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Mars Rover Colour Vision:

Generating the true colours of Mars

Dave Barnes

Head of Space Robotics

Aberystwyth University, Wales, UK

ASTRA 2013

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Contents

• Previous work – RCIPP

• The Mars White Point Problem

• Judd and the CIE Standard illuminants

• Creating Mars ‘standard’ illuminants

• Application of Mars illuminants:

Creating Mars natural-colour images

Chromatic adaptation examples

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Baseline Radiometric and Colourimetric Image Processing Pipeline (RCIPP) Developed

raw image

data

compress

uncompress flat/dark

correction

camera

response

correction

relative

reflectance CIE XYZ

CIE XYZ to

sRGB etc PanCam

image

PanCam

spectrum reflectance

ROI select

flats/darks

(temp,exp)

PCT

BRDF

data

ESA PSA

PDS format

R*

data

CIE

XYZ

data

SOC Operations

Surface Operations

camera radiometric correction

pixel → spectral radiance

(W∙sr-1∙m-2∙nm-1)

image radiometric and

colourimetric processing

ESA PSA

PDS format

Rover with

PanCam

and PCT

Mathcad

Java

LabVIEW

Presented at ASTRA 2011

ASTRA 2013

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Generating Rock Spectra AMASE 2009, BOCK01 Site

using calibration test target

(Arctic Mars Analogue Svalbard Expedition – AMASE)

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440 nm 560 nm

470 nm

510 nm

600 nm

660 nm

Narrow-band (10 nm) filtered input images

Image Natural Colour Correction

Output colour corrected image

in sRGB format

Clarach Bay 2010

ASTRA 2013

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The Mars White Point Problem

•Surface colour is dependent upon both surface

reflectance/absorption properties AND the surface

illumination.

•Getting the surface solar spectrum irradiance correct is

vital for planetary colourimetry.

•Associated with a solar spectral power distribution

(SPD) is its correlated colour temperature (CCT) which

determines the reference white point.

•If the CCT is wrong, then the white point will be wrong,

and hence the colours will be wrong.

•We need to be able to compute ‘typical’ Mars solar SPDs

based upon actual or desired CCT.

• We need Mars (standard) illuminants

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The Problems of Mars Surface Solar Irradiance Spectra for

Radiometric and Colourimetric Image Processing Typically past missions have applied a number of fixes to overcome this

problem. For example:

• Use AM0 solar spectrum and attenuate to mean Mars heliocentric distance, then

add attenuation factors to represent Mars atmospheric effects – not a good

solution!

• For Viking Landers a terrestrial standard illuminant C (6774K) was used – no

wonder Viking Landers had colour problems!

• MER processing has employed a constant correction using “… average bright

regions on Mars …” and referenced Maki 1999 Pathfinder IMP paper. These “…

average bright regions …” are not necessarily ‘typical’ and no spectra details are

given. Reference white point unknown.

As all science target reflectance spectra and natural-colour image products

are entirely dependent upon surface solar irradiance spectra, we need a better

solution – what can terrestrial solutions tell us?

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Example Standard CIE illuminants – Can the same be done for Mars?

Infinite number of daylight illumination conditions.

The use of ‘standards’ revolutionised colourimetry

(CIE – International Commission on Illumination)

References

Image courtesy

W. C. Lemons

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Judd et al 1964 chromaticity of daylight compared to the Planckian Locus

622 daylight solar

spectra measured at

different times of day,

different seasons,

different locations,

different weather

conditions

All spectra processed

using PCA to find

‘characteristic vectors’

Generated ‘typical’ daylight solar spectra as

a function of correlated colour temperature,

e.g.D65 (6500K), D75 (7500K) etc.

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(Between 4000K and 7000K)

(Above 7000K)

Mean SPD (S0) and two characteristic

vectors S1 and S2 computed from all samples Resultant daylight illuminant

The Correlated Colour Temperature (CCT) determines the reference white point

1.

2.

3.

4.

CIE (Judd)

Formulae

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Obtain Mars surface solar spectra by processing in situ calibration targets.

Currently working on Mars Pathfinder, and Phoenix. Include MSL when available.

