CHROMA TECHNOLOGY CORP®

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Executing highly optimized sputtered interferencecoating designs for colorimetry and light balance
filters using broadband monitoring
John D. Atkinson
Principal Product Engineer
Chroma Technology Corp.

CHROMA TECHNOLOGY CORP®

+1-802-428-2500 | 800-824-7662 | sales@chroma.com | www.chroma.com

Contents
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Color Theory
Optical filters
Sputter Coating Process
Optical Monitoring
Tri-Stimulus Filter Results
Light Balance Filters
Optical Modeling and Practical Application

CHROMA TECHNOLOGY CORP®

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Color – Human Visual Perception

Photo by Amanda Dalbjörn on Unsplash

https://specialized-cells.weebly.com/uploads/4/7/9/0/47901671/3818983_orig.gif

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Color – Human Visual Perception
Normalized (log)sensitivity
coneconesensitivity

https://specialized-cells.weebly.com/uploads/4/7/9/0/47901671/3818983_orig.gif

The plot of normalized conesensitivity shows three distinct
peaks ...
… but taking the log shows there
is significant overlap, and in fact
the L and M cones each coveralmost the entire visible
spectrum.
http://www.handprint.com/
http://www.handprint.com/

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Color – Human Visual Perception
Relative cone sensitivity

https://specialized-cells.weebly.com/uploads/4/7/9/0/47901671/3818983_orig.gif

The weighted response given therelative number of L, M, S cones in
the retina gives a more accurate
view of how our color vision isinfluenced by cone sensitivity.

http://www.handprint.com/L & M cones dominate, so our eyes are overall moresensitive to green light!
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Color – Human Visual Perception
In 1931 the Commission Internationale del´Eclairage (or CIE) gave us a construct tomodel color perception called the CIE 1931
Color Space. Essential to that theory are theTristimulus curves or Color Matching
Functions.

CIE 1931 XYZ Color Matching or Tristimulus Functions• ȳ(λ) or “y-bar” is identical to the spectral
luminous efficiency function V(λ) for
photopic vision• determines the overall sensitivity tobrightness or luminance.

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Human Spectral Sensitivity
photopic curveȳ(λ)

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• Peaks at about 555nm• Trails off toward the red and blue
• That means we are less sensitive to blue or
red photons than we are green!

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Human Spectral Sensitivity &Incandescent Lamp Spectrum
photopic curveȳ(λ)

we perceive
as yellow
incandescentlamp output

CHROMA TECHNOLOGY CORP®

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Human Spectral Sensitivity &Spectral Response of Silicon Photodiodes
photopic curveȳ(λ)

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Colorimetry & Photometry••

A colorimeter measures the color of an object, light source, or display as it would beperceived by the human eye.
A photometer measures the luminance, or intensity, again as it would be perceived with thehuman visual system.

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Colorimetry & Photometry•

Colorimetry by spectrophotometer is often the mostaccurate, but these devices are generally slower andmore expensive than colorimeters that employ thetristimulus method (described by German Industrial
Standard DIN 5033, part 6).

•Tristimulus-based colorimetry makes use of the CIE
color-matching functions, or tristimulus curves.

•In a tristimulus colorimeter, optical filters called
tristimulus filters are used to color the detector,
modifying the overall device response to mimic the CIE
color-matching functions (taking the detector’s QE intoaccount).

CHROMA TECHNOLOGY CORP®

z̄ (λ)ȳ(λ)

x̄ (λ)+1-802-428-2500 | 800-824-7662 | sales@chroma.com | www.chroma.com

Colorimetry & Photometry•In photometry, a V(λ), or photopic filter likewise
spectrally modifies the detector for conductingluminance measurements.

•In both tristimulus colorimeters andphotometers, the accuracy of the instruments
depends on how well the filters (combinedw/detector QE) match the CIE tristimulus curves.

•From ISO/CIE 19476:2014 comes the integral
term f1’, which is a measure of the quality of thespectral match, and therefore determines theinherent accuracy of the instrument.

CHROMA TECHNOLOGY CORP®

A low number for f1’ means acloser match and an inherently
more accurate instrument.

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Chromaticity- Color Quality Regardless of Brightness

CIE 1931 Chromaticity diagram, x y color coordinates
2D representation of the CIE XYZ Color SpaceCHROMA TECHNOLOGY CORP®

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Optical Filters
Optical filters are devices that transmit light of particular
wavelengths while reflecting or absorbing light of otherwavelengths, thus selecting what wavelengths getpassed on in the optical system.
Example: ‘red’ filter
Incident White Light

Transmitted Red Light

This spectral plot shows % transmission as a function ofwavelength and represents the percentage of energy that gets
through the filter -- what is not allowed to transmit is either
absorbed or reflected. T + A + R = 100%

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Optical Filters
Absorption glass ‘color’ filters

Dielectric interference coating on glass

rejection bands(primarily absorption)

rejection bands(primarily reflection)••

A multi-layer stack of very thin coatings is deposited
onto one or both sides of a glass substrate such thatparticular wavelengths of light interfere uponreflection at the multiple interfaces.
Stacks are typically alternating layers of two nonabsorbing materials, one with a ‘high’ index ofrefraction, and one with a ‘low’ index of refraction.

