Radiometry, apparent optical properties, measurements & uncertainties

Transcription

1 Radiometry, apparent optical properties, measurements & uncertainties Lecture 1: Introduction to traceable radiometric measurements Agnieszka Bialek IOCCG 2016 SLS, 26 th July, Villefranche-sur-Mer Queen's Printer and Controller of HMSO, 2016

2 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

3 Where does Ocean Colour measurement start? Picture courtesy of ESA Picture courtesy of NIST Picture courtesy of NASA

4 Context Satellite products quality assurance post launch Vicarious calibration RadCalNet Railroad Valley System vicarious calibration BOUSSOLE Cal/Val activities AERONET-OC AAOT

5 Outline SI System of Units and NMIs role Principle of radiometric measurement Radiometers and calibration

6 Famous quotes 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. William Thomson, Lord Kelvin of Largs ( ) Higgs boson 1960 idea CERN 2013 experiment (tentatively confirmed)

7 Metrology

8 International System of Units 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 Pictures in courtesy of BIPM

9 International System of units

10 Units definitions

11 Traceability Calibration must be linked to accepted national standards, via an unbroken chain of calibrations, preferably carried out by an approved calibration laboratory SIM CONVENTION OF THE METRE Key comparison of primary unit National Metrology Institutes ASIA/ PACIFIC auditing EUROMET Regional comparisons Transfer standards procedures Accredited Calibration Laboratories calibration Ensures compatibility with other instruments Ensures consistency of measurements over time Ensures measurement uncertainty is properly evaluated INDUSTRY

12 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

13 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) SI Coherent 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 m 2 per 1 s 3 so if you measure it optically, electrically, thermally, it s still a watt)

14 Electrical Substitution Radiometry a 100 yr old technology When thermometer temperature T=T o =T E then P o =P E Optical power = P o Absorbing black coating Copper disk Electrical Heater Power = P E Cryogenic cooler Absorbing cavity (~ ) Cooling improves sensitivity by 1000 X Principle of Cryogenic radiometry

15 Spectral Radiance and Irradiance Satellite Earth Imager

16 Traceability Cryogenic radiometer 0.01 % Reference photodiode 0.1% Filter Radiometer ~0.35 % Black Body ~0.5 % Standard Lamp ~0.7%

17 NIST laser based system Uncertainty ~0.2 % Brown, 2006

18 Radiometry Measurement of optical energy The Art of Radiometry, Palmer J.M. 2010

19 Inverse Square Law of Irradiance Palmer J.M, 2010 E Irradiance W m -2 I Intensity W sr -1 A A ω

20 Radiance invariance Throughput invariance (étendue) T = AΩ Palmer J.M, 2010 Assuming lossless beam propagation and no lens transmission

21 No ice cream cones in radiometry Palmer J.M, 2010

22 Basic radiance Snell s law n 1 sinθ 1 =n 2 sinθ 2 L 1 n 1 2 = L 2 n 2 2 Invariant across the lossless boundary n 2 law of radiance Zibordi &Voss, 2010 Palmer J.M, 2010

23 Radiometric properties of materials Palmer J.M, 2010 Reflectance, Transmittance, Absorptance and Emittance ρ, τ, α, ε

24 Radiometric properties of materials Basic relationships Energy conservation Basic assumption absence of wavelength shifting effects! (no Raman scattering, no luminescence) Kirchhoff law (at equilibrium)

25 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

26 BRDF bidirectional reflectance function Definition f (, ;,, ) I I R R ( ) de (, ) dl, ;,,, E R I I R R I I I I Measurement equation BRDF f r (,,,, ) d (,,,, ) lim I I R R I I R R 0 (,,,, ) R I I R R I ( ) cos R

27 BRF bidirectional reflectance factor BRF R I R lim 0 I (,, ) ;, BRDF i r ( ) cos R 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

28 Perfect Lambertian diffuser Reflects all radiance equally to all directions ρ = 1 BRDF(θ i, φ i, θ r, φ r ) = 1/π Lambertian source, radiance is independent of direction L θ, φ = constant I S θ = I I cos(θ s )

29 Reflectance configurations From: Schaepman-Strub at all 2006

30 Reflectance scale Reference reflectometer Williams, 1999

31 Radiometers Multispectral Stable and reliable Limited spectral information Hyperspectral Demanding characterisation necessary! Full spectral information Ehsani et al. 1998

32 Silicon detector

33 InGaAs detector

34 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.

35 Irradiance standards Lamps tungsten-halogen lamp ( FEL) 1 kw (~ 3000 K)

36 Typical FEL irradiance

37 Calibration Certificate example

38 Irradiance

39 Irradiance E λ, d = E λ, 500 mm 500 mm d 2

40 Radiance standards Lamp reflectance standard Integrating sphere

41 Reflectance Standard Spectralon Diffuse Reflectance Standard

42 Spectralon BRF Yoon et al. 2009

43 Radiance Radiometer Lamp - tile 45 Lamp Reflectance tile Shields L s E FEL 0:45 d d 2 cal 2 use

44 Radiance Integrating sphere

45 Calibration certificate example

46 Straight-line calibration function AERONET radiance calibration uses sphere with 5 different radiance levels ISO/TS

47 Calibration fit for purpose ACCURACY VS COST AND TIME REQUIRED

48 Above water radiometer system uncertainty Target 3% 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

49 Calibration fit for purpose Above water system uncertainties Source L WN Absolute calibration Sensitivity Change Correction t d ρ W Environmental effects Quadrature sum

50 MNIs recommend inter-comparison to ensure and validate the calibration measurements and its uncertainties. VALIDATION

51 In- situ inter- comparison Zibordi et al Reference sensor Comparison results

52 Quality check Sun inter-comparison BOUSSOLE EXAMPLE

53 QA/QC: intercalibration before deployment A fine example. V. Vellucci

54 QA/QC: intercalibration before deployment A bad example. Instrument sent back to factory for verification: collector replacement and recalibration. V. Vellucci

55 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)

56 Example results Stability

57 Example results Linearity

58 Example results Stray Light Measurement set up Supercontinuum laser Tuneable Filter Aperture Integrating sphere Order sorting filter used above 800nm Instrument

59 Example results Cosine response

60 In situ measurements Above water Johnson et al. 2015

61 In situ measurements In water Profilers BOUSSOLE ProVal Buoy system

62 In situ measurements In water and above water system require IOPs to derive water leaving radiance

63 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 )

64 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

65 Thank you The National Physical Laboratory is operated by NPL Management Ltd, a whollyowned company of the Department for Business, Innovation and Skills (BIS).