The most common type of CDs are indexes, but scanned images of original records are becoming popular.
Indexes are secondary sources but the scanned images provide a primary source of documents.
Look to see these become more prevalent. The difference is that these days, thanks to CD-ROM and Internet technology, you can find the information you need more swiftly and with greater convenience than before. A compact disk will hold 650 megabytes which means it provides the same space as about 460 high density floppy disks do. This amounts to 160,000 pages of text or 533 books of 300 pages each. This is due to development, reproduction and copyright fees.