Do you want to put your DVDs in your refrigerator or your hot water heater? Although it sounds silly, you could do so without damaging the disks. DVDs have an unusually wide operating temperature range (25°C to 70°C). They are made from transparent materials, such as acrylics, polyolefins, and polycarbonates. The 1.2-mm thick disk comprises two substrates bonded together. Each 0.6-mm substrate can contain one or two layers of data. The layer closest to the side from which it's read is called layer 0 (see the figure). The other layer is called layer 1. These layers are spaced quite closely together (55 µm), which is about 1/20th of the disk's overall thickness.
The optical pickup mounts the laser (or lasers) and associated electronic circuitry. The pickup is mounted on an arm that dynamically positions it under the disk moving in and out laterally, as required, so as to read different tracks. The pickups employ semiconductor red lasers at 635 or 650 nm. Because the data is stored at various depths, techniques have been devised to control the focus depth.