Optical density, which ExxonMobil measures with a transmission densitometer, is another 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.
Relevance to performance
Transmission densitometers have traditionally been used to evaluate the light transmission properties of photographic film, but the same test is now also used to 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%.
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.
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, and the light that is transmitted through the film is collected, measured, and logarithmically amplified; finally, 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||) = (||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.
|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|
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.