Apply PCA to

derive

Characteristic

Vectors

400 450 500 550 600 650 700 750 800600

400

200

0

200

400

600

800

CV1

CV2

CV3

CV4

Zero

CV1

CV2

CV3

CV4

Zero

Simonds Characteristics Vectors for Mars Sol-light

Wavelength

Co

mp

on

ents

of

Mar

s S

ol-l

igh

t D

istr

ibu

tio

n

Left

Right

400 450 500 550 600 650 700 750 800

400

500

600

700

800

900

1000

1100

1200

Example 560 nm Normalised Mars Solar Spectra Data

Wavenumber

Irra

dia

nce

1. Sample target (W & LG)

2. Determine spectral

irradiance from

radiance and pre-

launch reflectance

3. Fit cubic-spline

4. Normalise to 560nm

5. Repeat for 1st 50 sols

of data for all

missions

Example spectra

Currently only 50

spectra obtained

MER Example

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Obtained Mars surface solar spectra converted to CIE xy

coordinates and plotted on chromaticity diagram

Currently 50 Mars

solar SPDs obtained

representing:

Different sols,

Different times,

Different locations,

Different atmospheric

and dust conditions

Green

side

Purple

side

0.29 0.3 0.31 0.32 0.33 0.34 0.35 0.36 0.37 0.38 0.39

0.26

0.28

0.3

0.32

0.34

0.36

0.38

4200 K

4800 K

5500K

6500 K

Judd Chroma Locus

Planckian Locus

Mars Chroma Locus

IsoT-42

IsoT-48

IsoT55

IsoT-65

IsoT-75

Illuminant E

Spirit C. Temp.

Opp C. Temp.

Phx C. Temp.

4200 K

4800 K

5500K

6500 K

Judd Chroma Locus

Planckian Locus

Mars Chroma Locus

IsoT-42

IsoT-48

IsoT55

IsoT-65

IsoT-75

Illuminant E

Spirit C. Temp.

Opp C. Temp.

Phx C. Temp.

ALL Mars solar spectra chromaticity versus Planckian locus and Chromaticity Loci

CIE x

CIE

y

Note Spirit/Opportunity

difference

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Example derived Mars sol-light illuminants – Em()

400 450 500 550 600 650 700 750 8000.2

0.4

0.6

0.8

1

1.2

D65

M46

M48

M50

M52

M54

E

D65

M46

M48

M50

M52

M54

E

Wavelength (nm)

Rel

ativ

e Spec

tral

Irr

adia

nce

(no u

nit

s)

Using Mars chromaticity data,

xm and ym formulae derived

for specified CCT

Example xm and ym results

Example M1, M2, and M3 results

M1, M2, and M3 formulae

derived incorporating

xm , ym and zm

Using first 3 characteristic vectors (xm + ym + zm = 1) 1. 2. and

3.

4.

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Example of natural-colour Mars image:

“Heat Shield Rock” processed with illuminant M54

Use CIE reflective case:

Mars illuminant

included

Mars illuminant application

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Mars under illuminant M48

Because we know the illuminant we can use chromatic adaptation:

The scene illumination can be transformed to a

different reference white point

400 450 500 550 600 650 700 750 8000

1

2

Illuminant_Discrete

Illuminant_Continuous

Illuminant_E

Illuminant_Discrete

Illuminant_Continuous

Illuminant_E

Relative SPD for Illuminant

Wavelength (nm)

Rel

ativ

e SP

D (

no u

nit

s)

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400 450 500 550 600 650 700 750 8000

1

2

Illuminant_Discrete

Illuminant_Continuous

Illuminant_E

Illuminant_Discrete

Illuminant_Continuous

Illuminant_E

Relative SPD for Illuminant

Wavelength (nm)

Rel

ativ

e SP

D (

no u

nit

s)

Mars under illuminant D65: Noon Daylight

Illuminant M48 to Illuminant D65 Chromatic Adaptation

Mars terrain

on Earth! ASTRA 2013 16

400 450 500 550 600 650 700 750 8000

1

2

Illuminant_Discrete

Illuminant_Continuous

Illuminant_E

Illuminant_Discrete

Illuminant_Continuous

Illuminant_E

Relative SPD for Illuminant

Wavelength (nm)

Rel

ativ

e SP

D (

no u

nit

s)

Mars under illuminant F2: Cool White Fluorescent

Illuminant M48 to Illuminant F2 Chromatic Adaptation

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400 450 500 550 600 650 700 750 8000

1

2

Illuminant_Discrete

Illuminant_Continuous

Illuminant_E

Illuminant_Discrete

Illuminant_Continuous

Illuminant_E

Relative SPD for Illuminant

Wavelength (nm)

Rel

ativ

e SP

D (

no u

nit

s)

Mars under illuminant A: Incandescent / Tungsten

Illuminant M48 to Illuminant A Chromatic Adaptation

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Conclusion • The CIE Standard illuminants work has been applied to Mars

• The process and required formulae have been produced to create

Mars ‘standard’ illuminants

• Mars illuminants have been applied to:

Creating Mars natural-colour images

Chromatic adaptation

• Only 50 Mars solar SPDs have been obtained so far, but the work is

ongoing to increase this number to several hundred (with MPF, PHX

and MSL data – when available)

• Full characteristic vector data and formulae will be published in the

future literature:

Barnes D.P. “Spectral distribution of typical Mars sol-light as a function

of correlated colour temperature and its application to Mars

colourimetry”. In preparation.

ASTRA 2013