparison of primary unit

National Metrology InstitutesSIM ASIA/

PACIFIC

EUROMET

Regional

comparisons

CONVENTION OF THE METRE

Key comparison of primary unit

National Metrology InstitutesSIM ASIA/

PACIFIC

Ensures compatibility with other instruments

Ensures consistency of measurements over time

Ensures measurement uncertainty is properly evaluated

Measurement

Measurement – “process of experimentally obtaining one or more

quantity values that can reasonably be attributed to

a quantity”

Measurand – “quantity intended to be measured”

Quantity – “property of a phenomenon, body or substance,

where the property has a magnitude that can be

expressed by a number and a reference”

Measurement result – “set of quantity values being attributed to a

measurand together with any other available

relevant information”

Quantity value – “number and reference together

expressing magnitude of a quantity”

http://jcgm.bipm.org/vim/en/index.html

Traceability: why do we need it?

By linking back to a primary standard we provide a reference

for our measurements

This reference is (ideally) non-changing e.g. based on a

fundamental constant of nature (e.g. Boltzmann constant)

Therefore our measurements will be reliable and reproducible

in time (over decades)

SICoherent Stable

• Coherent: only conversion factor used is 1 so it doesn’t

matter how you get to a result, you’ll get the same answer

(i.e. a watt is 1 J per 1 second, but also 1 kg per 1 m2 per 1

s3 – so if you measure it optically, electrically, thermally, …

it’s still a watt)

Optical power = Po

Electrical Heater Power = PEAbsorbing black

coatingCopper disk

When thermometer

temperature T=To=TE then

Po=PE

Cryogenic cooler

Principle of Cryogenic radiometryAbsorbing cavity

(~ 0.99999)

Cooling improves

sensitivity by 1000 X

Electrical Substitution Radiometry – a

100 yr old technology

Satellite

Earth Imager

Spectral Radiance and Irradiance

Traceability

Cryogenic

radiometer 0.01 %

Standard Lamp

~0.7%

Filter Radiometer

~0.35 %

Black Body ~0.5 %

Reference

photodiode 0.1%

NIST laser based system

Brown, 2006Uncertainty ~0.2 %

Radiometry

Measurement of optical energy

The Art of Radiometry, Palmer J.M. 2010

Inverse Square Law of Irradiance

𝜔

I Intensity

W sr-1

A A

E Irradiance

W m-2

Palmer J.M, 2010

Radiance invariance

Throughput invariance (étendue) 𝑇 = 𝐴Ω

Assuming lossless beam

propagation and no lens transmissionPalmer J.M, 2010

“No ice cream cones” in

radiometry

Palmer J.M, 2010

Basic radiance

Snell’s law

𝐿1𝑛1−2 = 𝐿2𝑛2

−2

Invariant across the lossless boundary

𝑛1sin𝜃1=𝑛2sin𝜃2

𝑛2 law of radiance

Zibordi &Voss, 2010

Palmer J.M, 2010

Radiometric properties of

materials

Reflectance, Transmittance, Absorptance and Emittance

Palmer J.M, 2010

𝜌, 𝜏, 𝛼, 𝜀

Radiometric properties of

materials

Basic relationships

Energy conservation

Basic assumption – absence of wavelength shifting effects ! (no

Raman scattering, no luminescence)

Kirchhoff law (at equilibrium)

Reflectance 𝝆and Reflectance

factors

We have 9 kinds of reflectance and 9 equivalent reflectance

factors.

First defined in 1977 by Nicodemous to simply surface

scattering phenomena,

Assumptions:

Geometrical ray optics

A flat surface that is uniformly illuminated

Incident radiance depends only on direction

Surface has uniform and isotropic scattering properties

Nicodemous, 1977

BRDF bidirectional reflectance

function

R I I R R

I I R R I I R R0

I R

( , , , , )( , , , , ) ( , , , , ) lim

( ) cosr

BRDF

f d

R I I R R

I I R R

I I I

, ; , , ,( , ; , , )

,

( )

( )

IdL Ef

dE

Definition

Measurement equation

BRF bidirectional reflectance

factor

R I R

0I R

( , , )lim

( ) cos; ,i rBRF BRDF

the ratio of the radiance flux actually reflected by a sample surface

to that which would be reflected into the same reflected-beam

geometry by an ideal perfectly diffuse standard surface irradiated in

exactly the same way as the sample

Perfect “Lambertian”

diffuser

Reflects all radiance equally to all directions

𝜌 = 1

𝐵𝑅𝐷𝐹(𝜃𝑖 , 𝜙𝑖 , 𝜃𝑟 , 𝜙𝑟) = 1/𝜋

Lambertian source, radiance is independent of direction

𝐿 𝜃, 𝜙 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡

𝐼𝑆 𝜃 = 𝐼𝐼cos(𝜃𝑠)

Reflectance configurations

From: Schaepman-Strub at all 2006

Reflectance scale

Reference reflectometer

Williams, 1999

Radiometers

MultispectralStable and reliable

Limited spectral information

HyperspectralDemanding characterisation necessary !

