Light transmission is the percentage of incident light that passes through a film. ExxonMobil generally evaluates this property for OPPalyte films.
Relevance to performance
This property is significant to the performance of opaque films like OPPalyte. Customers expect these products to obstruct light transmission in a predictable way, which is important for the appearance of printed graphics and protecting light-sensitive packaged products. OPPalyte film light transmission values range from 15% to 50%, depending on the product design and film thickness.
NOTE: Metallized films are very effective at blocking light transmission, but the preferred test for these films is optical density.
What affects light transmission
In OPPalyte films, light transmission is determined mostly by pigment and cavitation characteristics, which are controlled by proprietary resin formulations and film-making process conditions. Since light transmission is usually a critical property for white opaque films, it is measured regularly. Product will be rejected if values are outside the specified tolerance limits.
A unidirectional perpendicular light beam is directed onto the film specimen, and a photo detector measures the total light transmitted by the specimen after it enters an integrating sphere. Commercial hazemeters are typically used for this testing, but ASTM D 1003 also allows for the use of a spectrophotometer, provided that it meets the procedure requirements. ExxonMobil uses the BYK-Gardner XL-211 haze-gard and haze-gard plus hazemeters consistent with ASTM guidelines.
The hazemeter when set up to measure light transmission will display the final result as a percentage to the nearest tenth.
|Light Transmission (%) =||Total light transmitted by specimen||x 100|
The same test equipment with a different setup is also used to measure and display haze.
Luminous transmission: Luminous transmittance refers to the same thing as light transmission. It is a more technical name used in the ASTM D 1003 test procedure title.
Haze: Haze is the scattering of light by a specimen, which results in a cloudy appearance or poorer clarity of objects when viewed through the film.
Translucent: Translucent is an attribute used to describe material that transmits light, but also diffuses it so that objects cannot be seen clearly.