封套平片是诸多缩微品中重要的一类。在对一般文件、档案、科技资料的缩微化中往往通过照像的方法将文件或资料逐页地拍摄到16毫米胶片上,一卷16毫米规格胶片的长度是30米,可容纳2500页上述的资料,也就是说,一卷胶片上容纳的将不止一份文件的资料,这样,使用、借阅、查找都很不方便。缩微平片则是以一张或数张平片容纳一份文件资料的构思而形成的,一张平片上一般可 Envelope plain film is an important category of many microforms. In the general document, file, technology and information in the miniaturization often through the method of photography of documents or documents on a single film to 16 mm film, a roll of 16 mm format film length is 30 meters, can accommodate 2500 pages of the above That is to say, it is inconvenient to use, borrow and search through more than one document on a roll of film. Microfiche is based on one or several pieces of flat paper to accommodate the idea of ​​a document formed, a flat film generally