How does the laser read or write a disc?
CD players use a near-infrared 780nm laser. The visible light spectrum is generally considered to be 400nm to 700nm; few people can see anything past 720nm. The drive shines a laser through the polycarbonate (plastic) on the “bottom” of the disc. This bounces off the reflective layer, passes back through the polycarbonate, and is read by a photosensor in the drive head. The index of refraction for polycarbonate is about 1.55, so laser light bends when it enters, allowing a much finer focus for the laser (from 800um at the bottom of the polycarbonate down to about 1.7um at the metal surface). This minimizes the effects of dust and scratches, because the effects of any surface gunk are reduced as the laser’s focus width is reduced. A 400um-wide piece of dust on the surface of a CD would completely block a laser focused down to 200um at the surface, but has little effect on a CD player. If the photosensor sees a strong beam — the CD standard requires the signal strength to be at least 70
CD players use a near-infrared 780nm laser. The visible light spectrum is generally considered to be 400nm to 700nm; few people can see light past 720nm. (DVD, by contrast, uses a visible red 635nm or 650nm laser.) The drive shines a laser through the polycarbonate (plastic) on the “bottom” of the disc. This bounces off the reflective layer, passes back through the polycarbonate, and is read by a photosensor in the drive head. The index of refraction for polycarbonate is about 1.55, so laser light bends when it enters, allowing a much finer focus for the laser (from 800um at the bottom of the polycarbonate down to about 1.7um at the metal surface). This minimizes the effects of dust and scratches, because the effects of any surface gunk are reduced as the laser’s focus width is reduced. A 400um-wide piece of dust on the surface of a CD would completely block a laser focused down to 200um at the surface, but has little effect on a CD player.
CD players use a near-infrared 780nm laser. The visible light spectrum is generally considered to be 400nm to 700nm; few people can see light past 720nm. (DVD, by contrast, uses a visible red 635nm or 650nm laser.) The drive shines a laser through the polycarbonate (plastic) on the “bottom” of the disc. This bounces off the reflective layer, passes back through the polycarbonate, and is read by a photosensor in the drive head. The index of refraction for polycarbonate is about 1.55, so laser light bends when it enters, allowing a much finer focus for the laser (from 800um at the bottom of the polycarbonate down to about 1.7um at the metal surface). This minimizes the effects of dust and scratches, because the effects of any surface gunk are reduced as the laser’s focus width is reduced. A 400um-wide piece of dust on the surface of a CD would completely block a laser focused down to 200um at the surface, but has little effect on a CD player. If the photosensor sees a strong beam — the
(2002/12/10) CD players use a near-infrared 780nm laser. The visible light spectrum is generally considered to be 400nm to 700nm; few people can see light past 720nm. (DVD, by contrast, uses a visible red 635nm or 650nm laser.) The drive shines a laser through the polycarbonate (plastic) on the “bottom” of the disc. This bounces off the reflective layer, passes back through the polycarbonate, and is read by a photosensor in the drive head. The index of refraction for polycarbonate is about 1.55, so laser light bends when it enters, allowing a much finer focus for the laser (from 800um at the bottom of the polycarbonate down to about 1.7um at the metal surface). This minimizes the effects of dust and scratches, because the effects of any surface gunk are reduced as the laser’s focus width is reduced. A 400um-wide piece of dust on the surface of a CD would completely block a laser focused down to 200um at the surface, but has little effect on a CD player. If the photosensor sees a strong
