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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
2
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
ASTRA 2013
3
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
4
Generating Rock Spectra AMASE 2009, BOCK01 Site
using calibration test target
(Arctic Mars Analogue Svalbard Expedition – AMASE)
ASTRA 2013
5
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
6
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
ASTRA 2013
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?
ASTRA 2013 7
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
ASTRA 2013 8
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.
ASTRA 2013 9
(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
ASTRA 2013 10
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
ASTRA 2013 11
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
ASTRA 2013 12
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.
ASTRA 2013 13
14
Example of natural-colour Mars image:
“Heat Shield Rock” processed with illuminant M54
Use CIE reflective case:
Mars illuminant
included
Mars illuminant application
ASTRA 2013
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)
ASTRA 2013 15
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
ASTRA 2013 17
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
ASTRA 2013 18
19
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