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Online Spectrophotometry and Layer Design Tools
in Solar Control Coatings
For esthetical reasons architects like to have a
defined and constant color on the glass. Color
of a flat glass pane is the only critical effect for
the one, who is looking at a building and
therefore the primary reason for complaints.
The functional part of the architectural glass –
solar control - is of course more and more
important but not visible. Especially with all
debates about reducing CO2 emission, with
increasing energy prices, architects have to
think about energy saving, when heating or
cooling houses.
By sputtering coating layers on glass or on
flexible polymers (foils), these become an
important part of solar control in architecture
and automotive. Applying this process, the
color of the glass may change with viewing
angle, especially when using double (or
multiple) silver layer coatings. If the production
process is unstable - and the glass panes
mounted on a building front will have different
colors - complaints of clients will cause
tremendous costs. An On-Line Color Control
will ensure a stable process.
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Applications
Low-E Glass
In regions where weather is typically cold,
energy shall be kept inside; this is provided by
appropriate coating and window glass
arrangement.
Picture 1
Modern online spectrophotometers have to
take care of that problem. It is not enough to
measure with one specific condition. Just like a
pedestrian, who walks around the house and
thus changes his viewing angle, the
measurement device has to measure the
spectral curves with different illumination and
different viewing angles.
Not only in the architectural field is color of the
pane important but also in the automotive
industry. Modern cars have also solar control
glases which change colors by the viewing
angle. While the car is approaching to an
observer, the viewing angle and the color are
changing. See picture 2.
Solar Protection Glass
On the other hand there are regions with
strong sun shine. Here the glass shall keep the
sun´s infrared energy out of the building to
keep the rooms cooler. For this reason the
glass is coated with specific layer structures,
which are able to reflect the infrared radiation
to the outer space. These layers will also
influence the color perception of the glass
surface. Using On-Line spectrophotometry, the
color effect of coating layer structure can be
monitored during the production process. This
helps to ensure constant product quality.
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Online Spectrophotometers
of GretagMacbeth GmbH
Glass in automotive and architectural
applications is typically a non-scattering
material. If we look at a glass pane, we always
see the mirror image in the glass typically the
sky, or/and we are looking through the pane.
For this reason we have to measure the glass
in reflectance and in transmission to describe
precisely the visual impression.
The transmission measurement is quite easy
to handle: Light is sent through the glass pane
and on the other side a detector receives the
light that passed the glass. This light is
directed
to
a
high
resolution
spectrophotometer.
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The reflectance measurement of glass is
different to the measurement of other surfaces
(which typically have high scattering). Only
laboratory instruments which have a sphere
geometry (d:8°, specular component included)
can measure the pane, but just with one
viewing condition. Only light which is reflected
on the surface at 8° is measured. All other
angles are neglected.
The better technology is to have a
measurement directly in the specular direction.
With the instrument ER 56 PA(T) from
GretagMacbeth GmbH light is sent at 15° and
at 60° to the specimen. On the other side
at -15° and -60° the light is registered
sequentially and directed to a high resolution
spectrograph. This allows quality control for
steep angles and flat angles.
In the following Picture 3 you can see the
ER 56 PA built in a flat glass pane coating
production line. The sonar sensor in front of
the instrument detects the glass pane and
triggers the color measurement.
instrument in reflectance mode and with 250
mm distance in transmission mode.
During the measurement the sample is
illuminated by a white light (Xenon flash lamp,
daylight quality) for approx. 1 / 1000 sec. In the
specular angles of 15° and 60° the reflected
light is registered sequentially and directed to a
high-resolution spectrograph. In Picture 4 you
can see a principal drawing of the geometries.
The two lamps illuminate the sample with an
angle of 15° and 60°. On the other side are the
optics which direct the light to the
spectrograph.
Picture 4
Parallel to the sample measurement a
reference measurement of the lamp is done by
a second identical spectrograph (dual-beam
system design).
Optionally, transmisson measurement is
performed at the same time with an other
module placed at the reverse side of the glass
pane.
In each spectrophotometer light signal is
dispersed into 401 wavelength ranges (330 nm
up to 730 nm) via a corrected, concave grating.
An integrated photo diode cell array with more
than 400 detectors measures each wavelength
range. The result is a true spectral resolution of
1 nm. The measurement signals are amplified
and digitized with high resolution. A fast
processor in the measure-ment head
processes them into corrected spectral
reflectance values.
From
these
401
reflectance
values,
colorimetric readings for any illuminant or
standard observer can be calculated (e.g.
CIELAB for D65 / 10°).
The transmission measurement is done at the
same measurement spot with the same
principle.
Picture 3
2.1
Spectral
the ER 56 PA(T)
measurement
with
The spectrophotometer ER 56 PA(T) is
typically used on glass coating lines after the
vacuum
coaters,
directly
between
transportation rollers. There is no need to
destroy the glass pane for sample preparation.
The ER 56 PA(T) is a compact On-Line
spectral color measurement system with
special measurement geometry for specular
reflectance using two viewing angles and with
an
optional
transmission
measurement
module.
The glass pane is measured in a non-contact
way in 10 mm to 22 mm distance to the
Automatic calibration of the instrument
includes the calibration of intensity for each
wavelength, and the absolute wavelength
calibration for excellent measurement accuracy
and long-time stability.
The measuring instrument is controlled by a
computer (PC), to which the measurement
data are transferred. Besides the RS 232 there
is a RS 422 interface, allowing distances up to
1000 m between the measuring system and
the evaluation computer. A galvanic separation
of the data interfaces guarantees stable
operation
in
a
real-world
production
environment.
