Sharp DV-500D, Technical Manual

Page: 7 / 117

7
DVD-Series
λ
2. Comparison of CDs and DVDs
Item CD DVD DVD(CD)
Track Pitch 1.60
µ
m 0.74
µ
m 1.60
µ
m
Minimum Pit 0.87
µ
m 0.40
µ
m 0.87
µ
m
Laser Beam Wavelength 780nm
(Infrared Light)
650nm
(Red)
650nm
(Red)
Opening of the Object Lens 0.45 0.6 0.38
Laser Focus 1.8
µ
m 1.0
µ
m 1.7
µ
m
*CD mode of DVD
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
Φ
=
NA
λ
(laser beam wavelength) = 650nm
DVD
Φ
=
0.6
= about 1
µ
m CD(DVD)
Φ =
0.38
= about 1.7
µ
m
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:
4. Longer Track, Smaller Pit
650nm 650nm
NA =
f
a
a
Disk
f
Laser Beam Focus
0.74 Track Width
Minimum Pit
0.4