CHROMA TECHNOLOGY CORP®

•••

Metals and other inorganic and organic
compounds are melted in with the glass upon
manufacturing.
These materials absorb some wavelengths oflight (convert light energy to heat), whiletransmitting others.
Many of these ‘dyes’ contain non-ROHS compliant
substances, e.g. lead, cadmium.

+1-802-428-2500 | 800-824-7662 | sales@chroma.com | www.chroma.com

Optical Filters
Dielectric interference coating on glass
Clear
opticalglass

Absorption glass ‘color’ filters
1mm
OG590

coating

2 pcs of absorption glass~10um coating on glass

•
••

•

Can theoretically reach 100% transmission in passband.
The number of layers and their individual thicknesses
determine the spectral characteristics.
The thicknesses of the layers are controlled to a very highprecision, and in this way the band-shape can beengineered.
Overall coating thickness is typically < 20μm, and can bedeposited onto clear glass substrates as thin as 0.5mmCHROMA TECHNOLOGY CORP®

2mm
BG40

•
•

•

The peak transmission is determined by how much thematerial absorbs and the thickness of the glass.
The pass-band is generally broad, and not easily controlled
because there are only so many materials with uniqueabsorption characteristics that can be used.
To effectively “block” unwanted wavelengths, anabsorption filter must be thick (2-3mm); if two pieces areneeded to define the edges of the pass-band then theoverall thickness of the filter can be several mm thick.
+1-802-428-2500 | 800-824-7662 | sales@chroma.com | www.chroma.com

Optical Filters
Dielectric interference coating on glass
Clear
opticalglass

coating~10um coating on glass

Absorption glass ‘color’ filters
1mm
OG5902mm
BG402 pcs of absorption glass

With high energy, high precision sputter depositedcoatings, it is possible to get a durable “all-in-one” filter
coating on a single side of relatively thin glass.

CHROMA TECHNOLOGY CORP®

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Sputter Coating Process – Main Components
Light Source
VacuumChamber
Substrate SpinnerRotates Continuously
VacuumPump

Glass
Substrate
Substrate Heater
Magnet Assembly(Magnetron Sputtering Cathode)Ion Gun (O2+)
Optical monitoring system(measures transmission of lightthrough coating during deposition)

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Sputter Coating Process – Deposition Primary Components
Thin Film Stack –alternating high and low
index layers
High-precision layer
thickness control

Ar+

Ar+

Low-index Deposition
Cloud
High-index Deposition
Cloud
Argon (Ar+) Ions
Accelerated
Accelerated toto Target
TargetMetal TargetHigh-index sputter cathode

Silicon/Metal Target
Low-index
Low-index sputter
sputter cathodecathodeCHROMA TECHNOLOGY CORP®

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Sputter Coating Process – Sputtering Basics
GLASS SUBSTRATE
CoatingWith reactive sputtering, oxide
coatings can be made by introducing
oxygen gas which reacts with themetal/semiconductor during
deposition.
Ion-assist is used to deliver energy to
the growing film in order to promote
void-free growth and enhance film
durability.

Argon (Ar+) Ions
accelerated to target

ElectricField

Primary
Magnetic
Field

Sputtered atoms
ejected fromtarget

Metal Target

MagnetAssembly
Magnetron Sputtering CathodeA negative high voltage is applied to the metal target andionized argon gas is accelerated toward the target
thereby dislodging metal material.

Vacuum chamber pressure/pumping performance, gas flows, temperature, temperature,
cathode currents and voltages, ion-gun operation are all critical parameters for repeatablecoating execution, stable refractive index, and consistent film structure.
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Optical Monitoring Systems
lamp
chopper

vacuum windowlight path throughsubstratevacuum window
steering optics
monochromator/spectrometer, detector

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Optical Monitoring Systems Traditional Single λMonitoring

lamp
chopper
vacuum window

vacuum window
steering optics
monochromator/spectrometer, detector

incomingbeam
https://www.chemicool.com/definition/monochromators.ht
ml

Dep recipemλ = 500nm---------------------------------------------------------

L-layerstart end

trace info and layer cut points.

Deviations represent
thickness errors!
But what’s happening tothe full spectrum?

