CD-ROM discs, DVD discs, and Blu-ray Discs look the same to the naked eye, but are drastically different when you compare the discs at the sub-microscopic level and explore the laser technologies that read their optical information. When it comes to Optical Storage, there are 3 general types:
CD-ROMs (read-only-memory) are commercial grade discs and use a metallic data layer. This layer is created using a molding machine that stamps pits (depressions) and lands (flat surfaces) into a polycarbonate substrate base. The metal layer is then applied to the base, creating the data layer. CDs are generally used for data and file storage.
DVD-ROMs (read-only-memory) are commercial grade discs and also use a metallic data layer. This layer is created using a molding machine that stamps smaller pits (depressions) and lands (flat surfaces) than CD-ROMs. DVDs are of the same dimensions as compact discs (CDs), but store more than six times as much data. DVDs are mainly used for video and data storage.
Blu-ray Disc (BD), sometimes called “Blu-ray,” is an optical disc storage medium designed to supersede the standard DVD format. Its main uses are for storing high-definition video, PlayStation 3 video games, and other data, with up to 25 GB per single layered, and 50 GB per dual layered disc. The disc has the same physical dimensions as standard DVDs and CDs.
Light Waves Light is made up of a stream of particles called photons. However, light also behaves like a wave.
The wavelength of light is the distance from the crest or trough of one wave to the crest or trough of the next wave.
Lasers A laser is a device that produces a powerful beam of light. Laser light is different than white light. White light is made up of many different wavelengths. Laser light is made up of only one wavelength. As a result, laser light is only one color. Unlike the light waves in white light that spread out, the waves in laser light are all in step .
This chart shows that the range of wavelengths for each color in the visible spectrum is different. Another thing to note is that the wavelengths for laser light are in Nanometers.
Nanometers are a billionth of a meter. Optical Media is read by lasers being reflected off the pits or bumps of the Disc, creating a digital signal. A wonderful resource for kids talking about Nanotechnology is the Nanooze! page.
When the reflected signal laser bends away from the sensor the digital signal is OFF (0).
Optical Media is read by lasers being reflected off the pits or bumps of the Disc, creating a digital signal.
When the reflected signal hits the optical pickup (sensor), the digital signal is ON (1).
Each wavelength offers one piece of data to a reflected light in an on/off binary 1/0 type of data.
Red has the widest range and the longest wavelengths. The laser diodes used in CD players and CD-R/-RW burners are infrared lasers with a wavelength of 780 nm.
A different red laser diode produces the beam needed for the smaller pit sizes of DVDs. In order to decrease pit sizes further to pack more information on a disc, engineers have to move beyond red lasers into the Blue-violet laser range for Blu-ray. This is why Blu-rays cannot be viewed with the same technology as CDs and regular DVDs.
Each wavelength offers one piece of data to a reflected light in an on/off binary 1/0 type of data. So the smaller the pit sizes means an increased frequency or an increased amount of pieces of information available on a disc.
Optical Media use the information from lasers reflected in the pits in order to retrieve data. The incredibly small dimensions of the pits make the spiral track on Optical media extremely long. If you could lift the data track off a CD and stretch it out into a straight line, it would be 0.5 microns wide and almost 3.5 miles (5 km) long!
The fundamental job of the Optical Media player is to focus the laser on the track of pits (or bumps since it’s coming from the opposite side). The pits/bumps reflect light differently than the “lands” (the rest of the aluminum layer), and the opto-electronic sensor detects that change in reflectivity. The electronics in the drive interpret the changes in reflectivity in order to read the bits that make up the bytes.
The smaller the distance between tracks or the pits, the smaller the wavelength of light required. Likewise the smaller the pit, the smaller the wavelength of light required.
The combination of pit sizes, distance between tracks, and wavelength of lasers is why Blu-rays cannot be viewed with the same technology as CDs and regular DVDs.