Sharp DV-500D Technical Manual page 7

2. Comparison of CDs and DVDs
Item
Track Pitch
Minimum Pit
Laser Beam Wavelength 780nm
Opening of the Object Lens
Laser Focus
3. Decreasing the Laser Beam Focus Wavelength
The size of the laser beam focus Φ has small λ, When NA increases, it becomes smaller.
It is determined using the following formul:
NA (Opening of the object lens) = DVD : 0.6
CD(DVD) : 0.38
λ
Φ =
NA
λ
(laser beam wavelength) = 650nm
650nm
DVD Φ = = about 1µm CD(DVD) Φ =
0.6
NA shows the brightness of the lens. When the focal distance of the lens becomes shorter and
the diameter increases, the NA becomes bigger. It is determined using the following formul:
1a
NA =
f
4. Longer Track, Smaller Pit
CD
1.60µm
0.87µm
650nm
(Infrared Light)
0.45
1.8µm
650nm
0.38
f
a
Disk
Laser Beam Focus
0.74 Track Width
Minimum Pit
0.4
7
DVD DVD(CD)
0.74µm
0.40µm
650nm
(Red)
0.6
1.0µm
*CD mode of DVD
= about 1.7µm
DVD-Series
1.60µm
0.87µm
(Red)
0.38
1.7µm