According to Abbreviationfinder, CD stands for compact disc. Optical discs have a protected internal layer, where the bits are stored by means of different technologies, and in all of them these bits are read thanks to an incident laser beam. This, when reflected, allows to detect microscopic variations of optical-reflective properties that occurred as a consequence of the recording made in writing. An optical system with lenses directs the light beam, and focuses it as a point on the disk layer that stores the data.
Engraving during manufacture
A CD can be burned by molding during manufacture. Using a nickel mold (CD-ROM), once a multimedia application has been created on a computer’s hard drive, it must be transferred to a medium that allows copies to be made for distribution.
CD-ROM applications are distributed on 12 cm diameter compact discs, with the information recorded on one side. The manufacture of these discs requires having a “clean” room, free of dust particles, in which the following processes are carried out. A layer of high resolution photosensitive material, of the type used in the manufacture of microchips, is applied to a finely polished optical grade disc. On this layer it is possible to engrave the information thanks to a laser beam.
Once the transcription of all the information to the disk is finished, the data it contains is in a latent state. The process is very similar to developing a photograph. Depending on the areas that the laser has accessed, the layer of photosensitive material hardens or becomes soluble when certain baths are applied.
Once the different baths have been completed, a first copy of the disc is available that will allow the others to be printed. However, the film that contains the information and is attached to the glass plate is soft and fragile, which is why it is essential to protect it by means of a thin metallic coating, which gives it both hardness and protection.
Finally, thanks to a combination of optical and electrochemical processes, it is possible to deposit a nickel layer that penetrates into the gaps and adheres to the magnetic film applied first on the glass layer. In this way, a matrix or “master” disk is obtained, which allows thousands of copies of the CD-ROM to be printed on plastic afterwards. Once these copies have been obtained, it is possible to screen print on the ultraviolet filtering lacquer layer of the discs images and information, in one or more colors, that allow it to be identified. All this, logically, on the side that does not contain the information.
The manufacture of CD-ROMs for a multimedia application concludes with the packaging of the discs, which is necessary to protect them from possible damage. A booklet containing information on the use of the application is added to the box.
Finally, the cellophane wrapping guarantees the user that the copy they receive is original. These manufacturing processes currently allow production rates of up to 600 units per hour on a single machine.
Steps followed by the head to read a CD
- A coherent beam of light (laser) is emitted by an infrared diode towards a mirror that is part of the reading head, which moves linearly along the surface of the disk.
- Light reflected from the mirror passes through a lens and is focused on a point on the surface of the CD.
- This incident light reflects off the aluminum layer, passing through the polycarbonate coating. The height of the projections (pits) is the same in all and is selected very carefully, so that it is just the wavelength of the laser in the polycarbonate. The idea here is that the light that reaches the plain (land) travels 1/4 + 1/4 = 1/2 of the wavelength (in the figure it is seen that the wave that goes to the area without overhang half a period ago, bounces and does another half period, which returns a wave half period ½ out of phase when it goes to the height of the ledge), while when the light bounces off a ledge, the signal bounces with the same phase and period but in the opposite direction. This makes a property of optical-physics to be fulfilled that says a signal that has a certain frequency can be canceled by another signal with the same frequency, and same phase but in the opposite direction, for that reason the light does not reach the photoreceptor, it destroys itself. The value 0 is given to any succession of projections (when the light does not reach the photoreceptor) or non-projections (when the light arrives out of phase ½ period, which has crossed almost without problems the light beam going in the other direction, and has reaching the photoreceptor), and we give the value 1 to the change between salient and non-salient, thus having a binary representation. (Change from light to no light at photoreceptor 1, and continuous light or non-continuous light 0). which has crossed the beam of light that goes in the other direction almost without problems, and has reached the photoreceptor), and we give the value 1 to the change between salient and non-salient, thus having a binary representation. (Change from light to no light at photoreceptor 1, and continuous light or non-continuous light 0). which has crossed the beam of light that goes in the other direction almost without problems, and has reached the photoreceptor), and we give the value 1 to the change between salient and non-salient, thus having a binary representation. (Change from light to no light at photoreceptor 1, and continuous light or non-continuous light 0).
- The reflected light is routed through a series of lenses and mirrors to a photodetector that collects the amount of reflected light.
- The light energy of the photodetector is converted into electrical energy and by means of a simple threshold the detector will decide whether the point indicated by the pointer corresponds to a protrusion (pit) or a flat (land).