What IS a 35mm Digital Cartridge?

Sometimes we will get an microfilm scanning inquiry about a "35mm cartridge" that needs to be converted into digital image, such as PDF or TIFF. However there is no such thing as a 35mm roll film cartridge; only 16mm microfilm cartridges exist. Therefore "35mm digital cartridge" is a misnomer.

How can you tell the difference between 16mm and 35mm roll film? The best was is to take a ruler and measure across.

Anyway, 16mm digital cartridges have a hard plastic casing that protects the microfilm inside. Popular brands include Kodak microfilm cartridges and 3M cartridges.