Read Sharp DV-560H (serv.man6) Service Manual online
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:
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
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