“Full” spectral information

Ehsani et al. 1998

Silicon detector

http://www.hamamatsu.com/resources/pdf/ssd/s12698_series_kspd1084e.pdf/

InGaAs detector

http://www.hamamatsu.com/resources/pdf/ssd/s12698_series_kspd1084e.pdf/

Calibration

“operation that, under specified conditions, in a first step,

establishes a relation between the quantity values with

measurement uncertainties provided by measurement

standards and corresponding indications with associated

measurement uncertainties and, in a second step, uses this

information to establish a relation for obtaining a

measurement result from an indication.”

http://jcgm.bipm.org/vim/en/index.html

Irradiance standards

Lamps tungsten-halogen lamp ( FEL) 1 kW (~ 3000 K)

Typical FEL irradiance

Calibration Certificate example

Irradiance

Irradiance

𝐸 𝜆, 𝑑 = 𝐸 𝜆, 500 mm500 mm

𝑑

2

Radiance standards

Lamp –reflectance standard

Integrating sphere

Reflectance Standard

Spectralon® Diffuse Reflectance Standard

https://www.labsphere.com/site/assets/files/1828/spectralon_targets.pdf

Spectralon BRF

Yoon et al. 2009

Radiance

Lamp

Radiometer

ShieldsReflectance tile

45°

Lamp - tile

2

0:45

2

FEL cals

use

E dL

d

Radiance

Integrating sphere

Calibration certificate example

Straight-line calibration function

AERONET radiance calibration uses sphere with

5 different radiance levels

ISO/TS 28037

http://www.npl.co.uk/science-technology/mathematics-modelling-and-simulation/products-and-services/software-downloads

ACCURACY VS COST AND

TIME REQUIRED

Calibration fit for purpose

Above water radiometer system

uncertainty

G. Zibordi et al., “AERONET-OC: A Network of the Validation of Ocean Color Primary

Products”, Journal of Atmospheric and Oceanic Technology, 2009, vol. 26

Target 3%

Calibration fit for purposeAbove water system uncertainties

Source 𝐋𝐖𝐍

443 667 443 667 443 667 443 667 443 667

Absolute calibration 2.7 2.7 1.4 1.4 0.7 0.7 0.3 0.3 0.0 0.0

Sensitivity Change 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

Correction 2 1.9 2 1.9 2 1.9 2 1.9 2 1.9

𝐭𝐝 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5

𝛒 1.3 2.5 1.3 2.5 1.3 2.5 1.3 2.5 1.3 2.5

𝐖 0.8 0.4 0.8 0.4 0.8 0.4 0.8 0.4 0.8 0.4

Environmental

effects 2.1 6.4 2.1 6.4 2.1 6.4 2.1 6.4 2.1 6.4

Quadrature sum 4.5 7.8 3.9 7.4 3.7 7.3 3.6 7.3 3.6 7.3

1.05 3.2

3.4 4.9

VALIDATION

MNIs recommend inter-comparison to ensure and validate the

calibration measurements and its uncertainties.

In- situ inter- comparisonZibordi et al. 2012

Reference sensor

Comparison results

BOUSSOLE EXAMPLE

Quality check – Sun inter-comparison

• A fine example.

QA/QC: intercalibration before deployment

V. Vellucci

• A bad example.• Instrument sent back to factory for verification: collector replacement and recalibration.

QA/QC: intercalibration before deployment

V. Vellucci

Why do we have these problems?

Due to other instrument characteristics.

Stray light, temperature, linearity, cosine response,

immersion coefficient

REMEMBER!

Calibration is valid only under specified conditions, (during

calibration)

Example results

Stability

Example results

Linearity

Example results

Stray Light

Supercontinuum laser

Measurement set up

Tuneable Filter

Integrating

sphere

Instru

ment

Aperture

Order sorting

filter used

above 800nm

Example results

Cosine response

In situ measurementsAbove water

Johnson et al. 2015

In situ measurementsIn water

Buoy system

Profilers

ProVal

BOUSSOLE

In situ measurements

In water and above water system require IOPs to derive

water leaving radiance

Not all measurements are SI

traceable

In the absence of SI traceability the community agreed

standard is used.

Such measurements are still done to the very high

standards and accuracy,

Example:

Mauna Loa Observatory in Hawaii (calibration of Direct

Sun measurements by comparison to AERONET “master”

instrument )

Summary Metrology view

SI traceability – ensures the valid measurements

Calibration link instrument output readings to physical

values

SI traceability especially important to radiometry, as these

measurements are used calibration and validation of

satellite sensors

Instruments characteristics influence theirs properties and

performance

Thank you

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