The shiny side and dull side only appear on polycarbonate (PCTE) membranes, because polyester (PETE) membranes are shiny on both sides. With PCTE membranes, most users opt for the shiny side up, because it is the smoother surface.

This is particularly important in microscopy for optimum sample observation and for avoiding the necessity of continuously refocusing the microscope. However, in applications where you are trying to attach or adhere cells, it is better to use the dull or matte side.

It is fairly easy to determine the shiny side by simply holding the membrane up to the light and observing which side is more reflective. All filters disks are packaged with the same orientation—shiny side up. A blue paper is used to separate the membranes.