This document summarizes research into using sub-wavelength gratings for large-gamut color and spectral displays. It describes how nanostructure gratings can select narrowband primaries from RGB backlights or a single white backlight. An optimization process selects combinations of primaries to maximize color gamut volume. Initial test results show the approach exceeds sRGB gamut and matches colors from a high-end display. While prototype efficiency is low, the technique offers potential for high-quality large gamut displays using existing manufacturing processes.

1. CIC24, November 7 – 11, 2016, San Diego, California
Large-gamut color
and spectral display
using sub-wavelength gratings
Peter Morovič†, Ján Morovič†,
Francesco Aieta‡, Marco Fiorentino¶,
Charles Santori§, David Fatal‡,
†HP Inc, Spain, ‡LEIA 3D,
¶Hewlett Packard Labs, §Verily

2. © Copyright 2016 HP Inc.
Outline
❖ Background
❖ Nanostructure gratings for displays
❖ Optimally selecting spectral primaries
❖ From RGB backlights
❖ From a white backlight
❖ The HANS imaging pipeline for displays
❖ Test setup & results
❖ Conclusions

3. © Copyright 2016 HP Inc.
Background
❖ Displays are used ubiquitously in creating and consuming digital content
❖ Windows to remote events
❖ Previewing objects before production
❖ Range of their colors and spectra is very important
❖ Manufacturing: potential to replace or significantly reduce the production of prototypes
❖ Entertainment: experiences that are more vivid, life-like, intense, and convincing
❖ As a component of an object (e.g., e-ink smart-shoes): overall consistency of appearance
❖ Current displays: white backlight + absorptive filters / colored backlights
❖ Achieves limited gamut ← filters relatively broadband, incapable of approaching
spectral locus
❖ Display with enhanced gamut → multiple narrowband backlights, but displays using
more than 3/4 primaries have prohibitive cost & size
❖ Multiple narrow band light sources (e.g., lasers (Brazas and Kowarz, 2004), quantum
dots (Kwak et al., 2012)) or notch filters, either absorptive or interference) → costly +
limited resolution imposed by the manufacturing and assembly of filters

4. © Copyright 2016 HP Inc.
Nanostructure gratings for displays
Spectral selection
related to period of
grating.
Diffuser extends
field of view.
Arbitrary number of
sub-pixels can be
defined, each with
different selectivity.
Gratings can be tiled,
manufactured using
large-scale
lithography.

5. © Copyright 2016 HP Inc.
Selecting filtered primaries: RGB backlights
Candidate LED spectra (normalized) Candidate LED spectra (absolute lumens)

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Selecting filtered primaries: 10 nm candidates
Candidate filtered primary spectra
25 primaries
Choose 9 out of 25
2,042,975 combinations
Add threshold at 20 lumen:
22 primaries
497,420 combinations

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Selection of primaries
Optimal for spectral gamut
Volume: 2.47x1020
Worst case: 7.43x1014
Optimal for CIELAB gamut
Volume: 1.59x106
Worst case: 763
White point: brightest 9–primary combination
Optimal for CIEXYZ gamut
Volume: 1.97x108
Worst case: 7.20x103

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Optimized color gamut
Spectral locus
HP DreamColor
wide-gamut 3 LED
HP 12-ink printer
Optimally-selected
9 narrow-band
primaries

9. © Copyright 2016 HP Inc.
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Filtering a single, white backlight
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Thorlabs Warm White LED 25 candidate primaries Gamut-maximizing choice of 9

10. © Copyright 2016 HP Inc.
0 0.1 0.2 0.3 0.4 0.5 0.6
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Spectral locus
sRGB
Optimally-selected
9 narrow-band
primaries (from RGB)
Optimally-selected
9 narrow-band
primaries (from W)
Optimized color gamut

11. © Copyright 2016 HP Inc.
The HANS imaging pipeline for displays
Each pixel
consists of NxN
sub-pixels.
Intensity can be
controlled
continuously
(e.g., via an LCD).
Static mask also
possible → lower-
cost, static
displays (e.g.,
point of sale,
posters, …).

12. © Copyright 2016 HP Inc.
The HANS imaging pipeline for displays
❖ 9 sub-pixel primaries → 2
9
=512 display primaries
❖ Color → find combination of 512 primaries that
convexly combine to it
❖ Traverse all polyhedra formed by 512 primaries’
colorimetries
❖ Select one enclosing polyhedron (e.g., tetrahedron)
❖ Barycentric coordinates of enclosing polyhedron are
convex weights with which corresponding primaries
need to be combined
❖ Use error diffusion, or PARAWACS (see paper at end
of this session) to spatially distribute

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Test setup
SEM micrograph
of test sample
Backlight Grating
TSR
Displayed
test patches

14. © Copyright 2016 HP Inc.
Results

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Measured patches
9primaries2pairs
backlight

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Gamut & additivity

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Conclusions
❖ These are first, proof-of-concept results done under prototype
conditions
❖ Measurements show clear signal that grating color filters provide
primaries from white backlight that significantly exceed the standard,
sRGB gamut
❖ Greens close to the HP DreamColor display with RGB backlights
❖ Brightness and energy efficiency are expect it to be relatively low in this
configuration
❖ We only use light diffracted vertically
❖ More efficiency can be achieved using LCD shutter to modulate
light diffracted at other angles.
❖ Combination of nanotechnology
and advanced color analysis techniques offers new platform for
high quality and scalable large color gamut displays

18. © Copyright 2016 HP Inc.