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Definition
Relevance to performance
What
affects the optical density of metallized films
Test
principles
Related
Terminology
Definition
Optical density,
which ExxonMobil measures with a transmission
densitometer, is a representation of a
material's light blocking ability. The optical
density scale is unitless and logarithmic, and
it enhances the data resolution for materials
that transmit only a small fraction of
incident light. ExxonMobil uses optical
density measurements with metallized films
only.
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Relevance to performance
Transmission
densitometers are traditionally used to
evaluate the light transmission properties of
photographic film. The same test is now used
to also represent the thickness of the
aluminum layer of vacuum-metallized films.
Since layer thickness affects important
performance-related properties like film
barrier, light transmission, and appearance,
optical density provides critical data for
process control of the metallizing process.
Depending on the
product design, ExxonMobil metallized films
have optical density values ranging from 2.0
to 3.0, which is equivalent to light
transmission values of 0.1% to 1.0%.
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What affects
the optical density of metallized films
Metallizing process
parameters and base film characteristics
control the aluminum layer thickness and
uniformity, and therefore, the optical
density.
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Test principles
ExxonMobil
data are provided by commercial densitometers
that meet standard conditions defined by ANSI.
A unidirectional, perpendicular light beam is
directed onto the film sample. The light that
is transmitted through the film is collected,
measured, and logarithmically amplified. The
densitometer calculates and displays the
optical density value. ExxonMobil uses the
Tobias TBX transmission densitometer and
Macbeth models TD903 and TD932.
The
optical density values represent the following
calculation and relationship to % light
transmission.
| Optical
Density
(unitless) = |
log10( |
Incident
light |
)
=
log10( |
100 |
) |
| Transmitted
light |
Light
transmission (%) |
Optical
density values are reported to two decimal
places. Table 11 compares optical density
values to light transmission values in the
range that exists for ExxonMobil metallized
films.
| Optical
density |
2.00 |
2.10 |
2.20 |
2.30 |
2.40 |
2.50 |
3.00 |
| Light
transmission (%) |
1.0 |
0.8 |
0.6 |
0.5 |
0.4 |
0.3 |
0.1 |
Table
11: Light transmission values at various
metallized film optical densities
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Related terminology
ANSI |
ANSI stands for the
American National Standards Institute. |
Light
transmission |
Light transmission is
the percentage of incident light that
passes through a film sample. It can be
calculated from optical density values. |
Opacity |
Opacity is a common
paper measurement that describes a
substrate's light-blocking ability. A
perfectly opaque paper has an opacity
value of 100%. Because white opaque
films are replacing paper in some
applications, ExxonMobil measures TAPPI
opacity for some OPPalyte films. |
TAPPI |
TAPPI stand for the
Technical Association of the Pulp and
Paper Industry. |
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