Glass panes have to be measured on several
spots after the coating process for quality
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control. The ER 56 PA(T) can be placed
between the transportation rollers on a linear
track to measure in cross direction (CD) - left
side, center and right side - of the glass pane.
As soon as the pane is above the sonar
sensor, it stops and the ER 56 PA(T) performs
its measurements. To measure the next
position, the pane will move for a specified
distance. Then the procedure is repeated.
If the cross direction of the coating is quite
stable, one ER 56 PA(T) can be used on a
fixed position.
The result of each measurement is a spectral
curve from 330 nm till 730 nm for reflectance
and for transmission measurement. Picture 5
shows reflectance curves for 15° and 60°
viewing angles at a double silver layer coated
flat glass pane.
60
50
60°
15°
Reflexion %
40
30
20
10
0
300
350
400
450
500
550
600
650
700
750
800
Wellenlänge nm
Picture 5
The ER 56 PA(T) can be used in the laboratory
as a desktop instrument, too. Mounted on a
support frame it can measure even large
architectural samples. The results correlate
very well to the On-Line Measurement Mode
and show exactly, what the human eye sees
under these conditions (see Picture 6 –
desktop).
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Picture 6
2.2
Other online instruments from
GretagMacbeth for glass or film
measurements
Like the ER 56 PA(T), the ER 54 PA(T) can
measure glass in reflectance and transmission.
The difference is, that the reflectance
measurement is taken with just one geometry:
+15°:-15°. A transmission measurement mode
is optional, too.
The ER 55 PA is applied only for
measurements in transmission mode. This
instrument determines color (L*, a*, b*), light
transmission and non contact correlated haze
– all readings are collected online. It can be
used for glass, but also for plastic films – like
interlayers in laminated glazing (see Picture 7).
Picture 7
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Layer Design
The variation of color with changing observer
position should be taken into account in the
design of architectural glass coatings. If the
number of layers and the possible variation in
available optical constants in a coating line
allow to satisfy both the functional demands of
the coating (such as heat insulation or
protection from solar radiation) as well as
wishes for a certain color impression, the
designer has some freedom to make choices.
Due to the complexity of modern coatings their
design is usually based on an optical model. If
the optical constants of the applied materials
have been carefully determined, and effects of
'layer interactions' that occur when depositing
a sequence of layers in a coating line are
understood and taken into account in the
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model, the performance of the final product
can be predicted with high accuracy. The
CODE software of W.Theiss Hard- and
Software contains all mechanisms needed to
investigate the optical properties of materials
like sputtered oxides, nitrides or metals and to
predict technical data of coatings such as U, g,
integrated transmission and reflection values
as well as color coordinates. It also assists the
designer with the computation of the color
variation with viewing angle. In an intuitive
manual design phase you can instantly watch
the color variation while modifying layer
thicknesses or optical constants with slider
motions (fig.?). Having defined the wanted
color variation as optimization target, the
program finally tries to optimize the coating
automatically. All computed technical data can
simultaneously be used as fit targets. A human
being as coating designer is still required, in
particular to control the automated optimization
by setting the relative weights of the individual
target quantities.
Screenshot with angle variation of color and
thickness sliders
Fig. ?
The design software GenetiCode of W.Theiss
Hard- and Software optimizes coatings using a
different approach: The user defines the
number of deposition stations (in most cases
these are sputtering cathodes) in the coating
line and the available materials for each
station. The wanted performance of the coating
is specified by a collection of target values like
in the CODE software discussed above.
Having defined the available deposition
hardware and the wanted coating properties,
the layer stack is then optimized using a
genetic algorithm, usually in overnight sessions
performing several evolutionary runs (fig ??).
Since GenetiCode is not blocked by any
design knowledge, it can be a valuable partner
of experienced coating designers, either
creating new, unusual solutions or confirming
existing approaches as good solutions of the
problem.
GenetiCode screenshot
Fig. ??
4 Layer Correction using On-Line
Spectrophotometry
The production of modern glass coatings
usually involves many sputtering cathodes in
the coating line – each one contributing a
certain amount of uncertainty about what is
produced at the moment. An optical production
control is required, providing online information
about the coating produced at present. In most
cases
transmittance,
reflectance
or
ellipsometry spectra are recorded, either inline
between groups of cathodes or directly behind
the coating line when the final products have
left the vacuum.
If the design of the coating has been done
taking into account details of the coating line it
is natural to use the optical model of the design
for production control as well. In the case of
CODE you can even take exactly the same
program. CODE can be used to analyze the
spectra recorded during production, driven as
OLE automation server by the software that
controls the production equipment, such as
LabView (a product of National Instruments).
Based on CODE, W.Theiss Hard- and
Software has developed the software package
BREIN which is dedicated to the optical control
of the production of multilayer coatings in large
coating lines. BREIN stands for BRight Eye
INspection network: It manages several
stations called 'Bright Eyes' (BE) which
analyze optical spectra taken at a certain
position in or after the coating line (fig ???).
Before doing the analysis for a product, each
BE picks up the results obtained by the BE
prior in the line. This way only the newly
deposited layers after the last BE have to be
analyzed - the computational effort for each BE
is usually quite low and can be done even on
slow embedded computers. The main BREIN
application collects the BE results and displays
the current status of the production. The
operators can use this information for making
decisions about changes of the production
conditions.
BREIN scheme
Fig. ???
5 Conclusions