The document discusses spectral printing versus colorimetric printing. Spectral printing aims to match the spectral properties of an original under any light source, while colorimetric printing matches color under a single light source. The document evaluates the benefits of spectral printing using a metric called MIPE that assesses color differences under multiple light sources. It finds that for spot colors and fine art prints on glossy paper, spectral printing significantly reduces color differences, but for other cases colorimetric printing performs nearly as well. It concludes that while spectral printing provides clear benefits in some specialty applications, it may not replace colorimetric printing in general anytime soon due to technical challenges.

1. Revisiting Spectral Printing:
A Data Driven Approach
P. Morovič, J. Morovič, J. Arnabat and J. M. García–Reyero
Hewlett Packard Company, Sant Cugat del Vallés, Catalonia, Spain
© Copyright 2012 Hewlett-Packard Development Company, L.P. The information contained herein is subject to change without notice.

2. Q: Is the extra effort
of spectral printing
worth it in practice?
- What are the benefits of spectral reproduction?
- What are the spectral constraints of multi-ink printing systems?
- How can we tell whether one spectral reproduction is better than another?
- How do the spectral matches of a spectral and colorimetric reproduction compare?
© Copyright 2012 Hewlett-Packard Development Company, L.P. The information contained herein is subject to change without notice.

3. Spectral printing: what are we all waiting for?
Spectral printing has clear benefits!
Light source 1
- Match original under any light source
- Match for all observers (human, animal)
- à reproduction changes with illumination
in the same way as the original does, for all observers O
P
(‘I can see what the Mona Lisa would look like in my living room’)
What is needed?
- Spectral content (little point in matching RGBs)
Light source 2
Light source 2
- Multi-ink printers (metamerism is needed)
Light source 2
- Spectral control of output Light source 2
(print properties need to be chosen based on expected spectra)
O
P
- Printers that span sufficient spectral gamut
(on a chosen substrate) to expected content
3
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4. Metameric/Colorimetric printing: where are we today?
What are colorimetric matches like?
Light source 1
- Designed to match color under single light source (typically D50)
- In high-end printers accurate to within noticeable differences*
(< 1DE)
O
P
- Repeatable, consistent (via calibration to keep systems stable)
BUT:
?
- Even under chosen light source there are color differences
Light source 2
Light source 2
- Even under other light sources there are (uncontrolled) color Light source 2
Light source 2
differences
P
- Assumption: for a spectral print, color differences under variety
O
P
of illuminants are lower than for colorimetric (metameric) print
* for in-gamut colors – same applies in spectral case
4
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5. How good is a print spectrally?
© Copyright 2012 Hewlett-Packard Development Company, L.P. The information contained herein is subject to change without notice.

6. The spectral use case
Many metrics defined in the literature
- Purely spectral, taking into account CMFs, focusing on metamerism between specific light
sources, looking at sets of color difference statistics per light source + combinations of these
BUT: how does an observer experience the goodness of a spectral match?
- Original – print ß observer
- Observer views original-print-pair under many light sources
(not arbitrary à database of measured lights)
- Under each light source they see certain levels of color difference
(à most accurately predicted by ∆E2000, with JND units)
- Surveying their experiences from under multiple light sources gives rise to
a variety of color difference magnitudes (à choice of relevant statistics)
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© Copyright 2012 Hewlett-Packard Development Company, L.P. The information contained herein is subject to change without notice.

7. MIPE: an experience–based metric
Color difference
with visually meaningful
units (~JND)
Paramers
(>0 ∆E even under
‘reference’ illuminant)
Non-parametric descriptive statistics
Median:
how close a match can be
expected for an arbitrary,
but realistic, light source
95th percentile – median: how much
this match varies
Light sources / illuminants
CIE standard and recommended illuminants Maximum:
how far apart the two can
+ measured light sources
get at worst
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8. How good a spectral match is a
metameric print?
© Copyright 2012 Hewlett-Packard Development Company, L.P. The information contained herein is subject to change without notice.

