Transparent Products
Gloss & Opacity of Films and Foils

Transparent films are used in thousands of different applications. Whether shrink film, blister or medical films – each application asks for specific behaviors in the material selection and processing conditions. Flower packaging should be very clear, protecting and presenting its content in the same way. Films for grocery bags are expected to diffuse the light. Consistent transparency can only be guaranteed if the key material and process factors are under control and a standardized sample preparation is used.

Gloss of films

Besides transparency, high quality films require defined reflection properties, regardless if they are brilliant glossy packaging or non-glare films for LCD use. The internationally standardized method for measuring gloss illuminates the sample under a defined angle and detects the reflected light intensity. At transparent materials, a part of the illuminating light penetrates the surface. The transmitted light is reflected at the rear surface within the material and is partly transmitted into the direction of the sensor.

This additional reflection is dependent on the background used and has a significant impact on the measurement. To minimize this influence, it is recommended to use a black, matte background, e.g. paper board, and it is important to always use the same background.

It is additionally challenging when the samples are very thin and don´t form a really flat surface under the gloss meter. Therefore, often a vacuum plate is used to make sure no air bubbles or wrinkles distort the measured gloss results.

Opacity

In some applications the opposite of transparency is required, e.g. shopping bags or diaper backings, which need to keep its contents private. This behavior is called opacity and controlled by use of color measurement. The spectro2guide includes the respective index to automatically calculate opacity.

Opacity is the ability of a thin, transparent material to hide the surface behind. It is also sometimes referred to as contrast ratio and hiding power. Opacity is expressed as the ratio of the reflectance when the material is backed by a black substrate to the reflectance when it is backed by a white substrate.

100 % opacity means complete hiding: no difference can be seen between the transparent material over black and white. For reproducible results, it is important to use always the same backing, therefore BYK-Gardner offers opacity charts which assure defined measurements. In the righthand graph the opacity of different types of sheet protectors were compared.