H-layerstart
end

predicted

actualcomputerH-layer

LH

transmissio
nlight path throughsubstrate

The monitor wavelength, mλ, is set
to optimize precision layertermination for accurate layer
thicknesses.
The dep recipe
contains predicted

CHROMA TECHNOLOGY CORP®

grating based (2D)
monochromator

detector(PMT or PD)

LH
H

H
LL

L-layertime 🡪🡪

How well the actual trace follows the predicted islargely dependent on signal noise and opticalmaterial constants, which in turn depend on
deposition process control and stability.

+1-802-428-2500 | 800-824-7662 | sales@chroma.com | www.chroma.com

Optical Monitoring System Broad Band
Monitoring (BBM)

grating based spectrometerw/ array detector

lamp
chopperarray detector

vacuum windowlight path throughsubstrateincoming
beam

entrance slit

grating

https://www.chemicool.com/definition/monochromators.ht
mlcomputer

vacuum windowsteering optics

Advanced software control
predicts the outcome, determines
the best fit given the layers not yet
With BBM a range of λ’s iscoated, and re-optimizes themonitored.
coating design in order to correct
This range could change
the outcome and achieve
dynamically during deposition as
improved spectral matching.
some narrower bandwidths are
more important at the beginning ofthe run, while other broader
bandwidths might become more
important later in the process.
monochromator/spectrometer, detector

CHROMA TECHNOLOGY CORP®

BBM----------------------------------------------------------

Re-opttransmissio
nDep recipeλ (nm)

How well the actual trace follows the predicted islargely dependent on signal noise and opticalmaterial constants, which in turn depend on
deposition process control and stability.
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Re-optimizing

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Tri-stimulus
FilterResults
target (shaded)
f1’ ~23% (303487), single λf1’ ~16% (360425), single λf1’ ~13% (353510), single λf1’ ~5% BBM, non-optimized process
f1’ ~4% BBM w/ re-optimization
f1’ ~3% BBM w/ re-optimizationand optimized coating process

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Tri-stimulus Filter Results
CIE X tristimulus filter, f1’ = 1.5%
targetactualCHROMA TECHNOLOGY CORP®

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LightBalanceFiltersfilter transmission
incandescent lamp output

f1’ scores
0.94%

1.44%
1.12%

1.67%

CHROMA TECHNOLOGY CORP®

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Optical modeling – Practical application••

Customer replaced absorption-glass based V(λ) filter with dielectric coating version
Although the dielectric filter gave better spectral match = more accurate luminancemeasurement, the reflective coating caused glare when viewing bright LCD display

Glare from uncoated rear surface
Glare from cover sensor reflection off CIE Y surface (facing sensor)
Glare from cover sensor reflection off CIE Y surface (facing source)

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Optical Modeling – Practical Application
Different CIE Y filter configurations - CAD models

Original - without AR
coated rear surface

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Tilted - relatively close to thecover glass, w/ AR coated rearsurface

Tilted - as far from sensor as
possible, practically against thelens, w/ AR coated rear surface+1-802-428-2500 | 800-824-7662 | sales@chroma.com | www.chroma.com

Optical Modeling – Practical Application
0° Tilt CIE Y Filter - No Rear AR coating
Ghost reflections
overlap with main
beam, resulting in arotationallysymmetric
defocused bluraround image.

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Optical Modeling – Practical Application
8° Tilt CIE Y Filter - Rear AR coating - Closer to Sensor
Ghost reflections offof cover glass reflect
at an angle, withlittle overlap of main
beam on image
plane.
Ghost reflections off
rear surface of CIE Y
filter would still beon-axis of main
image plane, butwith AR coating they
are assumed to benegligible.

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Optical Modeling – Practical Application
8° Tilt CIE Y Filter - Rear AR coating - Further from Sensor
Ghost reflections offof cover glass reflect
at an angle, withlittle overlap of main
beam and somemissing sensor
altogether.
Ghost reflections off
rear surface of CIE Y
filter would still beon-axis of main
image plane, butwith AR coating they
are assumed to be
negligible.

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Optical Modeling – Practical Application
Ghost reflections - Comparison

No tilt, ~6.9mm fromsensor

8° Tilt, ~8.3mm fromsensor

8° Tilt, ~15.8mm fromsensorResults in the 0° tilted sample show how a second back reflection is occurring at around four orders of magnitude lower.

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Thank you for Joining!
John Atkinson
Principal Product Engineer at Chroma Technology
jatkinson@chroma.com
Chroma.com

Located in southern
Vermont, Chroma
Technology is aleading manufacturer
and OEM supplier ofhighly precise opticalfilters using thin-film
coating technology.
Chroma was founded
in 1991.
Chroma is a 100% employee-ownedcompany,

CHROMA TECHNOLOGY CORP®

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