9. Test setup
matte
Printer: HP Designjet Z3100
- 10 inks: cMmYnNKRGB + glossy/matte black + gloss enhancer
- substrates: Hahnemühle Smooth Fine Art (matte), HP Premium
Instant-dry Photo (glossy)
- BUT: inks not optimized for spectral reproduction!
Measurement
- XRite i1 spectrophotometer, 400 nm to 700 nm at 10 nm intervals, glossy
45°/0° geometry
Original data sets
- Spot color: PANTONEs on three substrates: uncoated, matte and
coated, 1224 patches per substrate (3672 total samples; mixtures of
Pantone system’s 15 base inks)
- Fine art: 1168 measurements taken from multiple paintings
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10. How does MIPE relate to spectral dimensionality?
Glossy print spectra"
40"
35"
Degree of spectral
30" variance accounted for
does not relate well to
25"
MIPE (∆E2000)"
N=3, 99.83%, 24.4∆E" visual consequences.
20"
N=4, 99.92%, 17.6∆E"
15" 99.8% coverage still has
99.95, 11.3" 95th percentile MIPE of
10"
Median" 5 ∆E2000 and max. of
5" 95th percentile" 99.97, 5.9" 24 ∆E2000.
99.99, 3.1"
100, 2.4"
Maximum" 100, 1.4"
0"
97" 98" 99" 100"
% spectral variance accounted for"
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11. Results: colorimetrically derived print
How good a spectral match is the result of matching colorimetry?
Colorimetric ‘goodness’
Spectral ‘goodness’
- Broadly similar performance under D50 (for which colorimetry was matched) and all other
172 illuminants (color gamut differences a big contributor already)
- MIPE errors higher than D50 ∆Es (sanity check for maximum)
- Gap between MIPE and D50 ∆Es indicates room for improvement from spectral matching
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12. How about a real spectral print?
© Copyright 2012 Hewlett-Packard Development Company, L.P. The information contained herein is subject to change without notice.

13. Spectral printer control
0.9
0.8
matte
0.7
0.6
HANS
0.5
CIE y
- Take advantage of greater solution space with HANS 0.4
(kn linear v. n non-linear)
0.3
- Yule-Nielsen modified CIE XYZ space for convexity
0.2
0.1
- Full convex hull of Neugebauer Primaries (in YNN) – 0
i.e. set of arbitrary within-ink-limit convex combinations,
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
CIE x
0.9
is addressable à printable spectra
0.8
glossy
0.7
0.6
Simulation
0.5
CIE y
- In spectral-gamut samples: zero error
0.4
- Out-of-spectral-gamut samples: spectral gamut mapping (min. 0.3
Euclidean distance to YNN convex hull)
0.2
0.1
- Original reflectance à linear model basis à convex hull à MIPE
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
CIE x
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14. Results: bases and projections
1
PCA Basis 1
0.9 PCA Basis 2
PCA Basis 3
337 OOG
PCA Basis 4
0.8
3D
More color gamut ≠
PCA Basis 5
PCA Basis 6
0.7
greater spectral
relative reflectance (%)
matte
0.6
coverage at same
0.5
0.4
0.3 number of bases.
0.2
0.1
Greater spectral variety
0
400 450 500 550 600 650 700
wavelength (nm)
1
0.9
PCA Basis 1
PCA Basis 2
PCA Basis 3 483 OOG
à more bases needed
to characterize to same
PCA Basis 4
3D
0.8
PCA Basis 5
PCA Basis 6
level
0.7
relative reflectance (%)
glossy
0.6
0.5
0.4
0.3
Not same 3 bases!
0.2
0.1
0
400 450 500 550 600 650 700
wavelength (nm)
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15. Results: MIPE
Some maxima
Gamut mismatch is increased (MIPE v.
Increasing limit – improvements YNN min. dist.)
similarity
in-gamut
Spot colors - spectral
Fine art - spectral
Decreasing
gamut coverage
Colorimetric reproduction
Error reduced to
between quarter
and half
(BUT: repeatability)
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16. Conclusions
Spectral printing has clear benefits in some cases (e.g., Fine art on glossy), but:
- Spectral matching of colorimetric prints is often as good as or close to it (e.g., no benefit for
spot colors in this case)
- Maximizing the benefit of fully spectral printing requires highly accurate matching and high
repeatability
- Further improvements could be expected from ink–sets optimized for spectral printing
(although constrained options using manufacturable materials)
- Biggest benefits will be in specialty applications (e.g., Hersch ’11 security printing)
Is spectral going to replace colorimetric printing? Not anytime soon …
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17. Acknowledgements
Carlos Amselem
Óscar Martinez
África Real
Rafael Giménez
Johan Lammens
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18. Thank you!
© Copyright 2012 Hewlett-Packard Development Company, L.P. The information contained herein is subject to change without notice.