Focus depth and numerical aperture
With a track pitch as small as 0.74 and 0.34 ,um for DVD and HD DVD for- mats respectively and even smaller dimensions for the BD format, the laser spot size hitting the surface of the disc must be very small. It has to be small enough to distinguish between pits and avoid reading adjacent tracks. The spot diameter, W is directly proportional to the wavelength, ,1 and inversely proportional to the NA of the objective lens:
For DVD-video, NA = 0.6 and ,1 = 650 nm, the spot width is W = 650/ (2 X 0.6) = 0.541 ,um.
Similarly for HD DVD, W = 405/(2 X 0.65) = 0.311 ,um and for BD, W = 405/(2 X 0.85) = 0.238 ,um.
The effect of larger NA is to reduce the focus depth causing the readout to be more sensitive to disc thickness and other irregularities. Although the thickness of the substrates may be reduced as in the case of the BD for- mat, the optical head remains sensitive to disc warping which causes the disc to tilt. During readout, as the optical head moves across the disc, a changing tilt angle causes what is known as a skew error, which will disturb the read- ing of the data. For this reason, a tilt or skew sensor may be mounted on the surface of the optical unit to produce a skew error signal which is used to ensure that the optical head moves parallel to the disc surface.