IGCSE ICT 0417 – Storage Devices and Media: Optical Discs
Storage Devices and Media – Optical Discs
Learning Objective
Know and understand the characteristics, uses and handling of optical discs: CD, D \cdot D and Blu‑ray.
What is an Optical Disc?
An optical disc stores data as a series of microscopic pits and lands on a reflective surface. A laser beam reads the pattern by detecting changes in reflected light.
Suggested diagram: Cross‑section of an optical disc showing pits, lands and the laser read head.
Key Types of Optical Discs
Compact Disc (CD)
Digital \cdot ersatile Disc (D \cdot D)
Blu‑ray Disc (BD)
Compact Disc (CD)
Originally designed for audio, later adapted for data (CD‑ROM, CD‑R, CD‑RW).
Standard diameter: 120 mm
Data capacity: 700 MB (≈ 80 minutes of audio)
Laser wavelength: 780 nm (infra‑red)
Typical read speed: 1× = 150 KB/s
Digital \cdot ersatile Disc (D \cdot D)
Developed to store video and larger data files.
Standard diameter: 120 mm (same as CD)
Data capacity:
Single‑layer, single‑sided (D \cdot D‑5): 4.7 GB
Dual‑layer, single‑sided (D \cdot D‑9): 8.5 GB
Dual‑layer, double‑sided (D \cdot D‑18): 17 GB
Laser wavelength: 650 nm (red)
Typical read speed: 1× = 1.385 MB/s
Blu‑ray Disc (BD)
High‑definition video and large‑capacity data storage.
Standard diameter: 120 mm
Data capacity:
Single‑layer, single‑sided (BD‑R/BD‑RE): 25 GB
Dual‑layer, single‑sided (BD‑XL): 50 GB
Laser wavelength: 405 nm (blue‑violet)
Typical read speed: 1× = 4.5 MB/s
Comparison of Optical Disc Technologies
Feature
CD
D \cdot D
Blu‑ray
Typical Capacity
700 MB
4.7 GB – 17 GB
25 GB – 50 GB
Laser Wavelength
780 nm (infra‑red)
650 nm (red)
405 nm (blue‑violet)
Minimum Pit Size
0.83 µm
0.40 µm
0.15 µm
Standard Read Speed (1×)
150 KB/s
1.385 MB/s
4.5 MB/s
Typical Uses
Audio, small software
Video, software, games
HD video, large backups, 4K content
Compatibility
All CD drives
D \cdot D drives (backward compatible with CD)
BD drives (backward compatible with D \cdot D and CD)
Advantages of Optical Discs
Portable and lightweight.
Read‑only versions (e.g., CD‑ROM, D \cdot D‑ROM) are resistant to accidental modification.
Long shelf‑life when stored properly (up to 30 years for archival quality).
Low cost per disc for moderate‑size data distribution.
Disadvantages of Optical Discs
Limited capacity compared with modern flash or hard‑disk storage.
Susceptible to scratches, dust, and U \cdot damage.
Physical handling required – risk of loss or breakage.
Drive technology is being phased out in many new computers.
Handling and Care
Store discs in jewel cases, away from direct sunlight.
Handle by the edges or the centre hub; avoid touching the data surface.
Keep discs clean with a soft, lint‑free cloth; wipe from centre outward.
Do not expose discs to extreme temperatures (below 0 °C or above 60 °C).
Summary
Optical discs remain a useful medium for distribution of audio, video and moderate‑size data. Understanding the differences in capacity, laser technology and handling requirements helps learners choose the appropriate disc for a given task and maintain data integrity.