Optical Storage - Upgrading and Repairing PCs - page 566

drives to 12x, which equates to a 54MBps transfer rate. The time to read or write an entire single- or
dual-layer disc at various BD drive/media speeds is shown in Table 11.16 .
Table 11.16. BD Drive/Media Speeds and Disc Read/Write Times
Standard CDs use a 780nm (infrared) laser combined with a 0.45 numerical aperture lens, whereas
DVDs use a 650nm (red) laser combined with a 0.60 numerical aperture lens. Blu-ray uses a much
shorter 405nm (blue-violet) laser with a 0.85 numerical aperture lens. Numerical aperture is a
measurement of the light-gathering capability of a lens, as well as the focal length and relative
magnification. The numerical aperture of a lens is derived by taking the sine of the maximum angle of
light entering the lens. For example, the lens in a CD drive gathers light at up to a 26.7° angle, which
results in a numerical aperture of SIN(26.7) = 0.45. By comparison, the lens in a DVD drive gathers
light at up to a 36.9° angle, resulting in a numerical aperture of SIN(36.9) = 0.60. Blu-ray drives
gather light at up to a 58.2° angle, resulting in a numerical aperture of SIN(58.2) = 0.85. Higher
numerical apertures allow increasingly oblique (angled) rays of light to enter the lens and therefore
produce a more highly resolved image.
The higher the aperture, the shorter the focal length and the greater the magnification. The lens in a
CD drive magnifies roughly 20 times, whereas the lens in a DVD drive magnifies about 40 times. The
Blu-ray lens magnifies about 60 times. This greater magnification is necessary because the distance
between tracks on a BD is reduced to 0.32um, which is almost half that of a regular DVD. A
comparison of BD and standard DVD is shown in Table 11.17 .
Table 11.17. Comparison of BD and DVD Specifications