1. Chapter I, Digital Imaging Fundamentals: Lesson IV
Storage

2. As we saw in the last module, photographic digital images generate a lot of data. One 35mm negative scanned for Photo CD generates an 18 megabyte file.

3. To store that one image file on high density floppy disk, would take 14 disks.

4. Even a 120 Megabyte hard drive, could store only six of these images.

5. This is why optical media like Photo CD, with a storage capacity of over 650 Megabytes, is playing an increasingly important role in digital imaging. If this illustration were drawn to scale according to storage capacity, the compact disc would be 500 times as large as the high density floppy disk.

6. On magnetic media, such as floppy disks and hard drives, data bits are detected as changes in the orientation of magnetic particles.

7. On optical media, such as Photo CD, data bits are detected as optical changes in the pits and lands on the disc.

8. There are two basic types of compact disc: pressed media and write-once media. Write-once media is often referred to as CD-R media, for CD-Recordable. After the compact disc is written, it cannot be changed or erased by the user.

9. CD-ROMs, much like audio CDs, are typically pressed from a master by the thousands. CD-ROM is used for general data storage, such as catalogs, encyclopedias, and data bases.

10. With write-once or CD-recordable media, such as Photo CD, a CD writer is used to record images or data on the disc. The writer is hundreds of thousands of dollars less expensive than the mastering equipment used for audio CDs and CD-ROM.

11. Let's take a closer look at the cross section of a recordable CD
Let's take a closer look at the cross section of a recordable CD. The polycarbonate substrate is a high optical plastic which allows the laser to achieve a clean focus on an organic dye recording layer.

12. Pressed into the substrate is a spiral pregroove, which helps guide the laser around the disc. The wobbled track spirals counter-clockwise from the center of the disc. If this track were a straight line, it would stretch more than three and a half miles.

13. Yet these tracks are so narrow that more than 20 tracks could be located in one groove of an LP record.

14. The gold reflection layer, which has been sputtered onto the substrate, reflects laser light with high intensity.

15. During CD recording, the organic dye recording layer is microscopically deformed when heated by the laser. This creates pits.

16. And finally, a protective lacquer coating shields the gold reflection layer from scratches and wear. This makes CDs very tolerant of small scratches or dust particles on the disc.

17. In optical writers, the digital image's binary data is translated into pulses of laser light.

18. An optical system aims a high intensity light at a blank rotating disc
An optical system aims a high intensity light at a blank rotating disc. Where the digital encoding activates a focused laser burst, a less reflective surface is created. This is called a pit.

19. Where the laser remains off, a highly reflective surface remains
Where the laser remains off, a highly reflective surface remains. This is called a land.

20. The CD reader focuses a low-intensity laser beam onto the data layer and reads the reflected light.

21. Changes in light defraction indicate the beginnings of lands or pits
Changes in light defraction indicate the beginnings of lands or pits. The leading and trailing edges of a pit indicate ones. All areas between pit edges indicate zero.

22. The CD reader uses error correction to replace or estimate image data, lost in the event of severe scratches or large dust particles.

23. Module Review
Let's review what we've just learned. Select 1 or 2 on the image to indicate which type of laser light is used in the writer.